PB96-964401
                                EPA/ROD/R08-96/110
                                March 1996
EPA  Superfimd
       Record of Decision:
       Silver Bow/Butte Creek (Rocker Timber
       Framing & Treating Plant O.U.), MT
       12/22/1995

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                         RECORD OF DECISION
      ROCKER TIMBER FRAMING AND  TREATING  PLANT OPERABLE UNIT
     SILVER BOW CREEK/BDTTE AREA,  (Original  Portion)  NPL SITE

INTRODUCTION
The U.S. Environmental Protection Agency (EPA) and the Montana
Department of Environmental Quality (MDEQ)  present the Record of
Decision (ROD) for the Rocker Timber Framing and Treating Plant
Operable Unit  (Rocker OU) of the Silver Bow Creek/Butte Area
(original portion) National Priorities List (NPL) site.  The ROD
is based on the Administrative Record for the site, the Remedial
Investigation/Feasibility Study (RI/FS),  the Baseline Human
Health Evaluation, the Proposed Plan,  the public comments
received (including those from the potentially responsible
parties (PRPs) and local government),  and responses of EPA and
MDEQ to these comments.  The ROD presents a brief outline of the
RI/FS for the Rocker OU, actual and potential risks to human
health and the environment presented at the Rocker OU, and the
selected remedy for the Rocker OU.  EPA guidance was used in
preparation of the ROD.  The ROD has the following three
purposes:

     1.   To certify that the remedy selection process was
          carried out in accordance with the requirements of the
          Comprehensive Environmental, Response, Compensation and
          Liability Act  (CERCLA),  as amended by the Superfund
          Amendments and Reauthorization Act  (SARA),   (42 U.S.C.
          §§ 9601 et seq.), and to the extent practicable, the
          National Contingency Plan (NCP);

     2.   To outline the engineering components and remediation
          goals of the selected remedy;  and

     3.   To provide the public with a consolidated source of
          information about the site history,  characteristics,
          and risk posed by the conditions at the Rocker OU, as
          well as a summary of the cleanup alternatives
          considered, their evaluation,  and the rationale behind
          the selected remedy.

The ROD is organized into three distinct sections:

     1.   The Declaration functions as an abstract for the key
          information contained in the ROD and is the section of
          the ROD signed by the EPA Assistant Regional
          Administrator and the MDEQ Director;

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2.   The Decision Summary provides an overview of the site
     characteristics,  the alternatives evaluated, and the
     analyses of those options.  The Decision Summary also
     identifies the selected remedy and explains how the
     remedy fulfills statutory requirements.  The Decision
     Summary includes, as an Appendix, the final applicable
     or relevant and appropriate requirements (ARARs) for
     the site and waivers of any of these ARARs; and

3.   The Responsiveness Summary addresses public comments
     received on the Proposed Plan, the RI/FS,  and other
     information in the Administrative Record,  which were
     not responded to previously.

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DECLARATION

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          DECLARATION FOR THE RECORD OF DECISION

SITE NAME AND LOCATION
Silver Bow Creek/Butte Area (Original Portion)  NPL Site
Rocker Timber Framing and Treating Plant Operable Unit
Rocker, Montana

STATEMENT OF BASIS AND PURPOSE

This decision document presents the selected remedy for the
Rocker Timber Framing and Treating Plant Operable Unit of the
Silver Bow Creek/Butte Area site near Rocker,  Montana (the Rocker
OU).  The selected remedial action was chosen by EPA,  with the
concurrence of the Montana Department of Environmental Quality
("MDEQ" or "State"), in accordance with the requirements  of
CERCLA, as amended, and the NCP.  This decision is based  on the
Administrative Record for the site.  The State of Montana has
played a significant role during the remedy selection process for
this site and concurs with EPA on the selected alternative as
indicated by concurrence on the ROD.

ASSESSMENT OF THE SITE

Actual or threatened releases of hazardous substances at  and from
the Rocker OU, if not addressed by implementing the response
action selected in this ROD,  may present an imminent and
substantial endangerment to public health, welfare, or the
environment.

ROLE OF THE OU WITHIN THE SITE STRATEGY;

The Rocker OU is one operable unit in the Silver Bow Creek/Butte
Area (original portion) NPL Site.  The Rocker OU addresses the
geographic area surrounding and contamination associated  with the
former Rocker Timber Framing and Treating Plant near Rocker,
Montana.  This remedy decision presents the final decision for
the Rocker OU.  There is some overlap between this operable unit
and the Streamside Tailings operable unit where rail lines extend
through the Rocker OU,  where stream deposited wastes occur in a
depression between the rail lines in an area historically used to
store logs for the framing mill (Rocker stull storage area),  and
in the current 100 year flood plain of Silver Bow Creek,  which
flows through the Rocker OU area.  Both cleanups will be
coordinated to avoid duplication of effort.

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DESCRIPTION OF THE SELECTED
COMPOt
The remedy selected by EPA, with the concurrence of the State,
addresses surface soil, alluvium and fill, and groundwater
contaminated by wood treating compounds and mining waste in the
Rocker OU.  The EPA has selected the final remedy for the Rocker
(OU) after considering all written comments and oral testimony
received during the public comment period.  The remedy has been
modified from the proposed plan in response to public comment.
The changes that have been made in the remedy from the proposed
plan are considered significant; but are considered a logical
outgrowth of the public comments received.

The primary purpose of the remedy is to protect human health from
threats posed by direct contact with contaminated surface soils
or exposure to contaminated groundwater.  With respect to
contaminated groundwater, the primary objective is to prevent
contamination of. groundwater resources  (deep alluvium and
tertiary groundwater systems) under current use (or that have the
potential to be developed) by the community that are in hydraulic
connection with the Rocker OU arsenic plume.  An extension of
this objective is to make the groundwater resource available to
the community at the earliest opportunity.  The secondary
objective of the groundwater remedy is to reduce contaminant
concentrations in the arsenic plume and the shallow alluvial
aquifer to regulatory standards.

The Rocker remedy includes contingency measures to address the
arsenic contamination of the shallow alluvial aquifer where
remediation goals involve moderate uncertainty and may at a
future date dictate an ARARs waiver and/or establishing
containment goals.  EPA fully expects to meet the primary
groundwater remediation goal stated above.  Achieving this goal
is consistent with a major EPA theme of "pollution prevention".

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              (with, continc
The remedy selected utilizes treatment of the arsenic-laden
source materials that contribute to groundwater contamination and
surface soil hot spots to the maximum extent practicable, in
order to reduce mobility of the arsenic, in combination with
standard excavation, treatment, and on-site disposal
technologies.  Limited circumstances may occur where
solidification with cement may also be required to reduce arsenic
mobility to below Agency "characteristic" levels for hazardous
wastes prior to disposal.  The groundwater remedy includes
utilization of natural and enhanced arsenic attenuation
processes, and contingent hydraulic controls to contain and treat
any (unexpected) groundwater migration off-site.  Also, a portion
of the groundwater remedy includes a temporary well ban to
prevent development of the nearby groundwater and an alternative
water supply for the residents of the community of Rocker to use
while the well ban is in effect.  The remedy requires
institutional controls to limit future land uses (to prevent
residential land use).  Monitoring of the vegetative cover and
monitoring of groundwater is also required (to document trends in
water quality and determine if contingent remedies might be
needed and to assure protection of domestic water supplies).  The
estimated cost for this remedy is $5,400,000 (compared to
$7,340,000 for the remedy proposed in the proposed plan).  Each
component of the remedy is described in more detail below.
However, for a complete understanding of the remedy, please refer
to the subsequent Decision Summary part of the ROD.

o    Groundwater Source Material Removal and Treatment of Shallow
     Groundwater

     Arsenic groundwater "source material" is defined as soils
     and other substrate materials that previously have been
     contaminated with concentrated wood treating solutions and
     other arsenic waste,  and which continue to act as a source
     to ongoing groundwater contamination.  The area containing
     "source material" was  preliminarily defined in the
     feasibility study to be within the 10,000 parts per billion
     arsenic groundwater plume/ five feet deep into the saturated
     zone.  The selected remedy for "source materials"
     (approximately 41,000 cubic yards) is excavation,  subsequent
     chemical fixation utilizing complete mixing of iron sulfate,
     lime, and water with the arsenic contaminated media, and
     then backfilling the excavated area above the water table
     with this amended material to the extent practicable.
     Disposal of treated wastes will only occur in areas where
     iron has been added to the shallow groundwater beneath the
     waste repository as described below.  The addition of iron
     to adsorb and immobilize arsenic is considered by the Agency
     to be an innovative treatment technology.

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During remedial design, an on-site pilot-scale treatment,
disposal, and testing process will be implemented in order
to optimize amendment dose rates and confirm  (using EPA's
toxicity characteristic leaching procedure  (TCLP)) that
treated wastes will be below characteristic levels for
hazardous wastes.  Following iron treatment and lime
addition, limited volumes of highly concentrated wastes may
have arsenic concentrations higher than the 5 ppm specified
for "characteristic" hazardous wastes.  For these materials,
the remedy will include solidification, by cement addition,
prior to disposal on-site.  A testing program for the
duration of the remedy will be designed following the pilot-
scale testing.

The use of ferrous sulfate to fixate arsenic and render it
immobile is well documented in the literature and has been
validated in part by ARCO's test program at Montana Tech.
It should be emphasized that this process is consistent with
the administrative record developed for the Rocker OU and
the use of this treatment process is responsive to concerns
identified during the remedial investigation/feasibility
study and public comment period.

A better definition of the specific quantity and locations
of "source material" to be removed and treated will occur
after the Record of Decision, during the Remedial Design
phase of the Superfund process.  During these subsequent
sampling and analytical investigations, if  +3arsenic
"source material" is identified in addition to that defined
within the 10,000 ppb groundwater arsenic isopleth,  such as
at the old vat, other known treatment areas, and the off-
loading trench, this "source material" would also be
removed,  treated and disposed of in the OU repository.   If
additional "source material" repositories are required, in
excess of the volume available in the excavation/backfill
areas, an approved plan must be developed and implemented
consistent with the technologies developed in this ROD.

During the excavation of "source materials", care must be
taken to properly abandon any existing monitor wells that
would have to be removed as well as  minimizing the release
of pore waters from the saturated zone by utilizing proper
excavation equipment and associated removal techniques.   It
is very probable that the exposed groundwater in the
excavation would contain elevated arsenic concentrations.
Therefore, iron sulfate solution would be added to and mixed
with the groundwater and the pH would also be adjusted as
necessary.  The excavation would then be backfilled up to
the water table with washed gravel, properly compacted and
then covered with a filter blanket to maintain porosity.
This recently iron-enriched shallow groundwater can then

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move laterally and deeper into the lesser concentrated
portions of the plume, thus enhancing the rate of arsenic
attenuation in the plume.  However, it is recognized that
this process will be limited to the more permeable zones in
the aquifer and the effectiveness will diminish as iron
precipitates reduce aquifer permeability.  The area of
contamination is expected to continue shrinking as natural
attenuation continues and lower concentration groundwater
(from up gradient areas) continue to flow through the site.
Treatment of contaminated groundwater by such an in-situ
technology is considered an innovative technology by EPA,
and together with the innovative iron treatment of arsenic
wastes  (described above) is consistent with the statutory
preference for such remedies.

The excavated solids will have oversize materials removed
and disposed of at the local municipal landfill, consistent
with State and Federal solid waste disposal requirements.
The final site surface contours will be designed in such a
manner that 18-inches of additional non-contaminated cover
soil can be added to provide an adequate vegetative growth
zone and protective cover over the treated materials/hotspot
areas and promote proper surface drainage, and other ARARs
are met.  An adequate number of monitor wells would then be
completed into the permeable zone and into deeper portions
of the alluvial aquifer to permit ongoing groundwater
monitoring to document the trends in groundwater quality
improvement around the source removal area, within and
outside of the remaining arsenic plume.

Contaminated Surface and Near-Surface Soils

The surface and near surface soils outside of the source
area removal zone, to the site boundary, will be
systematically sampled and analyzed for arsenic
concentrations.   Sampling will not occur in areas being
remediated by the adjoining Streamside Tailings operable
unit.  The area utilized for the loading and off-loading of
the local recreational railroad will be included within the
area to be sampled and potentially remediated.  A soil
arsenic concentration of 380 parts per million (ppm)
corresponds to a one in 10,000 excess cancer risk for
trespassers,  recreationists or workers that frequent the OU
and who may be exposed via the direct contact pathway.
Soils greater than this concentration pose a risk exceeding
the EPA acceptable risk range. Those areas found to be
greater than 380 ppm arsenic but less than 1000 ppm will be
covered directly with 18-inches of growth media and
revegetated.

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Surface areas found in excess of 1000 ppm arsenic  (hot
spots) shall be excavated to a maximum depth of 18-inches.
The excavated highly contaminated soil will be treated in a
manner identical to the source "materials" utilizing iron
sulfate and lime (described previously).  Limited
circumstances may occur where iron-treated materials, when
tested using EPA's toxicity characteristic leaching
procedure  (TCLP), will exceed concentrations that would
classify the materials as a "characteristic" hazardous
waste.  A contingent  solidification  (by concrete addition)
treatment procedure is provided for in the remedy to address
this limited potential circumstance.  The resulting treated
wastes will then be disposed of on-site in an on-site
repository above the water table where groundwater has been
treated with iron below.  Excavated/covered areas will be
revegetated with appropriate species of draught resistant
grasses that are self-reproducing and that are consistent
with the remedial objectives of this ROD (minimizing surface
erosion and utilization of soil moisture).  The final site
contours must be compatible with the ongoing use of the
railroad corridor,  and promote good surface water run-on/off
control.

The excavation, treatment and on-site disposal of high
concentration soils and groundwater "source materials" will
prevent uncontrolled contaminant releases via surface and
groundwater pathways and will prevent direct contact with
this highly toxic chemical.  These aspects of the remedy are
consistent with the Streamside Tailings OU remedy.
Coordination between operable units will continue, which is
important with respect to excavation and disposal of wastes
from both operable units.

Institutional controls and monitoring will maintain the soil
cover and vegetative communities, and limit land uses that
would jeopardize the integrity of the cover.  Institutional
controls will also designate the area for continued
railroad/industrial use and specifically exclude residential
development as a future use (consistent with County planning
documents).

Well Ban and Alternative Water Supply

Given the hydraulic connections between the shallow and
deeper alluvium and the tertiary aquifer, EPA believes that
it is necessary to restrict shallow and deep groundwater
development in order to prevent the spread of the existing
arsenic plume into aquifers currently used at or near the
OU.  Therefore, during the term of the Rocker remedy, a
groundwater well ban will be implemented for new wells
within a one-quarter mile radius of the site in any of the

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designated three aquifer units.  The well ban will be
removed once sufficient evidence from the post monitoring
efforts determines that the arsenic plume has been
controlled sufficiently to abate the threat of further
migration.

To further reduce the possibility of ground water use and
contamination spread and to provide residents of the
community of Rocker adequate water to meet demands during
the period of the well ban, an alternative water supply will
be provided.  Current users of groundwater can continue to
utilize this resource.  Routine monitoring of the quality of
domestic/commercial groundwater supplies within the area of
the well ban shall be conducted.  The alternate water supply
and well ban together contribute to the Agency's objective
of preventing pollution of important water supplies
connected to the current area of contamination.

Contingent Remedy

In the unlikely event that plume migration occurs (laterally
or vertically),  additional hydraulic controls may be
implemented to contain the plume.  The contingent remedy
would be determined necessary if plume advancement is
detected in a lateral or vertical direction into surface or
ground water with arsenic concentrations below the 18 ppb
standard, that would result in long term arsenic
contamination that exceeds the State standard.

Groundwater Monitoring:

Water quality sampling and analysis for nearby existing well
users and for key monitoring wells developed for the Rocker
site will also continue on a seasonal/four times-per-year
frequency.  EPA, in consultation with the State, will make a
decision at the time of the 5 year review, or other
appropriate times, regarding: the need for contingent
remedies  (as described above),  or the removal of groundwater
restrictions, or other appropriate refinements to the
remedy.


Coordination With Streamside Tailings OU

For areas within the floodplain, the Rocker Remedy will be
coordinated with the Streamside Tailings OU proposed remedy
particularly with respect to waste repositories.
Contamination occurring along the railroad sidings within
the Rocker OU will be remediated to arsenic and metals
concentrations consistent with the recreational land use
projected as part of the Streamside Tailings OU remedy.

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This innovative remedial action breaks the surface, direct-
contact pathway for recreationists, trespassers, or workers that
may frequent the site.  It will also free up the site for future
use as an industrial site.   In addition, the remedy assures that
the primary groundwater remedial action objective of protection
of the quality and continued use of the tertiary aquifer, the
regionally preferred groundwater source, is achieved.

STATUTORY DETERMINATIONS

The selected alternative is  protective of human health and the
environment as described above.  The selected alternative will
comply with or achieve all Federal and State requirements  (i.e.,
ARARs), except where a waiver of such requirements has been
determined to be appropriate (see Appendix 1), and is cost-
effective.

The selected remedy uses permanent solutions to the maximum
extent practicable for this  site and utilizes the development of
alternate treatment technologies to the maximum extent
practicable.  It also satisfies the statutory preference for
remedies that employ treatment that reduces the toxicity,
mobility, or volume of contamination through treatment and that
use alternative treatment technologies.

Since hazardous substances above health-based levels will remain
onsite, a review will be conducted within five years after
commencement of remedial action to ensure that the remedy
continues to provide adequate protection of human health and the
environment.
William Yellowtail     °                               Date
Regional Administrator
United States Environmental Protection Agency
Region VIII
In concurrence:
        Wx/
Mark /£imonich                                     Date
Director
Montana Department of Environmental Quality
                                8

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DECISION SUMMARY

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                        TABLE OF CONTENTS


                                                             PAGE

1.   SITE NAME, LOCATION, AND DESCRIPTION	1

2.   OPERABLE UNIT HISTORY AND ENFORCEMENT ACTIONS   ...... 5

3.   HIGHLIGHTS OF COMMUNITY PARTICIPATION   	 6

4.   SCOPE AND ROLE OF THIS OPERABLE UNIT WITHIN SITE STRATEGY  7

5.   SUMMARY OF SITE CHARACTERISTICS  	 8

6    SUMMARY OF SITE RISKS	25

7.   REMEDIAL ACTION OBJECTIVES	29

8.   DESCRIPTION OF ALTERNATIVES  	  32

9.   SUMMARY OF COMPARATIVE ANALYSIS OF ALTERNATIVES  ....  36

10.  THE SELECTED REMEDY	46

11.  PERFORMANCE STANDARDS  	  53

12.  STATUTORY DETERMINATIONS	55

13.  DOCUMENTATION OF SIGNIFICANT CHANGES	59


REFERENCES	62

APPENDIX 1     ARARs FOR THE Rocker Timber Framing and Treating
               Plant OU

APPENDIX 2     RESPONSIVENESS SUMMARY FOR THE Rocker Timber
               Framing and Treating Plant OU


FIGURES:

1.1  Site Location Map	2

1.2  Rocker,  Montana Floodplain Derived by GIS 	  3

5.1  Total PAH Concentrations in Surface Soils 	 11

5.2  PAH Concentrations in Subsurface Soils  	 12

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                  TABLE OF CONTENTS  (Continued)


FIGURES:  (continued)                                         PAGE


5.3  Total PAH, 1992 Concentrations in Shallow Alluvial
       Monitoring Wells	14

5.4  Arsenic Concentrations in Surface Soils 	 15

5.5  Arsenic Concentrations in Subsurface Soils Above the
       Water Table	17

5.6  Arsenic Concentrations in Subsurface Soils at or
       Below the Water Table	 18

5.7  Cross Section Location Map for Arsenic - 1992 in
       Shallow Alluvial Monitoring Wells 	 20

5.8  Arsenic (ug/1) - 1992 in Deep Alluvial Monitoring Wells . 21

5.9  Arsenic (ug/1) - 1992 in Tertiary Sediments
       Monitoring Wells	22

5.10 Cross-Sect ions of Arsenic Plume (1992)	23

6.1  Conceptual Model for Potential Exposures  	27a


TABLES:

5.1  Summary of Previous Investigations Performed at the
       Rocker Site	9

6.1  Summary of Risk Estimates for surface Soil	27

6.2  Summary of Risk Estimates for Groundwater	27
                               111

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                      DECISION SUMMARY

1.  SITE NAME, LOCATION, AND DESCRIPTION

     Silver Bow Creek/Butte Area (original portion) NPL Site
     Rocker Timber Framing and Treating Plant Operable Unit -
     Butte, Montana

The Rocker Timber Framing and Treating Plant Final Remedial
Action operable unit (Rocker OU) is part of the Silver Bow
Creek/Butte Area (original portion) NPL site.  The Rocker OU is
located approximately 7 miles west of the community of Butte,
Montana (Silver Bow County) and adjacent to the community of
Rocker, Montana.  The OU consists of an area previously used for
the treatment, storage, and shipping of mine timbers using
creosote and arsenic (see Figure 1.1), involving both surface
soil and groundwater contamination.  The waste in the Rocker OU
also contains mine waste from upstream sources.

The general boundaries of the OU are a stream diversion on the
east end of the Rocker OU, and Silver Bow Creek to the north
(including a small storage area north of the creek).  The western
boundary includes the western limit of the rail siding used for
this OU and the full extent of groundwater contamination down
gradient from the Rocker Timber Framing and Treating Plant.
The boundary to the south is the rail line and a small off-
loading area.  The Rocker OU covers approximately 16 surface
acres.  This area contains contaminants from the former treating
operation mixed with mining waste transported down Silver Bow
Creek and/or that was brought in .for fill on the site.  This OU
does not include treating plant contaminants which may have
migrated into Silver Bow Creek.  Stream monitoring during the
Rocker remedial investigation did not demonstrate that there is
ongoing contaminant migration from the OU to Silver Bow Creek.

The topography of the site is variable as a result of extensive
fill that has been brought in to compliment the industrial
development of the site.  Prior to development, Silver Bow Creek
probably traversed the site just south of the creek's present
location,  with gently sloping stream terraces on either side.
Fill for railroad corridors now form the southern boundary, while
the eastern boundary is located along a historic stream
diversion.  In addition, the area where wood treating processes
occurred was filled approximately 15 to 18 feet deep, probably
with waste rock and cinders from the nearby mining operations.  A
small poorly drained depression in the east central portion of
the site (stull storage area)  probably is representative of the
original land surface in this area (Figure 1.2).

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                                                                 nt. Highway  15/90

                                                                 Caging Station
  Rocker Site
SOURCE: USGS 7.5-minute topographic quadrangle maps.
North Uulla and South Bulle (Montana-Silver Bow Co.) revised 1987
FIGURE 1.1
SITE LOCATION MAP
ROCKER TIMBER FRAMING AND TREATING

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      Rocker,   Montana
 Floodplain  derived  by  GIS
                   100-year
                   Floodplain

                   Woods
                  Building


                  Boundary of Rocker
                  Superfund  Operable Unil

                  Floodplain survey transecl

                  Paved road

                  Gravel  road

                  Railroad

                  Bridge

                  Fence

                  Wall

                  Guardrail
                 Figure  1.2
Base map digitized from aerial photographs of 1991  by
Horizons, Inc., of Rapid City.  The floodplain shown on
this map is the area where a surface model derived  from
(he 2-foot contours shown on this map is  lower than a
surface model derived from the 100-year flood elevations at
survey transects from a 1:9000 scale map  by CH2MHHI:
'Silver Bow Creek Flood Modeling Study.*

These data were obtained by NRIS from the sources named
above.  NRG does not guarantee the data  for functionality,
accuracy, or being free from errors.  The user assumes
responsibility to verify usability for theii purposes.


                  Scale of Feet

0         200        400        600        BOO
              Contour interval 2 feet
            Montana State Library

   I.NRIS  Natural Resource lnformalii>n Sy

            Map #96epa14  -  12/15/95

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Most of the site is currently used for a rail siding.  Rarus
Railroad has an active line and siding and two small storage
sheds at the western end of the OU.  The small community of
Fredericksburg lies to the south of the site, while the community
of Rocker is just north of Silver Bow Creek.  The eastern,
western and northern boundaries of the Rocker OU adjoin the
Streamside Tailings OU.

Natural resources associated with the Rocker OU identified to
date are described in the State of Montana's report on natural
resource damages entitled Rocker Groundwater Injury Assessment
Report.  Evaluation of natural resource issues by federal or
tribal authorities has not occurred.

Silver Bow Creek forms the predominant surface drainage feature.
Groundwater identified within the boundaries of the OU include
three distinct water bearing zones.  The shallow alluvium
(extending from ground surface to about 20 feet) has been
extensively contaminated from site activities.  This groundwater
zone has a low yield and water chemistry generally reflects the
sulfate type waters characteristic of Silver Bow Creek and Silver
Bow Creek alluvium.  This groundwater is intermittently recharged
from Silver Bow Creek; although at times discharge also is
released to surface drainage.  The deep alluvium (from about 20
feet below ground surface to the top of the Tertiary) has a
higher yield more suited to domestic groundwater development and
a very high quality bicarbonate type water chemistry.  This
groundwater zone is thought to be recharged from areas to the
south of the ou.  This part of the saturated zone has received
contamination down to about 40 feet in depth.  Both the shallow
and deep alluvium are primary porosity aquifers composed of
sorted silts, clays, sands and gravels.  The high yield tertiary
aquifer produces large (commercial) volumes of good quality water
(in excess of 100 gallons per minute)  from zones of
secondary/fracture permeability within a silty matrix (with
limited sand lenses).  The tertiary alluvium extends from 80 feet
below ground surface to as little as 13 feet on the western .end
of the OU where the tertiary alluvium ramps up.  This sedimentary
unit is known to be several hundred feet deep.  All of the
groundwater zones have been demonstrated to be in hydraulic
communication.  All of the groundwater zones are classified by
the State as potentially usable for domestic and commercial
consumption.  The deep alluvium and tertiary alluvium aquifers
are used currently for domestic and commercial use, and increased
usage is very likely as development occurs in the Rocker area.

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2.  OPERABLE UNIT HISTORY AND ENFORCEMENT ACTIONS

The Rocker Timber Framing and Treating Plant was constructed in
1909 and operated until the plant was closed in approximately
1957.  The Anaconda Company, predecessor in interest to the
Atlantic Richfield Company  (ARCO), owned and operated the site.
Initially, the facility treated mining timbers with a creosote
solution.  Subsequently, the facility began using arsenic
trioxide solutions for treatment, and this formulation became the
primary treatment process up to the final days of plant
operation.  Rocker wood treating wastes are also mixed with
contaminated tailings and other mining waste washed downstream to
Rocker from mining/smelting facilities in Butte.

During the approximate 48 year history of plant operation,
spilled process materials (arsenic trioxide powder), treated wood
chip residues, and dripped or leaked process solutions  (creosote
and caustic heated arsenic brines) have resulted in contaminated
soils throughout the plant site and significant groundwater
contamination.

The Rocker Timber Framing and Treating Plant operable unit is
part of the original Silver Bow Creek Superfund Site, that was
listed on the National Priorities List (NPL) on September 8,
1983.

In 1989, the State of Montana directed ARCO to remove
contaminated soils and debris with concentrations exceeding
10,000 parts per million arsenic.  Approximately 1,000 cubic
yards of contaminated material were removed to a licensed
disposal facility.  Areas involved in the removal action were
subsequently covered with approximately one foot of "clean" fill
material from a nearby off-site area.  Nevertheless, materials
exceeding the 10,000 parts per million (ppm) concentration have
been identified at three locations remaining on the site.

Investigations that have been conducted of the Rocker OU include:
investigations of surface waters upstream and downstream of
Rocker conducted for ARCO in 1987 and 1988 by Hydrometrics Inc.
In 1989, a Phase II investigation report, also developed by
Hydrometrics, was submitted.  The State's Phase I Remedial
Investigation for Silver Bow Creek addressed the Rocker OU
preliminarily.  In 1989, data was also collected along Silver Bow
Creek, including limited information pertinent to the Rocker OU,
by CH2M Hill for the State of Montana.  Under the State's
unilateral order, ARCO performed soil sampling to delineate soils
exceeding 10,000 parts per million in 1989.  The remedial
investigation of the Rocker OU began in the Fall of 1991, under
an administrative order on consent (Docket No. CERCLA-VIII-91-14)
between EPA and ARCO (August, 1991, and amended June, 1993).  A
historical data report was compiled by ARCO in 1991, as required
by the administrative order.  During the period 1991 through 1994

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several detailed investigations of the Rocker site progressed
with ARCO providing information regarding the nature and extent
of contamination in soils and groundwater as necessary, regarding
the Rocker OU.  In May of 1994, the Montana Bureau of Mines and
Geology conducted an important pump test that evaluated the
connection between the Rocker site and an adjacent commercial
well.  The interpretive report for this effort was submitted to
EPA in August, 1994.  This investigation/report provided
information that significantly influenced EPA's interpretation of
the potential impact of the Rocker OU to domestic water supplies
in the area.  All of these studies are described and interpreted
in the Final Remedial Investigation, 1995.  The Rocker Final
Feasibility Study, July, 1995, considered 7 alternatives to abate
the contamination problems and human health threats documented in
the Baseline Human Health Evaluation, 1995.

EPA's Potentially Responsible Party Search identified ARCO as the
potentially responsible party for the Rocker OU.  An action to
recover EPA's past and future response costs for the Rocker OU
and other portions of the Silver Bow Creek/Butte Area  (original
portion) NPL Site is currently pending in Federal district court.

3. HIGHLIGHTS OF COMMUNITY PARTICIPATION

EPA has conducted the required community participation activities
through: several public meetings discussing the development of
the RI/FS and Proposed Plan, a 30-day public comment period, a
formal public hearing, and a meeting shortly after the close of
public comment to present EPA's revised position regarding the
Selected Remedy in this ROD.  Specifically included with this ROD
is a Responsiveness Summary that summarizes public comments and
EPA responses, in consultation with the State.  In addition, EPA
implemented its Community Relation Plan throughout the conduct of
the RI/FS.  This involved interviews with local residents,
community leaders, and periodic fact sheets describing the status
of the CERCLA activities at the site.  EPA will continue to
involve and inform the public as the Remedial Action is
implemented.

The Baseline Human Health Evaluation for the Rocker OU was
completed and released to the public in March, 1995.  A fact
sheet and a public meeting was held to explain this report.  The
RI for the Rocker OU was released to the public in April,  1995.
Several public meetings were held as the RI was being developed
to keep the community current with the preliminary findings of
the investigations.  The Final report was discussed at two public
meetings.  Prior to the completion of the Rocker FS, two public
meetings were held to discuss the screening of technologies and
the development of remedial alternatives for detailed analysis in
the FS.  Rocker OU FS was released on July 13, 1995
contemporaneously with EPA's proposed plan.  The notice of
availability of the RI/FS and the Proposed Plan was published in

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the Butte newspaper, The Montana Standard, on July 13, 1995.  The
Proposed Plan was mailed to all individuals on the Rocker mail
route and to individuals and organizations who had previously
expressed an interest in the OU.  The formal public comment
period was open for 30 days and closed August 11, 1995.  During
the public comment period, two public meetings were held to
explain the RI/FS and proposed plan.  The public hearing was held
August 9, 1995 and a transcript of public comments was recorded.
Comments were also received in written form subsequent to EPA's
public hearing.  All comments have been recorded and specific
responses from EPA are provided in Appendix 2 of this record of
decision.

4. SCOPE AND ROLE OF THIS OPERABLE UNIT WITHIN SITE STRATEGY

EPA has identified ten OUs within the Silver Bow Creek/Butte Area
site.  These are: Rocker Timber Framing and Treating Plant,
Priority Soils, Non-Priority Soils, Active Mining Area, Mine
Flooding, Warm Springs Ponds Active Area, Warm Springs Inactive
Area, Mill-Willow Bypass ERA, Warm Springs Ponds Final Decision,
and Streamside Tailings.  EPA is the lead agency for remedial
activities at the Rocker OU and other OUs, and the State of
Montana is the lead agency for the Streamside Tailings OU.

The Streamside Tailings OU has a close relationship to the Rocker
OU because it includes an evaluation of the risk of mine wastes
within Silver Bow Creek, its associated flood plain deposits, and
railroad grade materials and potential spillage that extend
through the Rocker OU.  Care was taken during the planning stages
of these two projects to have the investigations compliment each
other both in terms of the remedial investigation and the risk
assessment.  No effect of the Rocker OU on Silver Bow Creek has
been documented.  The remedies for each of these OUs are being
determined contemporaneously and will continue to be coordinated.
Coordination between these two projects is particularly important
during remedial design with respect to waste disposal.

Remediation in the Rocker OU is considered a priority by EPA
because of potential risks to human health which would be caused
by the release of the contaminated waters to aquifers under
current use by residents and potential risks from direct contact
exposure to workers and trespassers to contaminated soils.
Remedial actions undertaken in the Rocker OU will address these
threats, and all other threats identified in the Baseline Risk
Assessment.

The remedy presented in this ROD represents the final remedial
action for the Rocker Timber Framing and Treating Plant OU.

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5.  SUMMARY OF SITE CHARACTERISTICS

REMEDIAL INVESTIGATION
The Remedial Investigation  (RI) for the Rocker OU was conducted
by the responsible party, ARCO, with EPA and State oversight.
The RI was conducted in accordance with CERCLA, the NCP, and the
EPA guidelines and in compliance with the Administrative Order on
Consent  (Docket No. CERCLA-VIII-91-14).  The Order became
effective on August 2, 1991 and field investigations continued
through 1995.

The purpose of the RI was to summarize site and background data
collection activities and results and to collect additional data
where necessary in order to satisfy the NCP requirements for an
RI (40 CFR Section 300.430(d)), in order to adequately
characterize the site for the purpose of developing and
evaluating effective remedial alternatives.

Previous Studies
The Rocker Timber Framing and Treating Plant was part of the
original Silver Bow Creek Superfund site, which was listed on the
National Priorities List (NPL) on September 8, 1983.  This NPL
listing was amended on July 22, 1987 to include large areas
around the city of Butte, Montana.  As part of the original site
listing and investigation,  preliminary investigations were
conducted that included the Rocker OU.  These prior studies, and
other data collection activities described earlier (Table 5.1)
were compiled by ARCO in order to determine if sufficient data
were available to characterize the nature and extent of
contamination at the Rocker OU.  This information was submitted
to EPA in a Historical Data Assessment Report.

Field investigations conducted during each field season from 1991
through 1995 encompassed all suspected sources of contamination
including: stream sediment,  surface and subsurface soil, surface
water, groundwater, potential underground storage tank, and a
vadose zone study.  This record of decision is supported by all
of the investigations that were conducted on the Rocker OU and
that were summarized and interpreted in the Final Remedial
Investigation and Feasibility Study; however,  only the more
salient aspects of these investigations are summarized here.

Both of the wood treating compounds used, creosote and arsenic,
were detected in the soils and groundwater at the Rocker site.
The extent of environmental contamination from these original
compounds is traced by their chemical and physical
characteristics and the history of use and handling at the Rocker
facility.

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      Table 5.1 Summary of Previous Investigations Performed at the Rocker Site
Public health and
environmental assesement
data summary report,
Rocker and Ramsay areas
(CH2M Hill 1989)
Investigation of potential
resources contamination
near Rocker (Hyrometrics
1988, 1989)
 Rocker Timber Framing
 and Treating Site soil
 removal conducted in 1989
 (Keystone 1991)
 Historical data assessment
 report (PTI, 1991)
 Remedial investigations
 (Keystone 1992 and PTI
 1992)
Groundwater quality
monitoring and offsite soil
sampling
Surface water and
groundwater quality
monitoring and soil
profiles
Soil removal
 Historical data review
 Groundwater, surface
 water, and soils data
 collection
Collect supplemental data for
public health and
environmental assessment.

Evaluate metals concentrations
in surface soils in and adjacent
to populated areas in Rocker
and Ramsay

Evaluate Rocker site and
Gimlet as potential sources of
metals and organic
contamination to Silver Bow
Ck.  Provide an understanding
of the type and extent of soils
and tailings. Groundwater
quality monitoring.

Removal of —1000 yards of
arsenic contaminated soil and
wood chips.


Assess the quality of historic
data generated during earlier
investigations.

 Groundwater, surface water
 and soils quality assessment.

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Creosote is a mixture of polynuclear aromatic hydrocarbon  (PAH)
compounds derived principally from distillation of coal tars. The
density of the prepared creosote can range from slightly denser
than to lighter than water.  Because it has a high adsorption
coefficient, creosote adheres to soil particles and is not
exceptionally mobile in the environment.  It is also resistant to
biologic and chemical decay, and thus is persistent.  Gradually,
creosote will breakdown into individual PAH compounds which are
mobile in groundwater but which also become susceptible to
biologic and chemical decomposition.  The biologic decomposition
of the PAH compounds becomes important as it consumes oxygen with
the potential to enhance arsenic mobility in a reducing
groundwater environment.  At the Rocker site wood treating
creosote compounds were detected in only a limited number of
borings and shallow groundwater samples in the immediate vicinity
of the historical treatment area, attesting to either its limited
use or subsequent decomposition.  Creosote is an important
contaminant at the Rocker OU primarily because it can have an
effect on arsenic mobility.  Concentrations found on the OU,
however, have not been detected at concentrations exceeding EPA's
acceptable carcinogenic risk range, nor non-cancer hazard index.

PAH Surface Soil Contamination
Creosote (PAH) contamination is present in the surface and
subsurface in a small area extending from the historical creosote
treating area and extending northwest toward Silver Bow Creek
(Figure 5.1).  Elevated PAH's were also detected in samples taken
along the railroad right-of-way across the south side of the site
and in the holding pond area on the east side of the site.  The
PAH contamination from the railroad surface samples are
interpreted to be incidental contamination associated with the
railroad track ties and not directly attributed to wood treating
processes at the site.  The holding pond area PAH contamination
indicates either an incidental contamination from site wastes or
unrelated PAH's from another source such as the nearby rail line.
Total PAH's in surface soils range from non-detectable quantities
to over 10,000 ppb.  The highest values were found in the
northwest portion of the site and on the east side of the holding
pond.

PAH Subsurface Soil Contamination
PAH's were detected in subsurface soils in essentially the same
areas as the surface samples.  Higher concentrations, up to
62,430 ppb, were found immediately beneath the creosote plant and
wood treating area (Figure 5.2).  No samples with PAH's above
10,000 ppb were found in the subsurface at the holding pond or
along the railroad ballast. At one location north of the
treatment plant adjacent to Silver Bow Creek, total PAH's of
22,887 ppb were found in one sample from a depth of 14 to 16
feet. Odors and oil sheens were also noted in this boring
suggesting creosote disposal may have occurred in this area.


                                10

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    TOTAL P|
        H
ROCKB1TNBB1FRAIMQ AND TREATMQ
 PUMTOPSWU IHT. ROCKER UT

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                                                 <--.-_      /r,r3HIH*yii.   •    ;    '     if*B«  ^
                                                    "^^ c-B»m-   ^fc/;i;v6-rtwi«/      J&
                                            •:..:•••     / M*^^M^^-...yf
                                               :   '      .  —^/Hi^^fi-  '^M^_
                                            '   '                 ^5'-J*1»i.^*Mirth./^
O - Told PAH Cone. Mow Detection Limit
• - Told PAH Cone 0.1-cl.J < 1C
• - Told PAH Cone. > to1 and < 102 ug/kg
A - Toto) PAH Cone. > to2 and < 103 ug/kg
• - Totd PAH Cone. > 103 and < to4 ug/kg
 ~ Total PAH Cone. > to4 and < 10s ug/kg
       FIGURE 5.2
    PAHCONCamUTONS
    MSUBSUFACE3018
ROOra TUB) FMIM *» 7BEATH9
 FUllTOPBttBlflMT.BOCKHlMT
ARCO

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PAH Groundwater Contamination
PAH's were detected in 7 shallow alluvial aquifer wells extending
from the treatment plant to the northwest.  The highest total PAH
concentration was 822 ppb in the well immediately downgradient of
the historical creosote treatment plant area  (Figure 5.3).

Summary of PAH Contamination
PAH's associated with the wood treatment operations using
creosote at the Rocker OU are present in the soils, aquifer
matrix and in low concentrations in the shallow alluvial aquifer.
The extent of PAH contamination is small and indicating either
minimal use of creosote or post disposal degradation and
flushing.  PAH's contribute to the overall environmental
contamination at the Rocker OU but are not significant relative
to observed groundwater concentrations of arsenic.

Arsenic Contamination
Arsenic contamination of the soils and groundwater at the Rocker
site is the primary contaminant of concern.  Arsenic trioxide
used in the treatment process at the Rocker OU was obtained from
the Anaconda Smelter.  Since its solubility in water is low, the
arsenic trioxide was dissolved into a heated, and very high pH
(13.4)  solution of caustic soda and water.  The resultant
mixture, containing about 6% dissolved arsenic as arsenic (III),
was used to treat wood timbers in a retort.  Environmental
contamination at the Rocker"OU from the arsenical wood treating
compounds is significant as a result of incidental spills of
arsenic trioxide powder and of the saturated arsenic solution,
onsite disposal of debris from the retort, and treatment solution
that dripped or washed off the treated timbers while they dried
or awaited shipment.  Contamination has been found in the surface
soils and at depth as well as in the groundwater.  Arsenic and
metals contamination from mine waste is also present at various
locations at the Rocker OU.
Arsenic Contamination in Surface Soils
Most of the arsenic contaminated surface soils with arsenic
greater than 10,000 ppm were removed in the 1989 interim  action
taken at the Rocker OU (Figure 5.4).  Only one sample collected
from the vicinity of the loading trench on the west central side
of the site and a sample from the railroad right-of-way
immediately south of the treatment area contained an arsenic
concentration greater than 10,000 ppm.  Numerous soil samples,
approximately evenly distributed over the site north of the
central railroad tracks,  had arsenic concentrations between 1,000
and 10,000 ppm.
                                13

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                                                                                          -^ "**#»   7f-^"^>'-;:: i••"'"" %   **  \
                                                                     f^-*"——-_*.~"7"'~?—•	.." ""r~-~->^.    /  -L&zhcj'!&'*?--       t-i           ,   '

                                                                      •-- — ..^::::.2^
                                                                                                                                                                    Hfl
           FIGURE S.3
     TOTAL PAH «2 GOHCaitRATIONS
   M SHALLOW ALUWAL MOHTOWS WB1S
   ROCKffl TIBffl FRAUN3 AKD TflaTHQ
     PLANT OPSUHE UW. HOCKffl, MT
                KEV. 1 2\3B\«5	
 d - Fom>«r/Exlillng Structure   5.05 - Totd PAH Concentration (mgA) - 1992
« - Slt« Boundary           • - Shallow Alluvial Monitoring Well
                      	 - PAH Concentration Contour
             GRAPHIC SCALE
             o     so    100
                ( a far)
               I bob - 100 R.
           CONTOUR IN1ERVAL - 2 FEET
ARCOO

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~;^S>  -xm   -«m   328   •;*!   ;:.:3!   -.5^3   :v\£l   >C-S! .  vi,3   ::^m
                                               ;(l    •     ;^-iff  /          •*/"<•    a«
                                              .'.':/     t 4 .1  ;:'i /   /.        •      /'. ;     z  '•
                                              i: •     ' I  I I  rM  I „           / :XV  .•   '•

                                              !l  I      t  i  ii'fJ Ja&          '   '  f; / t



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Arsenic Contamination in Subsurface Soils  (above the water table)
Arsenic concentrations in the subsurface soils between the
surface and the water table have generally lower arsenic
concentrations except at one location  (Figure 5.5).  A single
boring at the loading trench detected arsenic of 7,524 ppm at a
depth of 8-10 feet.  Arsenic concentrations in all other samples
in this sample depth grouping were less than 1,000 ppm.

Arsenic Contamination in Alluvium  (at and below the water table)
At several locations in the historical treatment plant area and
extending to the northwest arsenic concentrations in the alluvium
at and below the water table have elevated arsenic concentrations
between 1,000 and 10,000 ppm and including one sample with a
concentration of 42,900 ppm located in the historical treatment
plant  (Figure 5.6).

Arsenic Contamination in Groundwater
Arsenic mobility in groundwater at the Rocker OU is highly
variable depending on geochemical conditions in the aquifer
(available iron and oxidation/reduction potentials) and on the
oxidation state of the arsenic (As +III is more mobile than As
+V).  In areas of high iron and high oxidation potential, arsenic
rapidly adsorbs onto the aquifer matrix with iron oxyhydroxides
and becomes essentially immobilized. Under reducing conditions or
in the absence of iron, arsenic is much more mobile.  Both
geochemical conditions are likely present at various locations at
the Rocker site probably associated with PAH contamination or
areas of natural organic deposits where surface soils are
saturated.  However, the shallow alluvial groundwater is
generally considered to be oxidizing.  Unfortunately, two
significant groundwater producing strata with good quality water
are in hydraulic communication with the arsenic plume and have
the potential to become contaminated.

Concentrations of arsenic exceeding 100 ppb are present in all
three hydrogeologic units identified at the Rocker Site; although
arsenic migration into the deep alluvium (20 to 80 feet below
ground surface)  and the tertiary alluvium has been very limited.
The shallow alluvial aquifer has the highest concentrations
covering the largest area.   (refer to arsenic isopleth maps and
x-section)

Several areas of arsenic contamination are present.  The smallest
areas include the holding pond area on the east side and area
southeast (upgradient)  of the treatment plant area.  The elevated
arsenic in these areas may be due to either incidental disposal
of arsenic enriched wood treating chemicals or the result of
leaching mine waste materials or both.  A third arsenic plume was
detected in well RH-5 on the west side of the site.
                                16

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                                                                         APFflOXNATE
                                                                         AREA OF FU
                                                                                        APPROXMATE   '
                                                                                        AREAOFFtt   1
0 - Aratnic Conegntratlon B«tow D«t«ctfon Umlt
• - Arwofe Conetntrotlon D«t«cttd < 101 rag/kg
B - ArMnlc Concentration > 101 and < 102 mg/kg
A - Arwnlc Concentration > I02 and < 103 mgAg
• - Areenlc Concentration > 103 and < 104 mgfltg
 , - Areenlc Concentration > 104 and < 105 mgAa
                                               G - Exlltlng Structure
                                               D - Former Structure
                                              — ~ Site Boundary
              FIGURE 5.5
         ARSEMCCONCEKTRATIONS
M SUBSURFACE SOLS ABOVE THE WATER TABLE
   ROCKB1 TIEB) FRHWa A» TflEATMQ
     PLANT OPERABLE UNT. ROCKER UT

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 i;f  JS7
 £  Jv-
 •X  "-:
If
    f
    B
.a
P

                 O - Ar*«nlc Concentration Below Detection Limit       O - Exletlng Structure
                 • - Arsenic Concentration Detected  < ID1 mq/kq      D - Fonner Structure
                   - Arsenic Concentration > 10^ and < 102 mg/kg    — - Site Boundary
                 4 - Arsenic Concentration > 102 and < 103 m
                 »- Arsenic Concentration > ID3 and <104m
                   - Arsenic Concentration > 10* and < 105 nig/kg
              RGURE 5.6
ABSOK CONCEHIRATONS H SUBSURFACE SOLS
       AT OR BaOWM WATS TABLE
    ROCKffl TWER FRAUNQ AM) TREATW
      PLANT OPERABLE IHT.ROCKEaUT
             \ACAD\AKCOVOCKOMVW4-«mj  KV. I  3\24\»5
 II.

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The largest, and most concentrated arsenic plume, up to 25,700
ppb arsenic, is located in the shallow alluvial aquifer beneath
the center of the site and extending west and northwest from the
historical treatment area and the backfilled loading trench
(Figure 5.7). Definition of the northwest side of the plume is
obscured by some mixing with Silver Bow Creek and a lack of wells
at proper depths in this direction.  Concentrations within the
plume along the east to west flow line diminish rapidly down to
approximately 100 ppb over a length of approximately 300 to 400
feet.  The final investigation of the groundwater plume occurred
in the Spring of 1995 on the north side of Silver Bow Creek.
Concentrations of arsenic in all three hydrogeologic units were
less than 10 ppb, indicating the plume had not migrated beneath
Silver Bow Creek.

Elevated arsenic in the deep alluvial aquifer is restricted to
one well on the west side of the site where a concentration of
approximately 6,000 ppb was detected at a depth of 30 to 40 feet
(Figure 5.8).  The alluvial aquifer contains downward gradients
in this area and the most concentrated portion of the plume in
the shallow alluvium passes over the top of this location.

Knowledge of elevated arsenic concentrations in the Tertiary
sediments aquifer is limited to a single well (Figure 5.9 &
5.10).   This well is located high in the stratigraphic section
along the west side of the site where the Tertiary sediments ramp
up to near surface.  The shallow and deep alluvial aquifers
contain elevated arsenic concentrations adjacent to where the
sand bed yielding water to this well probably subcrops and the
arsenic enriched groundwater in this well is probably coming
directly from the alluvial aquifer to the east.   The extent of
the arsenic plume in this strata was also limited by the recent
Tertiary sediments wells located north of Silver Bow Creek where
arsenic concentrations were found to be less than 10 ppb in the
Spring of 1995.

Remedial Investigation Summary of Major Findings;
1.   Arsenic is present in soils across the site.  However, the
     highest levels of arsenic are fairly localized and coincide
     with past operations, generally diminishing with depth below
     ground surface. Concentrations increase abruptly near the
     water table.  The highest arsenic concentrations occur
     immediately above and below the water table, where they
     serve as the long term source of arsenic for groundwater
     contamination.

2.   PAHs in soils are localized.  The highest levels of PAHs
     coincide with historical creosote operations and diminish
     rapidly, both vertically and horizontally away from the
     source.
                               19

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       C: - Former/Existing Structure 1830  - ArMnic Concentration (ug/L) - 1992
      	Site Boundary              •  - Shollow Alluvtol Monitoring Well
      NS - Not Sampled            	Artente Concentration Contour
                                         Shollow Alluvial Monitoring Well
                      GRAPHIC SCALE
                      o      so      100
                              HEET )
                        1 boh -  100 ft
                    CONTOUR INTERVAL - 2 FEET
                   FIGURE 5.7
   CROSS SECTION LOCATION IMP FOR ARSEHC • S82
       N SNA110W AlUJVUL MOHTORNS WEOS
       ROCKffl TIBER FRAIM AND TBEATNQ
         PUNT OPERABLE IMT, ROCKER HT
\ACM>VWCOV*OCXatlV)09-4J».Q  KCV. 1  3\1\»5

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                       10 - Ar>«ile Cen«ntniU«i (ug/1) 1992

                         — Amnfc Conc«ntrat)on Contour

                       4- - DMO Akntal Monitorhj Well Ucollon
 MDEPAllNHLUOMrOHHa NELLS
Rocffi) lira nuum MD HEATM
 PUm OFBVHEIMT. HOOHl UT

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I
I

57:
            O - Fonnw/txlttlng Stnicturv     9.2 - Ara«itc Concentration (ug/))
            - - att BouMory            	Ar»«Mc Cenontrotlon Contour
                                          - Twtlary S«d1m«nt Monitoring VMI
                             1 tacb • 100 ft.

                        CONTOUR HTERVM. " 2 FEET
             M TERTMRY SBUBirS MOMTOW HBL8
              ROCXB) TICB) FRMMa A» 1BEATMQ
                FUNT OFBtAU tNT, nOOtB^UT

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                                                                                                                                                  SECTION AA - AA'
                     CC
                                     •H-M/40
WEST
T.
OK —
SSSO —
5540 —
SSSO —
SS» —
SJIO —
CC 5500 -
WEST „„,«

	 nn rl —
-fltt-M "' 51.4 1
51.5 47.2 I
~ ~~ -\_ W~7
X
•x
X
*v
X
• ~v
X
"^
\"
"BH^23 "JfH^
\% Si
S8:e2'
f?..9
*H-W/24
" -x '
'•-< RJf
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"x>-~.


•J«-" -


	 	 	 j
RH-28 RH-»
B5 15-'
<7.9 52.»
RH-30
IS

"" EAST
50- 0 W 100- 1W
SOU£ IH FEET
	 SMO ARSPdC (»/U
30OO (ltB2)
1310 (1991)
	 "* MM (HMS
StW (1M7)
	 5550
	 5S20
-25 », 32 «H-M "««» — "'" CC' '
	 woo EAST Vc«fv«cc\wc»OTi\nw-«wo «v, t s\A»
RHi-W
SCRKN AT DEPTH SECTION CC — CC* «JOT SWLE 1 - 30
5277 TO 52S7 .
PGURES.10—  CROSS-SECTIONS OF ARSENIC PLUME (1992)

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3.    There is no evidence to indicate that groundwater and/or
     soils from the Rocker OU are contributing arsenic or PAH
     concentrations to the streambed sediments or surface water
     in Silver Bow Creek.

4.    PAH-impacted groundwater is localized to the previous
     treatment area.  No impacts to surface water quality have
     been observed.  However, the creosote/PAH accumulations
     which occur in the same region as the highly contaminated
     .arsenic plume creates an oxygen demand which maintains
     arsenic in its reduced, more mobile form.

5.    Taken together, the more coincident arsenic/PAH .plumes
     provide a long term source of arsenic to the groundwater
     systems.

6.    Gross arsenic contamination in the upper portion of the
     shallow alluvial groundwater is predominantly attenuated
     over a distance of a several hundred feet by indigenous iron
     oxyhydroxides in the aquifer. The oxygenated and neutral pH
     conditions in groundwater on portions of. the site appear to
     be important factors in limiting desorption of the arsenic.
     However, hydraulic gradients are not static and with future
     development of groundwater resources these conditions can
     change.

7.    Although vertical downward hydraulic gradients appear to be
     present, they have not resulted in significant downward
     migration of arsenic except in limited areas (associated
     with the deep alluvium and Tertiary sediments aquifers).
     Geochemical conditions, principally iron in the shallow
     alluvium,  and hydraulic gradients have kept the arsenic from
     deeply penetrating the alluvial aquifer beneath the site and
     apparently have limited the spread of the arsenic plume.
     This is despite the apparent continued release of arsenic
     speciated as arsenite (arsenic III)  into the shallow
     groundwater beneath the site.  Maintaining an oxidized
     condition in the alluvial aquifer is critical to minimizing
     movement of arsenite species. Increases in groundwater use
     affecting hydraulic gradients and or changes in geochemistry
     in any of the three hydrologic units will result in a change
     in' the fate and transport of the arsenic from the Rocker
     Site.   More specifically,  if downward hydraulic gradients
     are increased, arsenic migration into two significant,  high
     quality aquifers (deep alluvium and Tertiary sediments)  will
     result.  The lack of iron in these particular groundwater
     resources can allow arsenic to migrate considerable
     distances.
                               24

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8.   The high concentration of arsenic  "source materials" and
     resultant concentration of arsenic in the shallow
     groundwater presents a threat to the current and future use
     of the deep alluvial aquifer and to the deeper, more
     productive portions of the Tertiary aquifer. Both of these
     threatened aquifers have low arsenic attenuation capacities
     and are aquifers currently used or planned for use by local
     residents and businesses.  The high concentration also poses
     a threat to potential uses of the shallow aquifer, which is
     classified by the State as a potential source of drinking
     water.

9.   Soil arsenic contamination also poses a threat to
     recreational users, trespassers, and workers.  Although
     residential use is not expected at the Rocker OU, there
     remains the potential for a health threat unless this use is
     restricted.

6.  SUMMARY OF SITE RISKS

The Rocker OU baseline risk assessment intentionally avoided an
evaluation of ecological risks.  Ecological risk has been
evaluated as part of the Streamside Tailings Operable Unit which
is immediately adjacent to, and up and down stream from, the
Rocker Operable Unit.  The Rocker risk assessment also did not
address the inhalation pathway, because it was determined that
this issue was more appropriately addressed in the context of the
more expansive Streamside Tailings risk assessment.  The review
of water quality information from both the Silver Bow Creek and
Rocker investigations do not indicate water quality degradation
in Silver Bow Creek as a result of the Rocker operable unit.  The
potential for future effects to Silver Bow Creek are not expected
when considering the selected remedy.

Rocker Human Health Evaluation:
The primary purpose of the Rocker baseline risk assessment was to
characterize the current and future potential human health risks
from contamination on site, assuming the site is not cleaned up.
The contaminants of greatest concern on this site are residuals
from the wood treating process, which primarily include arsenic
(originally applied as a caustic arsenic solution to the timbers)
and the components of creosote (polynuclear aromatic hydrocarbons
or PAHs).  The risk assessment also considered other metal
contaminants associated with the arsenic trioxide powders that
were used on the site and that are also present in mine waste
rock used for railroad fill and tailings that were deposited
adjacent to Silver Bow Creek OU and within the Rocker OU.

Human health risks from contaminants located on the Rocker OU
(wood treating by-products, and limited Streamside tailings and
mine waste) were evaluated to determine possible effects to
workers,  trespassers, and future potential residents of the

                               25

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Rocker site.  The risk assessment evaluated all reasonable
current and potential uses of the land and EPA determined that,
if institutional controls were placed on the site to prevent
residential use, occupational and recreational uses were the most
likely uses for the OU.  In addition, assumptions were made to
try to characterize exposure to individuals that would experience
the most risk from the OU.  For this reason, the EPA chose to
consider a future worker scenario that represented a normal work
day being spent on OU.  This scenario evaluated risks if the
Rocker OU was developed to accommodate some business venture at
this location in the future.  With the multiple railroad side
tracks and the flat adjacent area, the EPA considered it very
plausible that an industrial venture could locate on site at some
future date.  The remedy was revised however, following a
discussion with local County planning officials, residents and
businessmen to better reflect potential land uses.  County
planning documents propose a limitation on building within 100
feet of the Silver Bow Creek floodplain.  A close inspection of
the trail tracks on site indicate that this is a switching yard,
with little potential for loading/off-loading other than from the
northernmost rail line.  Owners of the major truck fueling
facility just north of the Rocker OU, indicated that they could
see no potential for further industrial/commercial development of
the Rocker rail siding.  This was in part due to the current
location of a similar type of facility nearby at the Butte Port
of Entry.

  The risk assessment also evaluated potential human health
concerns associated with exposure to the contaminated soils
beneath the cap to represent a condition where a landowner chose
to make improvements to the site that would move the clean fill
currently covering the contaminants.  Figure 6.1 presents the
conceptual model to illustrate the potential for exposures to
contaminants at the Rocker OU.

Risks have been evaluated considering both noncancer adverse
effects and the potential for cancers to develop as a result of
contact with site contaminants.  Tables 6.1 and 6.2 present the
cancer and noncancer risk estimates for soils and groundwater at
the Rocker OU.  Although 31 chemicals detected at the Rocker OU
and were evaluated as chemicals of potential concern, only
arsenic was concluded to be at concentrations posing sufficient
risk to require that a remedial action be taken.  One other
chemical, cadmium, was found in groundwater at two locations at
levels that exceeded the maximum contaminant levels  (MCLs).  Each
of these locations were in areas of known contamination from
sulfide materials associated with the Streamside Tailings OU
(railroad.fill and stream over bank deposits), and one of them
                                26

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Table 6.1
Summary of Risk Estimates for Surface Soil
Rocker Timber Framing and Treatment Plant, Rocker, MT
Surface Soil
Area
Exposed
Individual
Current Soil Conditions (Cover material In
On Soil Cover
Outside Soil Cover
Current Occupational
Current Trespass
Future Residential
Current Occupational
Current Trespass
Future Residential
Exposure
Route
Noncancer Hazard
Index
Average
Case
Reasonable
Maximum
Case
Excess Lifetime
Cancer Risk
Average
Case
Reasonable
Maximum
Case
jlace)
Ingestion
Ingestion
Ingestion
Ingestion
Ingestion
Ingestion
—
—
0.04
_.
—
8.2
0.04
0.03
0.28
7.5
6.2
55.8
—
—
3x10-6
—
_
5x10^
7x10-6
7x10-6
6x10-5
1x10-3
1x10-3
1x10-2
Potential Future Soil Conditions (Assumes cover material was never brought in)
Without Soil Cover
Current Occupational
Future Trespass
Future Residential
Ingestion
Ingestion
Inqestion
_.
_
3.4
3.1
2.6
22.9
—
—
2x10-4
6x10-4
6x10-4
5x10-3
Note:   Averge-case exposures are not quantified for occupational or trespass scenarios due to lack of data
        regarding central tendencies for these receptor types.
Table 6. 2
Summary of Risk Estimates for Groundwater
Rocker Timber Framing and Treatment Plant, Rocker, MT
Water-Bearing
Zone
Shallow Zone
Intermediate
and Deep Zones
Exposed
Individual
Future Resident
Future Resident
Exposure
Route
Ingestion
Inhalation
Ingestion
Inhalation
Noncancer Hazard
Index
Average
Case
243
<0.01
44
<0.01
Reasonable
Maximum
Case
443
<0.01
79
<0.01
Excess Lifetime
Cancer Risk
Average
Case
1x10-2

2x10-3

Reasonable
Maximum
Case
8x10-2

1x10-2

a.   Shallow wells include RH-1 thru 5, 7,9.10,11,13,15,17,29.21,23.24. 25,26.28.19,32.33,38,41, and 47.
     Intermediate wells include RH-8.12,14,16,18, 20, 22. 24, 27, 30, 31, and 34.
     Deep wells include RH-6, 35 thru 37, 40, 43, and 46.
b.   No volatile carcinogens were detected in groundwater.
                                                 27

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                                                   Figure 6-1

                              Conceptual site Model for Potential Exposures
                                Rocker Timber Framing and Treating Plant
Primary Sources of
Contamination
Residuum from former
wood treatment operations






Mine wast© rock, ballast
material, and concentrate
spills along railroad











Potential Exposure
Medium
Surface Soil
Subsurface Soil
Groundwater

Air (t)



Surface Soil
Subsurface Soil
Groundwater
Air 191











Potential Exposure
Routes
Ingestlon of surface soil
Dermal contact with surface soil
Ingestlon of nroundwater


Dermal contact with creek sediments


Ingestion of surface soil
Dermal contact with surface soil
Ingestion of groundwater
Inhnlnfinn of nlrHxvrw* rli ists


	



......



	
Potential Receptors
Residents(3)
Workers
Trespassers
Steam-side tailings from
upstream mining operations

i*-)

Silver Bow Creek surface waters
Creek sediments



Ingestion of surface waters
Dermal contact with surface waters
Ingestion of creek sediments
Dermal contact with creek sediments
= Pathway is or could be complete; data are available and pathway is evaluated quantitatively
= Pathway is or could be complete; data are lacking or pathway is judged to be minor.  Qualitative analysis only.
= Pathway is not complete; no evaluation required.
     (1)  Risks were considered for this pathway although this portion of the site is not specifically associated with the Rocker OU,
         which is intended to include only the areas affected by the wood treating operations.

     (2)  Health risks associated with exposure to creek sediments, surface water, and inhalation of airborne dusts were considered
         as part of the Stream-Side Tailings Operable Unit and are not quantified here.

     (3)  Residential risks are for hypothetical future conditions. Currently, residences do not exitst on the Rocker OU.

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was up-gradient from the Rocker wood treating operations.  For
these reasons, elevated cadmium concentrations in groundwater
should respond to the remedy selected for the Streamside Tailings
OU.  Arsenic remains as the contaminant focused on for
remediation for the Rocker OU.

Noncancer effects were developed by adding the ratios of known
exposure concentrations to known safe levels for all chemicals of
concern on the site.  Using this method, a value greater than
"one" would indicate a concern for noncancer potential effects.
Values greater than "one" were calculated for the reasonable
maximum exposure to current and future workers, current
trespassers to the site, and future potential residents.  Arsenic
is the main chemical of concern that influences the noncancer
hazard index.

When EPA evaluates the risk of developing cancer from
contaminants on a site, they consider individuals being exposed
to site contaminants for several years.  The potential adverse
effects of developing cancer are represented in the risk
assessment as a number of excess cancers that might be expected
for a population of people.  For example, the excess cancers
could be one in one thousand people.  This projection means that
over a long period of exposure to contaminants one person out of
a thousand might develop cancer as a result of that exposure.
This projected risk is over and above the one-in-four risk that
each of us has of developing cancer in our lifetime from other
causes.  EPA considers corrective action to reduce such risks if
the risk of developing cancer is greater than one additional
cancer per million people.  EPA's acceptable range for clean up
of contaminants can allow contaminant levels that correspond to
excess cancer risks up to one in ten-thousand excess cancers.

The excess cancer risks calculated for persons who might contact
contaminated soils outside of the existing soil cover are in the
range of one additional cancer per one thousand individuals for
current workers and trespassers (using reasonable maximum
exposure assumptions).  The evaluation of future use of the site
by workers and trespassers, assuming that the clean soil cap has
been disturbed, reveals a one-in-ten thousand excess cancer risk
(using reasonable maximum exposure assumptions).  For surface
soils, greater than 95% of the cancer and non-cancer risk is due
to the presence of arsenic.  No other contaminant was determined
to pose a risk outside of EPA's acceptable risk range.   The
remedy contained in this ROD addresses arsenic as the contaminant
that exceeds EPA's acceptable risk range.  An arsenic soil
concentration of 380 ppm arsenic corresponds to one excess cancer
per ten-thousand individuals.  The risks associated with arsenic
levels in cover materials at background levels will be
approximately one excess cancer per one-hundred thousand
individuals.
                                28

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The cancer risk projected for individuals drinking groundwater
from the shallow alluvium, and the intermediate/deep alluvium are
one person per 100 individuals and 2 persons per one thousand
individuals, respectively (using average exposure assumptions).
When considering the reasonable worst case exposure assumptions,
the shallow groundwater poses an excess cancer risk of 8 persons
per one hundred individuals; while the intermediate/deep alluvial
groundwater presents an excess cancer risk of one person per one
hundred individuals.  Arsenic contributes over 99% of the future
potential cancer risk of consuming groundwater from the shallow,
intermediate, and deep alluvial groundwater systems.  No other
contaminant detected on the Rocker OU poses an unacceptable
excess cancer risk.  It should be emphasized that no individuals
are presently exposed to contaminated groundwater at the Rocker
site.  The remedy is consistent with EPA's theme of pollution
prevention by preventing contamination of groundwater in current
use in the vicinity of the Rocker OU.  With the implementation of
the remedy, the arsenic groundwater plume will be contained and
is not expected to migrate any further toward existing
groundwater users.  In addition, the arsenic plume should
dissipate after source removal; however, the precise time frame
to attain the State arsenic standard of 18 ppm remains uncertain.

EPA has concluded that contaminated soils and groundwater at the
Rocker site may pose an imminent and substantial endangerment to
workers, trespassers, and future potential residents at or near
the Rocker site.  This conclusion provides the rationale for
requiring response actions at the Rocker OU.

7. REMEDIAL ACTION OBJECTIVES

EPA and the State's overall remedial action objective for the
Rocker OU is to reduce the current and potential human exposure
to contaminated soil and groundwater.  Consistent with this
overall objective, the Rocker remedy has been developed to meet
the following specific remedial action objectives:
Groundwater
          Attain groundwater standards (ARARs or other risk-based
          levels) for inorganic (primarily arsenic) and organic
          contaminants of concern for groundwater underlying and
          adjacent to the site, and protect human health during
          and after cleanup.  The State ARAR for arsenic in
          groundwater is 18 parts per billion.  Owing to the
          nature of the groundwater contamination, the aquifers
          of preferred use, and the quality/quantity of water
          available from water producing zones within the Rocker
          site, this remedial action objective is especially
          important in order to prevent further contamination of
          the two lower aquifers.
                               29

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Soils
          The State groundwater standard is also applicable to
          the shallow aquifer,  which is classified as a
          potential domestic water supply by the State.  The
          shallow alluvial aquifer yields significantly less
          water than other water bearing zones, is generally not
          developed as a water resource in this area and has a
          lower quality than the deeper water sources.
          Therefore, reducing contaminant concentrations in the
          arsenic plume and the shallow alluvial aquifer to
          regulatory standards is considered a secondary
          objective.

          Prevent release of contaminated groundwater to Silver
          Bow Creek that would result in a violation of surface
          water ARARs or other risk based contaminant levels.

          Prevent degradation of groundwater underlying and
          adjacent to the site.

          Prevent migration of contaminated site groundwater from
          areas where levels exceed groundwater standards into
          regions where levels are within groundwater standards.
          Prevent human exposure to inorganic (primarily arsenic)
          and organic contaminants in soils which exceed risk-
          based or other relevant levels.   Based on the Rocker
          Human Health Evaluation for the occupational and
          trespasser exposure scenarios,  the EPA, in consultation
          with the State has determined that soils exceeding the
          risk-based soil concentration of 380 parts per million
          arsenic (which represents a 1 in 10,000 excess cancer
          risk to workers or trespassers)  should be remediated to
          break this potential pathway.

          Prevent migration of contaminants that would impact
          surface water to the degree that would cause non-
          compliance with surface water ARARs or other risk-based
          levels.  The EPA, in consultation with the State, have
          concluded that surficial soils exceeding 1,000 parts
          per million arsenic have the potential over the long
          term to be released to surface water or groundwater,
          and therefore should be remediated.

          Prevent migration of contaminants from soils to
          underlying and adjacent offsite groundwater, such that
          it would fail to comply with groundwater ARARs or other
          risk-based levels.
                               30

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The USEPA Office of Solid Waste and Emergency Response  (OSWER) is
aware of the difficulty of restoring some aquifers to health-
based cleanup criteria as a result of a study they conducted to
evaluate the effectiveness of groundwater extraction systems in
achieving specified goals.  The findings indicate that
groundwater extraction systems were generally effective in
containing the contaminant plume and that these systems can
achieve significant contaminant mass removal.  However, although
the contaminant concentration decreased significantly after
initiation of extraction, they tended to level off at
concentrations above their cleanup goals (i.e., MCLs or state
standards).  Following source removal, the Rocker remedy relies
on continuing natural flushing of groundwater with lower arsenic
concentrations (concentrations in the shallow alluvial aquifer
have been observed to be less than 10 ppb in areas not affected
by the Rocker OU) and natural attenuation to be effective in
reducing arsenic concentrations in the current area of the plume.
The iron additions to the groundwater source area will also
contribute to enhanced attenuation processes for a limited area
around the source treatment.  Consistent with EPA guidance,
however, the Rocker remedy also contains contingency measures to
control the arsenic plume that may be implemented in the unlikely
event that they are needed.

Because EPA has projected moderate difficulty in meeting the
ARARs in a limited part of the groundwater system (only shallow
alluvium), the Remedial Action Objectives have been prioritized
according to the actual or potential use of these groundwater
zones.  The prime objective is to prevent pollution from reaching
the high quality lower aquifers which are currently used
(Tertiary groundwater system)  and that have the potential to be
used  (deep alluvium).  The source removal actions, in situ
groundwater treatment,  and the contingency measures to contain
the plume will meet the primary objectives previously
established.  Monitoring will document the effectiveness of the
remedy on the aquifer that has the least potential for
development (shallow alluvium) and the need for contingency plume
containment measures.  EPA will also evaluate the practicability
of meeting the State standard in the shallow alluvium during
remedy implementation.   This approach is consistent with EPA's
guidance regarding groundwater remedies and the Agency's theme of
pollution prevention.
                                31

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8.  DESCRIPTION OP ALTERNATIVES

Alternative 1:

This is the no action alternative.  If implemented, there will be
no further remedial action at the site.  All site features and
site contamination would remain as is.  Maintenance of the
existing soil cover as a requirement of the earlier removal
action would continue, as would natural attenuation of the
arsenic in the groundwater plume.  Because wastes will remain in
place, there will be monitoring of local groundwater wells every
5 years at a minimum.  Consideration of this alternative is
required by the NCP.

The No Action Alternative would include monitoring of current
conditions and the continuing use of specific institutional
controls.

Alternative 2

This alternative would require additional institutional controls
to protect against intrusion of the soil cover and maintenance of
the existing soil covers.  An alternate water supply would be
made available to the local residents during the term of the
remedy, or until ARARs are met, and institutional controls (well
ban)  would be implemented.  Subsequent reference to well bans as
an institutional control will be for the term described here.
Hot spot areas would be covered with soil of suitable thickness,
these areas revegetated, and institutional controls implemented
to prevent intrusion.  This alternative relies on natural
attenuation of arsenic for the plume.  Sediment control barriers
to control runon and runoff and dust suppression would be used to
control offsite migration of contaminants during construction of
the soil cover.  Long-term monitoring of the soil cover and
groundwater would be implemented.

Source Control:
     Implement additional institutional controls, maintain
     existing soil cover, and continue groundwater monitoring;

Plume Control:
     Provide alternate water supply, continue groundwater
     monitoring, add appropriate institutional controls, and
     continue natural attenuation;

Hot Spot Abatement:
     Cover hot spots with clean soil, revegetate, and add
     appropriate institutional controls.
                                32

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Alternative 3

Source materials  (soils and debris) would be excavated to the
water table, replaced with clean borrow materials, and
revegetated.  Excavated soils would be temporarily stored onsite
prior to transportation and disposal at a RCRA Subtitle C
disposal site.  An alternate water supply would be made available
to the local residents.  This alternative relies on natural
attenuation of arsenic for the plume.  Hot spot areas would be
overlaid with a clean soil cover and these areas revegetated.
Sediment control barriers and dust suppression would be used to
control offsite migration of contaminants during construction and
excavation activities.  Additional institutional controls to
prevent intrusion of the covers and well bans would be
implemented along with long-term monitoring of the groundwater
and soil cover.

Source Control:
     Conduct excavation of contaminated source material to water
     table, offsite disposal to hazardous waste site, replacement
     of excavated soils with clean backfill, revegetation, and
     additional institutional controls;

Plume Control:
     Provide alternate water supply, continue groundwater
     monitoring, add appropriate institutional controls, and
     natural attenuation;

Hot Spot Abatement:
     Cover of hot spots with clean soil, revegetate, and add
     appropriate institutional controls.

Alternative 4

Source materials  (soils and debris) would be excavated to
approximately 5 feet below the water table.  Iron in the form of
ferrous sulfate would be distributed in the excavation prior to
backfilling with clean borrow materials and revegetated.  This
would enhance the natural attenuation of arsenic in the saturated
zone.  Excavated soils would be temporarily stored onsite prior
to transportation and disposal at a RCRA Subtitle C disposal
site.  An alternate water supply would be made available to the
local residents.  This alternative relies on both natural and
enhanced attenuation of arsenic for the plume.  Hot spot areas
would be overlaid with a clean soil cover and these areas
revegetated.  Sediment control barriers and dust suppression
would be used to control offsite migration of contaminants during
construction of the cover.  Additional institutional controls to
prevent intrusion of the covers and well bans would be
implemented along with long-term monitoring of groundwater and
the soil cover.
                                33

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Source Control:
     Conduct excavation of contaminated source material five  (5)
     feet below water table, dewater, dispose offsite in a
     hazardous waste repository, add iron salt to groundwater,
     replace excavated soils with clean backfill, revegetate,
     implement additional institutional controls with
     enhancements;

Plume Control:
     Provide alternate water supply, continue groundwater
     monitoring, add appropriate institutional controls, and
     continue natural attenuation with enhancements;

Hot Spot Abatement:
     Cover hot spots with clean soil, revegetate, and add
     appropriate institutional controls.

Alternative 5

This alternative would be identical to Alternative No. 4 for the
source and hot spot areas.  However, plume remediation would
include the installation of wells to facilitate the injection of
iron (ferrous sulfate) to fix the arsenic associated with the
plume.  Additional institutional controls and long-term
monitoring are also the same as described in Alternative No. 4.

Source Control:
     Conduct excavation of contaminated source material five  (5)
     feet below water table, add iron salt, dispose offsite at a
     hazardous waste repository, replace excavated soils with
     clean backfill, revegetate, implement additional
     institutional controls with enhancements;

Plume Control:
     Provide an alternate water supply, inject iron via wells to
     promote enhanced attenuation, continue groundwater
     monitoring, and add appropriate institutional controls;

Hot Spot Abatement:
     Cover hot spots with clean soil, revegetate, and add
     appropriate institutional controls.

Alternative 6

Source soils and debris would be excavated to approximately
5 feet below the water table.  Large debris unsuitable for
backfill would be separated from the soils and disposed at a
nearby landfill.  Soils would then be mixed with ferrous sulfate
and cement and placed back in the excavation.  An alternate water
supply would be made available to the local residents.  This
alternative relies primarily on natural attenuation of arsenic
for the plume.  Hot spot areas would be overlaid with a clean

                                34

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soil cover and these areas revegetated.  Sediment control
barriers and dust suppression would be used to control offsite
migration of contaminants during construction and excavation
activities.  Additional institutional controls to prevent
intrusion to the cover and backfilled areas and well bans would
be implemented along with long-term monitoring of groundwater and
the soil cover.

Source Control:
     Excavation of contaminated source material five  (5) feet
     below water table, offsite disposal of debris unsuitable for
     backfilling, mixing of excavated soils with cement or other
     pozzolanic material and iron salt (ferrous sulfate),
     backfilling the amended soils, adding cover soil, and
     revegetation;

Plume Control:
     Provide alternate water supply, continue groundwater
     monitoring, add appropriate institutional controls, and
     continue natural attenuation with enhancements;

Hot Spot Abatement:
     Cover hot spots with clean soil, revegetate, and add
     appropriate institutional controls.

Alternative 7

This alternative would be identical to Alternative No. 4 for the
source and hot spot areas.  However, plume remediation would
include installation of a series of groundwater extraction wells
to collect plume waters.  Standard alkaline chemical and physical
treatment technologies would be used to remove arsenic from the
groundwater.  Treated waters would be reinjected to the
contaminated local aquifer through a series of injection wells.
Treatment sludges would then be disposed at an appropriate
landfill.  Hot spot areas would be overlaid with a clean soil
cover and these areas revegetated.  Sediment control barriers and
dust suppression would be used to control offsite migration of
contaminants during construction and excavation activities.
Additional institutional controls to prevent intrusion of the
soil covers and well bans would be implemented along with long-
term monitoring of groundwater and the soil cover.

Source Control:
     Conduct excavation of contaminated source material five (5)
     feet below water table,  offsite disposal to a hazardous
     waste repository,  iron salt addition to groundwater,  and
     replacement of excavated soils with clean backfill,
     revegetation and additional institutional controls;
                                35

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Plume Control:
     Provide alternate water supply, pump and treat sitewide
     contaminated shallow groundwater, effluent reinjection to
     site groundwater, treatment sludge disposal to appropriate
     offsite waste repository, groundwater monitoring;

Hot Spot Abatement:
     Covering of hot spots with clean soil, revegetation, and add
     appropriate institutional controls.

9.  SUMMARY OF COMPARATIVE ANALYSIS OF ALTERNATIVES

     In accordance with the NCP, nine evaluation criteria must be
used to evaluate legal, technical, and policy considerations that
are important for selecting remedial alternatives (40 CFR
Section 300.430(f)  (1)).  Seven of these evaluation criteria serve
as the basis for conducting the detailed analyses found in the
Feasibility Study and this section.  The two remaining criteria,
state acceptance and community acceptance, were evaluated during
the public comment period for the proposed plan and that
evaluation is reflected here.

The first two of the nine criteria are minimum, or "threshold,"
criteria that must be met by any selected alternative.  The next
five criteria are considered to be "balancing" criteria and are
important criteria in the selection of a remedial action.  The
last two, are considered to be "modifying" criteria.  The nine
evaluation criteria as defined in the NCP are as follows:

Threshold Criteria

(1)  Overall protection of human health and the environment.
     Alternatives shall be assessed to determine whether they can
     adequately protect human health and the environment, in both
     the short- and long-term, from unacceptable risks posed by
     hazardous substances, pollutants, or contaminants present at
     the site by eliminating, reducing, or controlling exposures
     to levels consistent with established remediation goals.
     Overall protection of human health and the environment draws
     on the assessments of the other evaluation criteria that
     follow, especially long-term effectiveness and permanence,
     short term effectiveness, and compliance with ARARS.

(2)  Compliance with ARARs.  The alternatives shall be assessed
     to determine whether they attain applicable or relevant and
     appropriate requirements under federal environmental laws
     and state environmental or facility siting laws,  or provide
     grounds for invoking a waiver.
                                36

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Balancing Criteria

 (3)  Long-term effectiveness and permanence.  Alternatives  shall
     be assessed for the long-term effectiveness and permanence
     that they afford, along with the degree of certainty that
     the alternative will prove successful.

 (4)  Reduction of toxicity, mobility, or volume  (TMV)  through
     treatment.  The degree to which alternatives employ
     recycling or treatment that reduces toxicity, mobility, or
     volume shall be assessed.

 (5)  Short-term effectiveness.  The impacts during the term of
     the remedy considering: risks posed to the community,
     impacts to workers, environmental impacts and the time until
     protection is achieved.

 (6)  Implementability.  The ease or difficulty of implementing
     the alternatives considering: technical feasibility,
     administrative feasibility, and the availability of services
     and materials.

 (7)  Cost.  The types of costs include: Capital costs, annual
     operation and maintenance  (O&M) costs, net present value of
     capital and O&M costs.

Modifying Criteria

 (8)  State acceptance.
     The state concerns that shall be assessed include the
     following:

     (a)  The state's position and key concerns related to  the
          preferred alternative and other alternatives; and

     (b)  State comments on ARARs or the proposed use of waivers.

 (9)  Community acceptance.  This assessment includes a
     determination of the components of the proposed remedy that
     are either supported or opposed by the affected community.

COMPARATIVE ANALYSIS BY CRITERIA

     The alternatives are evaluated against each other for  the
each of the nine evaluation criteria.  There is one commonality
to all of the alternatives:  Each alternative, with the exception
of the No Action Alternative and the selected remedy (a
combination of alternatives 2 and 6), uses the same action  for
managing hot spot areas.  Surficial soils that exceed the action
level of 380 parts per million arsenic (corresponding to EPA's
acceptable excess cancer risk of 10"4)  should be capped with


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18 inches of soil.  Thus, there is no difference among the
alternatives in terms of hot spots for the nine criteria.

(1) Protection of Human Health and the Environment

Human Health

     The No Action Alternative is not protective of human health
since no actions would be taken and arsenic would remain at high
levels in soils and groundwater.  All other alternatives  (the
action alternatives) except Alternative Two are judged to be
protective of human health to some degree.  This evaluation
varies as to long term permanence and reliability, because some
of the alternatives rely on institutional controls and natural
attenuation more than others.  CERCLA and the National
Contingency Plan state a preference for alternatives which rely
on engineering solutions rather than alternatives which rely on
institutional controls.  Accordingly, the alternatives provide a
progression in terms of an increasing reliance on engineering
technologies generally resulting in increasing protectiveness.
Alternative 2 relies solely on institutional controls/natural
attenuation and is considered the least protective.  Alternative
3 is more protective because it involves excavation of arsenic
source materials down to the water table with disposal off site.
Alternatives 4 through 7 involve removal of arsenic source
materials down to 5 feet below the water table surface with
disposal off site and each include iron additions in the source
area.  These engineering treatments are considered more
protective of human health.   However, alternatives 5, 6, and 7
each deal with reducing arsenic contaminant levels in the plume
surrounding the source area which provides an increasing level of
protection.  Of the plume control technologies, pump and treat
(alternative 7)  is considered most protective; although it is
uncertain what time frame would be required to attain the state
arsenic standard.

     Protection and short- and long-term risk reduction are
achieved with the provision of institutional controls and an
alternate water supply for Alternatives 2 through 7.  As long as
institutional controls remain in place and potable water is
supplied by the alternate water source to prevent consumption of
groundwater from any of the three aquifers, human health would be
protected by each of the action alternatives.

Environment

     The Rocker OU remedial investigation did not document an
impact of site contaminants to the sediments or surface waters of
Silver Bow Creek.  Therefore, this evaluation of "protection of
the environment" is not done in the context of biologic
receptors.  Rather, this criteria is evaluated with respect to


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the extent of groundwater contamination remaining following the
remedial action involved in each alternative, and its potential
to eventually impact Silver Bow Creek.  The No-Action Alternative
would not be protective of the environment.  Alternative 2 would
also not be protective of the environment, since no action would
be directly taken to reduce the contamination in the plume or the
source areas.  The remainder of the action alternatives would
achieve protection of the environment to varying degrees.

Alternative 3 would achieve limited protection of the environment
through removal of at least some of the source material  (the
unsaturated portion).  Alternative 6 is more protective than
Alternative 3 because saturated source material would be
addressed.  However, the uncertainties associated with pozzolanic
stabilization of the arsenic materials in the saturated zone may
limit the protectiveness of Alternative 6.  Alternative 4 is the
most protective of the environment for the alternatives that
address only source remediation.  The excavation and appropriate
disposal of the saturated and unsaturated materials would be very
effective at removing the source contamination.  In addition, the
iron salts added to the excavation prior to backfilling would
provide some attenuation of arsenic mobility in the plume.

Alternative 5 provides source removal, plus the potential to
provide a degree of protection in the plume area through
immobilizing arsenic in the groundwater and would be protective
of the environment.  Alternative 7 would be most protective of
the environment because it incorporates source removal, plus
removal of contaminants from the aquifers in the plume area.

(2) Compliance with ARARs

     There are a number of ARARs which apply to the alternatives.
The primary ARAR of concern at this site is the State of
Montana's standard of 18 fig/L for arsenic in the groundwater.
The 18 jug/L standard would not be met by any of the alternatives
in the short term.  Alternatives 5 and 7 include the
implementation of remedial actions specific to the plume area
that would reduce the time to meet ARARs, and are therefore more
likely to achieve compliance in a shorter time frame.
Alternatives 4 and 6,  through implementation of source control
actions,  could potentially meet groundwater ARARs in the long-
term, although there is uncertainty concerning the length of time
to achieve compliance (particularly in the fine textured silts
and clays in the shallow alluvial aquifer).  Because
Alternative 3 does not include removal of arsenic sources from
the saturated zone, it is not expected that ARARs can be met in
the foreseeable future.  Alternatives 1 and 2 would also not
likely meet groundwater ARARs at anytime in the foreseeable
future.
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There is no ARAR which applies to arsenic contamination in soils
at this site.  All other ARARs at this site can be met by all of
the action Alternatives  (2-7) .

(3) Long-term Effectiveness and Permanence

Long-term effectiveness and permanence are not achieved by
Alternatives 1 and 2.  Under Alternative 1, arsenic contamination
in the soil and groundwater at the site would remain unabated and
uncontrolled.  Similarly, under Alternative 2, arsenic in soil
and groundwater would remain.  The effectiveness of Alternative 2
is dependent upon the institutional controls that are put in
place.  Because of the coordination required among federal,
state, and local agencies, the possibility that institutional
controls can be changed or removed, and the long time period
before natural attenuation improves the groundwater
concentrations, institutional controls by themselves are not
effective or permanent.

The long-term effectiveness and permanence of Alternative 6 is
questionable because the pozzolanic reaction may not proceed
properly in the materials backfilled in the saturated zone of the
source area.  In addition, the relatively high pH associated with
the pozzolanic materials may actually tend to increase the
mobility of the arsenic, rather than decrease mobility.

Alternatives 3 and 4 include the excavation of the source and
provision of an alternate water supply.  With Alternative 4, the
deeper excavation into the saturated zone of the source area, in
concert with the addition of iron sulfate prior to backfilling,
provide a greater degree of effectiveness and permanence.  A
greater volume of contaminated source materials will be removed
from the site and properly disposed under Alternative 4 as
compared to Alternative 3, contributing to enhanced long-term
effectiveness and permanence.

Alternative 5 is judged to have greater permanence due to the
enhanced attenuation in the plume associated with injection of
iron salts.  However, the long-term effectiveness is uncertain
because of the anticipated difficulty in injecting iron salts
into the plume area such that all the arsenic will be
immobilized.  Arsenic immobilization in the heterogeneous aquifer
will be a function of the varying permeabilities of the aquifer,
effectiveness of the injection well layout, and/or short
circuiting of the treatment solutions.  Arsenic immobilization
will occur in the preferential flow paths and may be less
effective in the less permeable portions of the aquifer.  Field
evaluation could remove some of the uncertainties associated with
this remedial approach.
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Alternative 7 has moderate effectiveness and permanence because
of the questionable ability to extract arsenic-contaminated
groundwater over time.  Arsenic in the aquifer beneath the site
can be extracted from the areas of preferential flow, but it may
be problematic to remove the arsenic from the lower permeability
soils in the heterogeneous aquifer.  Arsenic concentrations in
groundwater are likely to decrease asymptotically over time and
the cost-effectiveness of operating the treatment system will
also decline.  It is assumed that the extraction wells at the
site will be "pulsed" (cycling the groundwater pumps off and on
to allow washing of the unsaturated soil column).  This may make
operation of the treatment system problematic unless sufficient
storage is provided for flow equalization.  The treatment process
to be used in Alternative 7 is a proven and effective method for
arsenic removal from water.

(4) Reduction of Toxicity, Mobility, or Volume (TMV) Through Treatment

Toxicity

None of the alternatives reduce the intrinsic toxicity of the
arsenic through treatment.  Even Alternatives 5 and 7, which
address treatment of the arsenic in the plume, do not reduce the
toxicity of the arsenic; rather, they reduce the mobility of the
arsenic.

Mobility

Alternatives 1 and 2, since they provide no actions for either
the source or plume, do nothing to decrease the arsenic mobility.

Alternatives 3 and 4 will reduce the mobility of the contaminants
in the excavated soils,  assuming that the wastes are placed in a
secure, lined facility.   The addition of iron salts prior to
backfilling  (Alternative 4) will tend to reduce the mobility of
the arsenic in the plume in the vicinity of the source removal
area.

Alternative 6 should result in decreased arsenic mobility in the
source area through addition of iron salts and pozzolanic
materials to the backfilled source soils.  However, the mobility
reduction is uncertain,  especially in the saturated zone, due to
the potential to actually increase arsenic mobility due to the
elevated pH associated with the pozzolanic materials that are
added to the soils.

Alternatives 5 and 7 will decrease arsenic mobility, both in the
source area and in the plume area.  The mobility reduction in the
source area soils will be similar to Alternative 4, and is
associated with soil removal and disposal offsite.  Alternative 7
will not intrinsically reduce the mobility of the arsenic within

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the plume.  However, the removal of arsenic, by pumping from the
preferential flow paths within the plume, will tend to limit
further downgradient migration of the plume.  Alternative 5,
through the injection of iron salts into the plume, will tend to
reduce mobility of the arsenic within the plume.  As long as the
injection wells are properly located, Alternative 5 should also
tend to reduce further downgradient migration of the arsenic in
the plume.

Volume

Alternative 6 will not reduce the volume of contaminants, and in
fact, will result in considerable increase in volume.  The
excavation and addition of cementing agents to the source soils
will result in bulking of the soils, and a net increase in volume
of up to 20 percent.

Alternatives 4, 5, 7, and to a lesser extent, Alternative 3,
would also result in an increase in volume of contaminated soil.
The process of excavating the source soils will also result in
bulking of these soils.  The likely increase in volumes would be
approximately 10 percent.

Alternatives 1 and 2 would not result in any net change in
volumes of contaminated materials.

(5) Short-term Effectiveness

Short-term effectiveness is judged upon potential risks to the
community, onsite workers, and the environment during remedial
action and the time until compliance with ARARs or protection of
human health is achieved.  There is some small degree of risk to
workers implementing each of the alternatives, with the exception
of the No Action alternative.

Alternative 2 has the shortest time to complete the remedial
action (alternate water supply and groundwater monitoring).
Alternatives 3 and 4 each have similar soil excavation
requirements in the source area and the provision of an alternate
water supply.   These two alternatives are essentially equivalent
in terms of risk to workers and impacts on the community during
implementation.

Alternative 6 will require the onsite stabilization of excavated
soil so the time to achieve protection is longer than that for
Alternatives 3 and 4 and approximately equal to Alternative 5.
Material handling during this alternative is greater than for
Alternatives 3 and 4 and has a higher risk of community,  worker,
and environmental exposure.
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Alternative 5 requires the construction of an  onsite  facility to
house the mechanical equipment necessary  to  supply iron salt to
the injection wells.  This alternative will  have  construction
time periods equivalent to Alternative 6.

Alternative 7 requires the construction of a treatment  facility
at the site.  The time to complete this remedial  action is  longer
than any of the other alternatives.  Soil excavation  during
remedial action presents the same risks as Alternative  4.
Installation of the groundwater extraction system and the
construction of the treatment system pose minimal impacts to site
workers.  Vehicular traffic and construction of the treatment
facility may cause some nuisance to the community during
implementation.

(6) Implementability

Technical Implementability

Alternatives 1 and 2 do not require any remedial  action, and
therefore are simplest from a technical implementability
standpoint.

All of the other alternatives  (3 through  7)  utilize standard
construction techniques, materials and equipment,  therefore,  they
present no unusual implementability concerns.  However,  both
Alternatives 5 and 7 would require additional  field testing to
refine design criteria prior to implementation.   Alternative 5 is
also more difficult to implement because of  the expectation that
injection wells will periodically plug, requiring ongoing
injection well construction.

The treatment plant associated with Alternative 7 will  likely
require equipment not available locally.  However,  the  technology
is standard, and reliable equipment is available  from a number of
vendors within the U.S.

Administrative Implementability

The No Action Alternative is the most easily implemented from an
administrative standpoint, since only future groundwater
monitoring would have to be implemented.  Essentially,  this
alternative is already implemented.

The administrative implementability of the remainder  of the
alternatives is primarily associated with implementation of
Institutional Controls, and the coordination required among the
various local, state, and Federal government entities.   The
Institutional Controls that must be implemented do not  vary
significantly among the action alternatives, since all  action
alternatives must have institutional controls  to  protect

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groundwater sources during and following remediation.  It is
judged that the primary differences among the action alternatives
would be associated with the length of time required to achieve
the cleanup goals.  In other words, it is likely that it will be
relatively easier, and more acceptable to the public, to
implement institutional controls for those alternatives that are
perceived to require lesser time to reach the cleanup goals.

Alternative 2 does not address remediation of either the source
or the plume, and would likely require thousands of years before
the arsenic would be naturally attenuated.  For this reason,
institutional controls would have to be maintained indefinitely
and it is judged that it would be very difficult to maintain
these institutional controls under this alternative from an
administrative perspective.

The alternatives that address only the source, and not the plume
(3, 4, and 6), will likely require hundreds of years before
natural attenuation would allow complete use of the site
aquifers.  These alternatives would require implementation and
maintenance of institutional controls during the attenuation
period.  There would be considerable resistance, on the part of
the agencies and the. public, to embark on a program of
institutional controls for a time period of hundreds of years.

It is likely that the institutional controls for Alternatives 5
and 7 would be more easily implemented, since both the source and
the plume would be addressed.  The time required for
institutional controls under Alternatives 5 and 7 (from 30 to
100 years)  would likely result in relatively less opposition to
the implementation of institutional controls for this period.

(7) Cost
A budget level cost analysis was performed for each of the
alternatives.  The analysis includes an estimate of capital
costs, annual operation and maintenance (O&M)  costs and the net
present worth (NPW)  of the alternative assuming a seven percent
discount rate and a 30 year period for ongoing O&M costs.
                               44

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Summary of Costs by Alternative
Alternative
1
2
3
4
5
6
7
Capital Cost
$ 0
1,253,.704
3,060,182
4,316,074
6,601,818
8,190,371
6,216,151
Annual O&M
Costs
$ 29,848
102,544
102,544
102,544
486,888
102,544
508,617
Net Present
Worth
$ 94,255
2,526,177
4,332,655
5,588,546
12,643,637
9,462,844
12,527,603
 (8)  State acceptance.

The State has been consulted during the process leading up to the
Rocker ROD.  Concerns expressed by the State have been addressed
during the course of document development resulting in State
concurrence on the principle documents supporting the ROD
including the Baseline Human Health Evaluation, Remedial
Investigation, and Feasibility Study.  Care has been taken during
the investigations for the Rocker OU to coordinate issues with
the adjoining State-lead Streamside Tailings OU, including the
final remedy selected for that OU.

The State concurred in the proposed plan for the Rocker OU.  The
State also supports the selection of this remedy, and concurs in
the selected remedy.

 (9)  Community acceptance.

Through advance consultation, EPA believes that the community has
accepted the remedy selected in the Rocker ROD.  Public comments
received during community comment period indicated that the
community supported innovative technology for the Rocker remedy,
if it was workable in a short time period.  The community also
expressed opposition to off-site disposal of waste on Smelter
Hill.  Primarily, the community emphasized the continued use of
groundwater by area residents and long term protection of
groundwater resources that they prefer to use.  A parallel issue
is the Community's need for an alternate water supply during the
term of the ban on additional development of groundwater
resources.
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10.  THE SELECTED REMEDY

The remedy selected by EPA, with the concurrence of the State,
addresses surface soil, alluvium and fill, and groundwater
contaminated by arsenic in the Rocker OU.  The remedy selected is
a variation of Alternatives #4 and #6 evaluated in the
feasibility study, and the preferred final remedy developed in
the proposed plan.  The EPA has selected the final remedy for the
Rocker OU considering all written comments and oral testimony
received during the public comment period.   The remedy has been
modified from the proposed plan in response to public comment.
The changes that have been made in the remedy from the proposed
plan are considered significant; but are considered a logical
outgrowth of the public comments received.  The rationale for
these changes are addressed in Section 13 of this record of
decision.

The primary purpose of the remedy is to protect human health from
threats posed by direct contact with contaminated surface soils
or exposure to contaminated groundwater through active cleanup of
relevant media.  With respect to contaminated groundwater, the
primary objective is to prevent contamination of groundwater
resources (deep alluvium and tertiary groundwater systems) under
current use (or that have the potential to be developed) by the
community that are in hydraulic contact with the Rocker OU
arsenic plume.  This purpose includes making the groundwater
resource available to the community at the earliest opportunity.
A secondary objective of the groundwater remedy is to reduce
contaminant concentrations in the arsenic plume and the shallow
alluvial aquifer to regulatory standards.

Consistent with OSWER Directive 9283.1-03, the Rocker OU remedy
includes contingency measures to address the arsenic
contamination of the shallow alluvial aquifer where remediation
requirements involve moderate uncertainty and may at a future
date dictate an ARARs waiver and/or establishment of containment
goals.  After the remedy has been implemented and with persuasive
monitoring data that allows EPA, in consultation with the State,
to conclude that it is technically impracticable to attain ARARs
in the arsenic plume present in the shallow alluvial aquifer, a
Technical Impracticability Waiver could be granted by the Agency.
This waiver cannot be granted, however, without convincing
evidence after source removal, that trends in decreasing arsenic
concentrations will not meet remediation requirements in a
reasonable time frame.  EPA's consideration of a Technical
Impracticability waiver,  following implementation of the remedy,
may be limited to the smallest extent of the groundwater system
(possibly limited to fine grained saturated materials in the
shallow alluvium) .  This portion of the groundwater system has
the lowest potential for development owing to low water yields.
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However, EPA fully expects to meet the primary remediation goal
stated above.  Achieving this goal is consistent with a major EPA
theme of "pollution prevention".

FINAL REMEDY (with contingency measures):

The remedy selected utilizes treatment of the arsenic-laden
source materials that contribute to groundwater contamination and
surface soil hot spots to the maximum extent practicable, to
reduce mobility of the arsenic in combination with standard
excavation and on-site disposal technologies.  The selected
remedy includes utilization of natural and enhanced arsenic
attenuation processes, and contingent hydraulic controls to
contain and treat any unexpected groundwater migration off-site.
Also, part of the groundwater remedy includes a temporary well
ban to prevent development of the nearby shallow and deeper
portions of the alluvial aquifer as well as an alternative water
supply for the residents of the community of Rocker.  The remedy
requires institutional controls to limit future land uses (to
prevent residential land uses), monitoring of the vegetative
cover, and monitoring of groundwater (to document trends in water
quality and determine if contingent remedies might be needed, and
to assure protection of domestic water supplies).  The estimated
cost for this remedy is $5,400,000 (compared to $7,340,00 for the
preferred remedy in the proposed plan).

o    Groundwater Source Material Removal and Treatment of Shallow
     Groundwa ter
     Arsenic groundwater "source material" is defined as soils
     and other substrate materials that previously have been
     contaminated with concentrated wood treating solutions and
     other arsenic waste, and which continue to act as a source
     to ongoing groundwater contamination.  The area containing
     "source material" was  preliminarily defined in the
     feasibility study to be within the 10,000 parts per billion
     arsenic groundwater plume/ five feet deep into the saturated
     zone.   The selected remedy for "source materials"
     (approximately 41,000 cubic yards)  is excavation, subsequent
     chemical fixation utilizing complete mixing of iron sulfate,
     and lime with the arsenic contaminated media, and then
     backfilling the excavated area above the water table with
     this amended material to the extent practicable and in
     compliance with solid waste requirements.  Disposal of
     treated wastes will only occur in areas where iron has been
     added to the shallow groundwater beneath the waste
     repository as described below.   The addition of iron to
     adsorb and immobilize arsenic is considered by the Agency to
     be an innovative treatment technology.  The excavation and
     treatment of high concentration soils and other substrate
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materials must reduce arsenic mobility to below levels that
would be characteristic in relation to designating the
material a hazardous waste to be eligible for on-site
repository disposal.

During remedial design, an on-site pilot-scale treatment,
disposal, and testing process will be implemented in order
to optimize amendment dose rates and confirm (using EPA's
toxicity characteristic leaching procedure (TCLP))  that
treated wastes will be below characteristic levels for
hazardous wastes.  Following iron treatment and lime
addition, limited volumes of highly concentrated wastes may
produce leachate with arsenic concentrations higher than the
5 ppm specified for "characteristic" hazardous wastes,
following the EPA toxicity characteristic leaching
procedure.  For these materials, the remedy will include
solidification, by cement addition, prior to disposal on-
site.  A testing program for the duration of the remedy will
be designed following the pilot-scale testing.

The use of ferrous sulfate to fixate arsenic and render it
immobile is well documented in the literature (in EPA's
administrative record) and has been validated in part by
ARCO's test program at Montana Tech (Chatham, 1995). It
should be emphasized that this process is consistent with
the administrative record developed for the Rocker OU and
the use of this treatment process is responsive to concerns
identified during the remedial investigation/feasibility
study and public comment period.

A better definition of the specific quantity and locations
of "source material" to be removed and treated will occur
after the Record of Decision, during the Remedial Design
phase of the Superfund process.  During these subsequent
sampling and-analytical investigations, if  +3arsenic
"source material" is identified in addition to that defined
within the 10,000 ppb groundwater arsenic isopleth, such as
at the old vat, other known treatment areas,  and the off-
loading trench, this "source material" will also be removed,
treated and disposed of in the OU repository.  If additional
"source material" repositories are required,  in excess of
the volume available in the excavation/backfill areas, an
approved plan must be developed and implemented consistent
with the technologies and ARARs specified in this ROD.
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During the excavation of "source materials", care must be
taken to properly abandon any existing monitor wells that
would have to be removed as well as  minimizing the release
of pore waters from the saturated zone by utilizing proper
excavation equipment and associated removal techniques.  The
excavated "source materials" will be placed on a nearby
drainage pad constructed of impermeable materials where free
liquids will drain back into the exposed excavation (in
conformance with appropriate ARARs).  It is very probable
that the exposed groundwater in the excavation will contain
elevated arsenic concentrations.  Therefore, iron sulfate
solution with iron concentrations approximately 10 times
greater than the arsenic concentrations (consistent with
Chatham, 1995) should be added to and mixed with the
groundwater and the pH should also be adjusted to between
7.0 and 8.0 with milk of lime as necessary.  The excavation
will then be backfilled up to the water table with washed
gravel, properly compacted and then covered with a filter
blanket to maintain porosity.   The resulting iron-enriched
shallow groundwater can then move laterally and deeper into
the lesser concentrated portions of the plume, thus
enhancing the rate of arsenic attenuation in the plume.
However, it is recognized that this process will be limited
to the more permeable zones in the aquifer and the
effectiveness will diminish as iron precipitates reduce
aquifer permeability.  The area of contamination is expected
to continue shrinking as natural attenuation continues and
lower concentration groundwater (from up gradient areas)
continue to flow through the site.  Treatment of
contaminated groundwater by such an in-situ technology is
considered an innovative technology by EPA, and together
with the innovative iron treatment of arsenic wastes
(described above) is consistent with the statutory
preference for such remedies.

The excavated solids will have oversize materials removed
that are unsuitable for chemical fixation and backfilling.
Such materials will be removed and disposed of at an offsite
landfill,  consistent with State and Federal solid and
hazardous waste disposal requirements.  The contaminated
materials separated from the oversize material will be
treated with iron and lime as described previously.  The
treated materials will be placed on the backfilled gravel
layer in the excavated zone (above the iron treated ground
water)  resulting in a net surface elevation somewhat higher
than the original surface.   The final site surface contours
will be designed in such a manner that 18-inches of
additional non-contaminated cover soil can be added to
provide an adequate vegetative growth zone and protective
cover over the treated materials/hotspot areas and promote
proper surface drainage, and other ARAR standards are met.
An adequate number of monitor wells would then be completed

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into the permeable zone and into deeper portions of the
alluvial aquifer to permit ongoing groundwater monitoring to
document the trends in groundwater quality improvement
around the source removal area, within and outside of the
remaining arsenic plume.

Contaminated Surface and Near-Surface Soils
The surface and near surface soils outside of the "source
material" removal zone, to the site boundary, will be
systematically sampled and analyzed for arsenic
concentrations.   Sampling will not occur in areas being
remediated by the adjoining Streamside Tailings operable
unit.  The area utilized for the loading and off-loading of
the local recreational railroad will be included within the
area to be sampled and potentially remediated.  A soil
arsenic concentration of 380 parts per million (ppm)
corresponds to a one in 10,000 excess cancer risk for
trespassers, recreationists or workers that frequent the OU
and who may be exposed via the direct contact pathway.
Soils greater than this concentration pose a risk exceeding
the EPA acceptable risk range. Those areas found to be
greater than 380 ppm arsenic but less than 1000 ppm will be
covered directly with 18-inches of growth media and
revegetated.

Surface areas found in excess of 1000 ppm arsenic (hot
spots) shall be excavated to a maximum depth of 18-inches.
The excavated highly contaminated soil will be treated in a
manner identical to the source soils utilizing iron sulfate
and lime (described previously).  Investigation derived
wastes stored in drums on site will also be treated in this
manner and disposed of consistent with State and Federal
solid and hazardous waste regulations.  Limited
circumstances may occur where iron-treated materials,  when
tested using EPA's toxicity characteristic leaching
procedure (TCLP),  will exceed concentrations that would
classify the materials as a "characteristic" hazardous
waste.  A contingent  solidification  (by concrete addition)
treatment procedure is provided for in the remedy to address
this limited potential circumstance.  The resulting treated
wastes will then be disposed of on-site in an on-site
repository above the water table where groundwater has been
treated with iron below.  Excavated/covered areas will be
revegetated with appropriate species of draught resistant
grasses that are self-reproducing and that are consistent
with the remedial objectives of this ROD (minimizing surface
erosion and utilization of soil moisture).   The final site
contours must be compatible with the ongoing use of the
railroad corridor, and promote good surface water run-on/off
control.
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The excavation, treatment and on-site disposal outside of
the flood plain of high concentration soils and groundwater
"source materials" will prevent uncontrolled contaminant
releases via surface and groundwater pathways and will
prevent direct contact with this highly toxic chemical.
These aspects of the remedy are consistent with the
Streamside Tailings OU remedy.  Coordination between
operable units will continue, which is important with
respect to excavation and disposal of wastes from both
operable units.

Institutional controls and monitoring will maintain the soil
cover and vegetative communities, and limit land uses that
would jeopardize the integrity of the cover.  Institutional
controls will also designate the area for continued
railroad/industrial use and specifically exclude residential
development as a future use  (consistent with County planning
documents).

Well Ban and Alternative Water Supply

A serious potential health threat at the Rocker OU involves
the opportunity for migration of arsenic into ground water
systems currently being used, and that have the potential
for continued development.  These ground water resources are
the water supply of choice for area residents and
businesses.  Given the hydraulic connections between the
shallow and deeper alluvium and the tertiary aquifer, EPA
believes that it is necessary to restrict shallow and deep
groundwater development in order to prevent the spread of
the existing arsenic plume into aquifers currently used at
or near the OU.  Therefore, during the term of the Rocker
remedy, a groundwater well ban will be implemented for new
wells within a one-quarter mile radius of the site in any of
the designated three aquifer units to prevent increased
ground water utilization that could influence the arsenic
plume migration.  The well ban will be removed once
sufficient evidence from the post monitoring efforts
determines that the arsenic plume has been controlled
sufficiently to abate the threat of further migration.
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To further reduce the possibility of groundwater use and
contamination spread and to provide residents of the
community of Rocker adequate water to meet demands during
the period of the well ban, an expanded capacity alternative
water supply will be provided.  Current users of groundwater
can continue to utilize this resource.  Routine monitoring
of the quality of domestic/commercial groundwater supplies
within the area of the well ban shall be conducted.  The
alternate water supply and well ban together contribute to
the Agency's objective of preventing pollution of important
water supplies connected to the current area of
contamination.

Contingent Remedy

In the unlikely event that plume migration occurs (laterally
or vertically), additional hydraulic controls may be
implemented to contain the plume.  The contingent remedy
would be determined necessary if plume advancement is
detected in a lateral or vertical direction into surface or
ground water with arsenic concentrations below the 18 ppb
standard, that would result in long term arsenic
contamination that exceeds the State standard.

Groundwater Monitoring:

Water quality sampling and analysis for nearby existing well
users and for key monitoring wells developed for the Rocker
site will also continue on a seasonal/four times-per-year
frequency.  EPA, in consultation with the State, will make a
decision at the time of the 5 year review, or other
appropriate times, regarding: the need for contingent
remedies  (as described above), or the removal of groundwater
restrictions, or other appropriate refinements to the
remedy.

Coordination With Streamside Tailings OU

The Rocker Remedy will be coordinated with the Streamside
Tailings OU proposed remedy particularly with respect to
waste repositories.  Contamination occurring along the
railroad sidings within the Rocker OU will be remediated to
arsenic and metals concentrations consistent with the
recreational land use projected as part of the Streamside
Tailings OU remedy.
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This innovative remedial action breaks the surface, direct-
contact pathway for recreationists, trespassers, or workers that
may frequent the site.  It will also free up the site for future
use as an industrial site.  In addition, the remedy assures that
the primary groundwater remedial action objective of protection
of the quality and continued use of the tertiary aquifer, the
regionally preferred groundwater source, is achieved.

11.  PERFORMANCE STANDARDS

Section 7 of this ROD presented the Remedial Action Objectives
for the Rocker OU.  These stated EPA1  (in consultation with the
State) overall remedial action objectives are to reduce the
current and potential human exposure to contaminated soil and
groundwater.  More detailed objectives for both soils and
groundwater were also conveyed.  The final determination of state
and federal ARARs for the Rocker OU is presented in Appendix 1.
The purpose of this section is to identify those key requirements
and ARAR standards which will measure the success of implementing
the remedial action.

For soils there is no federal or state chemical specific ARAR;
therefore, the clean up levels were set on health based
concentrations of arsenic that were within EPA's acceptable
excess cancer risk range, as determined by EPA's Baseline Human
Health Evaluation.  The arsenic concentration determined
acceptable for surface soils to address the direct contact
pathway for trespassers, recreationists, and workers is 380 parts
per million (ppm).  An additional criteria of 1,000 ppm arsenic
is  established for removal/treatment of contaminated surface
soils.  These highly contaminated materials pose a greater risk
of potential release of arsenic from the OU via surface erosion
and/or leachate migration to groundwater (if institutional
controls were to fail).  In addition,  the specific criteria for
"source material" excavation will be refined during remedial
design.  The performance standards are:

o    For groundwater, clean up levels are based on the state's
     standards for Class I and Class II groundwater, which for
     arsenic is 18 parts per billion (ppb).

o    Excavation of soils exceeding 1,000 ppm arsenic to a depth
     of 18 inches (outside of areas remediated during the
     Streamside Tailings OU remedy, including the rail lines,  or
     the Rocker "source material" excavation),  followed by
     replacement with a similar volume of uncontaminated soils
     suitable as a plant growth medium,  followed by revegetation.
     Excavated materials will be disaggregated,  treated with
     iron, and returned to an onsite repository above the water
     table in areas where groundwater has also been treated with
     iron.
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o    Cover surface soils where arsenic concentrations exceed 380
     ppm  (outside of areas remediated during the Streamside
     Tailings OU remedy, including the rail lines), with a
     minimum of 18 inches of uncontaminated soils  suitable as a
     plant growth medium, followed by revegetation.

o    Excavated soils will be tested on a routine basis,
     acceptable to the Agencies, to document that  excavation and
     treatment will decrease arsenic mobility to levels below 5
     parts per million arsenic, using  EPA's toxicity
     characteristic leaching procedure (TCLP).

o    Groundwater in all aquifers must meet the 18  ppb arsenic
     standard and all other standards for site constituents at
     appropriate points of compliance determined by the Agencies
     during remedial design.

o    A sampling and analysis program, will be conducted during
     remedial design which will provide better definition of
     "source materials" requiring excavation and treatment.
     Following the sampling and analysis program,  excavation and
     treatment of "source materials", expected to  continue
     releasing high concentrations of arsenic to groundwater,
     will be accomplished.  For areas where "source materials"
     are excavated, groundwater will also be treated with iron
     and iron/arsenic concentrations will be monitored so that
     iron concentrations can be maintained at optimum levels to
     attenuate arsenic in groundwater.

o    In the event that groundwater or surface water monitoring
     outside of the current area of arsenic groundwater
     contamination (above 18 ppb arsenic) reveals  that the
     arsenic plume has advanced laterally or with  depth, the
     Agencies will evaluate, select, and determine what
     appropriate plume containment measures must be implemented.

The narrative in this section describes what performance
standards will be met during or at completion of the remedial
action selected for the Rocker OU and the documentation that will
be maintained to verify compliance with these standards.  The
specific approach to document that performance standards are or
will be met are described below.  Detailed monitoring programs,
acceptable to the Agencies,  will be developed during the remedial
design phase of this project.  The remedial design phase of this
project,  will:

o    Provide sampling and analysis plans that are  consistent with
     the objectives of this ROD;
o    Conduct the final stages of investigation (sampling and
     analysis) to verify volumes of surface soils  or source area
     materials to be excavated and/or covered;
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o    Provide interpretive reports documenting final areas
     requiring excavation/covering consistent with this ROD;
o    Provide surface and groundwater monitoring plans in order to
     demonstrate compliance with the implementation of the remedy
     and long term trends with respect to groundwater quality,
     involving monitoring wells on or near the Rocker OU and
     nearby private wells utilized for domestic water supplies;
o    Conduct additional bench/field scale investigations to
     optimize the form and amount of iron, lime, cement additions
     to the various media (high concentrations soils, source area
     materials, and groundwater) that are being remediated
     consistent with this ROD;
o    Develop a monitoring and maintenance plan: for the
     soil/vegetative cover,  run on/run off as appropriate, and
     topographic features that isolate waste repositories from
     the floodplain;
o    Develop a revegetation plan that will provide a vegetative
     cover consistent with the long term objectives of
     controlling erosion, and utilizing moisture in the root zone
     (so as to minimize through-flow of moisture to groundwater);
o    Develop a plan that includes descriptions of required:
     equipment, materials, construction time frames, location of
     utilities potentially disturbed by pipeline construction,
     and required surface access agreements in order to install
     the alternate water supply and storage tank consistent with
     this ROD and a plan for implementation, in coordination with
     local land owners and authorities, for institutional control
     implementation, including the temporary groundwater well
     ban.

12.  STATUTORY DETERMINATIONS

Under its legal authorities, EPA's primary responsibility at
Superfund sites is to undertake remedial actions that achieve the
overall protection of human health and the environment.   In
addition, Section 121 of CERCLA establishes several other
statutory requirements and preferences.  These specify that the
selected remedial action for this site must comply with
applicable or relevant and appropriate environmental standards
established under federal and state environmental and siting laws
unless a statutory waiver is justified.  The selected remedy also
must be cost-effective and must utilize permanent solutions and
alternative treatment technologies or resource recovery
technologies to the maximum extent practicable.  Finally, the
statute includes a preference for remedies that employ treatments
that permanently and significantly reduce the volume, toxicity,
or mobility of hazardous wastes as their principal element.  The
following subsections discuss how the selected alternative meets
these statutory requirements.
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PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT

The selected remedy is protective of human health for the
following reasons:

o    Breaks the direct contact pathway for trespassers and
     workers potentially exposed to contaminated soils;

o    Protects current and future groundwater users by containing
     the existing groundwater plume, preventing increased use of
     aquifers to avoid direct contact and spread of the plume,
     treating contaminated soils and other material and ground
     water in the source area, and allows for natural attenuation
     of the arsenic plume.

COMPLIANCE WITH ARARS

The selected alternative will comply with the federal and state
requirements that have been determined legally applicable or
relevant and appropriate to the Rocker OU remedial action, as
described in Appendix 1.

Treated contaminated soils and sediments will be disposed of in
compliance with federal and state solid waste and reclamation
regulations through excavation, treatment and disposal above
treated groundwater.  The State remediation standard of 18 ppb
arsenic is the primary ground water ARAR for the Rocker OU
remedy.  As noted, Appendix 1 provides a list of all of the ARARs
for the selected remedy.

There is no basis nor need for an ARAR waiver at this time in
conjunction with the remedy.  However, the remedy is considered
to have moderate uncertainty when considering the potential to
achieve the State arsenic standard of 18 ppb in groundwater
moving through the fine grain shallow alluvium.  The remedy is
expected to achieve significant reductions in arsenic
concentrations within the current arsenic plume.  This action is
expected to meet the primary remediation goals of protecting the
quality of the deep alluvium and tertiary groundwater systems.  A
determination will be made following implementation of the remedy
whether the State standard can be met in a reasonable time frame
in the shallow alluvial groundwater system.   If it is found to be
technically impracticable from an engineering perspective to
achieve the State arsenic standard, an ARAR waiver for a limited
portion of the aquifer will be sought.  The saturated fine
grained shallow alluvium where this potential is greatest is the
groundwater resource that has the least likelihood of being
developed owing to its low yield and close proximity to ground
surface.
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COST EFFECTIVENESS

The selected remedy is cost effective compared to the other
alternatives evaluated.  Of the other alternatives considered,
including the preferred alternative in the proposed plan, the
cost of this remedy ($5.4M) falls mid-range between alternative 2
 ($2.5M) and alternative 7  ($12.5).  The cost of the preferred
alternative identified in the proposed plan was estimated to be
$7.34M.  The decreased cost of the remedy from the proposed plan
was a result of EPA's refinement of the remedy, based on public
comment, to dispose of excavated highly arsenic contaminated
materials on-site, after treatment.  EPA also evaluated the cost
of the remedy in terms of the high value of the groundwater
resource in this area.  Residents have expressed a preference to
use groundwater from the Tertiary alluvium aquifer because of the
increased cost associated with purchasing treated water from the
Butte municipal water supply.  The Tertiary alluvium aquifer has
been demonstrated to provide adequate water quality and quantity
to support commercial development  (which is likely in this area).
The state Natural Resource Damage Program conducted an evaluation
of the increased cost of purchasing Butte water, and concluded
that there is an increased cost of $607.00 per acre foot of water
over the cost of using groundwater supplies (Literature Review
and Estimation of Municipal and Agricultural Values of
Groundwater Use in the Upper Clark Fork River Drainage, 1995).
The Tertiary alluvium is recognized by residents and well
drillers to be the preferred groundwater producing zone in the
area between Rocker and Ramsey (several miles down stream).  The
remedy acknowledges the value of this groundwater resource to
area residents by maintaining as the highest priority, the
protection of the quality of the Tertiary aquifer.  The remedy
also provides protection to on-site workers and trespassers, and
returns the property to productive use.  In addition, consistent
with the remedial action objectives, the remedy strives to remove
the water well ban (for new wells within one quarter mile of the
groundwater plume) as soon as possible.  Accordingly, the costs
associated with this remedy are proportional to its overall
effectiveness.

UTILIZATION OF PERMANENT SOLUTIONS AND ALTERNATIVE TREATMENT
TECHNOLOGIES (OR RESOURCE RECOVERY TECHNOLOGIES)

The selected alternative uses permanent solutions and alternative
treatment technologies to the maximum extent practicable for this
site.  EPA in, consultation with the State,  has determined that
the selected remedy  provides the best balance in terms of long-
term effectiveness and permanence; reduction of toxicity,
mobility,  or volume achieved through treatment; short-term
effectiveness; implementability;  and cost, while also considering
the statutory preference for treatment as a principal element,
and State and community acceptance.


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From the evaluation of alternatives in the feasibility study, EPA
has concluded that arsenic source removal and treatment was
essential if improvements in groundwater quality were expected in
a reasonable time frame.  If the highest levels of arsenic and
creosote are left in the groundwater  (without treatment), EPA
concludes that these materials would present a long term source
of contamination to the three aquifers identified.  Following
excavation/treatment, of groundwater and solid materials from the
source area, with disposal of treated solids above the water
table, EPA concludes that this alternative treatment will
permanently adsorb arsenic with ferric iron oxy-hydroxide
precipitates that will remain stable under the environmental
conditions present at the Rocker OU.

Other technologies evaluated in the feasibility study did not
adequately address the removal/treatment of mobile arsenic in
pore waters contained in the fine grained materials within the
saturated zone.  Outside of the source area following excavation,
the remaining arsenic groundwater plume is expected to dissipate
quickly from natural/enhanced attenuation (resulting to some
extent from iron addition to groundwater in the source area) and
from the flushing of higher quality alluvial groundwater
containing less than the state standard of 18 ppb arsenic
entering the OU from an upstream direction.   However, the extent
to which the fine grained materials in the saturated zone may
slowly release arsenic to the groundwater outside of the source
removal area and the net effect on the quality of the shallow
alluvial aquifer is moderately uncertain.  The remedy includes a
plume containment contingency to protect the more valuable deep
alluvial and Tertiary aquifers in the unlikely circumstance that
plume migration occurs.

The selected remedy has been designed as a permanent solution.
Adherence to the performance standards for the remedy will ensure
the continued safety of the surrounding population, workers
implementing the remedy and environment.  Thus,  the selected
remedy meets the statutory requirement to utilize permanent
solutions and treatment technologies, to the maximum extent
practicable.

PREFERENCE FOR TREATMENT AS PRINCIPAL ELEMENT

The selected remedy combines several treatment approaches to
reduce the mobility of arsenic in both contaminated soils and
groundwater.  Contaminated surface soils (exceeding 1,000 ppm
arsenic)  and source area contaminated alluvium/fill materials
will be treated with iron, lime and cement as necessary to reduce
arsenic mobility.  In addition, iron additions to groundwater in
the source area will adsorb arsenic with iron oxy-hydroxides that
will precipitate; thereby capturing the arsenic in a form that
will remain stable under the environmental conditions present at
the Rocker OU.

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This satisfies the statutory preference for remedies that reduce
the toxicity, mobility, or volume of contamination through
treatment.

13.  DOCUMENTATION OF SIGNIFICANT CHANGES

CERCLA Section 117(b) requires an explanation of any significant
changes to the selected alternative as presented in the Proposed
Plan, which was made available for public comment.  In developing
the final remedy, five significant changes were made to the
Proposed Plan:

1)   Alternate Water Supply:
     New water users (within one quarter mile of the site) would
     be provided water from this source.  This was changed from
     the one half mile figure used in the proposed plan.  This
     distance was considered by EPA's geohydrologists to be an
     adequate area of protection that would prevent future
     groundwater development that might influence arsenic plume
     migration.  This determination was based on professional
     judgment and the geohydrologic information available for the
     Rocker OU.

2.   Change in Waste Treatment and Disposal:
     EPA revised their finding regarding the designation of
     Rocker wastes as listed hazardous wastes governed by RCRA
     Subtitle C.  This change is based on EPA's evaluation of the
     waste  listings under EPA's RCRA regulations, the lack of
     clear  documentation regarding waste sources, and site
     conditions.  The RCRA waste listings do not describe exactly
     the waste produced at the Rocker Wood Treating Plant.
     Accordingly, the listings do not apply to the waste.
     Additionally, the NCP Preamble states that where it is not
     possible to identify the exact source of wastes,  RCRA
     requirements need not be identified.  Here Rocker wood
     treating wastes are mixed with mining waste (fill brought in
     for railroad grades,  stream diversion,  and to raise the
     topography 5 to 8 feet under the Rocker Wood Treating Plant)
     to form an indivisible harm at the site, and the exact
     source cannot be identified.  Following excavation and
     treatment with ferrous iron, the excavated wastes will pass
     EPA's  TCLP test regarding the RCRA hazardous waste
     characteristic determination.  Therefore,  wastes will be
     disposed of in solid waste repositories on site.   This is
     consistent with the comments made by the community of
     Anaconda to not locate the waste on Smelter Hill.  In
     addition, Rocker residents indicated that a local repository
     was acceptable to them if an acceptable location could be
     found.
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3.   EPA's Universal Treatment Standards:
     The remedy will require that Rocker excavated and treated
     wastes to ultimately meet TCLP characteristic levels as a
     condition for on site disposal rather than the universal
     treatment standards described in the proposed plan.  The
     remedy was also revised to include solidification with
     cement, in the event that treated wastes exceed the TCLP
     standard of 5 ppm arsenic.  The cost estimate to add this
     component to the remedy, did not exceed the cost contingency
     factored into the original cost of the remedy.  Because it
     is expected that there will be very limited need for
     concrete addition, an adjustment to total cost of the remedy
     was not made.

4.   Well Ban:
     EPA's geohydrologists concluded that one quarter mile well
     ban around the arsenic plume would provide an adequate
     buffer so that future groundwater development would not
     influence arsenic plume migration.  The proposed plan used
     one half mile for the water well ban.

5.   Pump and Treat Contingency:
     EPA removed the pump and treat contingency for the Rocker
     remedy for the following reasons:

     o    It was considered unlikely that plume migration would
          be detected following implementation of the remedy;
     o    In the event that plume migration is detected, other
          plume containment alternatives are available,  without
          adding the significant additional expense of a
          treatment plant;
     o    It was considered to be not cost effective to install
          pump and treat technologies when natural groundwater
          flow and attenuation are expected to cause significant
          decreases in arsenic concentrations in groundwater; and

     o    The Proposed Plan recognized that over the long term
          the cost effectiveness of pump and treat to remove the
          arsenic remaining in very fine grained lenses of the
          alluvium would diminish.

These significant changes are considered to be a logical
extension of the comments received during the public comment
period.  During a follow up meeting with the major stake-holders
on this site (PRP, Communities, and environmental groups)  after
the close of public comment, EPA presented a revised position
that addressed public comments (the remedy contained in the ROD).
The representatives were largely supportive of the revisions to
the remedy, although the PRP sought clarification regarding the
precise volumes slated for source area removal.   EPA has
clarified this issue in the ROD with respect to the work to be
done during the remedial design phase of the project to better

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define  "source materials" to be removed during the remedy by
limiting areas of investigation to known areas of processing and
areas where the mobile form of arsenic (~l"++As)  is present.
Another minor adjustment was made in the ROD to update the
calculations made regarding excess cancer risk from exposure to
arsenic contaminated soils.  EPA revised the cancer slope factor
used in this calculation in June, 1995 from 1.75 mg/kg-day to 1.5
mg/kg-day. The changed slope factor altered the soil arsenic
concentration that poses an excess cancer risk of one in ten
thousand to 382 ppm.,  compared to 327 ppm from the Rocker
Baseline Human Health Evaluation, completed prior to the change
in the cancer slope factor.
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                            REFERENCES


This Record of Decision is supported by the Administrative Record
for the Rocker OU  (maintained in EPA's Helena, Montana Office)
and the following documents that have been cited in the text.

ARCO.  1995.  Rocker Timber Framing and Treating Operable Unit-
Final Remedial Investigation.

ARCO. 1995. Rocker Timber Framing and Treating Plant Operable
Unit Feasibility Study.

Chatham, William H.  April 1995.  Treatability Study Iron Flood
Method for In-situ Remediation of Arsenic at the Rocker Timber
Framing and Treating Plant Operable Unit, Phase I - Batch Tests

CHjM Hill.   1995.   Baseline  Human Health Evaluation for the
Rocker Timber Framing and Treating Plant Operable Unit.

Harper, T.R., and Kingham, N.W.  1992.  Removal of arsenic from
wastewater using chemical precipitation methods.  Water
Environment Research, Vol. 64, pp.  200-203.

Murray, Lament and Associates. 1993.  Supplement to:  Legal
Memorandum, dated April 2, 1992, Pertaining to Institutional
Controls at the Rocker Timber Framing and Treating Plant (Rocker)
Operable Unit.

State of Montana Natural Resource Damage Program. 1995.
Literature Review and Estimation of Municipal and Agricultural
Values of Groundwater Use in the Upper Clark Fork River Drainage.

State of Montana Natural Resource Damage Program. 1995.  Rocker
Groundwater Injury Assessment Report.

U.S. Environmental Protection Agency.  1988.  Guidance on
Remedial Actions for Contaminated Groundwater at Superfund Sites.
EPA/540/G-88/003.  December 1988.

U.S. Environmental Protection Agency. 1990.  Suggested ROD
Language For Various Groundwater Remediation Options (D).
EPA/9283.1-03(10/90). October 1990.
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              IDENTIFICATION AND DESCRIPTION OF
           APPLICABLE OR RELEVANT AND APPROPRIATE
REQUIREMENTS, STANDARDS, CONTROLS, CRITERIA, OR LIMITATIONS
    FOR THE SILVER BOW CREEK/BUTTE AREA SUPERFUND SITE -
                     ORIGINAL PORTION -
   ROCKER  TIMBER FRAMING AND TREATMENT PLANT OPERABLE UNIT

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                           INTRODUCTION

     Section 121(d) of CERCLA, 42 U.S.C. Section  9621(d),
certain provisions of the current National Contingency  Plan  (the
NCP), 40 CFR Part 300  (1990), and guidance and policy issued by
the  Environmental Protection Agency  (EPA) require that  remedial
actions taken pursuant to Superfund authority shall  require or
achieve compliance with substantive provisions of applicable or
relevant and appropriate standards, requirements,  criteria, or
limitations from State environmental and facility siting  laws,
and  from federal environmental laws, at the completion  of  the
remedial action and/or during the implementation  of  the remedial
action, unless a waiver is granted.  These requirements are
threshold standards that any selected remedy must meet.   See
Section 121(d)(4) Of CERCLA, 42 U.S.C. § 9621(d)(4); 40 CFR §
300.430 (f) (1).  EPA calls standards, requirements, criteria, or
limitations identified pursuant to section 121(d)  ARARs,  or
applicable or relevant and appropriate requirements.

     ARARs are either applicable or relevant and  appropriate.
Applicable requirements are  those standards, requirements,
criteria, or limitations promulgated under federal or state
environmental or facility siting laws that specifically address a
hazardous substance, pollutant, or contaminant, remedial action,
location, or other circumstance found at a CERCLA site.  Relevant
and  appropriate requirements are those standards,  requirements,
criteria, or limitations promulgated under federal environmental
or state environmental or facility siting laws that, while not
"applicable" to hazardous substances, pollutants,  contaminants,
remedial actions, locations, or other circumstances  found  at a
CERCLA site, address problems or situations sufficiently similar
to those encountered at the  CERCLA site such that their use is
well suited to the particular site.  Factors which may  be
considered in making this decision are presented  in  40  CFR
Section 300.400(g)(2).  Compliance with both applicable and
relevant and appropriate requirements is mandatory1

     Each ARAR or group of related ARARs is identified  by  a
specific statutory or regulatory citation, a classification
describing whether the ARAR  is applicable or relevant and
appropriate, and a description which summarizes the  requirements
and addresses how and when compliance with the ARAR  will be
measured (some ARARs will govern the conduct of the
implementation of the remedial action, some will  govern the
measure of success of the remedial action, and some  will do
     1   See CERCLA Section 121(d)(2)(A), 42 U.S.C.  § 9621

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both)2.  The descriptions  given here are provided to allow the
reader a reasonable understanding  of  each requirement without
having to refer constantly back  to the statute or regulation
itself and to provide an explanation  of how the requirement  is  to
be applied in the specific circumstances involved a this operable
unit.

     Also contained in this list are  policies, guidance or other
sources of information which are "to  be considered" in the
selection of  the remedy and implementation of the Record of
Decision (ROD).   Although not enforceable requirements, these
documents are important sources  of information which EPA and the
State  of Montana Department of Environmental Quality Sciences
(MDEQ)  may consider during selection  of the remedy, especially  in
regard to the evaluation of public health and environmental
risks;  or which will be referred to as appropriate in selecting
and developing cleanup actions3.

     Finally,  this list contains a non-exhaustive list of other
legal  provisions or requirements which should be complied with
during the implementation of this  ROD.

     ARARs are divided into contaminant specific, location
specific,  and action specific requirements,  as described in  the
NCP and EPA guidance.  For contaminant specific ARARs, ARARs are
listed according to the appropriate media.

     Contaminant specific ARARs govern the release to the
environment of specific chemical compounds or materials
possessing certain chemical or physical characteristics.
Contaminant specific ARARs generally  set health or risk based
numerical values,  or methodologies which,  when applied to site-
specific conditions,  result in the establishment of numerical
values.   These values establish the acceptable amount or
concentration of a chemical that may  be found in, or discharged
to, the ambient environment.

     Location specific ARARs are restrictions placed on the
concentration of hazardous substances  or the conduct of cleanup
activities because they are in specific locations.   Location
         40 CFR Section 300.435(b)(2); Preamble to the Proposed NCP, 53 Fed.
Reg. 51440 (December 21,  1988); Preamble to the  Final NCP, 55 Fed. Reg. 8755-
8757  (March 8, 1990).  The Atlantic Richfield Company (ARCO), the named liable
party for the site, argues that this NCP requirement is not consistent with
the CERCLA statute.  However, ARCO did not challenge the NCP in  the District
of Columbia Court of Appeals in a timely manner,  and therefore have waived the
right to assert this argument.  See Section 113(a) of CERCLA, 42 U.S.C.
Section 9613(a).


     3.   40 CFR Section 300.400(g)(3); 40 CFR Section 300.415(i); Preamble
to the Final NCP,  55 Fed. Reg. 8744-8746 (March  8, 1990) .

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specific ARARs  relate to the geographic or physical position of
the site, rather than to the nature of the site contaminants.

     Action  specific  ARARs are usually technology or activity
based requirements  or limitations on actions taken with respect
to hazardous substances.

     Only the substantive portions of the requirements are
ARARs4.   Administrative  requirements are not ARARs  and thus do
not apply to actions  conducted entirely on-site.  Administrative
requirements are those which involve consultation,  issuance of
permits, documentation,  reporting, recordkeeping, and
enforcement.  The CERCLA program has its own set of
administrative  procedures which assure proper implementation of
CERCLA.  The application of additional or conflicting
administrative  requirements could result in delay or confusion5.
Provisions of statutes or regulations which contain general goals
that merely  express legislative intent about desired outcomes or
conditions but  are  non-binding are not ARARs.6

     Many requirements listed here are promulgated as identical
or nearly identical requirements in both federal and State law,
usually pursuant to delegated environmental programs administered
by EPA and the  States,  such as the requirements of the federal
Clean Water  Act and the  Montana Water Quality Act.   The preamble
to the new NCP  states that such a situation results in citation
to the State provision as the appropriate standard, but treatment
of the provision as a federal requirement.  ARARs and other laws
which are unique to State law are listed in the State ARAR
section of this document.

     Only those state standards that are identified in a timely
manner and are  more stringent that federal requirements may be
applicable or relevant and appropriate.   To be an ARAR, a state
standard must be "promulgated",  which means that the standard is
of general applicability and is legally enforceable7.

     This document  constitutes MDEQ's and EPA's formal
identification  and  detailed description of ARARs for remedial
action at the Rocker  Operable Unit.   The ARARs analysis is based
on section 121(d) of  CERCLA,  42 U.S.C.  Section 9621(d); CERCLA
     4.   40 CFR Section 300.5.  See also Preamble to the Final NCP, 55 Fed.
Reg. 8756-8757 (March 8,  1990).

     5.   Preamble to the Final NCP,  55 Fed. Reg.  8756-8757 (March 8,  1990);
Compliance with Other Laws Manual,  Vol. I, pp. 1-11 through 1-12.

     6.   Preamble to the Final NCP,  55 Fed. Reg.  8746  (March  8, 1990) .

     7.   40 CFR Section 300.400(g)(4).


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Compliance with Other Laws Manual, Volumes I and II, OSWER Dirs.
9234.1-01 and-02  (August 1988 and August 1989 respectively);
various CERCLA ARARs Fact Sheets issued as OSWER Directives; the
Preamble to the Proposed NCP, 53 Fed. Reg. 51394 et seq.
(December 21, 1988); the Preamble to the Final NCP, 55 Fed. Reg.
8666-8813 (March 8, 1990); and the Final NCP, 40 CFR Part 300  (55
Fed. Reg. 8813-8865, March 8, 1990), and the substantive
provisions of law discussed in this document.
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FEDERAL ARARS

I.  FEDERAL CONTAMINANT SPECIFIC REQUIREMENTS

1.  Groundwater Standards - Safe Drinking Water Act  (Relevant and
     Appropriate)8

     The National Primary Drinking Water Standards  (40 CFR Part
141), better known as maximum contaminant levels and maximum
contaminant level goals (MCLs and MCLGs), are not applicable to
the Rocker operable unit area because the aquifer underlying the
area is not a current public water supply, as defined in the Safe
Drinking Water Act, 42 U.S.C. § 300f(4).  These standards are
relevant and appropriate standards, however, because there is
groundwater in the area which is a potential source of drinking
water, ground water use through private wells occurs in the area,
and the aquifer feeds Silver Bow Creek, which is designated as a
potential drinking water source.  In the identification of State
standards following this section, the State notes that the
subject aquifers are Class I and Class II aquifers which means
they have the potential for drinking water use, that State ground
water standards are applicable to the aquifer, and that State
nondegradation standards are also applicable.  This adds
considerable weight to EPA's determination to require cleanup to
identified ground water standards at the Rocker operable unit.

     Use of these standards for this action is fully supported by
EPA regulations and guidance.  The Preamble to the National
Contingency Plan clearly states that MCLs are relevant and
appropriate for ground water that is a current or potential
source of drinking water (55 F.R. 8750 - March 8, 1990), and this
determination is further supported by requirements in the
regulations governing conduct of RI/FS studies found at 40 CFR
Section 300.430(e)(2)(i)(B).  EPA's guidance on Remedial Action
for Contaminated Groundwater at Superfund Sites states that "MCLs
developed under the Safe Drinking Water Act generally are ARARs
for current or potential drinking water sources."  MCLGs which
are above zero are relevant and appropriate under the same
conditions  (55 F.R. 8750-8752 - March 8, 1990).  See also, State
of Ohio v. EPA. 997 F.2d 1520 (D.C. Cir. 1993), which upholds
EPA's application of MCLs and non-zero MCLGs as ARAR standards
for ground water which is a potential drinking water source.  EPA
notes that ARCO,  the identified liable party for the Rocker
operable unit, in its ARARs scoping document submitted to EPA,
agrees that MCLs and non-zero MCLGs are appropriate ARARs for the
Rocker operable unit.

     As noted earlier,  standards such as the MCL and MCLG
standards are promulgated pursuant to both federal and state law.
     8.  42 D.S.C.  Sections 300f et sea.
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Under the Safe Drinking  Water Act,  EPA has  granted  the State  of
Montana primacy in implementation and enforcement of  the Safe
Drinking Water Act.    Pursuant to the Public Water  Safety Act,
75-6-101 et.  seq., M.C.A.  and ARM 16.20.203 and .204,  the MCLs
specified in  40 CFR Part 141  (Primary Drinking Water  Standards)
are  incorporated.

Chemical        MCLG              MCL

A.   Arsenic     N.A.9              0.05 milligrams per liter
                                    (mg/1)10
B.   Barium      2.0 mg/111        2.0 mg/112
C.   Cadmium     0.005 mg/113      0.005 mg/114
D.   Chromium    0.1 mg/115        0.050 mg/116
E.   Copper      1.3 mg/117        1.3 mg/118
F.   Lead        N.A.19             0.015 mg/120
G.   Benzo-
    (a)pyrene    N.A.21             0.0002mg/l22
      9.  An MCLG and a revised MCL for arsenic may be promulgated by EPA in
the near future.  Such standards may be relevant to five-year reviews of the
remedy, conducted pursuant to section 121(c) of CERCLA.

      10.   40 CFR Section  141.11.

      11.   40 CFR Section  141.51.

      12.   40 CFR Section  141.62.

      13.   40 CFR Section  141.51

      14.   40 CFR Section  141.62.

      15.  40 CFR Section 141.51.

      16.   40 CFR Section  141.61.

      17.   40 CFR Section  141.51

      8.   40 CFR Section  141.80(c).  The requirement is an action level
rather than a simple numerical standard.

      19.   The MCLG for lead is zero, which is  not considered appropriate for
Superfund site  cleanups.

      20.   40 CFR Section  141.80(c), which establishes an action level  rather
than a pure numerical standard.


      21
          The MCLG for benzo(a)pyrene  is zero, and  is not considered
appropriate for CERCLA actions.

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H.  Ethyl-
   benzene     0.7 mg/123         0.7 mg/124
I.  Xylene     10.0 mg/125        10.0 mg/126

     Some of these standards  are also incorporated by Resource
Conversation and Recovery Act standards for ground water found at
40 CFR Part 264, Subpart F, which is incorporated pursuant to
State law at ARM 17.54.702.   The RCRA standards are the same or
less stringent than the MCLs  or MCLGs identified above.  Such
standards are relevant and appropriate standards for the Rocker
ou.

     These standards apply throughout the aquifers at and
surrounding the Rocker operable unit.  These standards will
govern the measurement of success of the remedial action, and,
when achieved along with other ground water standards identified
in the State ARAR identification section, will indicate the
completion of remedial action.   The arsenic standard will be used
in part to determine whether  contingency measures shall be
implemented at the Rocker ou,  as described in the ROD.
Compliance points for measurement of remedial action success and
for determining the implementation of contingency measures will
be determined during remedial design.  Remedial Design documents
will also establish which of  the above listed contaminants will
be actually monitored.

2.  Air Standards - Clean Air Act27 (Applicable)

     Limitations on air emissions resulting from cleanup
activities or emissions resulting from wind erosion of exposed
hazardous substances are set  forth in the action specific
requirements, below.
     22.  40 CFR Section 141.61.



     23.  40 CFR Section 141.51.


     24.  40 CFR Section 141.61.


     25.  40 CFR Section 141.51.


     26.  40 CFR Section 141.61.


     27.  Standards are promulgated pursuant to the Clean Air Act  - 42 U.S.C.
sections 7401 et  sea.


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3.  Surface Water - Ambient and Point Source Discharges
      (Applicable).

     CERCLA and the NCP provide that federal water pollution
criteria that match designated or anticipated surface water uses
are the usual surface water standards to be used at Superfund
cleanups, as relevant and appropriate standards, unless the State
has promulgated surface water quality standards pursuant to the
delegated State water quality act.  The State of Montana has
designated uses for Silver Bow Creek, and has promulgated
specific standards accordingly.  Those standards and their
application to the Rocker operable unit are identified in the
State ARAR identification section of this document.  These
standards will be applied to all contaminants of concern
identified in the Rocker operable unit Remedial Investigation,
to point sources created by the Rocker operable unit cleanup and
to ambient water quality in Silver Bow Creek.


II. FEDERAL LOCATION SPECIFIC REQUIREMENTS

1.  Floodplain Management Order (Applicable)

     This requirement (40 CFR Part 6, Appendix A, Executive Order
No. 11,988) mandates that federally funded or authorized actions
within the 100 year flood plain avoid,  to the maximum extent
possible, adverse impacts associated with development of a
floodplain.  Compliance with this requirement is detailed in
EPA's August 6, 1985 "Policy on Floodplains and Wetlands
Assessments for CERCLA Actions."  Based on EPA's analysis of the
current 100 year flood plain, most of the remedial measures to be
taken at the Rocker operable unit will not be within the current
100 year flood plain, including source material excavation and
subsequent re-disposal.   However,  if Rocker operable unit
measures are done within the 100 year flood plain and cause
adverse impacts, specific measures to minimize adverse impacts
may be identified following consultation with the appropriate
agencies.

     If the remedial action selected for the Rocker operable unit
is found to potentially affect the floodplain, the following
information will be produced: a Statement of Findings which will
set forth the reasons why the proposed action must be located in
or affect the floodplain; a description of significant facts
considered in making the decisions to locate in or affect the
floodplain or wetlands including alternative sites or actions; a
statement indicating whether the selected action conforms to
applicable or local floodplain protection standards; a
description of the steps to be taken to design or modify the
proposed action to minimize the potential harm to or within the
floodplain; and a statement indicating how the proposed action
affects the natural or beneficial values of the floodplain.

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2.  Protection of Wetlands Order  (Applicable)

     This requirement  (40 CFR Part 6, Appendix A, Executive Order
No. 11,990) mandates that federal agencies and the potentially
responsible party for the federally required activity avoid, to
the extent possible, the adverse impacts associated with the
destruction or loss of wetlands and to avoid support of new
construction in wetlands if a practicable alternative exists.
Section 404(b)(l), 33 U.S.C. Section 1344(b)(l), also prohibits
the discharge of dredged or fill material into waters of the
United States.  Together, these requirements create a "no net
loss" of wetlands standard.

       An examination of wetlands at the Rocker operable unit was
performed, in consultation with the U.S. Fish and Wildlife
Service.  Wetlands were identified on or near the Rocker operable
unit.  However, the remedial action for the Rocker operable unit
is not expected to have an impact on wetlands.  Further
documentation of this is required during remedial design.

3.  The Endangered Species Act (Applicable)

     This statute and implementing regulations (16 U.S.C.
Sections 1531 - 1543, 50 CFR Part 402, and 40 CFR Section
6.302(h)) require that any federal activity or federally
authorized activity may not jeopardize the continued existence of
any threatened or endangered species or destroy or adversely
modify a critical habitat.

Endangered species were evaluated during the Rocker operable unit
RI/FS.  No endangered species were identified for the Rocker
operable unit.  Therefore, no further action in compliance with
this ARAR is required, unless endangered species use is
subsequently identified and adverse impacts may occur.

4.  The National Historic Preservation Act (Applicable)

     This statute and implementing regulations (16 U.S.C. Section
470, 40 CFR Section 6.310(b), 36 CFR Part 800) require federal
agencies or federal projects to take into account the effect of
any federally assisted undertaking or licensing on any district,
site building, structure, or object that is included in, or
eligible for, the Register of Historic Places.  If.effects cannot
be avoided reasonably, measures should be implemented to minimize
or mitigate the potential effect.  In order to comply with this
ARAR, EPA, MDEQ, and the PRP may consult with the State Historic
Preservation Officer  (SHPO), who can assist in identifying listed
or eligible resources, and in assessing whether proposed cleanup
actions will impact the resources and any appropriate mitigative
measures.  Additionally, in April 1992, ARCO, EPA, MDEQ, SHPO,
the National Council on Historic Preservation, and local
governments entered into a Programmatic Agreement to ensure the

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appropriate consideration of cultural and historical resources in
a systematic and comprehensive manner throughout the Clark Fork
Basin, in connection with response actions at the four Clark Fork
Basin Superfund sites.  The results of the Programmatic Agreement
may provide additional consideration of the factors to be
addressed under this ARAR, and the two historical ARARs described
below.

     Cultural and historical resources were evaluated during the
Rocker operable unit RI/FS.  At this time, the selected remedial
action for the Rocker operable unit is not expected to impact the
identified resources.  This will need continued evaluation and
monitoring during remedial action implementation.

     Indian cultural and historical resources are also subject to
the protections of this act, and have not been addressed under
prior evaluations or the Second Programmatic Agreement.  Such
resources will need further evaluation and identification during
the remedial design process for the Rocker operable unit.

5.  Archaeological and Historic Preservation Act (Applicable)

     The statute and implementing regulations (16 U.S.C. Section
469, 40 CFR Section 6.301(c)) establish requirements for
evaluation and preservation of historical and archaeological
data, which may be destroyed through alteration of terrain as a
result of federal construction projects or a federally licensed
activity or program.  If eligible scientific, prehistorical, or
archaeological artifacts are discovered during site activities,
they must be preserved in accordance with these requirements.

6.  Historic Sites, Buildings, and Antiquities Act (Applicable)

     This requirement states that "in conducting an environmental
review of a proposed EPA action, the responsible official shall
consider the existence and location of natural landmarks using
information provided by the National Park Service pursuant to 36
CFR Section 62.6(d) to avoid undesirable impacts upon such
landmarks.  The Programmatic Agreement activities described above
should aid all parties in compliance with this ARAR.

7.  Migratory Bird Treaty Act of 1918,  as amended (Applicable)

     This requirement (16 U.S.C. Sections 703 et seq.)
establishes a federal responsibility for the protection of the
international migratory bird resource and requires continued
consultation with the U.S. FWS during remedial design and
remedial construction to ensure that the cleanup of the site does
not unnecessarily impact migratory birds.  Specific mitigative
measures may be identified for compliance with this requirement
during remedial design.


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8.  Bald Eagle Protection Act  of 1940, as amended (Applicable)

     This requirement  (16 U.S.C. Sections 668 et  seq.)
establishes a federal responsibility for protection of bald and
golden  eagles, and requires  continued consultation with the U.S.
FWD during remedial design and remedial construction to ensure
that any cleanup of the site does not unnecessarily adversely
affect  the bald and golden eagle.  At this time,  bald or golden
eagles  have not been identified at the Rocker operable unit and
no further efforts are likely  to be required.  However, specific
mitigative measures may be identified for compliance with this
requirement,  if bald or golden eagles are identified at the
Rocker  operable unit.

9.  Resource Conservation and  Recovery Act, as amended
(Applicable for the Treatment  Unit - Relevant and Appropriate for
the Re-Disposal Units)

     Any discrete waste units  created by the Rocker site cleanup
must comply with the siting  restrictions and conditions found at
40 CFR  § 264.18(a) and (b).
III.   FEDERAL ACTION SPECIFIC  REQUIREMENTS

1.  Air  Standards (Applicable)

     These  standards, promulgated pursuant to section 109 of the
Clean  Air Act,  are applicable  during conduct of the  remedial
action to releases into the air from any Rocker operable unit
cleanup  activities.

A. Lead  - No person shall cause or contribute to  concentrations
of lead  in  the ambient air which exceed 1.5 micrograms per cubic
meter  (mg/cm)  of air, measured over a 90-day average.

These  standards are promulgated at ARM Section 16.8.815 as part
of a federally approved State  Implementation Plan (SIP),  pursuant
to the Clean Air Act of Montana,  MCA 75-2-101 et  seq..
Corresponding federal regulations are found at 40 CFR Section
50.12.28
     tyn
         The ambient air standards established as part of Montana's approved
State Implementation  Plan in many cases provide more stringent or additional
standards.  The federal standards by themselves apply only to "major sources",
while the State standards are fully applicable throughout the state and are
not limited to "major sources".  See ARM 16.8.808  and 16.8.811-.821.  As part
of an EPA-approved State Implementation Plan, the  state standards are also
federally enforceable.  Thus, the state standards  which are equivalent to the
federal standards are identified in this section together. A more detailed
list of State standards, which include standards which are not duplicated in
federal regulations,  is contained in the State ARAR identification section.

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B.  Particulate matter that is 10 microns in diameter or smaller
 (PM - 10) - No person shall cause or contribute to concentrations
of PM - 10 in the ambient air which exceed:

- 150 micrograms per cubic meter of air, 24 hour average, no more
than one expected exceedence per calendar year.

- 50 micrograms per cubic meter of air, annual average.

These regulations are promulgated at ARM Section 16.8.821 as part
of a federally approved SIP, pursuant to the Clean Air Act of
Montana, MCA 75-2-101 et sea..  Corresponding federal regulations
are found at 40 CFR Section 50.6.

     Ambient air standards under section 109 of the Clean Air Act
are also promulgated for carbon monoxide, hydrogen sulfide,
nitrogen dioxide, sulfur dioxide, and ozone.  If emissions of
these compounds were to occur at the site in connection with any
cleanup action, these standards would also be applicable.  See
ARM 16.8.811, .814, .816, .817, and .820 and 40 CFR Part 50.

C.  Asbestos - Standards promulgated at 40 CFR Section 61.145 and
61.150 govern demolition and waste disposal for asbestos
demolition operations.  If asbestos is encountered during any
Rocker operable unit cleanup, these standards would be
applicable.

2.  Solid Waste  (Applicable), Surface Mining Control and
     Reclamation (Relevant and Appropriate), and RCRA (Applicable
     and Relevant and Appropriate) Requirements

     Upon further examination of the waste-types present at the
Rocker operable unit,  EPA has determined that RCRA requirements
are not applicable to the source material waste after treatment,
if the excavated and treated source material waste does not fail
characteristic criteria for hazardous waste found at 40 CFR Part
261 Subpart C, which is incorporated into applicable State law,
following treatment.  This determination is based on the limited
knowledge of the treatment plant process, the source of arsenic
at the site, and the mixture of various waste sources at the
Rocker operable unit.   Instead, the following solid waste and
other requirements described in 2.A, B., and C. are applicable or
relevant and appropriate to the re-disposal of Rocker operable
unit wastes.  RCRA requirements which are applicable to the
excavation and treatment of the excavated source material waste
prior to treatment, assuming the waste fails characteristic
criteria for RCRA hazardous waste, follow at 2.D.
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      A.   Requirements described at 40 CFR Sections  257.3-1(a),
257.3-3, and 257.3-4, governing waste handling, storage, and
disposal in general29.

      B.   Reclamation and closure regulations found  at 30 CFR
Parts 816 and  784,  governing  coal and to  a lesser extent, non-
coal  mining, are relevant and appropriate requirements.   These
requirements are also relevant and appropriate to the capping and
revegetation of  contaminated  soils at the site outside of the
excavation area,  and require  a minimum  18 inch soil growth medium
as  a  cap.30

      C.   RCRA  regulations found at 40 CFR Sections  264.116 and
.119  (governing  notice and deed restrictions), and  Sections
264.228(a) (2) (iii) (B), (C), and (D) and .251(c)f  (d),  and  (f)
(regarding run-on and run-off controls),  are relevant and
appropriate requirements for  the type of  waste planned for re-
disposal following treatment  at the Rocker operable unit.31

      D.   RCRA  regulations found at the  following regulations are
applicable to  the excavation  and treatment of the source
material:
      i.   Standards for Generators of Hazardous Waste

The RCRA regulations at 40 CFR Part 262 establish standards that
apply to generators of hazardous waste.   These standards include
§ 262.34 which allows for short-term on-site accumulation of
hazardous waste  in containers.   The substantive standards at 40
CFR Part 262 are applicable for any generation (including
excavation)  of hazardous waste.
      29.   Solid Waste regulations are promulgated pursuant to the federal
Solid Waste Disposal Act,  as amended by the Resource Conversation and Recovery
Act,  42 U.S.C.  6901 et sea.  They are applicable  regulations, although the
State of Montana has the lead role in regulating  solid waste disposal in the
State of Montana.

          The Surface Mining Control and Reclamation Act is promulgated at 30
U.S.C.  Sections 1201 - 1326.

      31.   As noted earlier, federal RCRA regulations are incorporated by
reference into  applicable  State Hazardous Waste Management Act regulations.
See ARM 17.54.702.  Use of select RCRA regulations for the active management
of solid waste,  including  mining waste, is appropriate when discrete units are
addressed by a  cleanup and site conditions are distinguishable from EPA's
generic determination of low toxicity/high volume status for mining waste.
See Preamble to the Final  NCP, 55 Fed. Reg. 8763  - 8764 (March 8, 1990),
CERCLA Compliance with Other Laws Manual, Volume  II (August 1989 OSWER Dir.
9234.1-02) p.  6-4; Preamble to Proposed NCP, 53 Fed. Reg. 51447  (Dec. 21,
1988),  and guidance entitled "Consideration of RCRA Requirements in Performing
CERCLA Responses at Mining Wastes Sites," August  19, 1986  (OSWER). Here,  site
conditions and  waste characteristics make the limited RCRA requirements cited
here  relevant and appropriate requirements.

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      ii.   Standards for Transporters of  Hazardous Waste

The RCRA  regulations at 40 CFR Part 263,  establish standards that
apply to  transporters of hazardous waste.   These standards
include requirements for immediate action for hazardous waste
discharges.  These substantive standards  are applicable for any
on-site transportation.

      iii.   Containers

The RCRA  regulations at 40 CFR Part 265,  subpart I, establish
standards  that apply to short-term on-site accumulation of
hazardous  waste in containers.32 These substantive standards are
applicable for any short-term on-site accumulation and treatment
of hazardous waste in containers.

      iv.   Miscellaneous Unit

The RCRA  regulations at 40 CFR Part 264,  Subpart X, establish
standards  that apply to miscellaneous units for the treatment,
storage,  and disposal of hazardous wastes.  These standards
include design and operating requirements  designed to protect
human health and the environment. These  substantive standards are
applicable for any on-site treatment or  storage of hazardous
wastes in  a miscellaneous unit.

      v.  Chemical,  Physical, and Biological Treatment

The RCRA regulations at 40 CFR Part 265,  subpart Q, establish
standards  that apply to chemical, physical,  and biological
treatment  in miscellaneous units. These  sbustantive standards are
applicable,  along with 40 CFR Part 264,  Subpart X, to any
chemical or physical treatment in a miscellaneous unit.

      vi.   Waste Piles

40 CFR Part  264,  Subpart L, applies to owners and operators of
facilities that store or treat hazardous waste in piles.33  The
regulations  include requirements for the use of run-on and run-
off control  systems and collection and holding systems to prevent
the release  of contaminants from waste piles unless certain
specified  criteria are met. These substantive standards are
applicable to any storage in waste piles at the site.
  32 A container is defined as any portable device in which a material is stored, transported, treated, disposed or otherwise
handled. § 260.10


  33"Pile" means any non-containerized accumulation of solid, nonflowing hazardous waste that is used for
treatment or storage. 40 CFR 5 260.10.


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3.  Point Source  (Applicable)

     If point sources of water contamination are created by any
Rocker remediation activity, applicable Clean Air Act and Clean
Water Act standards would apply to those discharges.  Specific
parameters for such discharges are not identified here, but are
reflected in the identification of State of Montana ARAR's
section of this document.  These regulations would also include
storm water runoff regulations found at 40 CFR Parts 121, 122,
and 125 (general conditions and industrial activity conditions).

4.  Dredge and Fill Requirements (Applicable)

     Regulations found at 40 CFR Part 230 address conditions or
prohibitions against depositing dredge and fill material into
water of the United States.  If remediation activities would
result in an activity subject to these regulations, they would be
applicable.

5.  Underground Injection Control (Applicable)

     Requirements found at 40 CFR Part 144, promulgated pursuant
to the Safe Drinking Water Act, allow the re-injection of treated
ground water into the same formation from which it was withdrawn
for aquifers such as the aquifer beneath the Rocker site, and
address injection well construction, operation, maintenance, and
capping/closure.  These regulations would be applicable to any
reinjection of ground water.

IV.  TO BE CONSIDERED DOCUMENTS (TBCs)

     The use of documents identified as TBCs is addressed in the
introductory portion of the ARAR identification.  A list of TBC
documents is included in the Preamble to the NCP, 55 Fed. Reg.
8765 (March 8, 1990).  Those documents, plus any additional
similar or related documents issued since that time, will be
considered by EPA and MDHES during the conduct of the RI/FS,
during remedy selection, and during remedy implementation.

V.  OTHER LAWS (Non-exclusive list)

     CERCLA defines as ARARs only federal environmental and state
environmental and facility siting laws.  Remedial design,
implementation, and operation and maintenance must nevertheless
comply with all other applicable laws,  both state and federal, if
the remediation work is done by parties other than the federal
government or its contractors.  The following "other laws" list,
both here and in the State ARAR section of this document, are
included to provide a reminder of other legally applicable
requirements for actions being conducted at the Rocker operable
unit.  They do not purport to be an exhaustive list of such legal
requirements, but are included because they set out related

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concerns that must be addressed and, in some cases, may require
some advance planning.  They are not included as ARARs because
they are not "environmental or facility siting laws."  As
applicable laws other than ARARs, they are not subject to ARAR
waiver provisions.

Section 121(e)  of CERCLA exempts removal or remedial actions
conducted entirely on-site from federal, state, or local permits.
This exemption is not limited to environmental or facility siting
laws, but applies to other permit requirements as well.

1.  The federal Occupational Health and Safety Act regulations
found at 29 CFR Section 1910.95 are applicable to worker
protection during conduct of RI/FS or remedial activities.  Such
requirements must be addressed in a Health and Safety Plan for
the remedial action implementation, and are independently
enforceable by OHSA.

2.  Off-Site Transportation of Hazardous or Contaminated Waste

     Any off-site transportation and disposal of waste or debris
would be subject to applicable laws and regulations.  Such
requirements are not analyzed in detail here.
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STATE OF MONTANA ARARS

As provided by Section  121  of  CERCLA,  42 U.S.C. § 9621, only
those state standards that  are more stringent than any federal
standard and that have  been identified by the state in a timely
manner are appropriately  included as ARARs.   To be an ARAR, a
state standard must also  be "promulgated",  which means that the
standards are of general  applicability and are legally
enforceable.

VI.  MONTANA CONTAMINANT  SPECIFIC REQUIREMENTS

1.   Surface Water Quality  Standards - Ambient and Point Source
      (Applicable)

     If a point source  is created by the Rocker operable unit
remediation, the following  standards are applicable:

ARM 16.20.604(1)(b)(Applicable) provides that Silver Bow Creek
(mainstem) from the confluence of Blacktail  Deer Creek to Warm
Springs Creek is classified "I" for water use.

The "I" classification  standards  are contained in ARM 16.20.623
(Applicable) of the Montana water quality regulations.  This
section states:

[T]he goal of the state of  Montana is  to have these waters fully
support the following uses:  drinking,  culinary, and food
processing purposes after conventional treatment;  bathing, ,
swimming, and recreation; growth  and propagation of fishes and
associated aquatic life,  waterfowl,  and furbearers; and
agricultural and industrial water supply.

ARM 16.20.604(1)(b) allows  a gradual attainment of WQB-7
requirements in already impacted  streams by  providing that point
source discharges be permitted at the  higher concentration of  (1)
the applicable standards  specified in  department Circular WQB-7,
(2) the site-specific standards,  or (3)  one-half of the mean
instream concentrations34  immediately upstream of the discharge
point.  This effectively  requires eventual attainment of the
Circular WQB-7 levels in  the stream, while allowing consideration
of the current, impacted  stream quality (a graduated reduction of
point source discharge  concentrations  based  on the mean instream
concentration where the stream is substantially degraded).

For the primary contaminants of concern,  the WQB-7 levels are
listed below.  WQB-7 provides  that "whenever both Aquatic Life
Standards and Human Health  Standards exist  for the same analyte,
     Mean instream concentration is the monthly mean instreahl concentration, as defined by the MDEQ Water Quality
     Bureau.


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the more restrictive  of these values  will  be used  as the numeric
Surface  Water Quality Standard."

Surface  water standards:

Arsenic:  18  /xg/135
Acenaphthene:  20  MSfA36
Barium:  1,000 /xg/1
Benzo [a] anthracene: 0.044  jzg/138
Benzo [b] f louranthene:  0.044 p.g/139
Benzo [k] Flouranthene:  0.044 /xg/140
Benzo [a] Pyrene: 0.02  /xg/141
Cadmium:  1.1 /xg/142
Copper:  12 /ig/143
Dibenz [a,h] Anthracene:  0.044 /xg/144
Iron: 300 p.g/145
Lead: 3.2 /ig/146
Manganese: 50 ug/147
Zinc: 110 /xg/148

I  classification  standards also include the following criteria:
  "   Human Health Standard.


  *   Human Health Standard.


  37   Human Health Standard.


  38   Human Health Standard.
   39   Human Health Standard.


   40   Human Health Standard.


   41   Human Health Standard.

   0   Chronic Aquatic Life Standard based on 100 mg/1 hardness.


   49   Chronic Aquatic Life Standard based on 100 mg/1 hardness.


   44   Human Health Standard.


   43   Human Health Standard.


   46   Chronic Aquatic Life Standard based on 100 mg/1 hardness.


   47   Human Health Standard.


   a   Chronic Aquatic Life Standard based on 100 mg/1 hardness.


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     1.   Dissolved oxygen concentration must not be reduced
          below 3.0 milligrams per liter.

     2.   Hydrogen ion concentration  (pH) must be maintained
          within the range of 6.5 to 9.5.

     3.   No increase in naturally occurring turbidity,
          temperature, concentrations of sediment and settleable
          solids, oils, floating solids, or true color is allowed
          which will or is likely to create a nuisance or render
          the waters harmful, detrimental, or injurious to public
          health, recreation, safety, welfare, livestock, wild
          animals, birds, fish or other wildlife.

     4.   No discharges of toxic, carcinogenic, or harmful
          parameters may commence or continue which lower or are
          likely to lower the overall water quality of these
          waters.

Additional restrictions on any discharge to surface waters are
included in:

     ARM 16.20.633 (Applicable), which prohibits discharges
containing substances that will:
          (a)  settle to form objectionable sludge deposits or
emulsions beneath the surface of the water or upon adjoining
shorelines;
          (b)  create floating debris, scum, a visible oil film
(or be present in concentrations at or in excess of 10 milligrams
per liter)  or globules of grease or other floating materials;
          (c)  produce odors, colors, or other conditions which
create a nuisance or render undesirable tastes to fish flesh or
make fish inedible;
          (d).  create concentrations or combinations of materials
which are toxic or harmful to human, animal, plant, or aquatic
life;
          (e)  create conditions which produce undesirable
aquatic life.

     ARM 16.20.925 (Applicable), which adopts and incorporates
the provisions of CFR Part 125 for criteria and standards for the
imposition of technology-based treatment requirements in MDEQ
permits.  Although the permit requirement would not apply to on-
site discharges,  the substantive requirements of Part 125 are
applicable,  i.e.,  for toxic and nonconventional pollutants,
treatment must apply the best available technology economically
achievable (BAT);  for conventional pollutants, application of the
best conventional pollutant control technology (BCT)  is required.
Where effluent limitations are not specified for the particular
industry or industrial category at issue, BCT/BAT technology-
based requirements are determined on a case by case basis using


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best professional judgment  (BPJ). See CERCLA Compliance with
Other Laws Manual, Vol. I, August 1988, p. 3-4 and 3-7.

Applicable for both surface water and ground water, § 75-5-605,
MCA, provides that it is unlawful to cause pollution as defined
in § 75-5-103 of any state waters or to place or cause to be
placed any wastes where they will cause pollution of any state
waters.  Because the Rocker operable unit excavated source
material waste will be treated and re-disposed of in a manner
such that releases will be minimized and further attenuated by
the iron addition to ground water immediately below the re-
disposed waste so that releases of contaminants above state WQB-7
standards beyond the immediate attenuation zone (the iron plug
immediately below the re-disposed solid waste) will not occur,
the selected Rocker operable unit remedial action is expected to
be in compliance with this requirement.  Remedial Design should
demonstrate this.

Section 75-5-308, MCA, allows MDEQ to grant short-term exemptions
from the water quality standards or short-term use that exceeds
the water quality standards for the purpose of allowing certain
construction or emergency environmental remediation activities.
Such exemptions typically extend for a period of 30 to 60 days,
but may be extended beyond this time frame for this action.
However, any exemption must include conditions that minimize to
the extent possible the magnitude of the violation and the length
of time the violation occurs.  In addition, the conditions must
maximize he protection of state waters by ensuring the
maintenance of beneficial uses immediately after termination of
the exemption.  Water quality and quantity monitoring and
reporting may also be included as conditions.  The addition of
iron to the ground water as part of the Rocker operable unit
remedial action qualifies for this exemption.  The exact nature
of application will be described in Remedial Design,  to further
indicate compliance with this standard.

     Because ground water at the Rocker operable unit does flow
into Silver Bow Creek, the standards identified above regarding
contaminant specific water quality parameters are also ambient
standards for the Rocker ou, and exceedances of these standards
from Rocker ou contamination must be prevented.  These standards
and the beneficial uses for Silver Bow Creek are considered
supported when the concentrations of toxic, carcinogenic,  or
harmful parameters in these waters do not exceed the applicable
standards specified in department Circular WQB-7 identified above
when stream flows equal or exceed the stream flows specified in
ARM 16.20.631(4)  (10 year 7 day low flow, i.e., minimum
consecutive 7 day average flow which may be expected to occur on
the average of once in 10 years).

     As noted in the Record of Decision explicitly, exceedances
of the arsenic standard in surface water from Rocker ou

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contamination may be the basis for contingency measures  to
intercept and control the groundwater plume.

2.   Groundwater Quality Standards

     In addition to the standards set forth below, relevant and
appropriate MCLs and MCLGs are included in the federal ARARs
identified above.

Montana Groundwater Pollution Control System  (Applicable)

ARM 16.20.1002 classifies groundwater into Classes I through  IV
based on the present and future most beneficial uses of  the
groundwater, and states that ground water is  to be classified
according to actual quality or actual use, whichever places the
ground water in a higher class.  Class I is the highest  quality
class; Class IV the lowest..  Based upon its  specific
conductance, the groundwater in and around the Rocker operable
unit, including the three aquifers described  in the ROD, should
be considered Class I or Class II groundwater.

ARM 16.20.1003  establishes the groundwater quality standards
applicable with respect to each groundwater classification.
Concentrations of dissolved substances in Class I or II
groundwater may not exceed the human health standards listed  in
department Circular WQB-7.  For the primary contaminants of
concern, these levels are listed below.  Levels that are more
stringent than the MCL or MCLG identified in  the federal portion
of the ARARs are set out in underlined type.

Groundwater Standards;

Arsenic; 18
Acenaphthene ; 20
Barium; 1.000 /xg/1
Benzo Fa] anthracene ; 0 . 044 /zg'/l
Benzo Tbl flouranthene; 0.044 /xg/1
Benzo Tkl Flouranthene; 0.044 jug/1
Benzo [a] Pyrene: 0.02 /*g/l
Cadmium: 5 /ig/1
Copper: 1.000 /zg/1
Dibenz fa. hi Anthracene; 0.044 jug/1
Iron; 300 fig /I
Lead: 15 pig/1
Manganese; 50 /xg/1
Zinc; 5000 /xg/1

     These standards apply throughout the aquifers at and
surrounding the Rocker operable unit.  These standards will
govern the measurement of success of the remedial action, and,
when achieved, will indicate the completion of remedial action
The arsenic standard will be used in part to determine whether

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contingency measures shall be implemented at the Rocker ou, as
described in the ROD.  Compliance points for measurement of
remedial action success and for determining the implementation of
contingency measures will be determined during remedial design.
Remedial Design documents will also establish which of the above
listed contaminants will be actually monitored.

ARM 16.20.1003 also requires that concentrations of dissolved or
suspended substances must not exceed levels which render the
waters harmful, detrimental or injurious to public health.
Maximum allowable concentrations of these substances also must
not exceed acute or chronic problem levels which would adversely
affect existing beneficial uses or the designated beneficial uses
of groundwater of that classification.

The 1995 Montana Legislature enacted several revisions to the
Montana Water Quality Statutes.  Except as reflected in the
analysis above, none of these changes has altered the application
of these water quality requirements to the Rocker operable unit.
One provision exempted from the permit requirements certain
discharges from a water conveyance structure or certain ground
water discharged to surface water, but these exemptions do not
apply if the discharged water contains "industrial waste."  See
§ 75-5-401;  MCA, as amended.  "Industrial waste" means a waste
substance from the process of business or industry or from the
development of any natural resource ..."  § 75-5-103(10), MCA.
Since the contamination found in the water in this operable unit
is industrial waste, these new exemptions would not apply here.

Because the ground water at the Rocker operable unit has not
demonstrated loading or effect on the surface water near the
site,  additional remediation beyond the above identified ground
water standards is not required.

3.  Air Standards - Montana Clean Air Act (Applicable)

     Limitations on air emissions resulting from cleanup
activities or emissions resulting from wind erosion of exposed
hazardous substances are set forth in the federal action specific
requirements and the action specific requirements,  below.
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VII.  MONTANA LOCATION  SPECIFIC REQUIREMENTS

1.    Solid Waste Management Regulations49  (Applicable)

Regulations promulgated under  the Solid Waste Management  Act,  §§
75-10-201  et seq..  MCA,  specify requirements that  apply to the
location of any  solid  waste management facility.50

Under ARM  17.50.505(1)  (formerly 16.14.505(1)), a  facility for
the treatment, storage or disposal of solid  wastes:

      (a) must be located where a sufficient  acreage of suitable
      land  is available for solid waste management;

      (b) may not be located in a 100-year  floodplain;

      (c) may be  located only in areas which  will prevent  the
      pollution of ground and surface  waters  and public and
      private water supply systems;

      (d) must be located to allow for reclamation  and reuse of
      the land;

      (e) drainage structures must be  installed where necessary to
      prevent surface runoff from entering  waste management areas;
      and

      (f) where underlying geological  formations contain rock
      fractures or fissures which may  lead  to pollution of the
      ground water or areas in  which springs  exist  that are
      hydraulically connected to a proposed disposal  facility,
      only  Class  III disposal facilities may  be approved31.

The unit or units created by the excavation  of the source
materials,  subsequent  treatment,  and  redisposal, are subject to
   49   Solid wastes are regulated pursuant to Article 17, Chapter SO while hazardous wastes are regulated pursuant to
      Article 17, Chapter 54. If material, including contaminated soils, remains characteristic after treatment, it must be
      managed pursuant to Article 17, Chapter 54.

   50   Under ARM 17.50.503 (formerly 16.14.503), solid wastes are grouped into two categories: Group II and Group
      in wastes. Group III wastes include wood wastes and non-water solids (including inert solid waste such as brick,
      dirt, rock and concrete and industrial mineral wastes which are essentially inert and non-water soluble and do not
      contain hazardous constituents). Group II wastes include decomposable wastes and mixed solid wastes containing
      decomposable material but excluding regulated hazardous waste. Pursuant to ARM 17.50.503, clean fill is not a
      waste. Wastes at mis operable unit not classified as hazardous wastes are Group II wastes.


         Group III wastes  consist of primarily inert wastes,  including
"industrial mineral  wastes which are essentially inert  and non-water soluble
and do not contain hazardous waste constituents."   ARM 17.50.503(1) (b)
(formerly 16.14.503(1)(b)). The highly contaminated and leachable Rocker
operable unit  waste  would not  qualify as Class III waste.

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these requirements, as such activity is obviously  active
management  and disposal of solid waste material  -  see footnote 36
of the Streamside Tailings operable unit Record  of Decision ARAR
identification.  The primary  excavation and re-disposal area
identified  in the Rocker operable unit ROD, which  is  outside of
the current 100 year flood plain,  is in compliance with these
requirements.   If additional  re-disposal units are required, such
as the Streamside excavation  areas suggested in  the ROD, remedial
design shall demonstrate compliance with these requirements.

The capping and revegetation  of contaminated soils above 380
parts per million arsenic without excavation also  can comply with
these requirements, because most if not all of that activity will
occur outside of the current  100 year flood plain.  Because the
Streamside  Tailings operable  unit extends through  the Rocker
operable  unit and addresses waste within the 100 year flood
plain, and  thus such wastes are not addressed here.

2.   Floodplain and Floodway  Management Act and  Regulations52
      (Applicable)

The Streamside Tailings operable unit remedial action is expected
to address  most if not all of the contamination  within the
current 100 year floodplain of Silver Bow Creek  at and near the
Rocker operable unit.  However,  if during Remedial Design, Rocker
operable  unit activities are  determined to be necessary within
the current 100 year flood plain,  the following  requirements
would be  applicable to those  actions.

The Floodplain and Floodway Management Act and regulations
specify types of uses and structures that are allowed or
prohibited  in the designated  100-year floodway and floodplain.
Since the SST Operable Unit lies primarily within  the 100-year
floodplain  of Silver Bow Creek,  these standards  are applicable to
all actions contemplated for  this operable unit.

A.   Allowed uses

The law recognizes certain uses as allowable in  the floodway and
a broader range of uses as allowed in the floodplain.
Residential use is among the  possible allowed uses expressly
recognized  in both the floodway and floodplain.   "Residential
uses such as lawns, gardens,  parking areas, and  play  areas," as
      The "floodway" is the channel of a watercourse or drainway and those portions of the floodplain adjoining the
     channel which are reasonably required to carry and discharge the floodwater of the watercourse or drainway.
     ARM 36.15.101(13).


     The "floodplain" is the area adjoining the watercourse or drainway which would be covered by the floodwater of a
     base (100-year) flood except for sheetflood areas that receive less than one foot of water per occurrence. The
     floodplain consists of the floodway and flood fringe.


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well as certain agricultural, industrial-commercial, recreational
and other uses are permissible within the designated floodway,
provided they do not require structures other than portable
structures, fill or permanent storage of materials or equipment.
§ 76-5-401, MCA; ARM 36.15.601 (Applicable).  In addition, in the
flood fringe (i.e., within the floodplain but outside the
floodway), residential, commercial, industrial, and other
structures may be permitted subject to certain conditions
relating to placement of fill, roads, floodproofing, etc.  § 76-
5-402, MCA; ARM 36.15.701 (Applicable).   Domestic water supply
wells may be permitted, even within the floodway, provided the
well casing is watertight to a depth of 25 feet and the well
meets certain conditions for floodproofing, sealing, and positive
drainage away from the well head.  ARM 36.15.602(6).

B.   Prohibited uses

Uses prohibited anywhere in either the floodway or the floodplain
are:

     1.   solid and hazardous waste disposal; and
     2.   storage of toxic,  flammable, hazardous, or explosive
          materials.
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ARM  36.15.605(2)  and  36.15.703   (Applicable53);  see  also  ARM
36.15.602(5)(b)   (Applicable).

In the floodway,  additional  prohibitions apply,   including
prohibition  of:

        1.     a  building  for living purposes or  place  of  assembly  or
               permanent  use  by human  beings;

        2.     any  structure  or excavation that will  cause water  to be
               diverted  from  the  established  floodway,   cause erosion,
               obstruct  the natural  flow of water,  or reduce the
               carrying  capacity  of  the  floodway;  and

        3.     the  construction or permanent  storage  of  an object
               subject to  flotation  or movement during  flood level
               periods.

§  76-5-402,   MCA  (Applicable).
       One commenter asserted that these regulations are not applicable to the Rocker OU. EPA and MDEQ have
       evaluated these arguments and have determined that these are applicable requirements. Under the NCP, 40 CFR §
       300.400(g)(l), EPA and MDEQ must make an "objective determination of whether the requirement specifically
       addresses a hazardous substance, pollutant, contaminant, remedial action, location, or other circumstance found" at
       the site. EPA and MDEQ have made the determination here that these requirements specifically address the
       hazardous substances and location involved and are applicable legal requirements.  While these prohibitions are '
       applicable requirements, exactly how these prohibitions  apply to specific mining wastes being addressed in this
       operable unit and the manner in which these prohibitions apply to specific actions requires some analysis. The
       floodplain management regulations include a version of  mis prohibition in three different provisions. ARM
       36.15.605(2) and 36.15.703, applicable to the floodway and the flood fringe, respectively, state this prohibition
       generally as noted above.  ARM 36.15.602(5)(b), applicable to the floodway, allows storage of materials and
       equipment under certain conditions, but provides "Storage of flammable, toxic, or explosive materials shall not be
       permitted."

       Neither the regulations nor the Floodplain Management  Act defines the terms disposal, storage, solid waste,
       hazardous waste, toxic materials or hazardous materials. In most contexts, the regulations are clear enough, by
       their plain meaning, to be easily implementable. As applied to the specific circumstances at this operable unit,
       however, these terms require some interpretation. The initial disposal of these materials does not constitute a
       violation of the regulations. However, actions taken to  actively manage these materials as part of the remedial
       action effectively trigger applicability of such requirements in certain circumstances.

       Summarized here, the agency's analysis has determined  that the arsenic pole treatment waste and tailings and
       mining wastes in the Rocker OU are included in the term solid wastes, as well as the terms toxic materials or
       hazardous materials, and that the prohibition on the disposal or storage of those wastes/materials within the
       floodplain applies to actions which constitute the active management/disposal of those wastes as part of the
       remedial action. The agencies further note that, if there were some jurisdictional prerequisite which were
       technically not met for applicability, the requirements identified here would be relevant and appropriate
       requirements as described for this remedial action. In such case, the agencies would apply these requirements as
       relevant and appropriate considering the factors set forth at 40 CFR § 300.400(g)(2)(i) through (viii).
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C.   Applicable considerations in use of floodplain or floodway

Applicable regulations also specify factors that must be
considered in allowing diversions of the stream, changes in place
of diversion of the stream, flood control works, new construction
or alteration of artificial obstructions, or any other
nonconforming use within the floodplain or floodway.  Many of
these requirements are set forth as factors that must be
considered in determining whether a permit can be issued for
certain obstructions or uses.  While permit requirements are not
directly applicable to remedial actions conducted entirely on
site, the substantive criteria used to determine whether a
proposed obstruction or use is permissible within the floodway or
floodplain are applicable standards.  Factors which must be
considered in addressing any obstruction or use within the
floodway or floodplain include:

     1.   the danger to life and property from backwater or
          diverted flow caused by the obstruction or use;

     2.   the danger that the obstruction or use will be swept
          downstream to the injury of others;

     3.   the availability of alternate locations;

     4.   the construction or alteration of the obstruction or
          use in such a manner as to lessen the danger;

     5.   the permanence of the obstruction or use; and

     6.   the anticipated development in the foreseeable future
          of the area which may be affected by the obstruction or
          use.

See § 76-5-406,  MCA; ARM 36.15.216  (Applicable, substantive
provisions only).   Conditions or restrictions that generally
apply to specific activities within the floodway or floodplain
are:

     1.   the proposed activity,  construction, or use cannot
          increase the upstream elevation of the 100-year flood a
          significant amount (^ foot or as otherwise determined
          by the permit issuing authority)  or significantly
          increase flood velocities, ARM 36.15.604 (Applicable,
          substantive provisions only); and

     2.   the proposed activity,  construction, or use must be
          designed and constructed to minimize potential erosion.

For the substantive conditions and restrictions applicable to
specific obstructions or uses,  see the following applicable
regulations:

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      Excavation of  material  from pits or pools - ARM
      36.15.602(1).

      Water diversions or changes in place of  diversion  -  ARM
      36.15.603.

      Flood control  works (levees,  floodwalls,  and riprap  must
      comply with  specified safety standards)  - ARM 36.15.606.

      Roads, streets,  highways  and rail lines  (must be designed to
      minimize increases in flood heights) - ARM 36.15.701(3)  (c).

      Structures and facilities for liquid or  solid waste
      treatment and  disposal  (must be floodproofed to ensure that
      no pollutants  enter flood waters and may be allowed  and
      approved only  in accordance with MDEQ regulations, which
      include certain additional prohibitions  on such disposal)  -
      ARM 36.15.701(3)(d).

      Residential  structures  -  ARM 36.15.702(1).

      Commercial or  industrial  structures - ARM 36.15.702(2).
VIII.      MONTANA ACTION SPECIFIC REQUIREMENTS

1.  Air Quality Standards54  (Applicable)  (Excavation, earth-
     moving, transportation, treatment system operation)

Dust suppression and control of  certain substances likely  to be
released into the air as a result of earth moving,
transportation, and similar actions may be necessary to meet air
quality requirements.  Certain ambient air standards for specific
contaminants and particulates are set forth  in  the federal action
specific section above.   Additional air quality regulations  under
the state Clean Air Act, §§ 75-2-101 et seq.. MCA, are discussed
below.

ARM 16.8.818.  Ambient air quality standard  for settled
particulate matter.   Particulate matter concentrations in  the
ambient air shall not exceed the following 30-day average:   10
grams per square meter.
     Each of the ambient air quality standards includes in its terms specific requirements and methodologies for
     monitoring and determining levels. Such requirements are also applicable requirements. In addition, ARM
     16.8.807 and 16.8.809, Ambient Air Monitoring; Methods and Data, respectively (Applicable), require that all
     ambient air monitoring, sampling and data collection, recording, analysis and transmittal shall be in compliance
     with the Montana Quality Assurance Manual except when more stringent requirements are determined by DEQ to
     be necessary.


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Montana has promulgated standards to regulate emissions of
certain contaminants into the air. The state emission standards
are enforceable under the Montana Clean Air Act, §§ 75-2-101 et
seq. ,  MCA.

The following air emission standards are applicable at the site:

ARM 16.8.1401.  Airborne Particulate Matter.  Emissions of
airborne particulate matter from any stationary source shall not
exhibit an opacity of 20 percent or greater, averaged over six
consecutive minutes.  This standard applies to the production,
handling, transportation, or storage of any material; to the use
of streets, roads, or parking lots; and to construction or
demolition projects.

ARM 16.8.1404.  Visible Air Contaminants.  No source may
discharge emissions into the atmosphere that exhibit an opacity
of 20 percent or greater, averaged over six consecutive minutes.
This standard is limited to point sources, but excludes wood
waste burners, incinerators, and motor vehicles.

ARM 16.8.1427.  Odors.  If a business or other activity will
create odors, those odors must be controlled, and no business or
activity may cause a public nuisance.

The Butte area, which includes Rocker, has been designated by EPA
as non-attainment for total suspended particulates, as well as
PM-10.  40 CFR § 81.327.  ARM 16.8.1401(4) requires that any new
source of airborne particulate matter that has the potential to
emit less than 100 tons per year of particulates shall apply best
available control technology (BACT); any new source of airborne
particulate matter that has the potential to emit more than 100
tons per year of particulates shall apply lowest achievable
emission rate  (LAER).  The BACT and LAER standards are defined in
ARM 16.8.1430.

ARM 26.4.761  (Relevant and Appropriate) specifies a range of
measures for controlling fugitive dust emissions during mining
and reclamation activities.  Some of these measures are relevant
and appropriate to control fugitive dust emissions in connection
with excavation, earth moving,  mixing, and transportation
activities conducted as part of the remedy at the site.

ARM 16.8.1103 requires sources for which air quality permits are
required to use best available control technology  (BACT) or to
meet the lowest achievable emission rate  (LAER), as applicable.
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2.     Water Quality  Statute and  Regulations  (Applicable):

a.  Nondegradation:  Section 75-5-303,  MCA35  states that  existing
uses of  state waters and the  level  of water  quality  necessary to
protect  the uses  must  be maintained and protected. 6

ARM  16.20.1011  provides  that  any  groundwater whose  existing
quality  is higher than the standard for its  classification  must
be maintained at  that  high quality  unless  degradation  may be
allowed  under the principles  established in  §  75-5-303,  MCA,  and
the  nondegradation  rules at ARM 16.20.706  et sea.
   55 As modified by Chapters 495 through 501, Laws of Montana, 1995.

   56   Pursuant to 75-5-317, MCA, the following sources of pollution are considered nonsignificant activities, and not
       subject to the nondegradation rules promulgated pursuant to 75-5-303:

       1) existing activities (as of April 29, 1993) mat are non-point sources of pollution;

       2) existing activities after April 29, 1993 when reasonable land, soil and water conservation is applied and existing
       and anticipated uses will be fully protected

       3) changes in existing water quality resulting from an emergency or remedial activity that is designed to protect the
       public health or the environment and is approved, authorized, or required by the department;

       4) the use of fluids, sealants, additives, disinfectants, and rehabilitation chemicals in water well or monitoring well
       drilling, development, or abandonment, if used according to department-approved water quality protection
       practices;

       5) discharges of water from water well or monitoring tests, hydrostatic pressure and leakage tests conducted in
       accordance with department-approved practices;

       6) short-term changes allowed under 75-5-308 (short-term exemptions)

       7) nonpoint sources mat cause short-term changes in existing water quality resulting from customary activities
       involving the use of water established by an existing water right or state permit;

       8) any other activity mat is nonsignificant because of its low potential for harm to human health and the
       environment in conformance with the new criteria required to be established in 301(5)(c).

       Although a number of the exemptions refer to nonpoint sources, it is important to note that EPA's definition of
       point source is quite expansive. The term point source is defined to include any discernable, confined, and discrete
       conveyance from which pollutants are or may be discharged. 33 USC §1362(14). EPA has proposed that
       discernable non-process discharges from mill tailings and other locations at a mine site are subject to point source
       permit requirements, see EPA draft Memorandum entitled, "Legal Bases to Expand the Clean Water Act
       Definition of Point Source in the Context of Mine Sites," from Dana J. Stotsky, Attorney, Air Water, and General
       Law Section,  Office of Regional Counsel to Mike Reed, Chief, Compliance Section, Region VIII and Melanie
       Pallman, Environmental Engineer, Compliance Section, Region VIE, dated October 28, 1991. Courts have also
       upheld EPA's expansive definition, see e.g.  Washington Wilderness Coalition v. Hecla Mining Co. 870 F. Supp.
       983 (E.D. Wash. 1994). Therefore, exemptions for nonpoint sources should not affect the implementation of the
       nondegradation rules to this remedial action to any large extent.

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b. Stormwater Runoff:

      a.  Pursuant  to authority under the Water Quality Act, and
      ARM 16.20.601  et  sea, and 16.20.1301 et sea..  Title 16,
      Chapter 20,  Sub-Chapter 6, and Title 16, Chapter 20, Sub-
      Chapter 13,  including ARM 16.20.1314, the Water Quality
      Division has issued general  stormwater permits for certain
      activities.  The substantive  requirements of  the following
      permits are  applicable for the following activities:

           (1) for construction activities: General  Discharge
           Permit  for Storm Water  Associated with  Construction
           Activity,  Permit No. MTR100000  (November  17,  1992).

           (2) for mining activities:  General Discharge Permit  for
           Storm Water  Associated  with Mining and  with Oil and  Gas
           Activities,  Permit No.  MTR300000 (May 18,  1993) ,57

           (3) for industrial activities: General  Discharge Permit
           for Storm Water Associated with Industrial Activity,
           Permit  No. MTROOOOOO  (October 26, 1994) ,58

Generally, the permits require the  permittee to implement Best
Management Practices (BMP) and to take all reasonable steps to
minimize or prevent any discharge which has a reasonable
likelihood of adversely affecting human health or the
environment. However,  if there is evidence indicating potential
or realized impacts on water quality due to any storm water
discharge associated with the activity,  an individual MPDES
permit  or alternative  general permit may be required.

c.  Groundwater Act (Applicable)  (Construction and  maintenance  of
      groundwater  wells)

Section 85-2-505, MCA,  precludes  the wasting of groundwater.  Any
well  producing waters  that contaminate other waters must be
plugged or capped,  and wells must be constructed  and maintained
so as to prevent  waste,  contamination,  or pollution of ground
water.
  37  This permit covers point source discharges of storm water from mining and milling activities (including active,
     inactive, and abandoned mine and mill sites) including activities with Standard Industrial Code 14 (metal mining).


  58 Industrial activities are defined as all industries defined in 40 CPR 122, 123, and 124, excluding construction, mining,
oil & gas extraction activities and stormwater discharges subject to effluent limitations guidelines. This includes wood
treatment operations, as well as the production of slag.


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d.   Substantive MDEQ Permit  Requirements  (Applicable)  (Point
      source discharges)

40 CFR Part 122,  Subpart C and ARM regulations set  forth
substantive requirements applicable to  permitted  discharges.
Although permits  are not required here, substantive requirements
including the requirement to properly operate and maintain all
facilities and systems of treatment and control are applicable.

3.   Solid Waste Management Regulations  (Applicable)  (Redisposal
unit or units)

ARM  17.50.505(2)  (formerly 16.14.505(2))  specifies  standards  for
solid waste management facilities, including the  requirements
that:

      i.   Class II landfills59 must confine solid  waste and
           leachate60 to the disposal facility. If  there is the
           potential for leachate migration,  it must be
           demonstrated that  leachate will only migrate to
           underlying formations which have no hydraulic
           continuity with any state waters;

      ii.   adequate separation of group  II wastes  from underlying
           or adjacent water  must be provided;61 and

      iii. no new  disposal units or lateral expansions  may be
           located in wetlands.

Because the excavated source material will be treated  to pass
RCRA leachability characteristic tests, and because the waste
will be placed on clean gravel material which will  be  situated
above iron treated ground water,  the Rocker ou remedial action
selected in this  ROD will likely meet requirements  i and ii
above.   Remedial  design should demonstrate compliance  with these
requirements.

 ARM 17.50.505 also specifies general soil and hydrogeological
requirements pertaining to the location of any solid waste
management facility.
  59  Generally Class II landfills are licensed to receive Group II and Group III waste, but not regulated hazardous
     waste. Class III landfills may only receive Group HI waste.


  60  Leachate is defined as a liquid which has contacted, passed through, or emerged from solid waste and contains
     soluble,  suspended, or miscible materials removed from the waste. ARM 16.14.502(25).


  61  The extent of separation shall be established on a case-by-case basis, considering terrain and the type of underlying
     soil formations, and facility design. The Waste Management Division of the Department of Environmental Quality
     has generally construed mis to require a 10-20 foot separation from groundwater.


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ARM  17.50.506  (formerly 16.14.506)  specifies design requirements
for  landfills.62 Landfills must be  designed to contain a
composite liner and  leachate collection system which comply with
specified criteria.

 ARM 17.50.511  (formerly 16.14.521)  sets forth general
operational and maintenance and design requirements for solid
waste management systems.  Specific operational  requirements,
specified in ARM 17.50.51163 are run-on and run-off  control
systems requirements,  requirements that sites be fenced to
prevent unauthorized access, and prohibitions of point source and
nonpoint source discharges which would violate Clean Water Act
requirements.

 ARM 17.50.530  (formerly 16.14.530)  sets forth the  closure
requirements for landfills.64 Class II landfills  must meet the
following criteria:

      i. install a final cover that is designed to minimize
      infiltration and  erosion.

      ii. design and  construct the  final cover system to minimize
      infiltration through the closed unit by the use of an
      infiltration layer that contains a minimum  18  inches of
      earthen material  and has a permeability less than or equal
      to the permeability of any bottom liner, barrier layer, or
      natural subsoils  or a permeability no greater  than 1 X 10-5
      cm/sec, whichever is less;

      iii.  minimize erosion of the  final cover by the use of a
      seed bed layer  that contains  a minimum of six  inches of
      earthen material  that is capable of sustaining native plant
      growth and protecting the infiltration layer from frost
      effects and rooting damage;

      iv. revegetate  the final cover with native  plant growth
      within one year of placement  of the final cover.65

ARM  17.50.531  (formerly 16.14.531)  sets forth post  closure care
requirements for Class II landfills.  Post closure care must be
conducted for a period sufficient  to protect human  health and the
  62  Landfills are defined as an area of land or an excavation where wastes are placed for permanent disposal, and that
     is not a land application unit, surface impoundment, injection well, or waste pile. ARM 17.50.502(25).


  0  ARM 17.50.511(1)0, 17.50.511(l)(k), and 17.50.511(1)(1).


  64  Closure means the process by which the operator closes all or part of the facility.


  65  ARM 17.50.530(l)(b) allows the department to approve an alternative final cover design if it achieves the
     reduction in infiltration and protection from erosion to a level at least as equivalent as the stated criteria.


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environment. Post closure care requires maintenance of the
integrity of the integrity and effectiveness of any final cover,
including making repairs to the cover as necessary to correct the
effects of settlement, subsidence, erosion, or other events, and
preventing run-on and run-off from eroding or otherwise damaging
the cover and comply with the groundwater monitoring requirements
found at ARM Title 16, chapter 14, subchapter 7.

As for the solid waste requirements of ARM sections 17.50.506,
.520, and .530  -.531 listed above, Section 75-10-206, MCA, allows
variances to be granted from solid waste regulations if failure
to comply with  the rules does not result in a danger to public
health or safety or compliance with specific rules would produce
hardship without producing benefits to the health and safety of
the public that outweigh the hardship.  If remedial design can
demonstrate compliance with the performance standards identified
in the ROD, including the performance standard relating to a
showing that leachate from treated solid waste will not leave the
solid waste unit and enter the surrounding aquifer, certain of
these requirements, most notably the liner and leachate
collection and  removal system requirements, will be subject to
variance in implementing the Rocker operable unit remedy.  The
scope and manner of applying the variance can be determined in
finalizing and approving of the remedial design.

4. Transportation of Solid Waste  (Applicable)

For solid wastes, § 75-10-212 prohibits dumping or leaving any
debris or refuse upon or within 200 yards of any highway, road,
street, or alley of the State or other public property, or on
privately owned property where hunting, fishing, or other
recreation is permitted.

ARM 17.50.523 (formerly 16.14.523) requires that such waste must
be transported in such a manner as to prevent its discharge,
dumping, spilling, or leaking from the transport vehicle.

5.   Reclamation Requirements (Relevant and Appropriate)  (Soil
     capping and excavation)

ARM 26.4.631 requires the prevention and minimizing water
pollution during reclamation activities.

ARM 26.4.633 states that all surface drainage from a disturbed
area must be treated by the best technology currently available
(BTCA).  Treatment must continue until the area is stabilized.

ARM 26.4.634 states that disturbed drainages will be restored to
approximate pre-disturbance configuration.

ARM 26.4.640 provides that discharge from sedimentation ponds,
permanent and temporary impoundments,  and diversions shall be

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controlled by listed devices, where necessary, to reduce erosion
and minimize disturbance of the hydrologic balance.

ARM 26.4.641 requires practices to prevent drainage from acid or
toxic forming spoil material into ground and surface water.

ARM 26.4.501 and 501A give general backfilling and final grading
requirements.

ARM 26.4.514 sets out contouring•requirements.

 ARM 26.4.519 provides that an operator may be required to
monitor settling of regraded areas.

ARM 26.4.638 specifies sediment control measures to be
implemented during operations.

ARM 26.4.702 specifies requirements for redistributing and
stockpiling of soil for reclamation.

ARM 26.4.703 specifies requirements for use of materials other
than or in conjunction with soil for final surfacing in
reclamation.

ARM 26.4.711 requires that a diverse, effective, and permanent
vegetative cover of the same seasonal variety native to the area
of land to be affected shall be established except on road
surfaces and below the low-water line of permanent impoundments.
Vegetative cover is considered of the same seasonal variety if it
consists of a mixture of species of equal or superior utility
when compared with the natural (or pre-existing) vegetation
during each season of the year.  (See also ARM 26.4.716 below
regarding substitution of introduced species for native species.)

ARM 26.4.713 provides that seeding and planting of disturbed
areas must be conducted during the first appropriate period for
favorable planting after final seedbed preparation but may not be
more than 90 days after soil has been replaced.

ARM 26.4.714 requires use of a mulch or cover crop or both until
an adequate permanent cover can be established.  Use of mulching
and temporary cover may be suspended under certain conditions.

ARM 26.4.716 establishes the required method of revegetation, and
provides that introduced species may be substituted for native
species as part of an approved plan.

ARM 26.4.718 requires the use of soil amendments and other means
such as irrigation, management, fencing, or other measures, if
necessary to establish a diverse and permanent vegetative cover.
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ARM 26.4.728  sets forth requirements  for  the composition of
vegetation on reclaimed areas.

ARM 26.4.751  requires measures to prevent degradation of fish and
wildlife  habitat.

ARM 26.4.761  contains measures for controlling fugitive dust
emissions during mining and reclamation activities.


IX.  OTHER LAWS

As explained  above in Section IV., these  laws are independently
applicable rather than ARARs for the  site.

1.   Surface  Water and Groundwater Act, 85-2-101 et. seq. MCA

Section 85-2-101, MCA, declares that  all  waters within the state
are the state's property,  and may be  appropriated for beneficial
uses.  The wise use of water resources is encouraged for the
maximum benefit to the people and with minimum degradation of
natural aquatic ecosystems.

2. Groundwater and Surface Water Appropriation

Parts 3 and 4 of Title 85, MCA, set out requirements for
obtaining water rights and appropriating  and utilizing water.
All requirements of these parts are laws  which must  be complied
with in any action using or affecting waters of the  state.

ARM Chapter 16,  Sub-Chapter 1, entitled Water Reservations,
implements the provisions in Parts 3  and  4  of Title  85,  MCA.

3. Groundwater

Section 85-2-516, MCA, states that within 60 days after any well
is completed  a well log report must be filed by the  driller with
the DNRC  and  the appropriate county clerk and recorder.

4. Controlled Ground Water Area:

Pursuant  to section 85-2-507 MCA, the State may grant either a
permanent or  a temporary controlled ground water area. The
maximum allowable time for a temporary area is four  years.66

Pursuant  to 85-2-506 MCA,  designation of  a  controlled groundwater
area may  be proposed if i) excessive  groundwater withdrawals
  66  If a temporary controlled ground water area is granted, the statute requires the State agency responsible for the
     petition to commence studies to determine the designation or modification of a permanent controlled ground water
     area.


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would cause contaminant migration; ii) groundwater withdrawals
adversely affecting groundwater quality within the groundwater
area are occurring or are likely to occur; iii) groundwater
quality within the groundwater area is not suited for a specific
beneficial use.

5.   Occupational Health Act, §§ 50-70-101 et seq.. MCA.

ARM § 16.42.101, along with the similar federal standard in 29
CFR § 1910.95, addresses occupational noise.

ARM § 16.42.102, along with the similar federal standard in 29
CFR § 1910.1000 addresses occupational air contaminants.

6.   Montana Safety Act

Sections 50-71-201, 202 and 203, MCA, state that every employer
must provide and maintain a safe place of employment, provide and
require use of safety devices and safeguards, and ensure that
operations and processes are reasonably adequate to render the
place of employment safe.

7.   Employee and Community Hazardous Chemical Information Act

Sections 50-78-201, 202, and 204, MCA, state that each employer
must post notice of employee rights,  maintain at the work place a
list of chemical names of each chemical in the work place,  and
indicate the work area where the chemical is stored or used.
Employees must be informed of the chemicals at the work place and
trainad in the proper handling of the chemicals.

8.  Montana Asbestos Control Act

     The provisions of the Montana Asbestos Control Act, Sections
75-2-501 et seq..  MCA, and implementing rules establish standards
and procedures for the accreditation of asbestos-related
occupations and control of the work performed by persons in
asbestos-related fields.  If asbestos is encountered at the
Rocker ou,  these health and safety standards would be
independently applicable.
                              A-38

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               RESPONSIVENESS SUMMARY ORGANIZATION
     During the public comment period for the Rocker operable
unit proposed plan, EPA received both oral and written comments
from the public, local government, and the potentially
responsible party for the Rocker operable unit, the Atlantic
Richfield Company  (ARCO).  The attached Responsiveness Summary
reprints the written comments received in total and response to
each substantive point raised in each written comment.  A
transcript of all oral comments submitted during the public
hearing on the proposed plan are included in the administrative
record for the Rocker operable unit and were considered by EPA
and the State of Montana Department of Environmental Quality in
reaching a decision for remedial action at the Rocker operable
unit.  However, the oral comments were duplicative of the written
comments and are not responded to separately in this Responsive
Summary.

     The written comments and EPA's response to those comments
are presented showing public comments first, local government
comments next, and ARCO comments last.

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                                         COMMENTS OF                   	
               THE CITIZENS' TECHNICAL ENVIRONMENTAL COMMITTEE
 COMMENT:
 The Citizens' Technical Environmental Committee (CTEC) wishes to add the following comments, and two resolutions passed
 by CTEC members present at our monthly meeting on August 10th, 1995, to comments being solicited on the Proposed Plan
 for the Rocker Timber Framing and Treating Plant Operable Unit.

 RESPONSE:
 No Response Required

 COMMENT:
 First,  CTEC would like to express its appreciation for the hard work mat you and the staff at EPA's Montana office have put
 into this project. The preferred alternatives put form by the EPA provide a good effort to attempt to remediate the site.

 RESPONSE:
 Comment Noted - Thank You.

 COMMENT:
 The technical assistants with CTEC have genuine concern about the hydraulic connectivity between unimpacted aquifers and the
 arsenic plume. There is also concern about the overall picture, chemically speaking, when iron additions are introduced into the
 plume, and the effectiveness of those additions to reach and adsorb the arsenic in me plume. We believe, however, that with
 close monitoring, that the iron-flooding technology proposed by ARCO should be given a chance, and further advances in studies
 dealing with arsenic and ground water problems be considered along the way.

 RESPONSE:
 Comment Noted - EPA disagrees that ARCO's proposal is implementable, but has adopted some aspects
 of ARCO's proposal.

 COMMENT:
 It is hoped that the innovative technology will prove successful, but milestones for meeting standards must be defined up front.
 Resolution #4, passed by members of CTEC, supports the ARCO proposal, in principle, with stipulations. Resolution #2 supports
 the decision to send removed material to Smelter Hill, if a removal is deemed necessary.

 RESPONSE:
 Comment Noted - ARCO's proposal is not accepted, as explained in EPA's response to ARCO's proposed
plan  comments.  In response to community concerns and cost-effectiveness issues, EPA has not chosen
 the Smelter Hill repository.

 COMMENT:
 In evaluation of the nine criteria, in accordance with the NCP, we believe mat the Rocker community's support of the ARCO
proposal sends a message to all involved in what the community wants. Therefore we support the effort to try the innovative
 technology first, and follow through with the EPA's remedy, in whole, if the standards are not met in five years.

 RESPONSE:
 Comment Noted - EPA has attempted to include community concerns fully in the selected remedy.

 Respectfully submitted,

 Shannon Wilson
 Senior Technical Assistant, CTEC

 Enclosures
                                                                                     CTEC Comments 1

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                                     COMMENTS OF CTEC

 COMMENT:
                               Proposed Resolutions Before the Membership
                                           August 10,1995
                                            Resolution #4

 ROCKER OPERABLE UNIT - ARCO PROPOSAL

 Whereas, the Rocker Operable Unit is contaminated with arsenic and must be remediated to protect human health and the
 environment and,

 Whereas, community support of a remedy is essential to successful implementation of a remedy;

 The Citizens' Technical Environmental Committee issues the following resolution in regard to the ARCO proposal:

 1.      CTEC supports the use of innovative technologies for remediation of the Rocker Operable Unit, under the following
 stipulations:

 a.      Benchmarks for success (i.e., standards within a timeframe) must be clearly stated within the Record of Decision
 (ROD). This includes clear indication of how long the in-situ treatment will be conducted before a determination is made whether
 it has been a success or failure. CTEC believes that mis should be no longer than five years.

 RESPONSE:
 As can  be noted in EPA's  response to ARCO comments, EPA has  serious concerns regarding the
 implementability of ARCO's proposed remedy  and the  time frames projected  for the associated
 investigations. ARCO has not been specific regarding how the iron flood technology will be intimately
 mixed with aquifer materials.  In earlier discussions, they suggested utilizing several trenches cut across
 the site to introduce the iron to the shallow aquifer and admitted that this technique did not replicate the
 complete mixing experiments  conducted at Montana Tech.   EPA feels that if the iron flood cannot be
 successfully introduced into the aquifer matrix, it is highly probable that its effectiveness will be marginal
 at best.  For this reason, EPA feels that it must reject the ARCO plan as submitted as the primary remedy
 component at the Rocker Site.

 EPA considers two elements of the selected remedy to be innovative: 1.) The iron addition to attenuate
 arsenic in the ground water, and 2.) Iron addition to contaminated soils and fill excavated from the site.
 The contingent remedy, should  it be deemed necessary, focuses on plume  containment.  The placement
 of coarse gravel in  the iron treated water table, was included with the remedy to allow on-going iron
 attenuation within and down gradient from this area.  The high permeability could  also allow pumping
 of ground water from the down gradient fringe of the arsenic plume into the iron-rich zone where arsenic
 would be  attenuated.   The  selected remedy  can be  implemented  the  first field  season following
 implementation of a consent decree or unilateral order with the potentially responsible party (ARCO).

 COMMENT:
 b.       Frequent monitoring (monthly) of the wells utilized for human consumption needs to be done within the immediate
 vicinity, to ensure arsenic concentrations are within safe drinking water standards, and to provide early warning of plume
 advancement.

 RESPONSE:
 Quarterly monitoring of key wells is contemplated at this time; however,  the details of a monitoring
program will be developed during the remedial design phase of the project.


                                                                                  CTEC Comments 2

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COMMENT:
c.      The measure of success should meet, at least, me federal MCL standards of 50 ug/L within the entire area of the
arsenic plume.

RESPONSE:
The State standard of 18 ug/l arsenic in the ground water is the objective for the Rocker remedy.  Trends
in water quality will be developed in order to assess the projected time frames till the State standard is
achieved.

COMMENT:
d.      The ROD needs to state what will happen if the arsenic plume advances, and how ARCO will handle this.

RESPONSE:
In the ROD, contingent plume containment actions can be implemented, in the event that plume migration
occurs.

COMMENT:
e.      If the in-situ treatment does not prove successful, ARCO needs to follow through with the EPA's preferred remedy.
This would not only include removal of source contamination, but the other measures stated in the EPA preferred remedy.

RESPONSE:
EPA's final remedy utilizes a combination of arsenic source removal,  treatment  and on-site disposal,
combined with hot spot remediation and enhanced and natural attenuation for  shallow groundwater
remediation.

COMMENT:
2.      CTEC believes that the Rocker Water & Sewer District must obtain a legally binding agreement with ARCO before
the ROD is issued, addressing that ARCO will  follow through with their proposed plan, and other stipulations in the ROD, no
matter what happens with Superfund reauthorization.

RESPONSE:
This is not an issue addressed by the laws that direct Superfund clean up activities.   This issue is more
appropriately addressed to the Rocker  Sewer and  Water District.

COMMENT:
                               Proposed Resolutions Before the Membership
                                           August 10,1995
                                            Resolution #2

ROCKER OPERABLE UNIT -  SMELTER  HILL REPOSITORY SITE

Whereas, the Preferred Alternatives for the remediation of the Rocker Operable Unit, developed by the U.S. EPA calls for the
removal of soil contaminated by arsenic and,

Whereas, removal of contaminated soil from the Rocker Operable Unit, if it is necessary, to prevent further contamination of
groundwater and,

Whereas, to protect human health and the environment, contaminated soils must be placed in a repository and,

Whereas, contaminated soils must not be placed in an unimpacted area and,

Whereas, the Smelter Hill site is already an impacted area;


                                                                                    CTEC Comments 3

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The Citizens' Technical Environmental Committee, supports the U.S. EPA's proposed repository site of Smelter Hill.

RESPONSE:
Comments noted - Thank you.  EPA has  considered ARCO's and citizen group comments to EPA's
Proposed Plan and as a result, EPA has chosen to modify the final remedy primarily by elimination of
the need to transport the excavated soils from Rocker to the Smelter Hill Repository. Rather, EPA's final
remedy will involve removing the contaminated source materials, placing washed gravels into the exposed
ground-water after  additional iron  compounds have been added to help accelerate the attenuation of
arsenic in the shallow groundwater system, and then treating the excavated materials with additional iron
and cement (if necessary), and then backfilling above the water table on-site for final disposal.  The
source materials will have fixated arsenic with a dramatically reduced permeability to minimize any
leachate production.  The disposal area will be outside of the 100-year floodplain. This approach reduces
capital costs and utilizes an already impacted site as a disposal area.
                                                                               CTEC Comments 4

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                                         COMMENTS OF THE
                           CLARK FORK-PEND OREILLE COALITION
COMMENT:
Thank you for providing the Clark Fork-Pend Oreille Coalition with the opportunity to comment on the Proposed Plan for the
Rocker Timber Framing and Pole Plant Site. While we support the general concepts of EPA's plan, we believe it must be
modified to effectively protect human health and die environment the at Rocker site.

RESPONSE:
Comment Noted

COMMENT:
Our comments will detail the changes that we believe must be made to assure that die clean up plan protects the high quality
aquifers below, and adjacent to the site. They will focus on four sections of the proposed plan: 1) the Alternate Water Supply;
2) Contaminated Soil Removal; 3) Groundwater Source Area Removal; and 4) the Proposed Contingency Measures.

M> response  required

COMMENT:
The Coalition supports EPA's commitment to provide an alternate water supply for existing and future users in the Rocker area
To meet these demands, the proposed plan calls for upgrading the current six inch water supply line from Butte. However, the
plan does not identify the source of mat water; nor does it indicate that the Butte Silver Bow County  government has agreed
to forfeit any of its water so that Rocker's water needs can be met.

We urge EPA to identify the source of this alternate water supply prior to signing the Record of Decision (ROD). Specifically,
we recommend that EPA:  1) estimate Rockers' existing water needs,  including water for fire suppression, 2) estimate the
Unreasonably expected" future needs, and 3) secure the water needed to satisfy these demands.

The Clark Fork basin is over-appropriated. In addition, the Big Hole basin, another potential water source, is also experiencing
increased demands. We believe the  water supply issue must be resolved  now since EPA is relying  on relatively unproven
technologies to remediate groundwater contamination at the site. If the water supply issue cannot be resolved prior to the ROD,
the Coalition requests that EPA select a remedy mat utilizes proven technologies - i.e. complete source removal, and pumping
and treating of contaminated groundwater.

RESPONSE:
During implementation of EPA's final remedy, an interim  well  ban will be required which  in turn  will
require the addition of an alternative water supply for additional residential and community needs. With
ongoing monitoring, it is estimated that the development  of local deeper groundwater resources will
reduce the problems  of long term surface water appropriations.

COMMENT:
The Coalition supports EPA's idea  of removing contaminated soils from the site  and placing mem in a RCRA-approved
repository. However, we disagree with  the criteria used to determine what soils should be removed,  and the strategy for
revegetating these areas.

The Proposed Plan calls for removing surface soils with concentrations exceeding 1,000 ppm arsenic  down to  a depth of 18
inches; backfilling these areas with 18 inches of growth media; and revegetating them.

We believe that all contaminated soils exceeding the 1,000 ppm arsenic concentration should be removed, not just the upper 18
inches. These soils continue to contaminate ground and surface waters in the project area and must be removed to protect human
health and the environment. In addition, we believe mat a minimum of 24 inches of growth media should be used in the removal
                                                                          Clark Fork-Pend Oreille Comments 1

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 areas. Re-establishment of vegetation is a critical component of the proposed plan - increasing the depth of growth media will
 increase the likelihood successful revegetation.

 RESPONSE:
 As can be noted in the final remedy, EPA has been sensitive to many of the proposed plan comments and
 has relocated the final repository for excavated, treated source material soils to the Rocker site itself,
 outside the 100-year floodplain.  The addition of cement to solidify the materials to prevent erosion and
 leachate production has also been included as a contingency for excavated soils and fill (in the event iron
 treatment does not pass EPA's "characteristic" TCLP test. EPA believes that removal of other hot spots
 to an 18-inch depth is appropriate because any minor leachate production to the shallow groundwater
from such residuals mil be attenuated either naturally or through the enhanced attenuation resulting from
 iron additions to ground water.  In addition, the 18-inch revegetated soil  cover is adequate in this
 climate to prevent significant infiltration of rain or snowmelt to minimize any leachate production.  The
 cover soil, with appropriate institutional controls regarding future land use is protective of human health
 and the environment.

 COMMENT:
 The Coalition supports source removal as the best way to improve water quality in the shallow aquifer, and to protect water
 quality in the deeper groundwater system. However, we do not believe EPA has adequately addressed the sources of arsenic
 in the proposed plan.

        •       The plan calls for removing all solid materials from the groundwater plume that exceed a concentration of
                10,000 ppb arsenic because EPA considers that these materials are the primary source of contamination to
                the shallow aquifer. The state water quality standard for arsenic is 18 ppb, yet EPA will only remove solid
                materials in areas that exceed 10,000 ppb - nearly three orders of magnitude higher than the state standard.
                The Coalition believes a more conservative estimate should be used to delineate "source materials".
                Specifically, we suggest 1,800 ppb - or two orders of magnitude higher than the standard - be used as the
                cutoff value.

 RESPONSE:
 Given the complexities of wood treating materials and secondary mineralization, EPA intends to better
 define source materials  to groundwater during the remedial design phase of the project.   The  10,000
ppb isocontour was chosen primarily to provide preliminary  source definition for purposes of the
feasibility study.

 COMMENT:
        •       The plan also calls for using iron salt additions to stabilize the arsenic plume. EPA must assure that the iron
                salts are uniformly distributed throughout the entire >  18 ppb arsenic plume. Yet the cleanup plan only calls
                for adding iron salts to the free-draining pore water, and to groundwater "exposed" during removal. If this
                unproven technology is to have any chance for success, EPA must modify the proposed plan to assure
                uniform distribution of the iron salts throughout the entire arsenic plume.

 RESPONSE:
 Given the physical and chemical  conditions in the shallow aquifer,  there are no methods to fully
 guarantee the distribution of iron salts throughout the shallow aquifer.  Therefore, EPA has chosen a
 method to introduce the iron in such a way as to enhance the rate of attenuation in areas where arsenic
 concentrations are clearly elevated and provide future iron supply for additional attenuation capacity as
flow gradients move the iron downgradient.  The iron rich, oxidized shallow groundwater must be utilized
 to attenuate arsenic where  enhanced enrichment is not possible.
                                                                      Clark Fork-Pend Oreille Comments 2

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COMMENT:
        •       Finally, it is not clear how the EPA will "project whether or not the 18 ppb arsenic standard will be achieved
                within a reasonable time frame." This prediction is absolutely critical because it will determine all future
                remedial actions at me site. Therefore, we urge EPA to clearly identify the methodology they will use to
                make this projection.

RESPONSE:
Response: Again, given the physical and chemical conditions at the site, even with aggressive measures
to remediate the shallow groundwater system as is demonstrated in the final remedy, there is no way to
make specific time determinations on when the aquifer may federal or state cleanup goals.  This issue will
be closely monitored.

COMMENT:
The Coalition recommends mat EPA retain the pump and treat option as a contingency measure, and that the other measures
be removed from further consideration. We are absolutely opposed to the notion that clean water should be pumped into the
contaminated aquifer to help dilute the arsenic plume. Clean water supplies are scarce in the upper Clark Fork and dilution of
contaminants is not an acceptable use of these critical resources. In addition, we believe the aquifer grouting concept is highly
suspect and would be next to impossible to successfully implement. Therefore, we urge EPA to retain the pump and treat
contingency and to drop the other measures from consideration.

RESPONSE:
EPA has retained a contingency measure regarding capture and pump back,  in the unlikely event that
substantial and unacceptable movement of contamination occurs.  Source removal is the best  response
to the  situation, and EPA's remedy provides for that.

COMMENT:
Also, we urge  EPA to include a reopener clause in the ROD that requires ARCO to implement these contingency measures if
the iron salt additions do not stabilize the arsenic plume - regardless of any changes in the federal Superfiind law. If ARCO is
unwilling to sign such an agreement, we request that EPA select a cleanup plan that emphasizes complete removal of all
contaminated soils and pumping and treating all groundwater supplies that exceed applicable state and federal standards.

RESPONSE:
EPA intends to maintain its maximum authority under Superfund law to assure that this site  is property
and satisfactorily remediated.

COMMENT:
That concludes our comments on the Rocker Proposed Plan. We hope our comments will be helpful and will be glad to discuss
them with you  in the future.

RESPONSE:
Comment Noted - Thank you.
                                                                       Clark Fork-Pend Oreille Comments 3

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                                        COMMENTS OF THE
                      CONFEDERATED SALISH AND KOOTENAI TRIBES
                                  INTRODUCTORY COMMENTS

COMMENT:
The Confederated Salish and Kootenai Tribes (Tribes) wish to commend you for your efforts in developing the Proposed Plan
for the Rocker Timber Framing and Treating Plant Operable Unit of the Silver Bow Creek/Butte Area Superfund Site. Your
efforts in providing technical information to the Tribes is especially appreciated.

RESPONSE:
Comment noted - Thank You.

COMMENT:
The Tribes offer die following comments on the Proposed Plan in accord with the Tribes fiduciary responsibilities as a trustee
of natural resources associated with the entire Clark Fork River Basin which, of course, includes the Silver Bow Creek/Butte
Area Superfund Site.

The Tribes wish to first underscore the fact that the Tribes retained, in accordance with the 1855 Treaty of Hellgate, rights to
hunt, fish, graze, and gather natural resources in the Clark Fork River Basin, including the Silver Bow Creek drainage, which
is a portion of the Tribes' aboriginal and ceded territory. The rights of access, egress, use, and enjoyment of natural resources
are incidental to the Treaty-retained rights. The fishery of the Clark Fork Basin is of particular concern to the Tribes as is,
obviously, the habitat and environment supporting the fishery.

The Tribes have, in addition, and since time immemorial, depended and relied on the resources of the Clark Fork River Basin,
including the Silver Bow Creek and Anaconda areas for cultural and spiritual renewal. As a direct consequence of that reliance,
die Tribes have identified Traditional Cultural Properties in die area, as that phrase is used in die National Historic Preservation
Act.

RESPONSE:
Above comments noted.

                                     TECHNICAL  COMMENTS

COMMENT:
The Proposed Plan outlines a set of actions which would be undertaken for die purpose of remediation of the contamination at
die Rocker Timber Framing and Treating Plant Operable Unit (Operable Unit). These actions contain components directed
toward the provision of an alternative water supply; groundwater source area removal; groundwater arsenic plume attenuation;
and die removal and capping of contaminated soils.

RESPONSE:
No response required.

COMMENT:
The provision of an  alternative water supply would serve new users within a one-half mile radius of die Operable Unit, and
would serve as a contingency supply for present groundwater users within die one-half mile radius should a release of arsenic
occur at me Operable Unit.

RESPONSE:
The final remedy includes a well ban for aquifers within a 1/4-mile radius of the  site.  An alternative
water supply will also be provided.


                                                                      Kootenai and Salish Tribes Comments 1

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 COMMENT:
 The Proposed Plan contemplates the removal of materials (sources) which contribute major amounts of soluble arsenic to the
 groundwater plume. These materials are described as being generally situated within the 10,000 micrograms of arsenic per liter
 iso-concentration line of the groundwater plume. This material extends from above to five feet below the phreatic surface. The
 removed materials would be transported and disposed of in a RCRA Title C facility to be constructed on Smelter Hill.  Prior
 to filling the excavation, iron (II) sulfate would be added to die excavation to increase attenuation of dissolved arsenic. A well
 ban within the one-half mile area around the  Operable Unit would be put in place for the duration of the remedial action, in
 order not to perturb hydraulic gradients controlling contaminated groundwater plume behavior. Non-engineered controls (Institu-
 tional Controls) in the form of land use restrictions may be required to prevent disturbance of the excavated area.

 RESPONSE:
 The above comments are correct except that in the final remedy, the well ban is 1/4-mile for the aquifers
 identified during the remedial investigation and the excavated soils will be treated and solidified (as
 necessary) and disposed ofon-site.

 COMMENT:
 The effect of natural and enhanced arsenic plume attenuation will be assessed over the first one or two, five-year review cycles
 which are mandated where  contaminants are left in place. According to the Proposed Plan, pump and treat actions for the
 groundwater plume may be needed to "...better achieve project remedial action objectives".

 RESPONSE:
 Source removal and natural and enhanced attenuation is the means used to remediate the alluvial aquifer.

 COMMENT:
 The removal and capping of contaminated soils outside of the arsenic source excavation area would entail two components.
 Contaminated surface soils exceeding 1000 milligrams of arsenic per kilogram of soil (parts per million) would be excavated
 to a depth of 18 inches. The excavated material would be disposed of in a RCRA Subtitle C facility on Smelter Hill at Anaconda.
 Surface soils exceeding 320 parts per million arsenic would be capped with  18 inches of clean fill. This cover material would
 be culturally manipulated and amended such that a suitable substrate for drought-tolerant, native grass species would result. An
 assemblage of Institutional Controls would be put into place to prevent disturbance of the cap, prohibit residential development,
 and provide for the repair of the cap should it be disturbed.

 RESPONSE:
 The above is correct except that an action level of380ppm arsenic to be used and that excavated soils
 will be disposed ofon-site utilizing treatment and solidification technologies.

 COMMENT:
 Given the uncertainties and technical difficulties associated with in situ groundwater treatment and the pumping and treating of
 the contaminated groundwater plume, the remedial actions offered in the Proposed Plan seem reasonable. These actions,
 including the provision of an alternative water supply during the remedy; source removal; treatment; and monitoring and periodic
 assessment of remedial action progress, should provide a phased approach to eventual attainment of remedial action goals for
 die contaminated groundwater plume.

 RESPONSE:
 The final remedy differs from the Proposed Plan in that the excavated source soils will be treated and
 disposed ofon-site  utilizing iron fixation and solidification technology.

 COMMENT:
 The Operable Unit is situated within the Streamside Tailings Operable Unit. The same inorganic contaminants occur within each
 operable unit,  although the distribution and concentrations of contaminants may differ within the operable units. The remedy
proposed for the Rocker  Operable  Unit is focused primarily on the contaminants posing risks to human  health in soils and
 groundwater resulting from the operation of die Timber Framing and Treating Plant. The remedy for the Streamside Tailings
                                                                        Kootenai and Salish Tribes Comments 2

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 Operable Unit is focused on railroad bed contamination and human health and environmental risks posed by tailings/contaminated
 soils and contaminated surface and groundwater.

 RESPONSE:
 Comment noted and agreed  with.

 COMMENT:
 The Proposed Plan for the Rocker Operable Unit recognizes the different emphasis in the response for the two operable units.
 The Proposed Plan also recognizes the need for consistency between the two response actions. The Proposed Plan points out
 that the investigations for the two operable units were complementary. The Proposed Plan states that the remedies for the two
 operable units are being coordinated.

 RESPONSE:
 Comment noted and agreed  with.

 COMMENT:
 While the need for coordination and complementarity between the two operable units is obvious and is clearly common knowl-
 edge, the Proposed Plan does not provide a structure or set of procedures which, if in place and complied with, would insure
 such coordination and complementarity. There is a need for phasing of the remedial actions, as is recognized in the Proposed
 Plan.

 RESPONSE:
 Comment noted and agreed with.

 COMMENT:
 There should be  a  separate effort undertaken to formulate and  design the structure and procedures necessary to effect
 coordination between the two remedial actions. The outcome should identify a set of activities which will insure complementarity
 and consistency of response bom within and between the two operable units.  The remediation goals of each action should be
 assured by the endeavor.

 RESPONSE:
 The need for coordination is clear.   This will be addressed in the Remedial Design phases of both
projects.

                                             CONCLUSION

 COMMENT:
 The foregoing comments are submitted on proposed actions which may have significant impact on the Tribes, who are one of
 several governmental natural resource trustees in the Clark Fork Basin. As noted above, in several pieces of recent correspon-
 dence to EPA, and in comments submitted to EPA and to the Montana DEQ on the Streamside Tailings Operable Unit Proposed
 Plan, the areas which will and may be affected by the Proposed Plan, constitute an important fraction of the Tribes ceded and
 aboriginal territories subject to the 1855 Treaty of Hellgate and other protections.

 RESPONSE:
 Comment noted.   EPA  intends to keep the Tribes fully informed at this site.

 COMMENT:
 The Tribes offer these comments in a cooperative and collaborative spirit with me goal of achievement of the greatest degree
 of cleanup. They are offered with the explicit expectation mat, as efforts proceed and plans and designs are initiated, the Tribes
 will be consulted in a meaningful manner in a timely fashion on a government-to-government basis.
                                                                       Kootenai and Salish Tribes Comments 3

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RESPONSE:
The comments are appreciated, and EPA will strive to maintain such a relationship with the Tribes.

COMMENT:
Such consultation must include, without limit, the recognition of the presence of the Tribes' Traditional Cultural Properties;
specific consultation on fisheries and water-related issues, and general technical consultation. In this regard, the matter of the
selection of the Anaconda Area as a present and possible future repository for hazardous wastes without the involvement of the
Tribes in the site selection process is a matter of immediate relevance and concern.

RESPONSE:
Comment noted.  The Smelter Hill site is no longer the repository for treated contaminants.

COMMENT:
We greatly appreciate the opportunity to comment, and again thank you for your efforts. We look forward to discussing these
comments with you.

Sincerely,
Marion Yoder            Phil Tourangeau
Tribal Attorney           Clark Fork Coordinator

cc:      Mr. Brian Antonioli, MT DEQ
        Mr. James Ford,     " "
                                                                      Kootenai and Salish Tribes Comments 4

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                                          COMMENTS OF
                                        SHANNON WILSON
Oral  Comments submitted by  Shannon Wilson, Senior Technical Assistant,  CTEC, addressing  the
Proposed Plan for the Rocker Timber Framing and Treating Operable Unit.

COMMENT:
I am Shannon Wilson, the Senior Technical Assistant for the Citizens' Technical Environmental Committee (CTEC), and am
making comment on the some of the issues that CTEC will be looking at during our monthly meeting tomorrow night. CTEC
will be submitting additional written comments this week after consensus from our membership, indicating support for particular
plan.

RESPONSE:
Comment Noted

COMMENT:
Alternate water supply: Rocker has a need for the expanded water service, however CTEC's staff is concerned about the
additional loss of water from the Big Hole River. The additional water supplied by the Butte water system could be detrimental
to the health of the Big Hole River during years of low precipitation.  However, any additional stresses need to be avoided when
trying  to contain the arsenic plume. This is why the plume needs to be dealt with aggressively, to ensure mat the community
of Rocker will have the water they need eventually to help facilitate growth.  So every effort should be made to make the
alternate water supply temporary.

RESPONSE:
EPA's final remedy includes well ban for a period while effectiveness of the remedy is being monitored
and evaluated.  Further, an alternative water supply will be provided to make water available during the
interim period and to reduce the threat of plume migration. EPA has conferred with Butte/Silver Bow
County government and does not believe this will cause significant or harmful depletion of the Big Hole
River. EPA believes that the final remedy selected utilizes the most aggressive plume remedy components.

COMMENT:
Groundwater source area removal; CTEC's staff agrees that in order to control the release of arsenic into the ground-water, the
source needs to be dealt with. There is concern about accelerating the release of arsenic into the surrounding ground-water once
excavation begins. With monitoring in place, detection of a release should kick in a pump and treat solution to help maintain
the gradient towards the source removal area. A well ban within the immediate area is essential for the term of the remedy,
however we would like to see mis as only a temporary ban, as  the levels of contamination to start to decline in the shallow
aquifer.  After source removal, iron additions to the system should be used, but should not be considered as an only option.
After initial adsorption by the iron, studies have shown that As(V) could be slowly released from coprecipitates, as crystallite
growth caused desorption of As(V). Also, an increase of pH can cause desorption of the arsenic.1 However, with most of the
source material removed, this problem should be minimized.

RESPONSE:
See previous  comment.  EPA agrees that source removal and subsequent groundwater treatment  are
important components of the final remedy and are necessary to achieve clean up of the site.  The well ban
will be temporary. Potential problems with excavation and arsenic mobility will be addressed through
the addition of iron to the groundwater.
    1    Fuller, C.D., Davis, J.A., Waychunas,  G.A., "Kinetics of arsenate adsorption and coprecipitation,"
Geochimica et Cosmochimaica Acta, Vol. 57, pp.  2271-2282.

                                                                                     Wilson Comments 1

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COMMENT:
Analysis of pumping test data requires an appreciation of all the facts that can affect the drawdown data. Precipitation that fell
during the period of the pump test on the Town Pump well may have masked a delayed response to the pumping.  Although
drawdown was not observed in die shallow wells, a decrease in water levels in eight of the deeper system wells was noted.
Separation of die aquifers by a aquitard still can permit water and contaminants to move between aquifers, depending on die
hydrostatic head in die aquifer. The Montana Bureau of Mines and Geology, in their report of die pump test, did not believe
me aquifer was stressed enough.  With die constant pump rate of 100 gpm being performed for the test, it is feasible with
development in Rocker that levels much more than this could be demanded of die aquifer. We believe with further development,
migration of arsenic  into the high quality waters of the deep aquifer is very likely.

RESPONSE:
EPA believes that the interpretation of the pump test data  and other weight of evidence support  the
connection of the arsenic plume migration pathway to the tertiary aquifer and  CTEC's concerns here.
EPA appreciates  the CTEC candor and support. More detailed responses on this issue are presented in
the response to ARCO comments on the proposed plan.
                                                                                       Wilson Comments 2

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                                          COMMENTS OF
                                        MARY KAY CRAIG
RE:  ROCKER O/U COMMUNITY ACCEPTANCE

COMMENT:
Over the past three years, I have attended most every public meeting on the Rocker Arsenic contamination, whether hosted by
EPA or by the TAG-funded Citizens' Technical Environmental Committee of which I have been president for the past year.
I am familiar with the site and its history.  I am aware that a new Town Pump well proved to be amazingly productive; thus,
giving the folks at Rocker reason to believe they could provide a far lower cost water for development man new residences and
industry  could get by hooking into the county water system.  As a member of a Butte-Silver Bow County committee on
development of a water quality district, I learned that of the three entrances to the city, the  Rocker entrance is likely to
experience greatest growth.

RESPONSE:
Comment Noted

COMMENT:
I spoke at your oral public comment hearing July 9th, 1995 at Rocker. I was surprised by the last minute agreement between
the Rocker Water and Sewer District and ARCO as discussed at mat hearing.  Since then I have become familiar with the offer
made by ARCO to the Rocker Water and Sewer District and have some additional comments:

I personally favor removing the source of contamination quickly and thoroughly, if possible.  I believe the data you have
developed clearly shows interaction between the upper shallow aquifer and the deeper, highly productive aquifer from which
the community of Rocker should be able to provide water for growth and development.  I believe you should require the
contamination above the groundwater be removed immediately without hesitation as a first step in your cleanup decision.  I
believe the excavated contaminated soil should be placed in a repository on Smelter Hill along with omer arsenic contaminated
material from the Anaconda area.

RESPONSE:
The final remedy continues to require that source materials be excavated, treated and solidified to reduce
continued movement of arsenic into the shallow alluvial aquifer.  Based upon many comments, EPA now
believes that on-site disposal of these materials is more cost effective but maintains equal protectiveness.

COMMENT:
I am aware of the issues involved in removal below the groundwater table by traditional technologies, and the contention mat
contaminants might spread during removal. I support use of innovative technologies when they are reasonably timed. One
innovative technology that has not yet been considered is liquid nitrogen injection to freeze the contamination in place and allow
removal in a safe manner. The World Trade Center in New York City was built with many stories underground, below the
groundwater table, by use of mis method. This technology was suggested by Montana Tech students during the comment period
for remedy at the Montana Pole Treatment Site in Butte two years ago. Please check out the viability of use of mis method in
order to be done with the site quickly.

RESPONSE:
Use of ground-freezing  techniques utilizing liquid nitrogen or flooding  or  piping with chillers and
ammonia are extremely expensive.  These very specialized techniques are  generally utilized on sites to
stabilize the surrounding soils where structural components such as foundations for additional buildings
or structural caissons are utilized.  EPA believes that use of the proper excavation techniques to minimize
subsurface disturbance and subsequent additional iron additions will be adequate to deal with short term
impacts to groundwater caused by excavation in the shallow alluvial aquifer.
                                                                              Mary Kay Craig Comments 1

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 COMMENT:
 From everything I have been able to discern about the Preferred Remedy, it could take up to 30 or 50 years to determine if the
 iron injection innovative technology works. I do not believe this to be a reasonable time frame, particularly when the people
 of Rocker are hoping to have a clean groundwater source. The ARCO proposal to Rocker Water and Sewer District seems to
 be better, with a five year proving period at most.  Yet, if that technology doesn't work, the proposal is that the Preferred
 Remedy you have selected would then kick in — once again, are we looking at 30 to 50 years from men? Please be very specific
 in your ROD as to what kind of timing is involved.  Please consider the concessions made by the people of Rocker in order to
 get ARCO's five year timing — they did not pursue other avenues that they could have sans the ARCO proposal.

 RESPONSE:
 Given the  sites' physical features and complicated arsenic geochemistry, EPA  recognizes that it  is
probable that no other technologies exist that can clean the site up any faster without massive negative
 environmental impacts and grossly excessive costs.   EPA guidance requires that balance of cleanup
 effectiveness and cost in conjunction with being protective of human health and the environment.  Further,
 EPA believes that the final remedy does that and that in a short period of time, use of the deeper tertiary
 aquifer will be available to the residents of Rocker.   ARCO's proposed remedy would not do this, but
 would take a considerable time period for implementation and evaluation.  Given the problems with the
 implementability and effectiveness of the ARCO proposal,  EPA agrees that it  is not acceptable.

 COMMENT:
 At the oral comment hearing, Rick Larsen from Butte-Silver Bow responded to my concerns about draw-down of the Big Hole
 for use in Rocker's growth by saying that the B-SB government had assured him mere would be no impact on the Big Hole with
 a new 12-inch pipe to Rocker. He said that the B-SB government would use more water from the Moulton Reservoir to offset
 that coming from the Big Hole.  Please obtain this assurance in writing from the Butte-Silver Bow owned water company and
 refer to it in your ROD.

 RESPONSE:
In the time since the close of public  comment,  EPA did not secure the assurance that you indicate is
appropriate.  However, there has been additional discussion regarding this issue with Butte/Silver Bow
 County and EPA will seek these assurances during remedial design, before the alternate water supply is
 implemented.  Please realize that the alternate water supply is  also  intended to be temporary, in that
 residents should be given access to tertiary groundwater as soon as the remedy can be demonstrated to
be effective.

 COMMENT:
Due to its having been a processed arsenic, imported for industrial use,  I believe the Arsenic at Rocker to be highly bioavailable
as compared to other arsenic in the headwaters, and certainly more dangerous man "background" arsenic, if mat is inorganic
arsenic. Because of this and the connection between arsenic and skin and other cancers, I believe you must stay with your 338
ppm EPA standard for cleanup, not allow the up to  1,000 ppm ARCO favors.

RESPONSE:
 The final remedy recognizes the importance of the 380 ppm action level and believes it to be Julfy justified
at the Rocker OU.  See EPA's Baseline Human Health Risk Assessment and EPA's responses to ARCO
comments on the Baseline Human Health Risk Assessment.

COMMENT:
"Background  arsenic" should not be considered in determining  cleanup  levels.  I question whether any comparison with
"background  arsenic" is valid.  First,  as stated above, the arsenic in the  O/U is processed—highly bioavailable—while
"background arsenic" levels normally refers to that naturally occurring in an area. Secondly, mere is some question as to
whether the "background" arsenic is, indeed,  natural.  As Albert Molignoni pointed out at a meeting a couple of weeks ago,
Rocker is  not a mineralized area, so it would seem  odd to have background levels of arsenic there.  Also, many people I've
spoken with believe it is unnaturally present, having  been blown across from the Anaconda Smelter.  Prevailing winds blow in

                                                                              Mary Kay Craig Comments 2

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that direction and the entire area between Butte and Anaconda has been continually denuded of trees in the memories of anyone
living today.  It is only since the Anaconda smelter was shut down over a decade ago, that we are now finally seeing lush growth
of junipers and the pines that follow them across that same stretch.

RESPONSE:
The consideration of background arsenic did not play a role in the determination of EPA's final remedy.
Based on recent data, EPA believes that the cleanup standard for arsenic is above background.

COMMENT:
Any T. I.  Waiver request by ARCO is frivolous: I do not believe a Technical Impracticability waiver should ever be employed
at this site. Given its small size, the fact that it is adjacent to and may adversely interact with the Streamside Tailings O/U, and
the probability that a technology such as injection of liquid nitrogen is safe and cost effective, I believe you need make no
contingency reference to the possibility of a T.I. waiver in your ROD.

RESPONSE:
EPA agrees that a 77 waiver is inappropriate to consider at this time.  However, given the difficulty of
groundwater cleanups and the recognition that the Rocker OU remedy's ability to meet the State standard
of 18 ug/l is  considered moderately uncertain, it  is appropriate for EPA to consider contingencies
(particularly to maintain the quality of the adjoining uncontaminated aquifers).  In the event that the
remedy as proposed and the contingency measures are unable to attain the State standard in  a reasonable
time frame, a 77 demonstration may be appropriate.  Recognize, that if this is the outcome from this
remedy, the primary objective of protecting the tertiary and deep alluvial groundwater on this site should
still be accomplished.

COMMENT:
Misleading information from ARCO on safety  of aquifer: I remain concerned about the information given by Sandy Stash of
ARCO at a recent public meeting at Rocker. She indicated that Rocker residents and/or the water and sewer district could drill
into the deep aquifer with no adverse effect.   Certainly,  the data you have gathered speaks otherwise, showing the distinct
likelihood that they could cause induced infiltration of the contaminants into these new wells they might drill. Please assure that
mis data from ARCO has not been relied on by the Rocker Water and Sewer District to where they may find themselves with
a more severely contaminated area than at present.

RESPONSE:
EPA's final remedy is protective of the deeper alluvial aquifer. EPA also disagrees with ARCO's view
of the hydrology in the area.  A water well ban  is included for new wells  within one quarter mile of the
arsenic plume.  Wells into the tertiary aquifer would not be allowed until  the remedy is demonstrated to
be effective.

COMMENT:
I concur with other issues discussed in the CTEC comments — specifically regarding the need for ARCO to not be allowed to
back out of the cleanup if Superfund law changes and with regard to the need for very specific criteria for measuring die success
or failure of the iron injection innovative technology if you decide to allow it to be employed.

RESPONSE:
See comment provided to CTEC.

COMMENT:
Thanks, Mike, foryour serious commitment to a good cleanup of the Rocker O/U. You have patiently explained the information
on this complex site and have gone out of your way to attend our meetings and respond to our questions.  We appreciate it.

RESPONSE:
Comment Noted - Thank You.
                                                                             Mary Kay Craig Comments 3

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                                          COMMENTS OF
                                        JOY ITTYCHERIAH
 COMMENT:
 The background level of arsenic in natural waters should be defined clearly before remediation starts to determine if the method
 of remediation is effective in reducing pollution or potential pollution.

 RESPONSE:
 The site is technically very complex.   The selection of the remedy was based upon the selection of the
 best,  cost  effective   technologies, given site conditions and other geochemical factors, not  on the
 determination of "background "groundwater concentrations.  It is important to note that the last round
 of water quality samples from all three aquifers identified on the site that are considered outside of the
 contaminated zone had  arsenic concentrations  less  than 8 ug/l.   Monitoring  will  continue  after
 remediation to monitor the success of the remedy and it further actions are ultimately deemed necessary,
 additional actions could be undertaken.

 COMMENT:
 ARCO should implement a remediation method mat not only meets but exceeds EPA and Montana regulations to prevent further
 remediation to the  same  site.  This should be done quickly using currently available technologies to prevent groundwater
 pollution from occurring.

 RESPONSE:
 EPA believes its choice of a final remedy is very aggressive and quickly implementable and offers the best
 choice for site remediation, given our understanding of the system.

 COMMENT:
 Unless monitoring wells are placed adjacent to currently existing wells and continuous monitoring (24-hour) of groundwater for
 Arsenic is performed, residents using well water may ingest Arsenic-contaminated water before, during or after remediation (if
 the remediation is ineffective).  Therefore, all affected residents using well water should be provided with an alternative source
 of water immediately and the increase in cost to use this new water source should be subsidized by ARCO.

 RESPONSE:
 EPA will require routine  water quality monitoring to assure that Rocker residents are clearly protected.
 An alternative water supply is being provided in the final remedy.

 COMMENT:
 If ARCO's alternative of iron treatment to stabilize the Arsenic in the plume is chosen, then ARCO must provide proof after
 completing remediation that the iron stabilization was effective and will prevent Arsenic from entering the aquifer except in
 naturally-occurring concentrations. However, to determine remediation effectiveness, exact details of the treatment plan and
 expected outcomes should be provided to the EPA and the representatives of the Rocker community prior to start of remediation
 and before final approval of the remediation method to be used at this site. Mobilization of any other naturally-occurring
 elements into the aquifer should also be monitored during the addition of iron both in the testing and the  "real" application phases
 to prevent any unexpected contamination of groundwater.  Finally, even though  iron hydroxide may be naturally-occurring at
 mis site, its concentration is determined by nature and the environmental effects  of the addition of large concentrations of mis
 or any  other iron salt should also be evaluated prior to final approval of the remediation method.

 RESPONSE:
 ARCO's plan will not be implemented since it is not considered either implementable nor effective or
protective (see EPA's comments to the  ARCO plan).   With regard  to your concerns relative to  iron
 oxihydroxides and its  affect regarding attenuation of arsenic, this issue will be closely monitored as well.

                                                                                   Ittycheriah Comments 1

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COMMENT:
If ARCO's method to add Iron salts is ineffective, then a) a manmade channel should be built to move the water around die
current location of die Arsenic plume - OR - b) a temporary dam should be built and water pumped around die area of die
Arsenic plume. This will allow die ground to dry up (if kept covered to prevent additional moisture, i.e.,  snow and rain, from
entering this zone) and then die area can be excavated and treated similarly to other hot spots as defined in die EPA proposal.
However, it is presumed dial either of tiiese two water-channeling methods are probably cost-prohibitive when compared to
treatment with iron salts (if shown to be effective) and should be implemented in case of an environmental emergency or if other
methods are ineffective.

RESPONSE:
Thank you for your suggestions.  EPA believes that its final remedy is the most suitable is remediating
the site given the technical  complexities that must be considered.
                                                                                      Ittycheriah Comments 2

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                                            COMMENTS OF
                                        JOHN SONDEREGGER
Re: Rocker Site Proposed Plan

COMMENTS:
As you may recall, the reason that I was quite late for the public meeting at the Butte Community Center was because our
Hydrogeology Field Camp had started. That ran through August 1st. After that I had a previous commitment to my
mother, daughter, and grandson (latter two live in New York) to visit Wisconsin and meet up with them.  Upon returning to
Montana, I was scheduled to be in the field with a grad student at the Mike Horse Mine.  What is included is an overview
of the written comments that will be mailed no later than Monday, August 14th. This will be very quick and probably
incomplete.

1.      I have only agreed with removal of "hot spots" ABOVE the water table.  Apparently you have misunderstood me.

2.      The in-situ treatment of dissolved arsenic with an iron solution should be given a fair chance.

3.      You and the EPA consultants have repeatedly ignored the ability of naturally occurring ferric hydroxide coatings
        on the minerals in the alluvial units to attenuate arsenic.  The document on feldspar attenuation of arsenic, which I
        gave to  you last spring, has likewise been ignored.

4.      The proposed response actions are duplicative and unnecessarily expensive.  If you are going to have an alternate
        water supply, why not put EVERYONE on it? ARCO offered to investigate the use of Silver Lake water at one
        meeting.  What is proposed does  not discuss that. Did ARCO withdraw from that position of providing the
        alternate water supply? Secondly, if you do disturb the alluvial material below the water table, the odds of
        needing a pump back system in operation are, in my opinion, considerably greater than with  the in-situ treatment
        approach. Finally, why removal  and an alternate water supply? Covering would provide the surface exposure
        protection.

My usual fax  source (the Montana  Tech library) is closed for the weekend. I will try sending mis using the two different fax
programs on my computer system at home. Please accept my apology if you receive two copies of mis.

RESPONSE:
No detailed response are provided here since the narrative above provides an overview of more
detailed comments by Dr.  Sonderegger to follow.  Responses will be provided to the  detailed
comments.
                                                                                     Sonderegger Comments 1

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                                             COMMENTS OF
                                         JOHN SONDEREGGER
COMMENT:
As a geologist, my first comment is that you have the choice of either an "all" or a "sort of fixed" approach. These materials
are located within a floodplain. Eventually a 100, 1,000, or 10,000 year flood event will move all of this material downstream,
dispersing it over a much broader area and effectively reducing the concentrations. Unfortunately, this is just one part of the
bigger problem associated with past mining activities in the Butte-Anaconda area. If the materials are removed, will the new
storage location be stable under the precipitation conditions that generate a 100, 1,000 or 10,000 year flooding event? Ideally,
one would hydraulically "mine" all of me mining  associated wastes from the headwaters of Yankee Doodle down to Warn
Springs and put these materials up on the Tertiary bench in a series of well-designed impoundments. This might give you the
best possible "all"  case. Almost anything less constitutes a series of "fixes" that may or may not be compatible. The presence
of continuing mining activity in the Butte vicinity  pretty much precludes the possibility of the extreme "all" approach.
Philosophically, the standards and approaches used on Superfund sites throughout the Butte-Anaconda area should be consistent.
Please note that you have left me railroad related arsenic sources (which are hydraulically up gradient) in place to be dealt with
under the streamside tailings action (page 11, 2nd paragraph of conclusions section). The use of multiple operable units may
make management of the clean-up activities easier,  but it does lead to possibility of upstrcam/upgradient activities
re-contaminating cleaned-up areas as well as opening the door to inconsistent performance standards.

RESPONSE:
Comment Noted: Since it is a  broad and of philosophical nature, no response is necessary.

COMMENT:
Having gotten the  "nothing stays in place forever" diatribe  off of ny chest, I'd like to comment upon technical problems with
the proposed plan. The statement at the bottom of page 2:

         "The remedial investigation for Rocker concluded that arsenic could be quite mobile owing to the lack of
        iron to attenuate arsenic in these ground water resources."

continues to ignore the geologic setting, the travel time calculations of the RI report, the available data presented in the report.
and the results of various scientific investigations. Because the  proposed plan is based heavily upon the perceived risk of
arsenic-contaminated groundwater and the effect upon human health, it is essential that these facts be addressed in calculating
risk to the populace.

RESPONSE:
The proposed plan addressed the lack of iron for attenuation in the tertiary aquifer system to be available
to attenuate arsenic as it moved from the well RH-6 toward the tertiary system.  As you recall, there was
a clear hydraulic response at RH-6 when the Town Pump well pump test was conducted.

COMMENT:
Starting with the calculated flow rates from the RI (p. 3-30) for the shallow alluvial aquifer, the report suggests that a geometric
mean hydraulic conductivity of 6.5 feet per day and an average velocity of 0.2 feet per day (73 feet per year) are reasonable.
We've had 40 to SO years at a minimum for the arsenic plume to migrate: this would yield a plume length of 2,920 to 3,650
feet it there were no attenuation or dispersion. Yet the plume migration is only 400 to 500 feet, using the  18 ppb dissolved
concentration limit (bottom of p. 7 of the proposed plan document). Neglecting dispersion, this requires a retardation factor of
roughly 7.3 (plume migration is about 10 feet per year to have reached 400 to 500 feet in 40 to 50 years). Something has been
taking up (attenuating) arsenic as the water moved through the alluvial aquifer for the past 40 to 90 years. If it isn't iron, it may
be feldspar, although I suspect mat this problem results from your consultants not discriminating adequately between dissolved
and particulate iron (especially ferric hydroxide coatings) in the alluvial aquifer.

RESPONSE:
EPA clearly  recognizes in the RI and FS documents the past and present attenuation of arsenic V on iron


                                                                             Sonderegger Comments (2nd Set) 1

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oxyhydroxides in the oxidizing environment of the current shallow alluvial aquifer on the Rocker site.

COMMENT:
Ferric hydroxide is quite insoluble as long as conditions are not reducing. The weathering of the mineralized Butte Quartz
Monzonite has been occurring for far longer than mining operations in Butte. 1 feel fairly safe in assuming that a significant
amount of the alluvial materials in the Silver Bow Creek drainage will show some minor ferric hydroxide coatings resulting from
the weathering of pyrite and other iron-bearing minerals in the drainage upstream. I believe that these coatings are the reason
that the plume has migrated far less rapidly than the groundwater itself.

RESPONSE:
EPA agrees with the comment.

COMMENT:
The absence of a plume thousands of feet long containing ppm levels of arsenic shows that natural attenuation is working at this
site. The fact mat the groundwater meets the new state standard (18 ppb) within 500 feet of the major source input indicates mat
the natural attenuation mechanisms are working quite nicely. I have included some references on ferric hydroxide and feldspar
attenuation of dissolved arsenic at the end of this comment.

RESPONSE:
EPA agrees with the comment.

COMMENT:
On page 8,  the 4th item under Remedial Action Objectives for Groundwater is impossible without immediately implementing
a pump-back program. Plume migration occurs because the sediment has a limited uptake capacity for the contaminant. Nearer
the source, the sediments are in approximate equilibrium with a higher concentration of the contaminant. Removal of the source
(even complete removal)  does not prevent continued plume migration because the sediment that has attenuated the contaminant
is trying to  re-equilibrate with "clean" water. While desorption of the contaminant nay be much slower than the adsorption
process, it does occur at a finite rate, requiring some downgradient nitration of the plume as released contaminant is re-adsorbed
by previously unimpacted sediment.

RESPONSE:
On page 8 of the EPA Proposed Plan, no mention is found of the  Remedial Action Objectives.  It is found
on page 5 however.  Assuming the comment is related to the "prevent any degradation of the groundwater
underlying and/or adjacent to the site " the following response is provided.  EPA's intent was to preclude
the exacerbation or  movement of the contaminated shallow ground  water from  its generally defined
location at the site.  The remedy utilizes source removal and addition of iron to the exposed groundwater
to enhance the rate of attenuation in the shallow groundwater system.

COMMENT:
It would make sense to try the addition of dissolved ferric iron to  try to catch' the majority of the dissolved arsenic migrating
in a downgradient direction as an alternative to a pump-back system (see the conclusions of the National Research Council on
the effectiveness of pump-and-treat systems: what is proposed instead is testing the feasibility of an in situ treatment).

RESPONSE:
EPA agrees that the addition of iron to the  groundwater will be  beneficial.

COMMENT:
I  am concerned about excavating below the water table. Documentation of the effects of this type of removal have not been
widespread in the scientific literature. I suspect that mis is because the fines (which commonly carry the highest concentration
of contaminants) tend to be lost and migrate down gradient. Even scientists don't like to advertise their failures, and I suspect
mat these types of action have generated more failures than successes, fl way to approach a compromise would be to require
that removal under alternative 3 be done at the time of year when the water table is at its lowest phase in the annual cycle.


                                                                          Sonderegger Comments (2nd Set) 2

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RESPONSE:
EPA believes that contaminated sediments can be successfully excavated 5-feet or more into the water
table.   Techniques are available that minimize the "breaking up" of the material which adds to the
dewatering problems and drainage problems  that must be  considered.  EPA acknowledges that the
exposed groundwater mil likely increase in arsenic contamination due to the removal but the impacts can
be more than offset by the addition of the extra iron to better attenuate what is present and to act as an
additional source of iron downgradient as the flow advances.

COMMENT:
In summary, the conclusions about arsenic mobility in mis environment used to reach the Proposed Plan Preferred Final Remedy
are invalid. I believe that the recommended action constitutes a costly "overkill" for mis problem. Alternatives 2 or 3 are the
only reasonable alternatives, in my opinion.

RESPONSE:
Comment noted.  EPA does not agree and explains its rationale for remedy selection in the ROD itself.
Specific comments relating to arsenic mobility are addressed in EPA's response to ARCO's RIDisclaimer
and in EPA's response to ARCO's comments on the proposed plan.
                                                                   Sonderegger Comments (2nd Set) 3

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John L. Sonderegger, Ph.D.
Certified Ground Water Professional No. 261
Montana Monitoring Well Constructor No. 16
Selected References

Fuller, C. C., Davis, J. A., and Waychunas, G. A., 1993, Surface chemistry of ferrihydrite: Part 2.
       Kinetics of arsenate adsorption and coprecipitation:  Geochimica et Cosmochimica Acta, v. 57,
       p. 2271-2282.

National Research Council, 1994, Alternatives for Ground Water  Cleanup: Nat. Academy  Press,
       Washington D.C., 314 p.

Prasad, Gur, 1994, Removal of Arsenic(V) from Aqueous Systems by Adsorption onto Some Geological
       Materials: in Arsenic in the Environment Part I: Cycling and Characterization, J. O. Nriagu, ed,
       Wiley-Interscience, NY, NY, p. 133-154.

Vircikova, E., Molnar, L., and Lech, P., 1994, The kinetics of the solubility of amorphous Fe(in)-As(V)
       precipitates: in Separation Processes: Heavy Metals, Ions and Minerals, The Minerals, Metals
       & Materials Society, p. 247-255.

Waychunas, G. A.,  Rea,  B.  A., Fuller,  C.  C., and Davis,  J. A.,  1993, Surface  Chemistry of
       ferrihydrite: Part I. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate:
       Geochimica et Cosmochimica Acta, v. 57, p. 2251-2269.

RESPONSE:
References and credentials noted.
                                                                  Sonderegger Comments (2nd Set) 4

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                                              COMMENTS  OF
                              ROCKER SEWER AND WATER DISTRICT
COMMENTS:
         The Rocker Water & Sewer District met in special session in an attempt to get some public input before voicing the
Board's position regarding groundwater contamination in our District and Arco's proposed cleanup.
EPA submitted their proposed plan on July 27, 1995 and it appears it involves extensive removal.  Arco disagrees with EPA's
plan involving removal of the contaminated material.  Albert feels that every attempt possible should be made to protect the
aquifer for our community.
         Lou Eveland asked if a ban was placed on water wells would it include both shallow and deep wells.  Sandy Stash
informed her that it would include those wells in the very, very, shallow aquifer, close to the site. She also informed the group
that she does not see any reason why people in mis area should preclude drilling wells or using current wells unless the well
was drilled right in the contaminated area. She feels we should do another huge pump test.  Sandy Stash asked Mike Bishop
(EPA representative) if EPA would be willing to do  another extensive pump test.  Mr. Bishop doesn't feel a pump test is
necessary.
         Albert asked if it would be feasible to have a community water supply and abandon the Big Hole line and, if so, would
we be protected in the future for an alternate water supply.  Ms. Stash said Arco would not be responsible for our community's
future water supply but felt the community well would be a good, long-term, source of water.  She suggested a very deep well
because they are confident the current water contamination wouldn't occur in a deep well.
         Larry  Braunbeck informed the group mat he has done some research on Arco's proposal and learned that the in situ
technique (injecting iron into arsenic) Arco is proposing is new and is basically unproven. Lou Eveland asked Mike Bishop if
the EPA would continue testing  well water if we went with Arco's proposal.  Mike informed her they EPA would. Frank
Weitzel asked Sandy Stash how  long the site would have to be monitored before Arco would know if the in situ  technique
worked.  Ms. Stash said it would take from 3 to 5 years. If it didn't work after this time frame, Arco would go to removal.
         Ray Palmer asked Mike Bishop if he drills a well now and in the future it is banned, does he have to incur the expense
of running the water line into his house?  Mr. Bishop said that he wouldn't be responsible; EPA would.  Albert suggested the
District take the stand mat if the cleanup systems fail Arco will guarantee the funds to furnish our District with a water supply.
Ms. Stash said Arco won't dig up the waste and guarantee funds for a future water supply because funds would be spent on
digging.  If they didn't do the digging men they would talk with the District regarding a guarantee.
         Jack King stated mat he feels the people in mis area should be protected by the Big Hole water line immediately in
the event Arco's plan fails we would have a water source.  Gary Swanson suggested visiting with Butte Silver Bow regarding
the enlargement of the line and if it is, in fact, possible. Mr. Swanson will confer with the Water  Company.
         Jim Manning made a motion to represent the Rocker Water and Sewer District's stand regarding the cleanup:

         1.       A new 12" (twelve inch) water supply line from the Big Hole River water line and a 30,000 (thirty thousand)
                 gallon storage tank must be installed immediately by Arco to provide Rocker Water and Sewer District with
                 a sufficient water supply.

         2.       The District agrees to  allow Arco to attempt to cleanup the current water on the site with their innovative
                 technology if  it can meet Federal drinking water standards.  The time frame to remedy the situation will be
                 3-5 (three to five) years. If mis technology fails Arco will then institute the EPA and the State of Montana
                 preferred alternative.

         3.       Every attempt possible will be made to maintain clear water and protect the present and future water supplies
                 in our District.

         4.       The County Water and Sewer District of Rocker will be allowed to put in a water supply well 1/4 (one
                 fourth) of a mile from the site and testing of the water quality for its purity will be Arco's responsibility until
                 the site is cleaned up.

         5.       If all cleanup attempts fail there will be a contingency payable to  the  District from  Arco which  will
                 supplement the District water users for the higher cost of water used from the Big Hole River water line.
                                                                        Rocker Sewer & Water District Comments 1

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        6.      If new water supply wells in the District become contaminated from the site, Arco agrees to reopen the
                whole cleanup project.
        Kenny Zeller seconded the motion.  The motion carried by a unanimous vote of the Board. Sandy Stash informed the
 group that any agreements with Arco would be done through a contract.

        Kenny Zeller made a motion to allow David Myers to hook onto the water and sewer line. Glen Eveland seconded
 the motion.  Motion carried.

        Glen Eveland made a motion to adjourn the meeting.  Kenny seconded the motion.  Meeting adjourned at 9:50 p.m.
                                                      Sincerely,
                                                      Shirley Dunks

                                                      Secretary
RESPONSE:

EPA appreciates the efforts of the Rocker Sewer and Water District.  The process that this group went
through to become apprised of the issues and invite public input has been commendable.  EPA has made
every effort to take into consideration the views of the community.  Please review EPA's responses to
other commentors and compare the points made above with the final remedy.  It is EPA's understanding
that the community leaders support the revised final remedy found in this record of decision after it was
presented to them personally after the close of the public comment period.
                                                                Rocker Sewer & Water District Comments 2

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                                           COMMENTS OF
                                MATT VINCENT & RICK LARSON
 COMMENT:
 After reviewing the EPA's Proposed Plan, ARCO's Proposed Plan, and numerous public comments, the personnel at the
 Health Department have the following comments.  If you have any questions or comments regarding these comments, please
 feel free to contact either Rick Larson or myself.

 RESPONSE:
 Comment noted -  Thank You.

 COMMENT:
 The EPA's Proposed Plan for the Rocker Timber Framing and Treating Plant (Rocker OU) has many subjects which need to
 be addressed before a Record of Decision can be accepted. The areas of conflict that arise are mostly closely related to
 impracticable treatment and removal; and inconsistencies with the ARCO plan.  The ARCO plan appears to have a lot more
 technically meritable support than does the EPA's plan.  The areas of conflict are, point by point:

        •        Background (As) compared to MDEQ Drinking Standards (As):  Rocker site data consistently shows that
                 the natural background concentration for arsenic is —30 ug/L. The MDEQ drinking water standard for
                 arsenic is 18 ug/L. To comply with the MDEQ limit, arsenic concentrations at the site would have to be
                 reduced to around half of the naturally occurring background concentration. This would mean mat
                 regardless of the removal, the 18 ug/L limit could not be met. An ARAR waiver is the solution to this
                 inevitable problem.

 RESPONSE:
 The issue of background did not play an important part in EPA  reaching a final remedy; however, it
 should be noted that the most recent sampling of all three aquifers identified on the Rocker OU in
 areas thought to be not impacted by the Rocker Plant had arsenic concentrations below 8 ug/l.  This
 issue will receive additional attention during the remedial design and  remedial action stages  of the
project.  However, EPA will not advocate that clean up occur to levels below pre-disturbance
 background levels. EPA does not believe  an ARAR waiver is justified at  this time, but will consider
 this issue after remedial implementation.

 COMMENT:
        •       Possible Soil Disposal in Smelter Hill Repository: Hauling up to 50,000 cubic yards of arsenic
                contaminated soil by rail from Rocker to Smelter Hill is a bad idea.  Cheryl Beatty,  Anaconda - Deer
                Lodge Chief Executive has stated that die residents of Anaconda-Deer Lodge do not want the Rocker
                waste transported to their county.

 RESPONSE:
EPA has modified the Proposed Plan to dispose of treated excavated source soils on-site.

 COMMENT:
        •       As migration within the aquifer: One of the driving factors for the EPA's proposed removal and
                treatment of the Rocker arsenic plume is its migration into uncontaminated aquifers used for drinking
                water. Since the time the Rocker Plant's operation initiated in 1909 until the present, the arsenic
                contamination has not migrated  deeper than twenty feet into the shallow aquifer. This lack of migration
                is most definitely due to a tight, nonporous aquifer (nearly confined, for the most part) and natural
                attenuation processes. The fact mat no contamination has  migrated into the deeper, potable aquifer over
                the last eighty-five years makes it hard to believe that a potential  migration risk would be more
                                                                                Vincent/Larson Comments 1

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                pronounced in the future.  This potential risk can be more efficiently addressed through regular
                monitoring and institutional controls, rather than costly removal with uncertain risks.

RESPONSE:
The data suggests that contamination has clearly migrated to depths up to 40 feet and towards the
tertiary aquifer near well RH-6.  The tertiary aquifer has not as yet been heavily stressed, but with
further development in the area increased movement could and probably would occur.  Removal of the
source area is necessary and justified.

COMMENT:
        •       Impracticality of Pump-and-Treat remedy: Significant data performed by ARCO shows that the
                hydrogeology and geochemistry of the site aquifer incapacitates arsenic's ability to migrate. The
                effectiveness of the EPA's pump-and-treat contingency plan depends on the release of arsenic from the
                aquifer, therefore making it an impracticable technology for removing arsenic from the site.

RESPONSE:
EPA is well aware of the limitations of pump-and-treat technology as it relates to this site.  Long term
water quality monitoring will be required to monitor the effectiveness of the final remedy.  Should low
probability, unanticipated plume movement occur, the ROD identifies a contingency of containment
technologies, such as capture and pump back, which are practical and implementable.

COMMENT:
        •       In-Situ Remedies for the Rocker OU:  The EPA's Proposed Plan does not consider the use of an in-situ
                remediation at the site.  An in-situ  remedy would be far less costly and dangerous (short-term) than the
                full scale removal/pump-and-treat technology proposed by the EPA. ARCO's in-situ remedy would
                enhance the natural attenuation of arsenic. An experiment performed at Montana Tech's Geochemistry
                Department has proven die effectiveness of an in-situ iron stabilization technology for arsenic on samples
                from the Rocker OU. Stabilizing the arsenic in place would allow for the arsenic to attenuate to natural
                background levels thus reducing the long-term risks at the site.

RESPONSE:
EPA has deep concerns that the ARCO proposed remedy is not implementable.  If it can't be
effectively implemented, it will also  not be protective or effective.  Please see EPA's comments to the
ARCO plan.

COMMENT:
        •       EPA/ARCO conflict of Fe content at the Rocker OU:  The EPA state's in the Proposed Plan the "no iron
                is available (on site) to attenuate the arsenic". Whereas ARCO's Proposed Plan states "the alluvial
                aquifer has an  abundant supply of iron which is the primary source for arsenic attenuation." Dr. Bill
                Chatham of the Mt Tech Chemistry Department maintains that his research indicated that there is
                sufficient iron present to fully adsorb the arsenic contamination. It is his position that the iron flood
                technology proposed by  ARCO will ensure mat there is sufficient iron present to immobilize the arsenic.

RESPONSE:
The intent was to state that once in  the tertiary aquifer, there is no iron to attenuate the arsenic.  Also
see EPA's comments to Dr.  John Sonderegger, on this same subject.

COMMENT:
        •       Alternate Water Supply: The primary potential human health concern for the Rocker OU is based on the
                potential consumption of contaminated water.  Presently, no individuals are exposed to contaminated
                groundwater at the site.  If the potential for consumption of arsenated water is the driving factor in
                determining the EPA Proposed Plan, it would be much easier and less costly to provide an upgrade to the


                                                                                Vincent/Larson Comments 2

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                current water supply line from die Butte water system to meet all current and potential expanded water
                needs for Rocker residents.  While the Butte Water Division has the capacity to supply water to Rocker
                it is vital that the Butte-Silver Bow Water Utility Division be allowed to review and control banning all
                new wells within the contamination zone, would be the most protective solution for human health risk
                problems associated at the Rocker site. Any existing well users within the zone of contamination would
                be adequately protected from consumption of contaminants by proper monitoring of the groundwater
                upgradient from their wells. In the event of a  detected contaminant plume, these users would then be
                connected to the upgraded Butte water line.  A water storage tank would also be provided within the
                Rocker community to meet their fire suppression needs.

 RESPONSE:
 EPA is concerned that development of additional  wells without some level of control may place
 additional hydraulic influences on the plume and  cause additional migration. Additional water
 supplies should be provided to the new residential and community needs for the community to alleviate
 this pressure, especially in light of the well ban.

 COMMENT:
        •       Institutional Controls: A Groundwater Control Area will be necessary at the Rocker OU site to prevent
                use of shallow, contaminated groundwater. Long-term sampling and analysis at the site could be
                performed by Butte Silver Bow Water Quality District personnel.

 RESPONSE:
 Response: Comment Noted.  EPA and ARCO  will consider the use of Butte Silver Bow Water Quality
 personnel for sampling/monitoring.

 COMMENT:
 The EPA Proposed Plan does not provide a sufficient amount of conclusive data to be considered feasible; especially when
 considering the $7.34 million price tag.  EPA openly admits the uncertainty which is associated with the effectiveness of
 their plan:  "None of the (EPA) alternatives reduce the intrinsic toxicity of arsenic through treatment. Even alternatives 4
 and 7 (part of the Proposed Plan), which address treatment of arsenic in the plume, do not reduce the toxicity of the
 arsenic."  This leads one to believe that, when considering the increased short-term risks associated with the EPA Proposed
 Plan and the uncertainty involved with its ability to achieve long-term risk reduction, it is just not a practical remedy.

 RESPONSE:
 The site is very complex and there is no "easy* solution.  ARCO's proposed plan is clearly deficient
 in that it is not implementable, hence it will not be effective.  EPA has considered comments and have
 made changes to the Proposed Plan to the final remedy that are implementable and reliable, and
 more cost effective, without sacrificing protectiveness to human health and the  environment.

 COMMENT:
 The ARCO plan of an in-situ remedy coupled with monitoring, revegetation, preventive institutional controls, and a
 contingent water supply for Rocker residents seems to be the most cost-effective and reliable alternative.  Also, the short-
 term risk associated with ARCO's idea is nil when compared to the EPA's Proposed Plan. The EPA states that removal
 construction may increase the mobility of the arsenic rather than decrease it—one more reason not to endorse the EPA plan.

 RESPONSE:
 EPA disagrees. ARCO's plan is not implementable or  cost effective, and is not reliable.  Short-term
 risks associated with the selected remedy  can be managed effectively.  Arsenic  mobility during
 excavation will be addressed through the  addition of iron  to groundwater and careful monitoring.
 EPA's selected remedy appropriately balances the nine selection criteria and the  statutory and NCP
 mandates—ARCO's does not.  The community of Rocker supports EPA's selected remedy and, from
follow conversations, we conclude Butte/Silver Bow County does as well.  See EPA's response to
 ARCO's comments for more detail.


                                                                               Vincent/Larson Comments 3

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                         ARCO COMMENTS REGARDING
              EPA's  Baseline Human Health Risk  Assessment
COMMENT:
"SOIL WITHOUT COVER" SCENARIO


In the risk assessment, the primary approach used to evaluate risks associated
with site soils is the calculation of  exposures  and risks for three  sets  of
exposure point concentrations: soil concentrations  in samples from the soil cover
placed over much of the site during a removal action completed in 1989 (referred
to as  "on soil  cover"  in  the  risk assessment),  soil  concentrations in samples
from site areas that were  not  covered during the removal action (referred to as
"outside  soil cover"), and soil concentrations in original site soils that are
currently beneath the soil cover  (referred to as "without soil cover" to reflect
the hypothetical risk assessment scenario that these soils might be exposed at
the  site surface  at  some point  in  the future).  (Risks  associated with  the
concentrations  observed   at  individual   sampling  locations are  presented  in
appendices to the risk assessment.) While this approach may provide an indication
of the potential risk levels associated with the specified categories of soil,
these exposure point concentrations are  unlikely to be representative of actual
patterns  and levels of exposure. Instead, potential  receptors  are  likely to be
exposed to some combination  of these categories of soil, and resulting exposure
levels and risks .will similarly reflect a combination of the levels calculated
in the risk assessment.


In particular, potential  receptors are unlikely to be exposed only to the soils
outside or beneath the cover,  but instead will likely be exposed to these soils
in conjunction with exposures to  the soil in the  cover. As  a  result,  the risk
estimates derived  in  the  risk assessment for areas  outside the soil  cover  or
exposures that might occur without the soil cover overstate  risks that are likely
associated with either current or future exposures.  For example, an individual
who currently has  occupational or recreational exposures at the site  would  be
unlikely  to have exposures only to covered areas or  only to uncovered areas  of
the site. Instead,  exposures are likely to occur across the  site and the exposure
point concentration reflecting such activity patterns would be an area weighted
average of the concentrations reported  in soils that are currently at  the site
surface.


Because a much greater proportion of the  sampled site area is currently covered,
concentrations that better reflect likely activity patterns would  be  closer  to
the  "on  soil  cover"  exposure point  concentrations  presented  in  the . risk
assessment than to  the  "outside soil cover" concentrations.  Risk estimates would
also be correspondingly lower.


Similar concerns exist regarding exposures that might occur  in the future. In the
risk assessment, the "without soil cover" risk calculations are presented as a
means of evaluating risks  and  exposures  that might occur if the soil cover were
disturbed in the future by  factors such as erosion  or excavation  (e.g.,  as a
result of  construction activities). As  presented in the risk assessment,  the
exposure point concentrations  reflect a  scenario in which the entire soil cover
is removed from the site, directly exposing all  soils currently beneath the
cover.  In actuality, if erosion or excavation occurred,  only some portion of the
soils currently beneath the cover  would  likely be exposed.  In addition,  soils
with higher concentrations that are currently beneath the cover would likely be
mixed with the lower concentration soils present  within the cover.  This mixing
would reduce the soil  concentrations found beneath the cover. Because  exposures


                               ARCO  Human Health Evaluation  Comments  1

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are likely to involve only a portion of the soils beneath the cover and because
mixing of soils would occur, the exposures and risks associated with hypothetical
future  exposures to  soils currently  beneath the  cover  are  likely  to  be
significantly less than those presented for the "without soil cover" case  in the
risk assessment.

At a minimum,  the risk  assessment should provide more  explicit explanations
and/or instructions  to the risk managers in order for the "without soil  cover"
risk assessment  to be used appropriately in the decision making process. For
example, there is a strong likelihood of the soil cover remaining intact and that
appropriate institutional controls and long-term maintenance requirements written
into the Record of Decision would make a complete, or even partial, breach  of the
soil cover very unlikely. Without this type of qualification some risk managers
and most  of the  public would  be  left with  the idea the "soil without  cover"
scenario is likely to occur.

RESPONSE:
In  establishing  data groupings  for  the  risk  assessment,  EPA
recognized that combining data  from samples  collected outside the
soil   cover  with   those   collected   on   the   soil   cover   would
inappropriately bias the exposure point  concentration  for the
combined group.  As stated on page 4-5 of  the risk assessment,  this
bias was  due  to  the  disproportionate  number of samples  (with
respect to area)  collected  from each area.   EPA recognizes the
uncertainty associated  with segregating exposure from  these two
areas due to potentially integrated exposures that could occur, but
felt  that it  was more important  to  avoid  the unnecessary bias.
However,  even  if the  areas were  area-weighted  with respect  to
exposure concentration, this would not change  the conclusions  of
the risk assessment.  Even if it were assumed that the soil  cover
constituted 75 percent of the potential site area (exposure unit),
the aggregate  risk  would still  substantially exceed EPA's point  of
departure  of  1x10-4  excess cancer risk.   EPA also recognizes the
uncertainty associated with assuming that current soils beneath the
cover could be exposure point concentrations on the surface in the
future,  and this is  stated explicitly in the  uncertainties section
of the Baseline Human Health Risk Assessment.  The risk analysis  of
this data group represents a bounding estimate that accounts  for a
situation  where the  soil  cover is  scraped off  the surface during
construction activities at certain areas,  exposing concentrations
equivalent  to those  measured beneath the cap.   In addition, the
soil  cover  was  installed  without  any  regulatory  approval  or
oversight,  the depth of  the soil  cover  is  uncertain and likely
feathers out  along  the  edges.    Current  or  future exposures  to
"under  cover"  concentrations in these areas are not  unreasonable.
                           ARCO Human Health Evaluation Comments 2

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COMMENT:
ARSENIC CONCENTRATIONS IN INDOOR DUST

Humans are exposed to contaminants  in soil not only while outside, but also by
coming into contact with indoor  dust that has been contaminated by soil brought
into the building. Very often metal concentrations in indoor dust are found to
increase  more  slowly   than  soil concentrations.  Thus   when  soil  metal
concentrations  are  high  total  exposures  may   be  overestimated  if  dust
concentrations of metals are  not  accurately  measured  or predicted.  In  the
baseline  risk  assessment for the   Old  Works/East  Anaconda  Development  Area
(OW/EADA)  operable unit  in Anaconda  (ICAIR 1993) , EPA relied on data from a study
performed in current residential areas of Anaconda (Bornschein 1993)  to model the
relationship between arsenic concentrations in soil and  arsenic concentrations
in indoor dust.  The resulting equation was used to predict indoor dust arsenic
concentrations,  and 50 percent of the daily intake of soil and dust by workers
was assumed to be from indoor dust.  The  equation is:

      €„„„= 0.15 x C^a + 40 ppm.

This relationship should also be applied in assessing exposure of site workers
or future residents to soil  arsenic at the Rocker site.


RESPONSE:
The relationship  between  indoor  dust  and  soil  concentrations of
arsenic  established for  the Anaconda site has not been established
to  be  applicable  to the  Rocker  site.   The relationship is site-
specific,  depending  on  arsenic  source, mineralogy,  meteorology,
concentration  versus  particle  size  distribution,   etc.     ARCO
provided no  site  specific information on this  topic,  which is
unlike the Old Works/East Anaconda Development OU process.   In lieu
of  this  site-specific information for  Rocker,  no  distinction is
made  in  the  risk   assessment  between  soil  and  dust   arsenic
concentrations.
COMMENT:
REDUCED BIOAVAILABILITY OF ARSENIC  IN SOIL


The  risk  assessment   discusses   uncertainties  that  exist   regarding  the
bioavailability of arsenic at the site; however, no quantitative adjustments were
made to reflect the reduced bioavailability  of arsenic  in soil.  Assumptions
regarding bioavailability of metals in soil and dust can significantly influence
risk estimates. The U.S. Environmental Protection Agency1 s (EPA) toxicity values
for arsenic ingestion are based on exposure  to arsenic dissolved  in water.
Because absorption of metals in soil and dust is generally less than that of
metals in water or food, risk assessment calculations  should account for these
differences by applying a bioavailability  adjustment  factor  to  either the
toxicity factor or to the intake estimate.  Arsenic absorption from Anaconda soil
samples has been examined in  two animal  models: rabbits and  monkeys (Freeman et
al. 1993, 1994) .

In the rabbit study,  the average absolute bioavailability of the arsenic in the
test soil  was found  to  be  28 percent,  which, when compared with the absolute
bioavailability of the  dissolved soluble arsenic delivered using  oral gavage


                              ARCO Human Health Evaluation  Comments 3

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 (i.e., 59 percent),  yields an average relative bioavailability adjustment factor
of approximately 0.50 (i.e., 0.48) .  The monkey study included indoor dust as well
as soil.  In the monkeys,  the  average absolute bioavailability of the soluble
arsenic delivered by oral gavage was 66.8 percent,  that of the arsenic in soil
was 13.4 percent, and that of  the arsenic in dust  was 19.2 percent. These data
yielded average relative bioavailability adjustment factors of 0.20 for arsenic
in soil and 0.28 for arsenic in dust. Monkeys are  more similar physiologically
to humans than are  rabbits; therefore,  the data obtained from the monkey study
provide the most appropriate estimates of the bioavailability adjustment factor
to use when evaluating  risks  associated with exposure to arsenic in  soil and
dust.

The arsenic in soil from Anaconda is derived primarily from smelter emissions,
and therefore,  is expected to differ in mineralogic  form from the arsenic present
in the soil at Rocker.  Recent studies  by PTI  Environmental  Services  suggest,
however, that  even  soluble arsenic salts mixed with soil  are associated with
reduced bioavailability after  a period of weathering.  PTI  has developed an in
vitro bioaccessibility test that may be  used to  compare the dissolution rates of
arsenic from various soil samples in the gastrointestinal tract. PTI has found
that arsenic in soil from several arsenical pesticide formulation facilities
exhibits dissolution rates as slow or slower than those of arsenic from Anaconda
soil. These data suggest that arsenic which has been in the soils at Rocker for
40 years or more is  likely to be less than 50 percent as bioavailable as arsenic
dissolved in water.  For that reason a bioavailability adjustment of 0.5 should
be made when assessing risks of exposure to arsenic  in Rocker area soil.

RESPONSE:
The bioavailability  of arsenic is recognized in the risk assessment
on page 7-2  as  a source  of uncertainty.    The bioavailability  of
arsenic  from  soil  was  assumed to  be   equal  to  that  in  the
toxicological  studies  from  which the toxicity values  for arsenic
were  derived,   and   this  is  the  basis   for  EPA's  use  of  the
bioavailability  factor.  This was due to the lack of site-specific
information  quantifying  the  reduced  availability  of  arsenic
following soil ingestion.  The information  cited in the comments is
site-sped fie  information related to the  very  different type  of
mining waste found at that site.  As stated on page 5-8 of the risk
assessment,  since the bioavailability of arsenic  from  soil at the
site  requires  understanding of the chemical form,  particle  size,
matrix  type,  etc.,  and  since  these  data  are lacking,  the  risk
assessment  made no  adjustment  for  bioavailability.    This  is
consistent with  EPA's position  in The Clark Fork  Position Paper on
the Bioavailability  of Arsenic (1994),  which states:

      "It  is  also   recommended   that   an  assumption   of   100%
     bioavailability   be   applied   at   sites    where   arsenic
      contamination   is   associated   with   the   application   of
     pesticides/herbicides, wood  treatment processes and/or fossil
     fuel  combustion."

Even if a  bioavailability correction  factor  of 50* were applied to
the soil and groundwater risks  for the Rocker site,  the total  risk
estimates would still substantially exceed EPA's point of departure
of 1x10-4  excess cancer risk.
                            ARCO  Human Health Evaluation Comments  4

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COMMENT:
UNCERTAINTIES IN ARSENIC TOXICITY

The  risk  assessment does not  include  a discussion of  the  uncertainties that
surround EPA1s standard toxicity factors for ingested arsenic. Numerous lines of
evidence suggest that toxicity factors currently used in risk assessments by EPA
to  evaluate  the  toxicity  of  ingested  arsenic  overestimate  toxic  effects,
particularly at the relatively low levels  associated with exposures in the U.S.
The carcinogenic slope factor  (CSF) and reference dose (RFD) for ingested arsenic
were derived by EPA from an ecological epidemiological study  of the incidence of
skin cancer and blackfoot disease in a Taiwanese population with elevated levels
of arsenic in their drinking water (Tseng et al. 1968; Tseng 1977) . EPA's Science
Advisory Board, in commenting  on EPA's draft Drinking Water Criteria Document on
Inorganic Arsenic  (Loehr and  Ray 1993; U.S. EPA  1993), urged  that  the agency
conduct an  in-house quantitative risk assessment for  cancers  other  than skin
cancer that accounts for potential nonlinearities in the dose-response curve and
the high background arsenic concentrations in the Taiwanese populations compared
to U.S. populations.  New epidemiological  analyses of the Taiwanese populations
and  new  data on the  dietary  sources  of arsenic  in these populations  provide
further evidence that the present CSF is likely to overestimate risks for U.S.
populations.

Recently, Quo and coworkers conducted a large-scale ecological  epidemiological
study  (including, but not limited to,  areas studied by Tseng et al.  [1968]  and
Tseng  [1977]) evaluating the relationship between arsenic exposure and cancer in
11.4 million people living in  243 townships in Taiwan (Quo et al. a,b, in press;
Guo  1994,  pers. comm.). Quo  et al.  (a,b,  in press;  Quo   1994, pers.  comm.)
evaluated  10  exposure categories  of arsenic in  drinking water   (' 0'  mg/L
[undetectable], trace,  0.01  mg/L, 0.02 mg/L,  0.03-0.04 mg/L,  0.5-0.08  mg/L,
0.09-0.16 mg/L, 0.170.32 mg/L, 0.33-0.64 mg/L,  and above 0.64 mg/L).  Evaluation
of the dose-response relationships for bladder  cancer, kidney-transitional cell
carcinoma, renal cell carcinoma, and skin cancer incidence per 100,000 people vs.
arsenic concentrations  in  well water  used by those populations  demonstrated
statistically  significant  increases only in  the  highest  exposure  level  for
bladder and skin cancer (Guo 1994, pers. comm.).  Three additional studies also
suggest a threshold for carcinogenic effects of  ingested arsenic  (Brown and Chen,
in press; Chiou et  al. 1993; Hsueh et al.  1993) . Taken as a whole, these studies
indicate that the dose-response relationship for carcinogenic effects of arsenic
may reflect either a threshold or a nonlinear relationship,  with exposures less
than 10~225 ug/day being either noncarcinogenic or of relatively lower potency
than high levels.

The strongest mechanistic evidence of nonlinearities in the dose-response curve
for  arsenic comes  from metabolism studies  that  indicate that methylation  of
arsenic to less toxic, more rapidly excreted species provides the primary means
of arsenic  detoxification  (Vahter 1983;  U.S.  EPA 1988; Thompson 1993).  This
metabolic  pathway   is located  primarily  in  the  liver.  Metabolism  involves
sequential addition of methyl  groups to yield monomethyl arsenic acid (MMA)  and
dimethyl arsenic acid  (DMA)  through enzymes known as methyl  transferases. Each
successive addition of a methyl group reduces toxicity by approximately an order
of magnitude, as reflected in decreased acute toxicity and genotoxicity.  Because
the metabolism  of  arsenic  depends upon enzymatic processes, the  relationship
between arsenic exposure and  internal  dose will be  inherently  nonlinear,  with
enzyme saturation  at high  arsenic levels resulting in diminished ability  to
detoxify  (i.e., methylate) arsenic.  When the  capacity  of this  detoxification
mechanism is overwhelmed, the  potential for  interactions of inorganic  arsenic
with  target tissues,  such as the  skin, lungs,  and  liver,  increases  with
increasing levels of  circulating unmethylated  arsenic.  This means that higher
exposure levels of  arsenic will be relatively more  efficient  at inducing adverse
effects than  lower levels.  Toxicological and  epidemiological  studies  provide
evidence of  impaired arsenic  detoxification at relatively high (0.4-0.6  mg/L)
levels of  arsenic.  It  should be noted that  the  levels at which methylation

                               ARCO Human Health Evaluation  Comments  5

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appears to be impaired are comparable to those observed by Guo to be associated
with increased risk of  bladder  and  skin cancer  (Guo et  al.  a,b,  in press; Guo
1994, pers. comm.).

Although the level of arsenic exposure in the populations studied by Tseng et.
al (1968) and Tseng (1977)  is poorly characterized, available evidence suggests
that arsenic exposures have been underestimated. Accurately accounting for the
higher arsenic exposures in evaluating the dose-response  curve for arsenic would
result  in  a reduction  in the  CSF  and  an  increase in  the RfD for  arsenic,
reflecting the  reduced potency of  arsenic.  Two sources of data suggest that
arsenic exposures have been underestimated.

First, EPA's calculations for the recently verified RFD for arsenic were based
on  higher consumption  rates  for  water and  higher body weights  in  these
populations than were assumed in deriving the CSF, suggesting that arsenic intake
from water may have been underestimated in the CSF calculations. When the CSF for
arsenic was recalculated using  the exposure assumptions used by EPA in deriving
the  RFD,  the CSF was  reduced  from  1.75  (mg/kg-day)~1  to 1.13  (mg/kg/day)  '
(Valberg and  Beck 1994, pers.  comm.).  Because it  is  based on more  accurate
estimates of arsenic intake in the exposed population,  this revised CSF should
be used in estimating risks from arsenic exposure.

Second, new data on arsenic concentrations in food products from Taiwanese areas
studied by Tseng et.  al (1968) and Tseng  (1977) suggest that EPA underestimated
the intake of arsenic from food  in calculating the toxicity values for arsenic.
To more  accurately estimate arsenic concentrations in Taiwanese food,  five
samples of yams and three samples  of rice (two samples of polished rice and one
of rice grains) collected from the  areas of Taiwan with historically high arsenic
exposures,  along with collocated soil samples, were analyzed for both Total and
inorganic arsenic concentrations. The average inorganic arsenic concentration in
the yams was 0.15 mg/kg, while the  concentration in the two polished rice samples
was 0.118 mg/kg. No organic  arsenic was  detected  in yams,  and organic arsenic
concentrations in  rice comprised less  than 16 percent  of the total  arsenic
concentrations. When the inorganic arsenic concentrations in rice and yams were
used to calculate an alternate RFD, a 2.5-fold increase in the current RFD (from
3x10-4 to 8x10-4 mg/kg-day) was obtained. Incorporating  the observed inorganic
arsenic  concentrations  in Taiwanese yams  and  rice along with the  new  RFD
assumptions yielded a CSF estimate of 0.77 (mg/kg-day)1, which is 2.3-fold lower
than the current CSF. A paper presenting  these data has recently been submitted
for publication to the journal Environmental Health Perspectives. These data have
also been submitted to EPA's Integrated Risk Information System for consideration
in EPA's evaluations of revisions  to the arsenic toxicity factors.

The new epidemiological evidence for nonlinearities in the dose-response curve,
combined with  new evaluations  of dietary arsenic exposures in the  Taiwanese
population that formed the basis for the current toxicity values provides strong
evidence that the current CSF overestimates cancer risks associated with arsenic
ingestion by more than an order of magnitude.  This evidence that arsenic toxicity
is substantially overestimated by EPA should be considered when evaluating risks
and deriving cleanup  levels for  arsenic  in soil. Because arsenic is  the primary
contributor to both the cancer  and noncancer risk estimates calculated for the
site  (accounting for more than 99  percent of  the risk in some cases) ,  using
conservative assumptions to estimate arsenic intake (i.e.,  the risk assessment
assumes that  all  arsenic present at the site  is fully bioavailable)  and toxicity
is likely to yield overly conservative estimates of the  overall  risks  posed by
the site.
                               ARCO Human Health  Evaluation  Comments  6

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RESPONSE:
The uncertainties associated with the human toxicology of arsenic
and the  application of the derived slope  factor for arsenic are
described  in EPA's  Integrated  Risk Information  System database
 (IRIS).   This information is available  to risk managers.   These
uncertainties include the possibility of nonlinear dose-response,
the potential for detoxification at low dosages,  the relevance of
skin cancer rather than internal cancers, competing mortality from
black-food disease, and lack of reliable information on alternate
sources of arsenic  exposure.

The Draft  Drinking Water Criteria Document  on Inorganic Arsenic
 (EPA 1993) states  that  subsequent analysis of the Taiwanese data
for the potential risk of non-skin cancers from arsenic ingestion
indicates that the risks for internal cancers may be as high as 10-
fold higher  than  for skin tumors.   If this is in fact the case,
then  the risk  estimates  in  the Rocker risk assessment may be
underestimated.

It is well recognized that arsenic is methylated in vivo at lower
exposure  concentrations,  with saturation  of  this detoxification
mechanism occurring at higher concentrations.   The possible result
is a nonlinear dose-response.  However,  according to  (EPA 1993), it
is  unknown whether  the  dose-response  data  used to  develop  the
cancer slope factor for arsenic was below or above the saturation
point.  If the  doses in the Tseng et al.   (1968) study were above
the saturation point, then the slope factor would be overestimated
at lower dose exposures.  Conversely, If the exposure levels in the
Tseng et al.  (1968) study were below saturation,  then  the slope
factor would be too low at high  dose exposures.  Considering this,
the EPA Risk Assessment Forum concluded that:

     "While  consideration  of  these data   on  the  genotoxicity,
     metabolism, and pathology of arsenic has provided information
     on  the  possible mechanism  by which arsenic may  produce
     carcinogenic effects, a more complete understanding of these
     biological data in relation to carcinogenesis is needed before
     they can be factored  with confidence into  the risk assessment
     process"

An adjustment of the arsenic slope factor from  1.75 to 1.13 mg/kg-
day, as  ARCO suggests,  would alter  the conclusions  of  the risk
assessment.  However,  the ROD did recalculate the cancer risk based
on  EPA's June  1,  1995 revised slope  factor reported in  EPA's
Integrated Risk Information System (IRIS) data base.  The revised
slope factor  of 1.5 mg/kg-day corresponds  to  382 ug/g arsenic in
soil at the 1 x lOr4 excess cancer risk  level compared to 327 ug/g
calculated in the EPA, Rocker Human Health Evaluation (February,
1995).
                          ARCO Human Health Evaluation Comments 7

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COMMENT:
GROUNDWATER EXPOSURE ASSESSMENT

In the risk assessment,  groundwater concentrations were calculated for shallow
groundwater wells (defined as samples collected at a depth of less than 20 ft)
and a combination of intermediate and deep groundwater wells  (defined as samples
collected a depth of greater than 20 ft).  This  simplistic characterization
inaccurately reflects the distinction between  two  separate  (shallow and deep)
groundwater aquifers existing at  the site. This characterization is also not
consistent  with  data  collected from the  RI.  As a result,  the  concentrations
presented in the  risk assessment for "deeper groundwater"  (which  is used as a
drinking water supply in the  region) suggest a level of contamination that is
higher than is actually present in any groundwater sources that might be used for
a drinking water supply.

A more  accurate  characterization  of the  groundwater at the  site, which is
consistent  with  hydraulic  and chemical  information from  the  RI' s  field
investigation,  show two separate aquifers-a shallow alluvial and deeper tertiary
aquifer. The shallow aquifer is contaminated with arsenic only in a limited area
around the Rocker site and is not being used as a  drinking  water  source. Even
without  the arsenic  contamination,  the  hydraulic  conductivities and  salt
concentrations of this shallow aquifer make it extremely unlikely it will ever
be used for drinking water. The  deeper tertiary aquifer is currently a drinking
water source, yet no drinking  water wells completed in this zone have ever shown
any arsenic contamination.  Only one tertiary well, which is completed in the
upper portion of this zone, shows any significant arsenic, and this well probably
indicates an  isolated  condition.   Geological,  hydraulic,   and  chemical  data
collected provide multiple lines of evidence there is minimal connection between
the two aquifers,  if any. This means the real drinking water source near Rocker,
which is used in the risk assessment, should be  limited to the tertiary aquifer,
and only data from the RI for  that aquifer  should be used to establish exposure
point concentrations. At a minimum,  the risk assessment should recognize: 1) the
unlikely possibility of consumption  of water directly from the shallow aquifer,
2)  currently, all drinking water wells show no arsenic  contamination,  and 3)
conditions allowing the shallow contamination to migrate into the deeper aquifer
is highly unlikely.

RESPONSE:
The RI recognizes  that there are 3 aquifer units of importance-  the
shallow alluvial aquifer which is severely contaminated,  the deeper
alluvial aquifer which is considerably less impacted and the deeper
Tertiary aquifer system that is the principal source of groundwater
in the area.   Risk assessment guidance and the NCP require that  the
most  conservative  assumption,   (residential  groundwater use)  be
evaluated for  groundwater classified as potentially usable,  which
the shallow aquifer is according to State of Montana classification
standards.  For that  reason,  the mean chemistry shallow aquifer  ,
limited to a depth of  about 20-feet,  was   utilized for  one  set of
calculations.    The    next  scenario utilized  the  data from  the
intermediate  and   deeper wells,  having much  lower  contaminate
concentrations for the second  set of risk calculations.

The risk assessment  does recognize  the  lower likelihood of  future
consumption of shallow groundwater.  However,  since  a  hydraulic
connection has been  identified between   the contaminated shallow
alluvial system   and the  Tertiary aquifer  near  well  RH-6,   the
shallow groundwater risk  estimates represent  a higher end estimate


                             ARCO Human Health Evaluation  Comments  8

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of potential future migration and exposure to human receptors.   In
addition,  the State classifies  the shallow aquifer as potentially
usable,  and the  Baseline  Human  Health  Evaluation  follows  this
determination, in accordance with  the NCP.

COMMENT:
EXPOSURE POINT CONCENTRATIONS FOR SOILS

Exposure point concentrations established for soils in the risk assessment are
likely not representative of the actual  site conditions  for several reasons.
First, as reflected in Figure 4-2 of the risk assessment, soil concentration data
are not available  for certain portions of the  site  (particularly the eastern
portion). If sampling was focused on the areas where contamination was suspected
and, thus,  concentrations are  likely  to be lower in the  unsampled areas,  then
site-wide,  area-weighted average concentrations (for areas outside the cover)
would be lower than those presented in the risk assessment.

Secondly, Figure 4-2 also suggests that  the samples  from  outside  the  cover
include a disproportionate number of samples from areas located near the railroad
lines. Samples and locations near the railroad have been shown to have higher
concentrations than other soils  located outside the cover area, therefore,  the
risk assessment exposure point  concentrations  for soils outside the cover may be
inappropriately biased too high  due to samples  near the  railroad.

Lastly, the soil  exposure point concentrations used in the  risk assessment do not
represent the fact  that  a significant portion  of the arsenic in the  surface soil
at Rocker is  likely  from mine  tailings, not from arsenic treating operations.
This is significant because the bioavailability of arsenic from mine tailings has
been studied for the  region and is shown to be much less than 100%,  which is the
general assumption used in soils assessment. Appendices  to the  risk assessment
present the results of  risk  calculations performed  for soil concentrations
observed at individual sampling points at  the site. These results indicate risks
greater than EPA's target levels at most locations sampled  at the site (excluding
the concentrations observed in  the soil  cover). When making risk management
decisions for  the site,  EPA should recognize  some of the  information presented
in  Appendix  B regarding the  likely influence  both to mine tailings  and
wood-treating  activities on some of the higher  soil concentrations observed at
the site. Otherwise,  there may  be  some  decisions regarding  mine  tailings  at
Rocker which are inconsistent  with  decisions  made at other sites,  such as  Old
Works and Streamside Tailings.
RESPONSE:

As  previously  stated under  "SOIL WITHOUT  COVER  SCENARIO",  data
groupings were made  to avoid biasing the risk estimates resulting
in  over estimation.   Accounting for sampling  density  and  area-
weighting  the  risk  estimates would not likely have  altered the
conclusions of the risk assessment.
The RI designed and conducted by ARCO could not distinguish between
various sources of  arsenic  such as  sulfide mineral  versus spilled
wood   treating fluids  or  arsenic  trioxide.    No  site-specific
information  exists  identifying arsenic form  at specific locations


                             ARCO Human  Health  Evaluation Comments 9

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on-site.  As such,  the assumption arsenic was lOOZ bioavailable, in
accordance with previously cited guidance.
In  their sampling plan,  ARCO defined  the frequency  of samples
taken.  A number  of  samples taken near the railroad right-of-way
was presumably to be of  interest for  the  occupational scenario
which ARCO believed to be  an appropriate scenario for human health
risk evaluation.  The assessment  of the  risks from the former wood
treating plant were  recognized by EPA  to be  complicated  by the
potential influence of 1)  operations of the railroad right-of-way
that runs within  the southern portion of the  Rocker  OU,  2)  past
mining operations in the  area, and 3)  a  removal  action formerly
conducted at the site.  The railroad right-of-way may contribute to
the human health risk associated with this site since the ballast
materials were composed  of mineralized mine waste rock and arsenic
containing  concentrates  that  were reportedly spilled  along the
right-of-way.  Although the primary focus of the  risk assessment
was  on  the  wood treatment  residues,  any  human  health  risk
associated with contaminants within the railroad right-of-way are
included in the overall  site risk estimates.
                         ARCO Human Health Evaluation Comments 10

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                       RISK ASSESSMENT ISSUES

                Identified in ARCO's  RI Disclaimer
COMMENT:

Presented here  is an overview of ARCO's numerous  concerns  on  EPA's  Risk
Assessment.  As  opposed to all other risk assessments performed to  date in the
Clark Fork Basin, EPA chose not to significantly dialogue with ARCO on the Rocker
BRA. It is apparent there was little dialogue between the author's and other EPA
risk assessment  contractors, based on the number of inconsistencies with other
recent EPA risk assessments related to arsenic.  Provided here is a list of some
of those  inconsistencies and other concerns, which will be detailed in ARCO's
comments  for the BRA.
RESPONSE:

.Responses to the  summary comments below can  be found in  detail in
EPA's response  to ARCO's  Comments on  the Baseline Human  Health
Evaluation   for  the  Rocker  Timber  Framing  and  Treating  Plant.
Operable Unit.   EPA's Rocker OU Baseline Human Health Evaluation
was  coordinated with other Clark Fork Basin  OU risk assessments,
and   appropriate  distinctions  were  made.    Most notably,  site
specific adjustments at other OUs based on site specific  data were
not  followed here,  because of  the different  kind of wood treating
waste present at the Rocker OU and the lack of site specific data.
COMMENT:
           Available site concentration data was not utilized appropriately to
           represent realistic exposures.
RESPONSE:

See  comment  addressed  as  response   to  "Spoil  Without  Cover'
Scenario.
COMMENT:
           Default  indoor  arsenic  was  used  as  opposed  to  relevant
           regional-specific data available and used in Anaconda.
                           ARCO Human Health Evaluation Comments 11

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RESPONSE:

See  comment  addressed as response  to "Arsenic Concentrations  in
Indoor Dust".
COMMENT:

      •     Reduced bioavailability of arsenic in soils versus water in water,
           and  reduced  bioavailability  due from  arsenic  in  soils  being
           partially from mine tailings was not considered.
RESPONSE:

See  comment addressed as response to "Reduced Bioavailability  of
Arsenic in Soil".
COMMENT:

     •     Residential  scenario was overstated for a site located next to an
           active railroad.
RESPONSE:

This  comment  was  responded  to  previously,  and  the  issue  is
addressed in the fourth paragraph, page 1-2 of BRA.
COMMENT:

     •     Worker  protection  and  trespass  scenarios  used  conservative
           assumptions inconsistent with other EPA risk assessments.
RESPONSE:

The uncertainty expressed here was addressed on pages 4-18 and  7-1
of BRA.
                           ARCO Human  Health Evaluation Comments  12

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COMMENT:
           Uncertainties of arsenic toxicities recognized by the EPA's Science
           Advisory Board was not recognized in the  risk assessment.
RESPONSE:
This comment was addressed previously in response to "Uncertainties
in Arsenic Toxicology".
COMMENT:
           Risk assessment assumes groundwater consumption  directly from the
           shallow  aquifer  will  occur,  not  considering the  limitation  of
           exposure,  the  disconnection  of  shallow   from  deep  aquifer,
           geochemical attenuation,  and dilution  factors.
RESPONSE:

This  comment  was  addressed in response  to  concerns regarding
"Groundwater Exposure Assessment".
                           ARCO Human Health Evaluation Comments 13

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                                ISSUES FROM ARCO DISCLAIMER
                          FINAL REMEDIAL INVESTIGATION REPORT
                        ROCKER TIMBER TREATING OPERABLE UNIT
 COMMENT:
 This disclaimer was prepared by ARCO in response to the Final version of Remedial Investigation (RI) that was prepared by
 EPA and its consultants.  Since the Remedial Investigation will serve as the informational source for the development of
 Remedial Alternatives for the Feasibility Study (FS), it is necessary that reader of the Remedial Investigation be aware of certain
 controversial issues and conclusions. ARCO submits the attached disclaimer for the Final Remedial Investigation Report for
 Rocker Timber Framing and Treating Operable Unit due to changes made by EPA of which ARCO does not agree.  Specific
 page and section references with an explanation and basis supported by data collected under the RI will follow after mis
 summary.

 No Response Necessary

 COMMENT:
 Arsenic does exist in a shallow alluvial zone proximal to the previous wood treating facility and extends for a distance of a
 couple hundred feet. This shallow aquifer is inadequate itself as a water supply due to low conductivities (i.e., ability to pump
 adequate volumes) and water quality concerns not related to the Rocker plant or mining (i.e., nitrates from septic tanks, high
 dissolved solids and salts).

 RESPONSE:
 Area is correct that the shallow aquifer at the Rocker site is not highly productive and is susceptible to
 surface contamination sources.  While intuitively correct, Arco did not collect any data to allow them
 to make the statement about nitrate from septic tanks  The shallow aquifer is productive enough to provide
 water to individual, properly constructed, domestic wells, and is classified as a potentially usable aquifer
 by the State of Montana.

 COMMENT:
 The most significant item of concern with EPA's RI is the implied connection between the shallow, low quality, arsenic impacted alluvium and the older and
 deeper sediments of the Tertiary aquifer. Both the chemical and hydrologic information available from the field investigation demonstrate a very poor connection
 between the two aquifers. There was no demonstrated response to shallower wells when the deeper, Town Pump well was tested by the Bureau of Mines. After
 seven days of stressing the deeper aquifer system the shallower wells that are impacted by arsenic contamination did not respond. The water chemistry likewise
 showed two very distinct types of water, thus indicating a poor connection.

 RESPONSE:
 The conclusion that the Tertiary aquifer and the alluvial sediments are in hydraulic communication was
 made on the basis of converging lines of evidence.  These lines of evidence were described in the RI and
 were verbally provided to ARCO in numerous technical meetings and letters. The following is a summary
 of those lines of evidence.

 1.       The alluvial materials directly overlie the Tertiary sediments.  The contact between the two
formations has been described by  ARCO as  an  "incised paleochannel" (ancient  stream channel),
 consequently upwards  of 80 feet  of Tertiary sediments are in direct lateral contact with the  alluvial
 sediments along the paleochannel sidewall.

 2.       Geologic samples, well logs, and geophysical logs all failed to identify and characterize any
 laterally continuous confining bed separating the Tertiary sediments from the alluvial sediments.
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 3.      The Tertiary sediments are a complex deposit of volcanic tuffs and fluvial sediments. Individual
 beds are discontinuous and  well  to well correlation has proved  impossible.    Individual strata
 demonstrating high horizontal permeability have been detected in every boring that penetrated the Tertiary
 sediments.   The high permeability strata consists of both primary permeability from coarse  alluvial
 deposits, and secondary permeability from fractures. Laboratory tests of vertical permeability conducted
 on short, selected, competent pieces of cores removed from the Tertiary sediments are biased toward the
parts of the section with high cementation or clay content since  those materials are the only ones that
 would remain intact long enough for shipping and  testing.   This discontinuous,  complex bedding
 containing high permeability zones and no identifiable laterally continuous confining bed indicates higher
 bulk, in-situ vertical permeability and consequently, more vertical communication than is implied by the
 discrete selected laboratory derived  vertical permeabilities.

 4.      The  highly productive portions  of the  Tertiary sediments provide industrial quantities of
 ground-water to local users.  These fractured zones immediately underlie the Rocker Site as determined
 by both on-site and off site test wells installed by ARCO.  These permeable zones provide a mechanism
for discharging  (disposing of)  vertical leakage coming through the overlying sediments.   Thus as
 observed at the Rocker Site, 1) hydraulic gradients within the alluvium are predominantly down (only 2
 exceptions) and the gradients between the alluvium and the Tertiary sediments are consistently downward,
 2) vertical gradients between the alluvium and the Tertiary  sediments are in the same range as  vertical
 gradients within the alluvium.  This data indicates that the system is  not under static equilibrium as
 evidenced by the gradients.  The predominant source of recharge water in the Rocker hydrologic system
 is surface water, precipitation, and surficial ground-water.   The predominant zones of discharge are
 lateral flow in the shallow alluvium and the underlying fractured Tertiary sediments,  consequently a flow
 system with a significant vertical flow component is logical and is supported by RI data, including the
 vertical gradients and the vertical distribution of arsenic in the alluvium which shows an arsenic plume
moving laterally and vertically down away from the source area(s).

 5.      The differences in water chemistry noted between strata and locales on the Rocker site are
problematic to explain under any hydrologic interpretation.  One interpretation consistent with EPA's
 conceptual model is that under current ambient vertical flow conditions, the potential for communication
is present but the actual volume of water exchanged is limited by the combination of vertical gradients
and vertical permeabilities. Under current hydrologic conditions vertical water flow is limited thus water
 chemistry characteristics reflect sediment geochemistry influences.

6.     Temperature data collected by ARCO also suggests a vertically active hydrogeologic system as
discussed in previous responses. Elevated nitrate concentrations in samples collected by Montana Bureau
of Mines and Geology on observation  wells used in the Town Pump test indicate vertical movement of
surficial water into the Tertiary sediments from which these wells obtain water.

 7.     The only significant aquifer test conducted in the Rocker area was the Town Pump test performed
by others and notably without support by ARCO.  This test has proved invaluable in assessing the
hydraulics of the  Rocker groundwater systems.   The results of the Town Pump test  include  non
controversial observations of drawdown and more subtle suggestions of drawdown.  ARCO has focused
their objection to EPA's conclusion of vertical communication on the more subtle analyses, the limitations
of which have been fairfy presented by EPA in the RI and at technical meetings.  The following is a
summary of the interpretations made by EPA on the Town Pump aquifer test.
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       a.  Drawdown within the Tertiary sediments during the test was  widespread and relatively
       consistent over large lateral distances indicating the productive zone in the Town Pump well is
       prolific and laterally extensive.

       b.  Drawdown in  the Tertiary sediment wells  was essentially  the same regardless of well
       completion  depth providing direct evidence of vertical  communication within the  Tertiary
       sediments.

       c.  None of the RI wells installed into the tertiary sediments at the Rocker Site appeared to have
       encountered the productive horizon producing water at the Town  Pump well yet all these wells
       experienced clear drawdowns.  This provides additional, direct evidence of vertical communication
       in the Tertiary sediments.

       d.  Indications of drawdown impacts in the deep and shallow alluvial wells  are  not grossly
       obvious and the requests to ARCO to examine this data carefully have been unsuccessful.  ARCO
       has been very vocal in objecting to EPA's interpretations of this data but have not presented any
       rebuttal analysis.  The following is a summary of EPA's analysis and interpretations of this data.

               i.   No drawdown could be detected in any shallow alluvial well.  Pretest water levels in
               the shallow and deep alluvial wells indicate both sets of wells respond to precipitation
               and Silver Bow Creek stages.

               ii.   Vertical gradients between every shallow and deep alluvial well pair changed during
               the test, consistent with drawdown of the deep alluvial well, i.e. downward gradients
               increased at all of the well pairs having downward gradients before the test and in the
               one well pair with upward gradients the gradient decreased.

               in.  Comparison of water levels trends (hydrographs) for deep and shallow alluvial wells
               prior to, during, and after the pump was shut off, show deep water levels immediately
               and consistently show signs of hydraulic recovery (water levels stop falling and began to
               increase coincidentally with the end of pumping while the shallow wells continued to
               decline).

               iv.  A plot of all wells showing the rate of water level decline during the test versus
               distance from the pumped well shows an apparent relationship between distance and rate
               of change with the wells nearest the pumped well declining at a faster rate than those
              farther away.

It has always been EPA's position that while the hydrographs for the alluvial wells do  not show the
irrefutable drawdown shown by the  Tertiary sediment wells, they are consistent with other  lines of
evidence regarding vertical communication. Individually each observation is insufficient to reach a strong
conclusion, however, these interpretations were made independent of one another and as  a group of
observations,  they converge to the same conclusion that the deep alluvial wells felt the effects of the
drawdown imposed by the Town Pump well.

8.  ARCO has often repeated the comment that after 7 days of pumping,  no drawdown was observed in
the alluvial wells, therefore there is no connection.  In addition to the analysis presented in the previous
paragraphs, it is appropriate to address the issue of the pump test duration and well responses.  Under

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fractured, leaky, confined conditions like the Tertiary Aquifer, drawdown response in the aquifer occurs
very quickly (as observed in the Town Pump test) because water pressure (head) changes propagate
rapidly.  Hydraulic responses in overlying, unconfined alluvial aquifers is much slower to develop since
water  level changes in these aquifers requires the actual movement of water particles.  The  time-
development and the total amount of drawdown in the overlying alluvial  aquifer is a function of the
horizontal and vertical permeability  of the alluvial aquifer,  sources  of recharge, and the  vertical
permeability of the strata separating  the pumping zone from  the overlying materials. Consequently,
without a detailed hydrogeologic analysis of the total hydrogeologic system, the lack of obvious drawdown
in  the alluvial aquifer  (as interpreted by ARCO)  cannot be presented as  proof of no  hydraulic
communication.  This issue has been repeatedly brought up to ARCO without response.

9.  The elevated arsenic concentration in well RH-6 is direct evidence that the Tertiary sediments and the
alluvial sediments are in hydraulic communication.  While there remains some uncertainty as to  the
pathway the arsenic took to get into the Tertiary sediments at this location, there is no uncertainty that
it came from an arsenic plume present in alluvial sediments. The two pathways are vertical migration
from arsenic in the shallow ground-water immediately above the well, or laterally from the subcrop area
where contaminated alluvial groundwater abuts the sidewall of the paleochannel. In either case arsenic
from the alluvial system has entered the Tertiary sediments by hydraulic communication.

Summary.   The overall assessment of the hydraulic relationships within the alluvial sediments between
the alluvial sediments and the  Tertiary sediments, and within the Tertiary sediments, conducted by EPA
is based on a series of converging evidence and concludes that the Tertiary sediments are in hydraulic
communication with the alluvial sediments.  Under ambient hydraulic  conditions the Tertiary sediments
have been impacted only slightly. However, under potential future hydrogeologic development, vertical
gradients between the contaminated alluvial aquifer and the Tertiary Aquifer will increase which, because
they are hydraulicalfy connected, will put the deeper aquifer at risk to become impacted by arsenic from
the Rocker Site.

COMMENT:
The second major issue is mat EPA understates the fact mat the arsenic has moved only several hundred feet laterally and 20
to 30 feet vertically in a period of 40 to 80 years since the plant was operated. This is important to show  that the natural
geochemical mechanisms already in place are effectively immobilizing the arsenic. Alluvial materials along Silver Bow Creek
contain abundant iron that literally traps the arsenic in the shallow alluvium in a process known as adsorption. This is similar
to additives used in conventional water treatment plants. Arsenic movement in the system could have occurred for the most part
during operation of the plant some 40 to 80 years ago, and possibly has moved little since that time.

RESPONSE:
There needs to be a separation between descriptions of arsenic  concentrations and movement over time
and the current amount of arsenic in either the sediments or the groundwater.  Historical  movement of
groundwater was assumed to be about the same  as  what is measured and described by the current RI.
This movement, individual groundwater flow paths, and hydraulic connections are complex, (anisotropic
and inhomogeneous) but the physical characteristics have probably changed little in the last few years.
The amount of arsenic still in the sediments and the groundwater are the result of groundwater movement
during and since the termination of wood treatment  at the site.  The relative mobility of the arsenic is a
major concern and also complex,. Problems with the data set include decreasing frequency of sediment
sampling with depth, the random sampling of even the  shallow sediment  sampling, lack of seasonal
sampling, and, probably the most important, the analytical schedule and variability in arsenic and other
constituents in the two groundwater sampling rounds. These data would even  more clearly document
                                                                              ARCO RI Disclaimer 4

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processes described in the current RI but would not significantly change the conclusions carried into the
FS.

Adsorption by iron oxyhydroxide on the sediments is one of the major "natural geochemical mechanisms"
that removes and that can irreversibly immobilize a finite amount of arsenic (assuming pH remains neutral
and oxidation reduction potential remains oxidizing). Laboratory data on the sediments show that a large
amount of arsenic (V+) is contained on the sediments probably by adsorption to iron oxyhydroxide. This
arsenic reservoir will remain largely immobile unless the pH becomes acidic or the oxidation reduction
potential becomes reducing.  Limited observations of acidic conditions have been observed  in settings
thought to be associated with sulfide materials (tailings and/or railroad fill) that are being addressed by
the Streamside Tailings Operable Unit investigations. These materials are proposed to be removed from
areas where they  are saturated as pan of the remedy for that OU and therefore are not considered a
significant influence on arsenic mobility for the Rocker OU.

The alluvial sediments probably have sufficient iron oxyhydroxide to adsorb a part, maybe a major part,
of the dissolved arsenic  but not enough to adsorb the milligram per liter concentrations  of arsenic
currently in the groundwater plume.   The amount of iron oxyhydroxide in the  Tertiary sediments
(volcanics) is probably both highly variable and less than what is on the alluvial sediments.  Groundwater
in the volcanics may also be largely flowing along fractures which would not present the same surface
area that porous media provide in alluvial sediments.   These data suggest that the Tertiary sediments
(volcanics) do not have sufficient iron oxyhydroxides exposed to  the groundwater to control even as much
as the alluvial sediments. Further evidence of this is the absence of iron in groundwater taken from the
Tertiary sediments aquifer.

The arsenic plume in the groundwater of the alluvial materials is sufficient evidence that arsenic is mobile
at the site but there is more data supporting arsenic mobility in the groundwater at the site.  Groundwater
data indicate that there is about equal proportion of arsenic (III) and (V) in the wood treatment area and
higher arsenic  (III) in the downgradient periphery of the  arsenic plume.    These data support the
conclusion that arsenic is being adsorbed but, more imporiantfy, also indicate that arsenic is mobile
because arsenic (III) is poorly adsorbed except by aluminum oxyhydroxides at an alkaline pH (pH> 8).
Therefore, these data suggest that there is a reservoir of mobile arsenic within the wood treatment area
and that the more mobile arsenic species indicates that the arsenic is still mobile in the downgradient part
of the plume. Furthermore, it is difficult to believe arsenic is immobile when  the arsenic concentrations
of the two sampling rounds are compared because the arsenic concentrations (and concentrations of many
other parameters)  are considerably different.  These differences  indicate a relatively mobile groundwater
system that includes arsenic as one of the parameters being mobilized at the site.

Finally, there is little data developed to date that indicate that the arsenic has "moved little  since" the
"operation of the plant some  40 to 80 years ago".  The arsenic  on the sediments and in the groundwater
at the site today is probably a remnant of a much larger arsenic source generated by the wood treatment
facility when it was operating.  The  high arsenic concentrations found in the RI reflect this past high
source and the complex hydraulic conditions at the site.

COMMENT:
The following is a brief summary of other issues discussed in this disclaimer.  They will be discussed in detail following this
summary, including an explanation of ARCO's alternative interpretation of the issue.

The issues identified is this disclaimer have been organized topically as pertaining to Physical Characteristics of Site, Geology, Hydrology, Groundwater
Chemistry, Aquifer Disconnection, Arsenic Fate & Transport, and the Baseline Risk Assessment

                                                                              ARCO RI Disclaimer 5

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 No Response Required

 COMMENT:
 Generally, the physical characteristics of the site are presented in a manner that ignores its historical, current, and future land
 use as a railroad siding on the 100-yr flood plain. To assume that there would be future residential development on the site is
 not realistic given the area is predominantly in a flood plain and is adjacent to an active rail line.  Also, previous removal and
 capping actions are presented as being ineffectual without evidence to support such claims.

 RESPONSE:
 The RI and Risk Assessment(RA) clearly explain and acknowledge the historical and present uses of the
 site.  As regards future residential use, EPA Guidance requires that the highest possible beneficial use
 which can reasonably be expected to develop, which in this instance is a residential scenario, be analyzed
 as part of the RA. The RA couches the potential for residential development to be limited as long as the
 site remains as an industrial/commercial setting.  In this way EPA has evaluated the potential risks of
 residential  development (something that  has  occurred on abandoned  rail lines  nearby) and can
 incorporate that information about potential risk into  the final remedy.  In the case of the Rocker OU,
 the remedy includes institutional controls to limit the land use to industrial/commercial settings but does
 not require cleanup  to full  residential risk values.   This is not an unrealistic  evaluation  or an
 unreasonable remedy.

 Mapping of the floodplain provided by the  Natural Resource Information GIS group, based on previous
flood elevations determined by the State/CH2M Hill in 1988 for Silver Bow Creek, most of the Rocker OU
 is out of the 100 year flood plain.   This  information has been provided in the ROD and ARCO was
previously aware of it from the work done for the Streamside Tailings OU.

 As regards previous removal and capping  efforts, the RI data clearly show that there are still elevated
 concentrations of arsenic at the surface on portions of the site.  Further, the average thickness of the
 cover  soil was about 12-inches but around the edges where  the cover soil feathers in with the original
 topography, it is clear that the thickness is much less.  The RA also considers that future land use could
 involve disturbances that would breach the integrity of the  cap (e.g., excavation incidental to on site
 construction).

 COMMENT:
 Hydrologically, all evidence points toward an effective disconnect between the shallow alluvial aquifer where arsenic is found
 locally and die Tertiary aquifer,  the source of the Town Pump water supply. Ignored is the fact that much of the underlying
 alluvial aquifer has not been degraded by arsenic contamination. One Tertiary shallow well (RH-6) near the railroad load-out
 trench has elevated arsenic likely associated with nearby arsenic spillage, not aquifer migration There is no evidence that this
 monitoring well is hydraulically connected to deeper aquifer zones.  The hydraulic conductivities of the alluvial aquifer is an
 order of magnitude less than the  deep Tertiary aquifer, and preferential flow direction will be horizontal versus vertical. This
 means,  even if the two aquifers were connected, migration into the deeper zone would be very low.

 RESPONSE:
 This comment has numerous parts  which are separated as follows.

 See response above regarding the hydraulic relationship between the alluvial and Tertiary aquifers.

ARCO is correct that only one instance of arsenic contamination in the Tertiary aquifer (well RH-6) has
 been documented.  This is also discussed in previous responses.
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 The RI conducted by ARCO and the Town Pump aquifer test conducted by the Montana Bureau of Mines
 and Geology (on behalf of EPA), did not collect any data at RH-6 regarding potential hydraulic
 communication with deeper aquifer zones, therefore it is technically accurate, but misleading, to state as
 ARCO does, that "There is no evidence that this well is hydraulically connected to deeper aquifer zones."
 There is no direct evidence because it was not investigated during the RI.  Vertical communication within
 the Tertiary sediments, within which RH-6 is completed, is discussed at length in previous responses.

 ARCO oversimplifies the  vertical versus horizontal flow of water, and potential  migration of arsenic
 between the alluvial aquifer and deep Tertiary aquifer.  In the first place the flow  or flux, of water from
 one aquifer to the  next follows Darcy's  law (Q=KIA) where flow (Q) is directly proportional to the
 hydraulic conductivity (K), the hydraulic gradient (I) and the area (A) across which  the water flows. The
flux is not controlled by the contrast in hydraulic conductivities between the strata.  The potential extent
 and subsequent impact of arsenic plume migration into the deep Tertiary aquifer is function of these
parameters, plus; concentration of the invading plume, vertical versus horizontal movement of the plume
 within the Tertiary sediments between the alluvial aquifer and the deep Tertiary aquifer, and the vertical
 and horizontal zone of contribution yielding water to a deep Tertiary aquifer well. Arsenic entering at
 the top could be drawn deep  into the Tertiary sediments by vertical flow fields imposed by a deep
production well.  Little is  known of the lateral  extent and horizontal versus vertical gradients within the
 numerous discontinuous permeable zones in the Tertiary sediments to allow ARCO to state as a hard
 conclusion that preferential flow in the horizontal direction will limit vertical migration. It is equally as
 likely that preferred flow paths may serve onfy to offset the vertical movement of a plume in a stair step
fashion.

 COMMENT:
 The RI presentation of aquifer geochemistry has been incorrectly represented that water chemistry should directly correlate with
 saturated sediment chemistry. The mobilization and/or fixation of contaminates such as arsenic are largely controlled by the
 groundwater oxidation or reduction state in addition to the presence of iron or manganese in the system.  The use of non-
validated temperature data and suspect cation/anion ratios for rationalizing mixing of the upper alluvial Aquifer and the deeper
 Tertiary aquifer is technically insupportable.

 RESPONSE:
 The comparison between the  water chemistry and the sediment chemistry was discussed to determine if
acidic conditions at the site was a major contributor of arsenic  and other metals to  the groundwater.
 When sulfuric acid attacks the sediments, and there is little to no mobilization of the dissolved parameters,
the dissolved concentrations of parameters in  the  groundwater generally approximate their respective
abundance in the sediments. It is true that with transport away from the areas of sulfuric acid attack into
peripheral areas with different pH and oxidation reduction potential (ORP,  redox, Eh, etc.) then other
processes (primarily adsorption) will individually alter the relative concentrations  of each parameter in
the groundwater. Changes in each parameter's concentration generally reflect the processes occurring
in the groundwater system. Unfortunately, aluminum and silica are missing from the analytical data base
and this indirect method of trying  to determine the relative significance of acidic  conditions had to be
applied at the site.

 The comment on "invalidated temperature data and suspect cation/anion ratios for rationalizing mixing
of the upper alluvial Aquifer and deeper Tertiary aquifer is technically insupportable" is not sufficiently
clear to give a direct answer.  However, all of the  temperature data used in making interpretations and
descriptions in the text are  temperatures measured in the field by the ARCO engineering firm professionals
during the two sampling rounds. EPA assumes that the ARCO professionals properly collected the data
because there is no report or other written statement indicating otherwise.  If this  is not the case, EPA

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should have been notified of this and any  other incorrectly collected or analyzed data generated by
ARCO.

Without knowing what is "suspect" about the cation/onion ratios, EPA cannot respond to the "technical
insupportability" part of this comment concerning the ratios.  However, as stated above, if EPA has been
given incorrect data without a description of the inadequacy then ARCO must submit information to EPA.
The use of the ratios was prompted again by the available data and believed to be relevant.  If there is
a specific concern about one or more of the ratios or how they were applied then this needs to be clarified
and will be subsequently answered.

COMMENT:
Arsenic fate and transport discussion presented in the RI does not address the positive effect of naturally occurring attenuation
of die arsenic through iron fixation.  These processes have been effective in minimizing the migration of the shallow groundwater
plume. The streamside tailings distributed throughout the floodplain are also a source of arsenic mat may be misinterpreted as
originating from the Rocker site. Understanding the role of the tailings will be important when determining die actual spread
of arsenic from die Rocker plant.

RESPONSE:
The "positive effect  of naturally occurring attenuation of the arsenic  through iron" adsorption was
assumed to be operating at the site because the process is ubiquitous under near neutral pH and oxidizing
groundwater conditions and this is reflected in the RI text. However, the adsorbed phase is only immobile
under these groundwater conditions.  In addition to this currently immobilized amount of arsenic there
is a high concentration of dissolved arsenic forming a mobile phase plume in the groundwater that is of
concern for the RI. If the site conditions remain stable then the adsorbed phase may remain immobilized
but if the pH becomes more acidic or the groundwater becomes more reducing this adsorbed phase can
released and, probably significantly increase the dissolved arsenic in the groundwater.  Assuming
relatively stable conditions, the fate and transport text deals primarily with the mobile groundwater phase
and assumed near equilibrium adsorption conditions on the sediments.   Obviously, if adsorption was
totally effective in immobilizing all the available arsenic from the wood treating site there would be little
to no dissolved arsenic in the groundwater.

The streamside tailings are known to be a source of arsenic but are not believed to be misinterpreted as
arsenic originating at the site in the RI unless specifically mentioned in  the text.  The  understanding of
the role of the tailings is important (see the  above responses on creating acidic groundwater conditions
at the site). It is EPA's understanding that the streamside tailings will be dealt with as part of 'ARCO's
remedial action of the  streamside tailings and that the streamside tailings component will no longer
contribute arsenic, metals, acidic groundwater, or change the Eh of infiltrating groundwater after their
remediation.  The site-related arsenic concentrations are believed to be appropriately addressed in the
RI.

COMMENT:
The Baseline Risk Assessment was developed using numerous overly conservative assumptions and erroneous summaries of
available data.  Several critical factors and analyses in me risk assessment are not even  consistent with other risk assessments
related to arsenic performed by EPA in Montana. Regarding risk from groundwater, EPA assumes the public would be drinking
directly from me shallow aquifer. They do not recognize die limitations of me exposure scenario regarding lack of  shallow
aquifer usage and die disconnection with die deeper drinking water supply. To support conclusions regarding residential risk
scenarios, a complete the aquifer connection was advanced by these authors.  However, die lack of supporting information of
an aquifer connection combined with die unlikelihood of a residential development scenario at the site makes assertions of risk
factors unrealistic.
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 RESPONSE:
 As was noted in the earlier responses, EPA Guidance indicates that the highest potential beneficial land
 use that can reasonably be expected to develop should be examined as pan of the RA so that Risk
 Managers (RM) have an upper range to consider in their decision-making process.  The site remedy is
 aimed at occupational/industrial use, which ARCO admits is likely, with institutional controls to prevent
 residential development.   Groundwater at the site is classified by the State as potentially useable, and
 therefore must be remediated.  Significant use ofgroundwater exists very near the Rocker site.  Risks were
 calculated for ingestion of groundwater from different portions of the aquifer to aid the RMs in their
 analyses.  EPA and their consultants further believe that there is clear evidence to show the connection
 of the contaminated shallow alluvial aquifer with the deeper alluvium and the Tertiary system at the west
 end of the site, as explained previously. The risk assessment is consistent with other risk assessments for
 Clark Fork Basin operable units, and does not, in EPA's opinion, use overfy conservative assumptions.

 COMMENT:
 Questionable conclusions reached in the RI and risk assessment could be used incorrectly to direct a remedial alternative that
 could exacerbate die naturally attenuating arsenic conditions of the site.

 RESPONSE:
 See responses to specific comments provided throughout the responsiveness summary.

 COMMENT:
 EPA added language to the second to die last paragraph on page 1-4 that qualified the depth of die removal action of 12 inches
 as "nominal" and "concluded diis phase of die removal action." These language changes imply diat die previous action was
 ineffectual, and mat future removal  actions are imminent. The removal area has reduced die potential  for a direct exposure
 pathway and the successful vegetation enhanced evapotranspiration reducing die potential leaching of contaminated deeper soils.

 RESPONSE:
 EPA agrees that the removal  action and subsequent cover soil has considerably reduced the short term
 risks of exposure to portions of the site that were highly contaminated.   The interpretation that the
 language changes imply the removal action as ineffectual is ARCO's interpretation. The order issued by
 MDEQ was an  emergency action, and by its very nature,  implies that further work at the site is highly
probable.  See also EPA's response to prior issues on this topic.

 COMMENT:
 100-yr Floodplain and Future Land Use
 Figure 3-1 and the  Section 3.1 narrative depict the previous wood treatment area on die NE portion of the OU as lying outside
 die 100-yr floodplain.  The source for these boundaries were derived from its oversight contractor CH2M HILL in 1988. The
 Rood Boundary and Floodway Map developed under the National Flood Insurance Program in September 1979 clearly shows
 this entire area north of me railroad tracks to be in die 100-yr flood plain.  Later in Section 6.0, there are statements made
 regarding the potential for railroad line abandonment and consideration of future land use for residential.  Section  3.3 was
 modified by EPA to reflect this unlikely, residential scenario. In fact, this subsection does not indicate any such limitations to
 die development of the site such as die current industrial use and floodplain status.

 RESPONSE:
 The referenced  1979 Flood Boundary Map uses a technique to develop flood plain maps that are much
 less rigorous than the flood modelling study conducted by CH2M HILL in 1988 which is much more
precise and which was why it was used. Earlier responses defined why EPA believed it was appropriate
 to consider residential use for the site.

 COMMENT:
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 Exaggeration of Surface Features—Section 3.1
 Nearly a half page of narrative was devoted to a six-inch diameter hole (likely animal burrow) discovered in 1992 near the
 carpenter's shack as a potential source of recharge waters to for transporting contamination to the subsurface. These conclusions
 drawn on page 3-2, last paragraph, are misleading and inappropriate for this part of the RI.

 RESPONSE:
 The location of the hole was first raised by ARCO in its October 1994 draft RI Report.   The actual
 discussion of the hole consists of 3 sentences describing a potentially significant field observation made
 by both ARCO and EPA but which ARCO did not include in their version of the RI. The observation of
 evidence of surface drainage entering this hole was deemed potentially significant because the drainage
 area was large, included potential arsenic source material from the railroad siding, and was located
 immediately  upgradient of the highest arsenic concentrations found in the shallow groundwater. Wdle
 the  conclusions provided in the text regarding the hole being a possible mechanism for transport of
 arsenic into the shallow groundwater may not be ideally  presented in  this section,  it is a logical
 mechanism that is appropriate for discussion in an RI.  ARCO's pointed objection to this brief discussion
 is not accurate or warranted.

 COMMENT:
 Geological Stratigraphic Relations—Section 3.4
 While ARCO agrees that regional and local geology is complex, the characterization of the Tertiary sequence above me Lowland
 Creek Volcanics as Melrose Basin equivalents (Derkey and Bartholomew, 1988) at the site discounts the actual low energy lake
 beds and tuffaceous sequence found at the Rocker site and Town Pump vicinity.  ARCO further feels that the usage of the
 nomenclature, Tertiary Sediments undifferentiated (Tu) suggests a homogeneous aquifer interval from the surface rather than
 the vertical Stratigraphic variability observed at the site.  The Tertiary sediments underlying the Rocker site have been
 differentiated as softer silts and clays overlying denser Tertiary clays, silts and aquifer-bearing bedrock.

 RESPONSE:
 Approximately 4 pages of the RI were devoted to describing the geologic formations in the Rocker area
 in an effort  to properly relate  the specific site stratigraphy to the  regional geology as described in
 scientific publications by the  U.S. Geologic Survey and the Montana Bureau of Mines and Geology.  The
 Stratigraphic descriptions provided in the published works were compared by EPA to the Stratigraphic
 records of the RI test wells and  the Town Pump water well.  The description of the Tertiary sediments in
 the available literature match the Stratigraphic description of the material penetrated in these wells.  It
 is still EPA's opinion that the strata underlying the alluvial material at the Rocker Site is Tertiary Melrose
 Basin Sequence (most recent formal name applied) or equivalent Tertiary sediments, as discussed in the
 RI.

 ARCO may be  confusing the geologic connotation of  "undifferentiated" with "massive".  The term
 undifferentiated as used in the RI does not imply a single homogenous aquifer.  The Tertiary sediments
 were clearly described as complexly  bedded and the term undifferentiated was used to denote that the
 various strata within the formation could not be separated into definable stratum because of Stratigraphic
 complexity and lack of data.  See paragraph 3, page 3-7 of the RI which describes the Tertiary strata and
provides the  rational for using the term Tertiary undivided sediments or just Tertiary sediments.

 At the Rocker site, based on ARCO's RI data, the Tertiary sediments gradationally become denser, exhibit
 more cementation and fewer generally fewer fluvial beds with depth.  There was no  specific geologic
 contact separating the Tertiary sediments into two units as ARCO's comment implies.  This  was
 recognized by ARCO's geologic consultants in that no such contact was identified on the lithologic logs
 or in geologic cross sections presented in draft RI's. Subsequent deep drilling by ARCO on the north side
 of Silver Bow Creek, completed after the EPA modified RI was submitted,  confirms the gradational

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 changes  with depth in the Tertiary sediments.  While always subject to different interpretations, the
 geologic model described by EPA in the RI is supported by RJ data.  ARCO's differentiation of the
 Tertiary sediments into 2 definable units cannot be justified with the available data.

 COMMENT:
 The aquifer layer encountered in the Town Pump well, north of the Rocker site, cannot be confirmed to extending "to a depth
 of at least 155 feet" at the Town Pump site as reported in the RI on page 3-7, paragraph two.  It is only likely that the more
 productive aquifer, extends from the upper screened interval of around 100 foot below the surface to an undetermined depth
 below 150 feet.  Page 3-11, paragraph three indicates that geophysical logging and the driller's logs of the Town Pump wells
 show the first major density change associated with the bedrock aquifer at 100 feet. The deepest paleochannel sands terminate
 at the Rocker site at 80 feet in accordance to data from core holes along the channel axis (re:  C-4 and C-10).  The Tertiary
 aquifer lies deeper in the denser bedrock beneath the site not in the shallow Tertiary silts and clays outcropping near the surface
 to the west. In summary, the available evidence does not indicate a homogeneous, connected system and likely supports
 physically distinct geological formations under the Rocker site.

 RESPONSE:
 This is a multi-part comment by  ARCO that incorrectly draws on geologic descriptions in the RI and
 attempts to present a geologic and hydrogeologic model not founded in data.

 The text on page 3-7 was describing the thickness and stratigraphy of the Tertiary sediments on the basis
 of available data.  No where in the referenced paragraph does EPA use, or imply,  the term  "aquifer
 layer".  The  text states that the Tertiary sediments extend to a depth of at least 155 feet since that was
 the depth of first  Town Pump well which, as indicated by the  well drillers log and by direct personal
 communication with the driller by EPA, was still in what appeared to be Tertiary sediments as described
previously, see comment R-12 above.

ARCO's second comment in this paragraph concludes that the "Tertiary aquifer" is present beneath the
 Tertiary silts and clays underlying the site.  EPA  concurs that there are  fls) a discrete zone(s) of high
permeability In the Tertiary sediments and that these (this) zones or zone underlies the Rocker site. ARCO
goes on  to state  that the  data "does  not indicate a (hydraulicalty) connected system and supports
physically distinct geologic formations". EPA disagrees with this statement because there  is no RI data
to support it and the Town Pump aquifer test clearly shows direct, and immediate response in Rocker
monitoring wells completed in the Tertiary sediments, stratigraphically above what ARCO refers to as the
 Tertiary aquifer.  Well RH-36 on the Rocker site, completed in the Tertiary sediments at a depth that
places  it above  the bottom of the paleo-channel,  also experienced drawdown during the  Town Pump
aquifer test.  ARCO's hydrogeologic model, as summarized by this comment, is not based on the RI data
and, in fact,  is directly contradicted by RI data.

COMMENT:
Structural Setting-Section 3.4
Multitech's (1987) Rocker Fault does not fit with the discussion presented in the text.  It has been noted by the RI author's mat
this eastern normal fault has a downthrown western side on page 3-8. Unless this includes yet another western fault with the
downthrown side to the east that would from a structural graben (re: western paleochannel border), the age relations would make
the paleochannel older not younger than the Tertiary Undifferentiated (TU) sediments.  Also, the Tertiary age material on the
downthrown side of the fault is higher than on the up-thrown (eastern) side of the fault, which is contradictory to what would
be expected in such a scenario. Other, later, investigations have not indicated that this fault exists (i.e., Purdy and Rowan, 1990
and Rowan and Segal, 1989).

While a structural answer for the western side of the paleochannel is a possibility, discussion is not presented regarding the
likelihood that faults in poorly consolidated material are often clogged with fault gouge rendering low permeabilities (i.e.,
hydraulic barriers).

                                                                                  ARCO RI Disclaimer 11

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RESPONSE:
EPA's use of the term "fault block basin " in the last paragraph of Section 3-4 on page 3-8 is incorrect.
The correct term should be  "structural basin".  EPA did not mean to imply there was also a fault west
of the Rocker site to form the fault block.  If present as described by Multi-Tech, the Rocker fault east
of the Rocker site  with  the west side of the fault downdropped would form a wedge shaped structural
basin that could fill with alluvial sediments.  The wedge of alluvial sediments would thin to the west. The
eastern side of the "paleo-channel", a feature first named and introduced by ARCO, cannot be defined
with the  RI data.   Therefore, the purpose  of this discussion  was to introduce a possible structural
explanation for the easterly dipping contact between the Tertiary sediments and the alluvial materials at
the Rocker site.

ARCO's  comment  that under the scenario described in this paragraph stratigraphic age relationships
would be wrong is incorrect. In such a sediment filled structural basin,  (or even if it was a fault block)
stratigraphic age relationships are correctly maintained, i.e. younger sediments over older deposits.

There is no discussion in the text about the possible hydrologic effects of faults, either as low conductivity
barriers to flow or highly permeable conduits because it wasn't relevant to the subject of the paragraph.
 EPA does not believe  the complexities of defining and characterizing fault hydraulics  is relevant  to
ARCO's efforts at characterizing the groundwater hydraulics at the Rocker site.

COMMENT:
Number and Nature of Aquifer Units?
Page 3-17 mentions that there are three hydrostratigraphic units yet lists a fourth, unsaturated sediment and fill. While ARCO
concurs that there may be distinct hydraulic characteristics of vadose material, it should not be identified in  this context with
aquifer units.  Also the hydraulic characteristics of the Tertiary Sediments also called "Volcanic Aquifer" on page 3-23, are quite
variable and appear to be more like an aquiclude in its upper 100 feet of thickness.  The likely, water-bearing horizon in which
the Town Pump well is screened (110-152 ft) is not listed as a hydrostratigraphic unit in the listing on page 3-17. It is concluded
mat the driller's preference to screen this interval was motivated by mis zone being discrete and water-bearing versus the upper
alluvial material or possibly separated by an aquitard clay. Installation of a later well that was screened continuously to the
surface made considerably less water presumably due to  the presence of clays and silts in the upper zone.   This lower
hydrological unit at the Town Pump is believed to underlie die Rocker site but thin at comparable depths. Overlying this older
rock unit are either channel sands of the lower alluvial aquifer and the shallow alluvia aquifer unit mat is associated with Silver
Bow Creek and tailings deposits.

RESPONSE:
This is a multi-part comment including both RI specific comments and unrelated restatements of ARCO's
conclusions regarding the hydrogeology of the site.

ARCO is correct about the  inconsistency between text and the number  of hydrostratigraphic units
described.  The text should state there are four hydrostratigraphic units. EPA included the  discussion
of the vadose zone materials in this section as a matter of report structure and to provide a complete
description of the hydrogeologic system at the Rocker site which includes the vadose zone.

The water bearing horizon at the Town Pump well was not presented as a distinct hydrostratigraphic unit
because, 1) almost nothing was known about it's physical characteristics because of poor geologic logs,
and 2)  as described in  the  RI, it is one  of several highly permeable zones in the Tertiary sediment
hydrostratigraphic unit.

ARCO's discussion of the construction of the first Town Pump well suggests the well penetrated a distinct,
water bearing, geologic  strata that the driller chose to screen. This is not the case based on discussions

                                                                                ARCO RI Disclaimer 12

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 with the driller which indicated the lower part of the geologic formation was essentially the same as the
 upper part, only more cemented.   This is consistent  with all descriptions of the Tertiary sediments
presented in the RI.

ARCO's contention that the reason the second Town Pump well is less productive is that it tries to draw
 water from silts and clays in the upper  Tertiary sediments is incorrect and ignores the fact the well is
screened over a very long section that includes the same production interval as the first well.   Why the
second well does not seem to produce as much water as the first and why it was drilled so deep is open
for speculation but is not relevant to the Rocker RI.

It is not clear  what ARCO is referring to as the "lower hydrological unit at the  Town Pump".  It is
apparent that the highly permeable zone in the Tertiary sediments that the first Town Pump well draws
from is widespread, based on the large areal extent of drawdown observed during the Town Pump aquifer
test.  This zone apparently extends beneath the Rocker site as well.  There is no RI data available to allow
ARCO to make the statement that the "lower hydrogeological unit" thins at depth. The Tertiary sediments
contain highly productive zones but almost nothing is known about their physical properties including
extent, thickness and stratigraphy.

COMMENT:
Well RH-38 Suspect in Defining Potentiometric Surfaces.
Well RH-38 is screened in both the alluvium and Tertiary sediments, thus its usefulness in determining the potentiometric surface
for either formation would be misleading.

RESPONSE:
Well RH-38 is screened across a thin, but permeable  alluvial sand encountered near the base of the
alluvium.   The topmost Tertiary sediments included in  the open portion  of the  well are much less
permeable than the alluvial sands therefore EPA feels RH-38 provides valid potentiometric (water level)
data for the alluvial materials in this area.  Potentiometric data from this well provides evidence of
complex alluvial groundwater flow patterns on the extreme southwest side  of the Rocker site.  RI data is
not sufficient to better define this area  and  it has  been tacitly agreed  that further groundwater
characterization in this area of the site is not pertinent to describing the arsenic impacted, central portion
of the  site.

COMMENT:
Tertiary Wells RH-6, RH-48, and RH-37 Were Ignored when Mapping the Potentiometric Surface.
EPA omitted wells RH-6, RH-48, and RH-37 in defining the potentiometric surface of the Tertiary Sediment aquifer.  While
RH-37 may arguably be included in the alluvial aquifer system, per the EPA interpretation, the reason for omitting RH-6 and
RH-48 is that they are "...too high up in the Tertiary sediments...".  Since EPA maintains that there is a vertical hydraulic
connection, why should die  stratigraphic position be the basis for  exclusion?  The inclusion of these wells  would lead to a
potentiometric surface which trends to the northeast.  This direction does not parallel the potentiometric surface of the alluvium;
therefore, providing support for a hydraulic disconnection between alluvium and deep Tertiary.

RESPONSE:
EPA omitted wells RH-6 and RH-48 when preparing the potentiometric map for the Tertiary sediments
because they are screened in the  uppermost portion of the sediments, whereas the other 4 Tertiary
sediment wells are screened at comparable depths much deeper in the sediments. There is about 1.8 feet
difference in water levels between the shallow Tertiary wells and deeper Tertiary wells.  This difference
is much larger that the water level difference between the 4 deep wells and consequently the water levels
of the shallow  wells dominate the shape of the potentiometric map produced  when all wells are used
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 together.  The difference in water surface elevations between the 4 deep wells is very small and indicates
 a northwest flow direction, consistent with the regional flow direction.

 When combined, the resultant potentiometric map indicates a flow direction, with very high gradients,
 directly towards the Town Pump well from the contaminated Tertiary sediments at well RH-6. EPA does
 not believe this is an accurate picture of the flow direction in the deeper Tertiary sediments and chose
 not to present the combined map. IfARCO believes the combined map is more accurate then they should
 be  prepared  to respond to the issues such a map and flow direction  raises, namely 1)  Why is  the
potentiometric surface and flow direction so radically different from the regional pattern?, and 2) When
 will the arsenic at RH-6  get to the Town Pump  well?.  Answering these questions would require an
 extensive field investigation and analysis of the stratigraphy and hydrogeology of the Tertiary sediments.
 EPA does not believe this is warranted and that remedy implementation is appropriate given  the analysis
 at hand.

 COMMENT:
 Also, the Final RI states that "The lateral extension of the permeable sand bed in well RH-6 is not known.  None of the other
 wells drilled into the Tertiary sediments in the vicinity encountered  correlative sand of the thickness and permeability of that
 in RH-6. Based on steady slope of the drawdown curve...it appears mat the sand bed at RH-6 did not receive significant
 recharge during the [1987 Hydrometrics pump] test.  A steepening of the drawdown curve beginning near the end of the test
 suggests a lateral barrier (or less permeable portion of the sand bed) was encountered." This forgoing analysis by EPA suggests
 a notable lack of either vertical or horizontal hydraulic conductivity  in the Tertiary sediments in the vicinity of RH-6.  In the
 Section 7.0  Summary and Conclusions, it is stated that the permeable sand in RH-6 "...is believed to subcrop at the contact
 between the alluvium and up sloping Tertiary sediments within the deep alluvial aquifer arsenic plume on the western side of
 the  site." The conclusions drawn from the pump test and the forgoing statement are contradictory.

 RESPONSE:
 Most of this comment is an accurate summary of that portion of the RI addressing the hydraulics of RH-6.
ARCO's statement that EPA's analysis suggest a "notable lack of vertical or horizontal permeability in
 the Tertiary sediment. .."is incorrect. The aquifer test at RH-6 indicates the Tertiary sediments are quite
permeable in this area.

ARCO 's last statement is not quite correct. EPA's belief that the permeable bed in RH-6 subcrops  against
 the alluvial aquifer is consistent with the source of arsenic in RH-6 coming laterally from the  east as
 opposed to vertically from the overlying contaminated alluvium.  The fact that the shape of the drawdown
 curve from the pumping test did not indicate a recharge source was factually stated by EPA. The physical
properties and extent of the permeable bed at RH-6 are unknown.  Therefore, EPA chose not to speculate
 on  the shape of the drawdown curve and its implications regarding connection between the Tertiary and
alluvial sediments.  It is entirely likely that the alluvial sediments at the subcrop  area are less permeable
than the water bearing zone at RH-6 and therefore the implied barrier boundary from the drawdown curve
 could easily reflect that hydraulic conductivity contrast.   Again, if ARCO would like to  pursue the
hydraulics of the high flow zone at RH-6, EPA would not object.

 COMMENT:
Groundwater Chemistry
Temperature Data Interpretation
Certain data such as field parameters were not part of previous DSR's (re: temperature) and were procured by EPA oversight
personnel directly without an opportunity for ARCO review of the data. This data was not available when requested by ARCO
and may be  suspect. Further, this data was procured post-stipulation and has not been validated.  Assuming the temperature
data is valid, EPA's use of it to conclude an indication of aquifer connection is against conventional understanding of subsurface
temperature relationships. A normal temperature gradient in an area  not subject to thermal activity would not produce elevated


                                                                                ARCO RI Disclaimer 14

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 temperature data in shallow zones. If anything, elevated temperatures imply upward convection gradients versus downward flow
 gradients.

 RESPONSE:
 All of the temperature data used in making interpretations and descriptions in the text are temperatures
 measured in the field by ARCO's engineering firm professionals during the two sampling rounds.  EPA
 assumes  that ARCO's professionals properly collected the data because there is  no report indicating
 otherwise.  If this is not the case, EPA should have been notified of this and any other incorrectly
 collected or analyzed data generated by ARCO.

 Thermal  gradients were used to ascertain the relative "normality" of the temperature gradient in the
 groundwater at the site using a "conventional understanding of subsurface temperature relationships".
 A homogeneous alluvial groundwater system generally shows little  if any thermal changes or highly
 variable temperatures when sampling rounds are compared even at the shallow depths investigated at the
 site.  Changes in temperature, therefore,  was investigated to determine if there were discrete parts of the
 alluvial aquifer indicated by temperature differences.  For example, two discretely different alluvial
 groundwater systems may be adjacent and their differences may not only be reflected by the chemical
 characteristics of the groundwater but also by physical characteristics, like temperature.  Sources of
 recharge and potential lack of mixing was investigated by evaluating temperature differences. Elevated
 temperatures in this climate and shallow groundwater system could also result from localized recharge
from a ponded source.

 COMMENT:
 The Final RI report concludes that there is a wide and overlapping temperature range in the Alluvial and Tertiary sediment
 groundwater.  However, conclusions presented from Section 4.4.1.3 mat the variability in temperature range within each aquifer
 horizon equates to aquifer mixing is of questionable validity. The basis for EPA concluding that temperature is an indicator of
 groundwater mixing is developed from the narrow range of standard deviations that appear to increase with depth. This is not
 a reliable basis for determining groundwater mixing. The increase in standard deviation merely reflects an increase in the range
 of data.  This increase in data range could be due to other factors not related to groundwater mixing. For instance, a small
 number of samples from a not normally distributed sample population could yield a higher standard deviation.

 RESPONSE:
 The point of the temperature discussion is that:: "The increase in standard deviation" with depth "reflects
 an increase in the range of the " temperature "data".  Variable increases in  temperature ranges with
 depth is not a  "normal" condition in groundwater systems.  Groundwater temperature variability typically
 decreases with depth as near surface sources of recharge or mixing decrease and the geothermal gradient
 controls the temperature.  Broad changes in groundwater temperatures measured in essentially the same
 season suggest  considerable changes in  site hydraulic conditions within  the aquifer and, therefore
probably  considerable variability and mobility  of dissolved parameters in the groundwater.   Broad
 overlapping temperature ranges indicate that the same hydraulic conditions affect both pans of the aquifer
 and, thereby suggest that they are hydraulically connected. Lack of hydraulic connection would mean
 that, for example, perhaps only one of the two parts  of the aquifer would show a broad variation in
 temperature while the other is relatively stable. The highly variable one is typical of localized recharge
 sources and/orfluctuating sources both of which indicate probable hydraulic connection and considerable
 groundwater movement.   The stable  condition is typical  of confined (shallow, but particularly deep
 (several 100 feet), systems) or very large  (stable) aquifer systems.

 COMMENT:
 The Rocker Site is proximal to a hydrothermally altered batholithic intrusion and contains sediments and layers of volcanic
 origin.  Thermal mixing could therefore occur irrespective of hydraulic connection.

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RESPONSE:
This comment is confusing.  The thermal energy of the batholithic intrusive has been dissipated through
the millions of years  since its (and subsequent -volcanic) intrusion.   The  temperatures used in the
evaluation are measured, not calculated, temperatures from the groundwater chemistry.  Therefore, the
last sentence is confusing and technically incorrect.

COMMENT:
REDOX Potential for PAH Derivatives
Numerous citations have been made to  the reducing potential of the PAHs found at the site to alter As*5 to As*3, yet no
references or direct evidence were provided to verify mis deduction. Contrastingly, on page 4-21, under discussion presented
on Eh, it is noted mat the Eh measurements in TOC enriched zones did not exhibit corresponding low Eh indicative of reduced
conditions. Discussion was also presented to discredit the field Eh measurements. No where at the site are Eh measurements
in the range that would support As"1"3 as a stable phase.

RESPONSE:
The correlation between arsenic speciation, TOC, and Eh is confusing to those who have not worked with
these parameters in several different sites.  Arsenic speciation, i.e. the quantitative determination of the
two valence states, is technically sound (USGS methodology proven by  use at many different sites).
Dissolved arsenic speciation indicates that the reduced state (arsenic III) occurs in about equal proportion
to the oxidized state, (arsenic V) in several samples in the wood treatment area.  Therefore, the reduced
arsenic state (the arsenic species  used in the wood treatment solution) still occurs at the site.

A comparison of the  TOC (total organic carbon) concentrations in analyses of the two groundwater
sampling rounds in essentially the same season indicates that the TOC  concentrations are so highly
variable  that the analyses are of questionable accuracy.  Alternately, the shallow groundwater systems
are so hydraulicalfy connected that the dissolved constituents are readily mobilized through intricate
hydraulically  connected flow paths connecting localized sources that major changes in groundwater
chemistry occur with relatively minor changes in surfidal activities.   Other constituents show some
variability but their changes show sufficient spatially correlation with one another that they are probably
correct but TOC concentrations relationships appear to be random and, therefore, potentially incorrectly
analyzed.

There are too few Eh and arsenic speciation measurement pairs to fully evaluate the relationship between
the measured bulk groundwater Eh and the arsenic speciation. However, it is a well known fact that bulk
water Eh values and individual dissolved ion speciation couple concentrations (arsenic, iron, manganese,
etc.) can indicate oxidizing conditions instead of reducing conditions if the dissolved ion couple is not
controlling the bulk water Eh.  In  this case, it is obvious that the arsenic is not controlling the bulk water
Eh. A equilibrium calculation could be made to determine the Eh represented by the arsenic couple.  The
calculated Eh would probably be much lower (less oxidizing, more reducing) than the measured Eh
indicates.  This calculated Eh could be the Eh  of the groundwater at or, at least nearer, the Eh of the
arsenic source. Cherry, et al., 1979,  proposed using the calculated Eh of the arsenic couple to estimate
the true Eh of the groundwater because the oxidation of arsenic III to arsenic V is much slower than most
other dissolved ion couples.

(Cherry,  J.A., Shaikh, A. U., Tollman, D.E., and Nicholson, R. V., 1979, Arsenic species as an indicator
ofredox  conditions in groundwater: Journal of Hydrology, Vol. 43, pp. 373-392)
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 COMMENT:
 Low Concentrations of Metals in Tertiary Aquifer Support a Hydraulic Connection to Alluvial Aquifers.
 The final paragraph on "Other Metals" on page 4-32 states that "Zinc, copper and lead occur in low concentrations in the
 Tertiary sediment aquifer supporting hydraulic connection to the shallow aquifer". This statement tends to distort the reality
 that fill and streamside tailings material is in contact with the near surface outcrops of the undifferentiated Tertiary Sediment.
 The hydraulic properties of mis upper zone have not been substantiated nor has the background chemistry of the Tertiary
 sediment been discussed. In fact, Tertiary age mineralization at the nearby Bluebird complex is a more likely explanation, but
 have been discounted as sources for base metals. The relationship and distribution of base metals at the site has little bearing
 on the distribution of the arsenic.

 RESPONSE:
 Fill and Streamside tailings material is not in direct contact with undifferentiated tertiary sediment, only
 alluvial material is.  IfARCO believes that the hydraulic properties of the upper zone of the Tertiary
 sediments needs further characterization and a background chemistry of this zone needs to be discussed,
 these activities should have been performed by them during the conduct of the RI.   The relationship
 between the Tertiary sediments on the western side of the site and the Bluebird complex east of the site
 is an unknown.  IfARCO believed this to be an issue at the site, they should have  investigated and
 reported the results in the draft RI report.

 EPA believes that there is adequate documentation of the  Tertiary sediments at the site to continue with
 the FS and Proposed Plan, and stands by the cited RI statements.
COMMENT:
In section 4.4.2.1 the implications that low concentrations of metals in RH-40 and RH-6 support hydraulic connection between
the alluvial and Tertiary aquifers is erroneous/. The statemenfZinc decreases from a concentration of 33 ug/L in the shallowest
well (RH-40, 22 feet deep) to less than 10 ug/L below a depth of about 40 feet (RH-6)" is totally incorrect.  Well RH-40 is
screened at a depth of approximately 100 feet below ground level or about 81 feet below the Tertiary-alluvial contact.  Well
RH-6 is screened at a depth of about 29 feet or 10 feet below the Tertiary-alluvial contact.  Thus the apparent effort to show
a decrease in zinc concentration in ground-water with depth actually shows an increase in zinc with depth. Also RH-40 is located
adjacent to well RH-38, a shallow alluvial aquifer well which shows a zinc concentration of 5 ug/L.  These data do not support
the hydraulic connection of the alluvial and Tertiary aquifers.

RESPONSE:
ARCO is correct.  EPA misread the depth of well RH-40 on Table 2-4 and mistakenly used the depth of
well RH-38 (adjacent to RH-40 in Table 2-4). The zinc concentration in RH-40 appears anomalously high
and may not reflect actual in-situ conditions.  The cited paragraph should be stricken.  However, it does
not effect the overall site characterization or interpretation of the geochemistry at the Rocker site, nor
does it change the remedy  selection presented in this ROD.

COMMENT:
Low levels of metal concentrations would be expected in an area mat has significant deposits of these metals.  Copper, zinc,
and lead occur in ores of the Butte area.  Satellite occurrences of elevated concentrations of these metals would be expected to
proximal to the mined deposits. Thus these metals would be expected to occur in groundwater down gradient of these naturally
occurring metals concentrations.

RESPONSE:
// is true that "low levels of metal concentrations would be expected in an area that has  significant
deposits of these metals."   The problem is one of how "low" is defined and if anthropogenic activities
have caused these metals to be released in concentrations higher than what could be  called  "natural".
As described above in several responses, there is such a high level of variability of the dissolved arsenic,


                                                                                  ARCO RI Disclaimer 17

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metals, and major ions that a localized anthropogenic source, not a  regional background source, is
responsible for the metals concentrations.   Drilling log descriptions repeatedly indicate anthropogenic
source materials but neither indicate nor reference any natural occurrence of copper, zinc, arsenic, or
lead in the subsurface at the site.

COMMENT:
Groundwater Quality Conclusions
Several of the groundwater quality conclusions lack substantiation in Subsection 4.4.4.  The second bulleted item on page 4-43
states "...both calcium and sulfate increases [as TDS increases] as  bicarbonate decreases..."  ARCO concurs that calcium and
sulfate increase with TDS, however, observations indicate that bicarbonate also increases with TDS, though not as pronounced
as calcium and sulfate.

RESPONSE:
There  is a considerable difference in the quantitative increases in calcium sulfate over that of bicarbonate
but it  is  true that bicarbonate slightly  increases with TDS.   The actual percentage of bicarbonate,
however, generally decreases  with increase in TDS.

COMMENT:
EPA's implication that the Tertiary sediment aquifer sharing the chemistry of the alluvial aquifers lacks the perspective of the
spatial relations. For instance, the chemical evolution path depicted in Figure 4-17 shows a progression from wells RH-43 to
RH-40 to RH-46 to RH-6 to RH-48.  Well RH-43 lies directly north (approx. 350 feet) of RH-40 and RH-46 lies approximately
200 feet west of RH-43.  Wells RH-6 and RH-48 lie 10 feet south  and 150 feet southwest of RH-46.  With groundwater flow
to the northwest, as shown in Figure 3-19, the chemical progression along a single groundwater flow  is not supported.

RESPONSE:
The complexity  of the individual groundwater flow paths in establishing their hydraulic connection on a
sand by sand or sand to fracture basis is not possible because of the complex geologic setting at the site.
Individual groundwater flow paths were  not evaluated because there are not sufficient monitoring wells
completed in each discrete flow path across the site to  do this.   The RI does describe  the  major
groundwater flow paths resulting from the combination of many of the individual flow paths.

COMMENT:
Field or specific conductivity measurements clearly show a distinction between the Shallow and Lower Alluvial Aquifers and
significantly different than the Tertiary aquifer.

Specific conductance is a measure of the amount of dissolved and suspended solids in the system and can be used to determine
the level of mixing between aquifers. Page 4-18, paragraph three indicates that the mean value for the Upper Alluvial aquifer
is 516 pmho/cm and the underlying Lower Alluvial Aquifer has a mean of 220 ^mho/cm. Correspondingly the Tertiary aquifer
has a mean conductance of 768 pmho/cm.  This does not support a conclusion for aquifer mixing but aquifer isolation.

RESPONSE:
The mean specific conductance of the three parts of the aquifer system are probably related to the number
of monitoring wells completed in each unit, extreme values (both low and high) in one or a few of the
total samples, localized anthropogenic sources, and mixing between aquifers.  The  conductance values
correspond more to major ions than even the high arsenic concentrations which are present in both low
to very high specific conductance values.  The relevance of specific conductance to mixing is tenuous at
best, and may be misleading, without evaluating the concentrations of the major, minor, and trace ions
contributing to the specific conductance values.  Specific conductance is really a measure of the charged
ion response and, unless the suspended solids have  a  charge,  suspended solids  do not register or
contribute to a specific conductance  value.    This is one of the major difference  between specific
conductance and TDS.

                                                                                ARCO RI Disclaimer 18

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About the only  conclusions  that can be  drawn from the  above three  mean specific conductance
measurements is that, given the samples representing each mean, the upper alluvial aquifer and Tertiary
aquifer probably contain groundwater with  higher TDS and which are probably of poorer water quality
for domestic use than the lower alluvial aquifer.

COMMENT:
Aquifer Disconnection or Separation & Lack of Evidence for Aquifer Connection
ARCO and the EPA agree that some connection between the alluvial aquifer and the undifferentiated Tertiary aquifer may exist.
However, the Agencies' qualitative discussion strongly implies an intimate connection and that extensive contamination of the
Tertiary aquifer is possible. This implication is not supported by the data.  In fact, the data provides multiple lines of evidence
which supports only a lack of connection as summarized below:

The lack of connection between the Tertiary and alluvial sediments is demonstrated by the observed difference in static water
levels in each unit and the relative response of each unit during the MBMG pump test.

Average water levels measured in the Tertiary sediments are approximately 3 to 7 feet lower than water levels measured in the
alluvial sediments.  (Water levels in the Tertiary sediments range from elevation 5362 to 5363, or approximately 5362.5 at the
center of the Rocker Site Water levels in the alluvial sediments range from approximately 5365 to 5370, or approximately 5367.5
at the center of the site).  The water levels were measured between wells with vertical screen elevation differences of 30 to 40
feet, and vertical gradients are, accordingly on die order of 0.1 to 0.2. EPA implies that this high vertical gradient is evidence
of strong connection between the alluvial and tertiary  sediments, when in fact, the high vertical gradients are evidence of a lack
of connection.  If die Alluvial Aquifers were highly connected to the Tertiary Aquifer, then the water levels in the respective
aquifers would  equilibrate.  A strong vertical gradient could not be maintained and there would be strong seasonal variation
which is not the case.  Because the groundwater flow between the aquifers is impeded by lower permeability material  (re:
aquitard clays observed in core), water levels cannot equilibrate.

RESPONSE:
EPA does not imply  that high  vertical gradients is evidence of vertical communication.  EPA does not
agree with ARCO's contention that vertical gradients are proof-positive of a hydraulic disconnect.  Given
the characteristics of the hydrologic systems at the Rocker site—namely, a constant surface water source,
intervening alluvial sediments and a permeable  water yielding zone with a lower potentiometric head in
the underlying Tertiary sediments—vertical gradients consistent with overall recharge  and discharge
patterns.  ARCO is correct that the gradients reflect, the impedance of water through the system.  EPA
agrees that in a natural system vertical permeabilities are generally lower than horizontal permeabilities
which produces vertical gradients. EPA also agrees vertical gradients are an indication that the system
is dynamic as opposed to being under static conditions and that there is a constant flux, of water moving
vertically through the system.  Insufficient  data exists to examine the  seasonal trends of water levels
between the hydrogeologic systems at the site.

COMMENT:
•The lack of connection between die  Tertiary sediments and the alluvial sediments is further demonstrated by results of die
MBMG pump test.  The rapid development of die cone of depression over a radius of thousands of feet and the low storativity
indicated a confined aquifer. Furthermore, MBMG noted that calculations of leakance resulted in very low leakance values.

As noted in die RI, die response of wells screened in die Tertiary aquifer was similar, ranging from approximately 1/2 foot to
over a foot, even in wells separated 200 feet vertically. However die response of wells screened in me alluvial aquifer only  a
few feet above die Tertiary was, at best, very subtle.  Leakance is die amount of water coming from a confining layer. A low
value indicates very little water is coming from the confining layer. This could be due to either a dry confining layer or low
hydraulic conductivity.
                                                                                    ARCO RI Disclaimer 19

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RESPONSE:
EPA agrees that the  rapid, areatty extensive drawdown  response in most observation wells is an
indication of confined conditions and  low storativity which  is consistent with thin, fractured, water
bearing zone(s) in the Tertiary sediments as the post RI deep drilling by ARCO confirmed.  The similar
water level drawdowns in  wells vertically and laterally separated however are not consistent with the
aquifer being a single, thin, discrete horizon.  The MBMG report concludes that based on the range of
storativity values calculated, the Tertiary sediment aquifer ranges from confined to unconfined and was
therefore considered as a  highly  stratified unconfined or leaky confined aquifer.  Much is yet to be
learned about the hydraulics of the Tertiary sediments in the Rocker area.

The leakance analysis provided in  the MBMG report apparently consisted of analyzing the test data from
an observation well(s) by  using the analytical routine for  a leaky aquifer in a commercial  software
program.   This  routine uses an automated curve matching technique.  Examination of the one leaky
aquifer curve in the MBMG report shows the program produced a marginal curve match.  Determination
of leakage coefficients requires a strong  understanding of the physical characteristics of the hydrogeologic
regime being tested. Because of the poor to non-existent well logs, this degree of data was not available
to MBMG. Therefore, EPA does  not view the computer generated leakage coefficient as reliable.

COMMENT:
EPA cite me rate of change of upward gradients  in monitoring well pair RH-13/14 and the rate of change of downward gradient
in monitoring well pair  RH-1S/16, during the MBMG pump test as evidence of hydraulic connection between the tertiary and
alluvial sediments. EPA notes that the changes occur just when pumping begins and ends.  However, EPA does not note that
the rise and lowering of stream stage in Silver Bow creek coincided with the pump test. Stream levels rose at the start of the
pump test and started falling about the time the pump was shut down. The rise and fall in stream stage clearly resulted in a
corresponding rise and fall of water level  in the  shallow alluvial wells. However, the stream stage changes apparently did not
affect the water levels in the deep alluvial wells.  This would result in the observed decrease in upward gradient at the RH-13/14
well pair and the observed increase in downward gradient at the RH-15/16 well pair.  Thus, at least some, if not all of the
changes in gradients observed in  these alluvial well pairs during the pump test must be attributed to the affects of precipitation
and stream stage change and the degree of response of the alluvial aquifer to pumping of the Town Pump well is much more
ambiguous man EPA implies.

RESPONSE:
Stream stage in Silver Bow  Creek rose abruptly about 1 day into the test and fell slowly at a constant rate
until about 2 days after the pump  was shut off. Gradient changes, especially the apparent recovery of
the deep alluvial wells at the end of pumping, does not appear related to stream stage changes. EPA has
always stated that  the alluvial aquifer response  during the Town Pump aquifer test was subtle  and
insufficient if taken  by  itself to indicate communication.  As stated in response previously, analysis of
alluvial water levels during the Town Pump aquifer test were  independently consistent and fit the body
of evidence indicating vertical communication between the alluvial aquifer and the Tertiary sediments.

COMMENT:
Arsenic concentration in RH-6 cited by the EPA as an example of migration into the Tertiary Aquifer and a clear indication of
a high potential for contamination of the entire Tertiary zone.  However:

•   The well is close to a source trench and at same elevation as die arsenic in the alluvial aquifer to the east. The arsenic in
    RH-6 may be a result of contamination from this  relatively close source, not an indication of arsenic migration several
    hundred feet from the central plant area.

•   Flow in Tertiary still primarily horizontal based upon stratified stratigraphy, pump test results, etc. Therefore, arsenic in
    a shallow lens of the Tertiary zone is not likely to spread into deeper zones.
                                                                                ARCO RI Disclaimer 20

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 •   Well is shallow, near the alluvial interface, and is in laterally discontinuous sand.

 RESPONSE:
 EPA believes the arsenic in RH-6 is due to lateral migration from the Tertiary sediment/alluvium subcrop
 east/northeast of RH-6.  If the arsenic at RH-6 was coming vertically from the overlying alluvium the
 other metals (zinc, copper etc) found in RH-5 should also be in RH-6. Either way, the arsenic in RH-6
 indicates either lateral (EPA's view) or vertical (ARCO's view)  communication between the alluvium and
 the Tertiary sediments.

 The vertical and lateral discontinuity of individual strata and the high degree of vertical communication
 within the Tertiary sediments as evidenced by the drawdowns observed in the Town Pump aquifer test are
 considered evidence of potential migration from the upper Tertiary sediments into lower zones.  The
 lateral extent of the highly permeable water bearing zone in RH-6 is not known.  It can only be said that
 it was apparently not encountered in wells drilled to the south, north and northwest.

 COMMENT:
 Arsenic Fate and Transport
 Issue: Arsenic Migration Rate and Tertiary Sediment Contamination
 The rate of groundwater exchange and any arsenic migration is slow and limited by the stratification and anisotropy of the
 Tertiary aquifer.

 This is supported by the statement on page 4-8, last paragraph  of Subsection 4.2.1 that "in a natural phenomena with a deep
 subsurface source, the arsenic concentration would increase with depth." Mean values in sediment range from 11 to 16 mg/kg
 at 36 to 60 feet illustrating mis decrease.

 RESPONSE:
 The referenced section is describing arsenic concentration trends by depth with regard to the source of
 arsenic, shallow, anthropogenic or deep natural sources.   The RI concludes the available soil-arsenic
 data indicates the arsenic  is coming from the surface.   This  section of the RI does not evaluate the
 groundwater exchange and attenuation of arsenic moving through the Tertiary sediment system. At face
 value however, the text referenced by ARCO does indicate that vertical arsenic migration has occurred.

 COMMENT:
 Endangerment of Town Pump Well is Not Verified
 EPA implies that the Town Pump well is endangered to  arsenic contamination. However, in (later) (earlier) discussion it is
 stated that capture zone for Town Pump is open to east. Water entering the well likely comes from east of the Town Pump not
 south.  Further this flow is supported in the regional conceptual model (Figure 3-15 shows E-W flow).

 RESPONSE:
 On page 5-27, paragraph 2, EPA discusses potential endangerment  of the Town  Pump  well and the
 Tertiary sediment aquifer (s).  EPA acknowledges the effect of capture zones on contaminant migration.
However,  considering the limited knowledge of the Tertiary sediment hydrologic system, the indications
 of vertical communication between the alluvium and Tertiary sediments under ambient flow conditions,
 and the possibility of increased groundwater extraction from the Tertiary system,  EPA's  concern with
 the potential for degradation of the Tertiary aquifer (s) and the  Town Pump well is justified.

 COMMENT:
 Abundance of Iron and Manganese in Tertiary Material for Attenuation of Arsenic
 EPA suggests mat there is deep iron and manganese in the  Tertiary sediments, therefore, if arsenic migrates there, little capacity
 exists to attenuate it geochemically.  In fact, the first paragraph of section 3.5.1 Tertiary Sediments, states  "Black mottling
 (probably manganese) and what appeared to be iron cemented concretions or nodules attest to the mineralized nature of the

                                                                                ARCO RI Disclaimer 21

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Tertiary sediments."  Manganese and iron are particularly evident in the Tertiary material at RH-6 and the down gradient well
RH-48.  Since RH-6 is the only known well in the Tertiary material showing arsenic in groundwater above background
concentrations, the presence of iron and manganese in mis area, and down gradient, suggests arsenic would be attenuated. EPA
presumes incorrectly that the lack of iron and manganese in groundwater equates to low concentrations in sediment. The
presence of iron and manganese in sediment would attenuate arsenic.

RESPONSE:
EPA suggests that there is less iron and a different form of iron in the  Tertiary sediments than in the
alluvial sediments,  and, therefore there  is  less attenuation capacity in  the Tertiary  sediments.  Iron
cemented concretions are localized sources of high iron oxides.  Their number and association with either
sands or fractures in the Tertiary sediments part of the aquifer is more important than their general
occurrence.  Their occurrence suggests that dissolved iron was present during digenesis of the sediments.
They do not suggest nor can they  'attest to the mineralized nature of the Tertiary sediments."  This
conclusion could only be verified by analyzing these concretions and comparing them with analyses  of
iron  concretions from other environments.   Iron concretions are present all over the world in both
 "mineralized* and  nonmineralized areas.  Furthermore, iron in concretions is  generally aged iron
oxyhydroxides (iron oxides) which have a much lower adsorptive capacity than fresh iron oxyhydroxides.

Black mottling may be either organics or manganese in the Tertiary sediments.  "Without an analysis, this
distinction cannot be made.   There is little arsenic adsorptive capacity presented by either organics or
manganese over that of other materials  in the aquifer sediments.  Therefore, their  occurrence is  of
minimal importance to the fate and transport of arsenic. Both are also ubiquitous in worldwide sediments
and, therefore, neither "attest to the mineralized nature of the Tertiary sediments."

The presence of iron and manganese in the Tertiary sediments in the vicinity of RH-6 simply means that,
like other high arsenic areas at the site,  dissolved iron and, particularly manganese, occur with high
arsenic and this area is probably hydraulicalty connected to the other areas. As this comment suggests,
arsenic is being  attenuated by  the iron oxyhydroxides  which  have formed in this  vicinity but this
adsorption capacity is not sufficient to control the transport of arsenic in the Tertiary sediments beyond
the RH-6 well location.

COMMENT:
Arsenic Contouring is Not Supportable
Contouring of arsenic concentrations in Figure 4-37 in vicinity of the previous Framing Mill and Former Holding Pond appears
to be incorrect. The orientation of the depicted groundwater plume is north-south, however groundwater flow is to the northwest
to westerly direction. This modification by EPA is believed to support the hypothesis of a northerly oriented paleochannel,
which is not supported by flow indicated by die mapping of the potentiometric surface.  The implication  of this interpretation
is mat there are two separate sources: (1) southeast of RH-32 and (2) southeast of RH-26.

Also, EPA's contouring of the arsenic distribution indicates a wider spread man the data suggests. The contours suggest arsenic
migrating under (or through) Silver Bow Creek, when no groundwater or surface water data is available to support it.

RESPONSE:
ARCO is technically correct, to honor the potentiometric surface data the arsenic contours in this area
should be  extended to the west.  However, the arsenic concentration contours in this area are dashed,
indicating insufficient data to accurately draw them in.  The 100 ug/L contour was wrapped around the
two elevated points which happen to be oriented north and south.  There was no intent on EPA's part  to
relate the  orientation  of this contour line to the "paleo-channel".   The paleo-channel as presented by
ARCO in earlier interpretations exists primarily west of these wells.
                                                                                ARCO RI Disclaimer 22

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Regarding ARCO 's comment about separate arsenic sources, it is unclear what point is being made.  EPA
agrees that the elevated concentrations in these two wells suggests two separate sources; the former
holding pond and somewhere south and east ofRH-32 in an area where no RI data exists.

ARCO is over reacting to the extent and implications of dashed concentration contours that happen to
extend under Silver Bow Creek.  ARCO's comments again seem to confuse the total lack of data with "not
supported by data".  At the time the RI was prepared the only data to support the extent of arsenic
contour lines extending from the south to north sides of Silver Bow Creek was at DP-5.  More recent
groundwater analyses from the shallow alluvium, deep alluvium, and tertiary sediments, which became
available during the FS, revealed concentrations of arsenic less than 8 ug/l.  These data suggest the
arsenic plume does not extend under Silver Bow Creek.  However, this does not change the conclusions
of the FS or the selection of the remedy presented in this ROD.

COMMENT:
Conceptual Model Concerns
Under its summary description of me site in Section 3.7.5, EPA states that the upper alluvial and lower Tertiary aquifers are
in direct connection. There is no physical evidence to support a hydraulic connection between the upper Tertiary sediments (i.e.,
arsenic impacted RH-6) and the lower Tertiary aquifer. The impact of the upper Tertiary sediments is a result of the spatial
relationship of the westward up sloping outcrop of the Tertiary and local spillage along the load-out trench.  There is no
hydraulic evidence to support some deep aquifer connection with the potable and much deeper water supply.  The conceptual
model also does not address the strong attenuating characteristics of the sediment to retard arsenic migration.

RESPONSE:
The evidence for and line of thought leading to EPA's conclusions on the hydraulic connection between
hydrostratigraphic units  has been presented  logically in  the RI and reiterated and elaborated on in
responses to previous comments.  The conceptual model discussion in  section 3.7.5 presents only the
physical setting, it was not intended to address arsenic fate and transport which is presented in Chapter
5 and summarized in Chapter 7.

COMMENT:
Health Based Risk Assessment
Estimate of Reference or Baseline Arsenic Value
Page 5-8 under Section 5.3, Review of Anenic Geochemistry, deduce! "die probable anenic background for groundwater at the Rocker Site is between 10 and
20 pg/L." The preceding ientence cited natural values to 39 pg/L. The Streamiide RI reports several values over 50 pg/L for groundwater possibly associated
with tailings. ARCO believes general literature values are not relevant in an area where there is natural mineralization and sources other than the Rocker plant
contributing to arsenic concentrations.

RESPONSE:
The intent of this discussion in this paragraph is to acknowledge that an area with mineralized soils would
have higher background concentrations of arsenic in groundwater than in areas ofnonmineralized soils.
The different aquifer units in the area have distinctly differing arsenic concentrations, some influenced
by mineralized materials, others directly from  the wood treatment arsenicals and other not impacted
whatsoever.  The RI for the Rocker OU also did not deal directly with the issue of background arsenic
concentrations prior to anthropomorphic influences. However, it should be pointed out that the last round
of groundwater analyses taken from the  shallow alluvium, deep alluvium, and the tertiary groundwater
systems (north of Silver Bow Creek) had concentrations less than 8 ug/l arsenic.  This indicates that the
contamination of concern and associated cleanup levels are not related to background conditions.
                                                                                ARCO RI Disclaimer 23

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COMMENT:
Baseline Risk Assessment (BRA)
Presented here is an overview of ARCO's numerous concerns on EPA's Risk Assessment.  As opposed to all other risk
assessments performed to date in the Clark Fork Basin, EPA chose not to significantly dialogue with ARCO on the Rocker BRA.
It is apparent there was little dialogue between die author's and other EPA risk assessment contractors, based on the number
of inconsistencies with other recent EPA risk assessments related to arsenic.   Provided here is a list  of some of  those
inconsistencies and other concerns, which will be detailed in ARCO's comments for the BRA.

•   Available site concentration data was not utilized appropriately to  represent realistic exposures.

•   Default indoor arsenic was used as opposed to relevant regional-specific data available and used in Anaconda.

•   Reduced bioavailability of arsenic in soils versus water in water, and reduced bioavailability due from arsenic in soils being
    partially from mine tailings was not considered.

•   Residential scenario was overstated for a site located next to an active railroad.

•   Worker protection and trespass scenarios used conservative assumptions inconsistent with other EPA risk assessments.

•   Uncertainties of arsenic toxicities recognized by the EPA's  Science Advisory Board was not recognized in the risk
    assessment.

•   Risk assessment assumes groundwater consumption directly from the  shallow aquifer will occur,  not considering the
    limitation of exposure, the disconnection of shallow from deep aquifer,  geochemical attenuation, and dilution factors.

RESPONSE:
Response to  the summary comments above    can be found in detail in  EPA's  response to ARCO's
Comments on the Baseline Human Health Evaluation for the Rocker Timber Framing and Treating Plant,
Operable Unit.
                                                                                       ARCO RI Disclaimer 24

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                            SUMMARY OF ISSUES FOR DISCLAIMER
                              FINAL FEASIBILITY STUDY REPORT
                        ROCKER TIMBER TREATING OPERABLE UNIT
COMMENT:
Summary of Issues of Concern
ARCO strongly disagrees with many of me interpretations and evaluations of remedial alternatives for the Rocker Timber
Treating Operable Unit (OU) in the Public Comment Feasibility Study (PCFS) to which this disclaimer is attached.  ARCO
and the United States Environmental Protection Agency (EPA) have entered into an Administrative Order on Consent (AOC)
to perform the Remedial Investigation (RI)/Feasibility Study (FS) for the Rocker OU. This means that ARCO has agreed to
carry out investigation of the impacts of mining wastes within the Rocker OU and to evaluate remediation alternatives to
clean up the Rocker OU in accordance with the process and schedule defined in the AOC. This work is performed with
oversight, review and control maintained by the regulatory agencies.

RESPONSE:
No response necessary, except to note that wastes at the Rocker OU are both wood treating wastes
and mine wastes.

COMMENT:
Under the AOC for the Rocker OU, ARCO has the responsibility to carry out an investigation or FS in which an
appropriate range of remedial alternatives are evaluated for cleanup of the Rocker OU. The final product of the process is
the FS  Report.  Although this document was originally prepared by ARCO, the EPA has final review authority and any
modifications to the document mat are requested by the EPA must be incorporated by ARCO before the document is
submitted for public review.  ARCO has fully complied with modifications demanded by the EPA.  However, although the
PCFS is published under ARCO's name, ARCO strongly disagrees with some of the interpretations and evaluations of
remedial alternatives that are discussed in the report as revised by the EPA for distribution to the public.

No Response Necessary

COMMENT:
Because the document is published by ARCO but does not, in some instances, contain ARCO's technical interpretations and
evaluations because of modifications mandated by the EPA, the AOC gives ARCO the right to attach a disclaimer to the
PCFS which presents significant issues contained in the PCFS with which ARCO disagrees. This disclaimer is the primary
record identifying the portions of the PCFS which were revised as required by the EPA but with which ARCO disagrees.

RESPONSE:
No response necessary.  EPA has provided a detailed response to the RI Disclaimer separately.

COMMENT:
Site characterization information in Chapters 1 to 3 of the FS was taken from the Rocker OU Remedial Investigation (RI)
prepared by the EPA and its consultants. ARCO prepared a RI disclaimer (Attachment A, RI) which described in detail
ARCO's disagreement with the Agency interpretation. ARCO hereby incorporates by reference its RI Disclaimer to the
extent that RI issues are raised in the FS. Since the RI interpretation is repeated in the first few chapters of the FS, a
summary of issues from the previous ARCO  disclaimer are discussed below.

No Response Necessary
COMMENT:
Arsenic in Groundwater Presents no Real Risk
Arsenic does exist in a shallow alluvial zone proximal to the previous wood treating facility and extends for a distance of a


                                                                                    ARCO FS Disclaimer 1

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couple hundred feet this shallow, limited impacted aquifer does not present a real risk to human health or the environment,
presently or in the future. The primary basis for this conclusion is summarized here.  This shallow aquifer is inadequate as
a water supply due to low conductivities (i.e. inability to pump adequate volumes) and water quality concerns unrelated to
the Rocker plant or mining (i.e., nitrates from septic tanks, high dissolved solids and salts).

RESPONSE:
The parameters described in this comment were generally not studied during the RI, the conductivities
are adequate to supply individual households, and the State of Montana considers the shallow alluvial
groundwater to be a potential source of drinking water. EPA also believes that the extremely high
concentrations of arsenic in this aquifer and the obvious hydraulic connections demonstrated to other
aquifer units poses a threat of arsenic migration from this aquifer unit to the other aquifer units,
particularly in conjunction with current and future groundwater development in the area. In this way,
EPA  believes there is a clear risk to human health.

COMMENT:
The most significant item of concern with EPA's RI/FS is the implied connection between the shallow, arsenic impacted
alluvium and the older and deeper sediments of the Tertiary aquifer.  Both the chemical and hydrologic information
available from the field investigation demonstrate a very limited connection, if any, between the two aquifers. There was no
demonstrated response to shallower wells when the deeper, Town Pump well was tested by the Bureau of Mines. After
seven days of stressing the deeper aquifer system the shallower groundwater wells impacted by arsenic contamination did
not respond.  The water chemistry likewise showed two very distinct types of water, thus indicating that there is little
connection, if any, between the shallow and tertiary aquifers.

RESPONSE:
See EPA's response to the  comments in ARCO's RI disclaimer regarding the evidence for hydraulic
communication between the alluvial and Tertiary sediment groundwater systems.

COMMENT:
The second major issue is that EPA understates the fact that the arsenic has moved only several hundred feet laterally and
20 to 30 feet vertically in a period of 40 to 80 years since the plant was operated. Also, arsenic movement in the system
could have occurred for the most part during operation of the plant some 40 to  80 years ago, and possibly has moved little
since that time. This is important to show that the natural geochemical mechanisms already in place are effectively
immobilizing the arsenic.  Alluvial materials along Silver Bow Creek contain abundant iron that literally traps the arsenic by
adsorption in the shallow alluvium.  This is similar to additives used in conventional water treatment plants.

RESPONSE:
There needs to be a separation between descriptions of historic arsenic concentrations and movement
over time, and the current amount of arsenic in either the sediments or the groundwater.  Historical
movement of groundwater was assumed to be about the same as what is measured and described by
the current RI.  This movement, individual groundwater flow paths, and hydraulic connections are
complex (anisotropic and inhomogeneous) but the physical characteristics have probably changed little
in the last few years.  The amount of arsenic still in the alluvium and the groundwater is the result of
groundwater movement during and since the termination of wood  treatment at the site.  The mobility
of the arsenic is a major concern to EPA that has been addressed through a complex investigation
and interpretation of the data.  The data set has limitations, that make the interpretation more
difficult, such as: a decreasing frequency of alluvium sampling with depth, the random sampling of
shallow alluvium sampling, lack of seasonal sampling, and, probably the most important, the
analytical schedule and variability in arsenic and other constituents in the two groundwater sampling
rounds.   Additional data in these areas would more clearly document the processes described in the
current RI but  would not significantly change the conclusions carried into the FS.  The FS
appropriately interpreted the data in a manner that lead to the technology selected for the proposed

                                                                                ARCO FS Disclaimer 2

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plan and the ROD. The ROD accounted for the identified data limitations by providing for limited
data collection during the remedial design phase of the project to focus the remedy on only those
areas that exceed EPA's criteria: for waste removal (surface soil contamination exceeding arsenic
concentrations of 1,000 ug/g), soil cover (arsenic concentrations greater than 380 ug/g) or alluvium
source area removal.

Adsorption by iron oxyhydroxide on the saturated alluvium is one of the major "natural geochemical
attenuation mechanisms" that removes and irreversibly immobilizes a finite amount of arsenic
(assuming thatpH remains neutral and oxidation reduction potential remains oxidizing).  The alluvial
sediments probably have sufficient iron oxyhydroxide to adsorb a pan, maybe a major part, of the
dissolved arsenic but not enough to adsorb the milligram per liter concentrations of arsenic currently
in the groundwater of the plume.  The amount of iron oxyhydroxide in the  tertiary alluvium (volcanic
origin) is probably both highly variable and less than what is on the shallow and deep alluvial
sediments. Groundwater in the tertiary alluvium may also be largely flowing along fractures which
would not present the same surface area that porous media provide in shallow alluvial sediments.
These data suggest that the tertiary diluvium does not have sufficient iron oxyhydroxides exposed to
the groundwater to control arsenic even as much as the alluvial sediments.

Analyses of the alluvium from the groundwater show that a large amount of arsenic (V) is present,
probably resulting from adsorption to iron oxyhydroxide.  This form of arsenic will probably remain
largely immobile unless the pH becomes acidic or the oxidation reduction potential becomes reducing,
which is not expected under ambient environmental conditions.  EPA examined the potential for
attenuated arsenic to remobilize and found that there are limited deposits of sulfide materials in or
near the Rocker OU that have the potential to produce limited acidic conditions resulting from
oxidation of sulfide materials associated with the railroad or tailings in the near stream environment.
The remedy for the Streamside Tailings OU will remove sulfide tailings from the water table in areas
close to the Rocker remedy.  There are no circumstances known to be present within the Rocker OU
that will pose a significant threat to the remedy selected in the ROD. (see also responses that follow).

The arsenic plume in the groundwater of the  alluvial materials is sufficient evidence that arsenic is
mobile at the site but there is more data supporting arsenic mobility in the  groundwater at the site.
Groundwater data indicate that there is about equal proportion of arsenic (III) and (V) in the wood
treatment area and higher arsenic (III) in the downgradient periphery of the arsenic plume.  These
data support the conclusion that arsenic is being adsorbed but, more importantly, also indicate that
arsenic is mobile because arsenic (III) is poorly adsorbed except by aluminum oxyhydroxides at an
alkaline pH (pH> 8).  Therefore, these data suggest that there is a reservoir of mobile arsenic within
the wood treatment area and that the more mobile arsenic species indicates that the arsenic is still
mobile in the downgradient pan of the plume. Furthermore, it is difficult to believe arsenic is
immobile when the arsenic concentrations of the two sampling rounds are compared because the
arsenic concentrations (and concentrations of many other parameters) are considerably different.
These differences indicate a relatively mobile groundwater system that includes arsenic as one of the
parameters being mobilized at the site.

Finally, there is little data developed to date  that indicate that the arsenic has "moved little since" the
"operation of the plant some 40 to 80 years ago".  The arsenic on the  sediments and in the
groundwater at the site today is probably a remnant of a much larger arsenic source generated by the
wood treatment facility when it was operating.  The high arsenic concentrations found in the RI reflect
this past high source and the complex hydraulic conditions at the site.

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 COMMENT:
 Generally, EPA's presentation of the physical characteristics of the site ignores its historical, current, and future land use
 as a railroad siding on the 100-yr flood plain. EPA's assumption that there would be future residential development on the
 site is not realistic given the area is predominantly in a flood plain and is adjacent to an active rail line.  Also, previous
 removal and soil cover actions are presented as being ineffectual without very competent evidence to support such claims.

 RESPONSE:
 See responses to prior RI comments. Basically, the Rocker OU is not in the 100-year floodplain, and
 EPA's remedy requires institutional controls to prevent residential development and cleanup of soils to
 occupational/trespasser scenario levels.

 COMMENT:
 Hydrologically, all evidence points toward an effective disconnect between shallow alluvial aquifer where arsenic is found
 locally and in the Tertiary aquifer, die source of the Town Pump water supply and other local well water.  EPA ignores the
 very limited extent of arsenic in the underlying Tertiary aquifer. This disconnection is  supported by the following:

 1.       Pump test results indicate that the tertiary aquifer behaves as a confined system;

 2.       Shallow alluvial wells did not drawdown during the test;

 3.       A pressure head differential exists between the two systems; and

 4.       A deep alluvial well deep not show any significant drawdown.

 RESPONSE:
 EPA has addressed each of these points in detail in the response to ARCO's RI disclaimer.  The body
 of evidence, supported by actual RI data indicates hydraulic communication between the alluvium and
 Tertiary sediments under ambient hydraulic conditions and under the minimal hydraulic stresses
 imposed during the Town Pump test. The evidence for communication is sufficient to warrant
 concerns over migration of site contaminants into the regional water supply given time and additional
 hydraulic gradients imposed by future groundwater development of the Tertiary aquifer.

 COMMENT:
 One anomalous Tertiary shallow well (RH-6) near the railroad load-out trench has elevated arsenic likely associated with
 nearby arsenic spillage, not aquifer migration. There is no evidence that this monitoring well is hydraulically connected to
 deeper aquifer zones due to poor conductivities in the Shallow Tertiary Zone. The hydraulic conductivities of the alluvial
 aquifer are an order of magnitude less than the deep Tertiary aquifer, and preferential flow direction will be horizontal
 versus vertical. This means, even if the two aquifers were connected, migration into the deeper zone would be very low.

 RESPONSE:
 It is more difficult to  reconcile the arsenic concentration in groundwater from monitoring well RH-6
 as coming from  "arsenic spillage" than as simply part of the arsenic resulting from the wood treating
facilities.  The fact that the arsenic is present in these concentrations in  groundwater from this well is
 evidence of hydraulic connection between this monitoring well and groundwater from other  wells of
 comparable  depth.  This only requires horizontal, downgradient movement of the groundwater plume
 not vertical movement.  However, potential fractures in the Tertiary sediments may hydraulically
 connect the plume to deeper sediments, albeit currently relatively low probably because of both
 relative head elevations in  the different parts of the aquifers and possibly limited permeability.  The
 argument by ARCO that downward migration of arsenic has occurred from the shallow alluvium into
 the tertiary sediments is also in opposition with their other arguments that the shallow alluvium and
 the tertiary sediments aquifers are not in communication.


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COMMENT:
The RI presentation of aquifer geochemistry has been incorrectly represented that water chemistry should directly correlate
with saturated sediment chemistry.  The mobilization and/or fixation of contaminants such as arsenic are largely controlled
by the groundwater oxidation or reduction state in addition to the presence of iron or manganese in the system. The use of
nonvalidated temperature data and suspect cation/anion ratios for rationalizing mixing of die upper alluvial Aquifer and the
deeper Tertiary aquifer is not technically supportable and is inconsistent with several other stronger lines of evidence such as
distinct differences in major cations in the various depths of the aquifers.

RESPONSE:
The comparison between the water chemistry and the sediment chemistry was discussed to determine if
acidic conditions at the site was a major contributor of arsenic and other metals to the groundwater.
When sulfuric add attacks the sediments, and there is little to no mobilization of the dissolved
parameters, the dissolved concentrations of parameters in the groundwater generally approximate
their respective abundance in the sediments.   It is true that with transport away from the areas of
sulfuric acid attack into peripheral areas with different pH and oxidation reduction potential (ORP,
redox, Eh, etc.) then other processes (primarily adsorption) will individually alter the relative
concentrations of each parameter in the groundwater. Changes in each parameter's concentration
generally reflect the processes occurring in the groundwater system.   Unfortunately,  aluminum and
silica are missing from the analytical data base and this indirect method of trying to determine the
relative significance of acidic conditions had to be applied at the site.

The comment on 'nonvalidated  temperature data and suspect cation/anion ratios for rationalizing
mixing  of the upper alluvial Aquifer and deeper Tertiary aquifer is technically insupportable" is not
sufficiently clear to give a direct answer. However, all of the temperature data used in making
interpretations and descriptions in the text are temperatures measured in the field by the ARCO
engineering firm professionals during the two sampling rounds. EPA  assumed that ARCO's
professionals property collected the data because there is no report or other written statement
indicating otherwise. If this is not the case, EPA should have been notified of this and any other
incorrectly collected or analyzed data generated by ARCO. Without knowing what is "suspect" about
the cation/anion ratios, EPA cannot respond to the "technical insupportability" part  of this comment
concerning the ratios. However, as stated above, if EPA has been given incorrect data without a
description of the inadequacy then ARCO should have notified the Agency.   The use  of the
cation/anion ratios was prompted again by the available data that was believed to be valid and
relevant.

COMMENT:
The arsenic fate and transport discussion presented in the RI/FS does not address the migration limiting impacts of
naturally occurring attenuation of the arsenic through iron fixation, processes have been effective in minimizing the
migration of die shallow groundwater plume. The streamside tailings and natural soils distributed throughout the fioodplain
are also a source of arsenic that may be misinterpreted as originating from the Rocker site.  Understanding the role of the
tailings will be important when determining the actual spread of (or lack of) arsenic from die Rocker plant.

RESPONSE:
The "positive effect of naturally occurring attenuation of the arsenic through iron" adsorption was
assumed to be operating at the site because the process is ubiquitous under near neutral pH and
oxidizing groundwater conditions.  However, the adsorbed phase is only immobile under these
groundwater conditions. In addition to this currently immobilized amount of arsenic there is a  high
concentration of dissolved arsenic forming a mobile phase plume in the groundwater that is of
concern for the RI.  If the site conditions remain  stable then the adsorbed phase may remain
immobilized but ifthepH becomes more acidic or the groundwater becomes more reducing this

                                                                               ARCO FS Disclaimer 5

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 adsorbed phase can released and, probably significantly increase the dissolved arsenic in the
 groundwater.  Assuming relatively stable conditions, the fate and transport text deals primarily with
 the mobile groundwater phase and assumed near equilibrium adsorption conditions on the sediments.
 Obviously, if adsorption was totally effective in immobilizing all the available arsenic from the wood
 treating site there would be little to no dissolved arsenic in the groundwater.

 The streamside tailings are known to be a source of arsenic but are not believed to be misinterpreted
 as arsenic originating at the site in the RI unless specifically mentioned in the text. The
 understanding of the role of the tailings  is important (see the above responses on creating acidic
 groundwater conditions at  the site).  It is EPA's understanding that the streamside tailings will be
 dealt with as part of remedial action of the streamside tailings and that the streamside tailings
 component -will no longer contribute arsenic, metals, acidic groundwater, or change the Eh of
 infiltrating groundwater after their remediation.  The site-related arsenic concentrations are believed
 to be appropriately addressed in the RI.

 COMMENT:
 The Baseline Risk Assessment was developed using numerous overly conservative assumptions and erroneous summaries of
 available data.  Several critical factors and analyses in the risk assessment are inconsistent with other risk assessments
 related to arsenic performed by EPA in Montana. Regarding risk from groundwater, EPA assumes die public would be
 drinking directly from the shallow aquifer. They do not recognize the limitations of the exposure scenario regarding lack of
 shallow aquifer usage and the disconnection with the deeper drinking water supply. To support conclusions regarding
 residential risk scenarios, a complete connection  of the shallow and deep aquifers was advanced by these authors. However,
 the lack of supporting information of an aquifer connection combined with the unlikelihood of a residential development
 scenario at the site makes assertions of any real risk improbably high.

 RESPONSE:
 This same comment was  responded to earlier in the response to ARCO's RI disclaimer and in EPA's
 response to ARCO comments on the Baseline Risk Assessment.

 COMMENT:
 Also, exposure to and the bioavailability of arsenic were seriously overestimated.  For example, arsenic distribution in the
 sites' surface soils was incorrectly characterized to assume all came from the Rocker Plant, where in fact, much could be
 attributed to less bioavailable railroad bed tailings and natural soils. Risk levels estimated for arsenic by EPA (3-300
 mg/kg) are two orders of magnitude above risk levels found by the EPA at Old Works OU.

 RESPONSE:
 The RI conducted by ARCO was not designed to distinguish between sources of arsenic (either mineral
 related arsenic from concentrates, ores and tailings materials versus the arsenic trioxide powders used
for wood treating).  EPA believes most of the arsenic at the Rocker OU is from the wood treating
 operation. EPA used the appropriate bioavailability factor that corresponds to wood treating forms of
 arsenic (such as the arsenic trioxide used on the site).  For the Streamside Tailings OU, 80%
 bioavailability was used for sulfide forms of arsenic minerals. Even if this lower bioavailability level
 were applied to materials that might be sulfide minerals such as the railroad bed, the action  level
 would be still less than 400 mg/kg arsenic, compared to the selected cleanup level of 380 mg/kg.  This
 difference is small considering that all of the site was used for handling of oxide forms of arsenic and
 that for most arsenic contamination on the site the higher bioavailability is correct. However, EPA is
 not advocating a remedy that is over designed.  If during the remedial design phase of this project
 ARCO  would provide the appropriate bioavailability information that would allow EPA to make a
 bioavailability determination, EPA would consider this information with respect to the final
 implementation of the remedy.  Reduced bioavailability assumption at the Old Works/East Anaconda

                                                                                ARCO FS Disclaimer 6

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Development OU reflect site specific bioavailability studies relating to mine waste resent at that site
and do not apply here to a very different kind of waste.

COMMENT:
The narrative in Section 2.2 (page 2-1) depicts the previous wood treatment area on the NE portion of the OU as lying
outside the 100-yr floodplain.  The source for these boundaries was derived from its oversight contractor CH2M HILL in
1988. The Flood Boundary and Floodway Map developed under the National Flood Insurance Program in September 1979
clearly shows this entire area north of the railroad tracks to be in the 100 yr flood plain. Considering the floodplain status,
its proximity adjacent to active rail lines, relatively small size, likely future land ownership (active railroad, ARCO), and no
road access to the site, this area has poor development potential.

RESPONSE:
This comment was answered previously in EPA's response to ARCO's RI disclaimer comments.

COMMENT:
Synopsis of Remedial Investigation and Health Risk Assessment
Several RI and risk assessment items discussed in the summary at the beginning of this disclaimer were discussed in detail in
the ARCO RI Disclaimer (Attachment A, RI). Since RI and risk assessment information are summarized in this FS chapter,
specific responses to these issues are not discussed in detail.  Please refer to the ARCO RI Disclaimer, which is
incorporated by reference into this document, for a more specific response to interpretation of the RI and risk assessment.
ARCO's basis is presented in the attached risk assessment comments.  The issues listed below are in addition to issues
discussed in the previous RI disclaimer.

RESPONSE:
No response necessary — see EPA's response to RI disclaimer and to  ARCO's Health Risk Assessment
comments.

COMMENT:
Reduced Geochemical Conditions-Section 3.1.6.I/Section 3.1.6.2/Section 3.1.7/Section 3.18
In these sections PAHs are stated to cause the mobility of arsenic creating a reducing (oxygen deprived) condition.  ARCO
believes EPA's position ignores site specific conditions that limit the availability of necessary biological activity. For PAHs
to create a mobilizing condition, microbes would need to digest PAHs which would consume oxygen, thus creating a
reduced environment. However, the probability of biological activity capable of digesting PAHs within mis environment is
very low, especially given die presence of arsenic, which would impede the digestion of PAHs.  Even if reducing conditions
existed, iron as well as arsenic would be reduced. Mobilization would be limited because iron would precipitate out with
arsenic as solution reached oxidized areas. Therefore the mobility of arsenic would  be severely limited.

RESPONSE:
This is a mixture of many different aspects of what determines reducing conditions and mobility of
iron and arsenic.  There are several abiotic sinks for dissolved oxygen in the alluvial sediments.
There is a considerable amount of organic material, including wood,  in addition to  the PAHs in the
alluvial sediments (drilling log descriptions) that are actively undergoing oxidation probably both by
abiotic and biotic reactions.  Sulfides occur in the surfidal material and with depth in the alluvial
sediments.  The clay color suggests that several of them are also reduced and will be undergoing
oxidation. Finally, there are the dissolved metals and arsenic which are undergoing oxidation.  All of
these processes are occurring abioticalfy and to some degree or another probably also bioticalfy.
Bacterial processes more efficient reducing agents but not a requirement for establishing a reduced
environment.   The point that was being made in the text is that the PAHs probably traveled along the
same groundwater flow path as the arsenic and physicochemical attenuation of one may inversely
influence physicochemical stability of the  other.  Given all of the parameters that can reduce the
dissolved oxygen in the alluvial aquifer, the PAHs add to the list of reducing agents potentially
capable of retaining the reduced arsenic III species used in the wood  treatment process.  The reduced


                                                                                   ARCO FS Disclaimer 7

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state of the arsenic then allows the arsenic to be potentially less oxidizable and more mobile than if
PAHs were absent.

Iron would likewise be more mobile where the groundwater and sediments are reduced.  This is very
important because if iron is dissolved it does not precipitate as the iron oxyhydroxide to form an
adsorption media for arsenic.  When both are transported by groundwater to more oxidizing
environments, iron will precipitate first and form an adsorption media for arsenic depending on the
amount of dissolved iron, the degree of oxidation encountered in the downgradient part of the system,
and the arsenic spedation.  Arsenic is slow to oxidize and arsenic III is poorly adsorbed by iron
oxyhydroxide so it will be transported farther along groundwater flowpaths than iron.  Therefore, the
mobility of arsenic would not be as  "severely limited* as one would suspect.  The arsenic plume is an
example of the above, if arsenic adsorption by iron oxyhydroxide was totally efficient and the
groundwater were sufficiently oxidized, there would be little dissolved arsenic to form a plume.

For these reasons, the remedy is justified when considering source removal which includes not only
the high arsenic contaminated materials, but also the materials that may influence a reducing
condition at this OU.

COMMENT:
Identification and Screening of Alternatives
Remedial Action Objectives—Groundwater/Soils—Section
Federal and State ARARs for groundwater are stated as SO ug/L and 18 ug/L respectively.  These ARARs cannot be met at
the site regardless of the alternative chosen.  The Agencies' and their consultants stated mat groundwater ARARs would not
be met at mis site,  but that protection of human health and the environment was achievable. ARCO is submitting a
Technical Impracticability memorandum requesting a waiver from ARARs for the Rocker site, which will provide additional
support for this position.

RESPONSE:
The Agencies did not indicate that ARARs cannot be met at the Rocker  OU.  There has been
recognition in the FS that clean up of the shallow alluvial groundwater system will be difficult.
However, with removal of the source materials that continue to contribute to the groundwater
contamination, water quality will improve over time.  The question is how much time it will  take for
water quality to improve in the shallow alluvium to the level that it will meet the State standard of 18
ug/l arsenic.   This question will be reevaluated following the implementation of the remedy and a
considerable period of monitoring.  In addition, the primary and secondary objectives of the remedial
action is to prevent further degradation of the quality of the deep alluvial and tertiary sediments
aquifers.  This objective will be met in the short term.

In responding to ARCO's Technical Impracticability memorandum, EPA has concluded that  this  issue
cannot be concluded until the remedy is implemented and documentation is provided that ARARs
cannot be met within a reasonable time frame, as is suggested by EPA  guidance.

COMMENT:
Risk based soil concentrations for soils are given as 380 to  3.3 mg/kg for arsenic corresponding to a 3.3 in 10,000 to a 3.3
in 1,000,000 excess cancer risks respectively. This range is technically impracticable, as well as inconsistent with values
from other Clark Fork Superfund sites (i.e., Old Works Operable Unit Risk Assessment). Also, mis arsenic soil range is
typical of highly mineralized sediments in the local Butte area. Removal of soils at the 3.3 mg/kg would basically
encompass most of the native soils in Butte-Silver Bow County.  Even soils at the 380 mg/kg would encompass an area
outside of the Rocker site boundaries.
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 RESPONSE:
 The range of risk is predicated on the bioavailability of arsenic which is assumed to be 100% as
 described in EPA's Baseline Human Health Risk Assessment and EPA's response to ARCO's
 comments on the Baseline Human Health Risk Assessment.  An action level of 380 mg/kg has been
 chosen for the remedy corresponding to an excess cancer risk of one individual in 10,000.  EPA
 concludes that cover soils are available nearby to meet these requirements.  The cover soil brought to
 the site from the gun club are for the removal action were less than 30 mg/kg arsenic.

 COMMENT:
 Initial Screening of Remedial Technologies and Process Options—Section 4.S
 In-situ treatment of vadose and groundwater using iron flooding was eliminated from consideration for detailed alternatives.
 EPA assumes mat an in-situ remedy cannot be used for source control which is inconsistent with Agency resources (USEPA
 1995, NRC 1994) preferences for treatment.  In-situ remediation poses the least short-term risk for an active remedial
 scenario by not requiring worker risks to excavation operations and community' risks from the off site removal operations.
 It also is the only remedial technology which has the potential to enhance and accelerate the ongoing natural attenuation
 which presently has limited arsenic migration. Conversely, physical and chemical disturbances of the arsenic in the soil by
 removal actions could likely exacerbate and increase arsenic mobility. In addition, in-situ iron flood remediation is an
 innovative technology which shows promise based on batch testing performed by Montana Tech (Chatham, 1995) and
 similar studies throughout the country (Groundwater Technology, 1995). A more complete treatability study would be
 appropriate before final remedies are selected. This would be best achieved under EPA's time schedule by building a
 contingency for in-situ iron treatment in the ROD.

 RESPONSE:
 Section 4.5 of the feasibility study evaluated Process Options on the basis of technical applicability to
 site conditions.  Appendix C, Table C-l provided the more detailed screening analysis.  The results of
 the screening were summarized in Table 4-1.  In-situ treatment using adsorption (iron fixation) was
 retained for further evaluation in Section 4. Alternative S-9, described on page 4-39 of the feasibility
 study, includes the iron flooding technology applied to  the source area (unsaturated soil and
 groundwater within the 10,000 ug/L arsenic in groundwater isopleth).  Similarly, Alternative P-4
 utilizes in-situ treatment (iron flooding) to  treat groundwater in the plume area.

 Alternative S-9 was eliminated prior to detailed analysis because of questionable effectiveness based
 on site conditions.  The buried trench  materials in the source area would make the proper application
 of an iron flooding technique problematic.  This determination is in no way inconsistent with the
 National Contingency Plan (NCP) preference for treatment alternatives. Indeed treatment alternatives
for the vadose soil and the groundwater were retained and evaluated in detail in Section 5. Not
 recognizing the unique features of the  site  would be inconsistent with EPA guidance on conducting
feasibility studies.

 The commentor is incorrect in stating: "It [iron flooding] is the onfy remedial technology which has
 the potential to enhance and accelerate the ongoing natural attenuation which has limited arsenic
 migration. * On the contrary, there are ex-situ techniques for soil stabilization that will enhance and
 accelerate the ongoing natural attenuation that were evaluated in the feasibility study.  EPA agrees
 that excavation of the source area will increase short-term risks to workers and potentially mobilize
 some arsenic, but both of these issues  are  manageable  using proper worker protection and addition of
 iron salts into the excavation.  Management of short-term risks is feasible enough to allow realization
 of reduced long-term risks through source  excavation.  The remedy as selected meets the Agency's
preference for treatment and the use of iron as a soil and groundwater amendment is considered
 innovative.
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COMMENT:
Source Area—Section 4.6.1
Analysis of fee "source" in this section, defined the base case to be the 10,000 ug/L contour line.  This arsenic contour line
is arbitrary and subjective and not supported by the available data. A sensitivity analysis performed by ARCO at the request
of the EPA yielded a defined source based on a modified 20,000 ug/L contour which correlates well with the location of
previous operation areas, including the trench area north of the carpenter shop. In the ARCO sensitivity analysis, depth and
an arsenic groundwater concentration contour were selected after considering worker exposures (short-term risks), technical
difficulty  in removing saturated material, efficiency of arsenic removal, and economics associated with removal.

RESPONSE:
Development of any of the isocontours is somewhat subjective because of changing site conditions with
time.  It is incorrect however to say that one isopleth is less arbitrary than another simply because the
area is smaller and more well defined. All isocontours were based on the same database. EPA
believes the 10,000 ug/l figure provides a strong indication of the location of arsenic "source
materials. *

The purpose of the differing contours was to compare the costs of remedial alternatives as the area
comprising the "source" changed.  This analysis is presented in Section  5.5 of the feasibility study.
As described in the ROD, the  implementation of the remedy in the source area will not strictly adhere
to the 10,000 ug/L area. During source removal the source area soils will be excavated and disposed
based on further remedial design work, as described in the ROD.  Using a 10,000 ug/l isocontour
line in the feasibility study was simply a method for getting consistent cost estimates from one
alternative to another, and a reasonable  attempt to define a protective  'source materials" area which
•would cause continued groundwater contamination.

COMMENT:
Development of Remedial Action Alternatives—Section 4.8
Each of the excavation alternatives identified by EPA in its rewrite of the FS require transport of contaminated soil to a
RCRA Subtitle C TSD facility.  EPA's position is based on its belief that soils contain the listed wastes F034 and F035.
Based on this position, EPA has identified certain RCRA Subtitle C requirements as ARARs for fee Rocker OU. EPA's
current position on this matter is inconsistent with its earlier position that a RCRA Subtitle D facility would be suitable for
contaminated soils that passed a TCLP test and stabilized soils that initially failed a  TCLP test. ARCO believes that EPA
should reconsider this position for several reasons.  First,  several potential sources of arsenic, in addition to wood treating
solution residuals, have been identified at the Rocker site, e.g., tailings, railroad related materials,  and arsenic trioxide
powder "cold treater dust" used as a constituent of wood treating solution. These sources are outside the scope of F035.
EPA has not provided any basis for  determining which site soils contain F035 and which do not. Certainly, the presence of
arsenic in soil is insufficient for this  determination.  ARCO believes that EPA should not assume feat all Rocker site soils
contain F035 given these other potential arsenic sources and further believes feat any agency action based on feis erroneous
assumption would be arbitrary and capricious.

Second, fee "contained in" policy is  not a codified requirement and has been explicitly recognized by EPA and fee courts as
an interpretive statement closely related to fee "mixture" rule specified in 40 C.F. R. 261.3(a)(2)(iii).  See, e.g., "Land
Disposal Restrictions for Newly Listed Wastes and Hazardous Debris" Final Rule, 57 Fed. Reg. 37194, 37225 where EPA
recognized feat fee "contained in" principle has "served as an interpretive gloss on fee existing mixture and derived from
rules." ARCO believes feat, given EPA's failure to promulgate a mixture rule by October 1, 1994 as  required by fee
Chaffee Amendment, EPA's ability to invoke fee "contained in" principle as a basis for requiring Subtitle C management of
Rocker site soils is suspect and should not form a basis for EPA's remedial action decision at fee Rocker site. ARCO
believes feat EPA cannot rely upon an uncodified principle, which, in EPA's own words, is merely an "interpretive gloss"
on an invalid rule.

Third, ARCO believes feat EPA's position will not provide any additional protection of human health or fee environment
because there are no currently promulgated treatment standards for F034 or F035 wastes.
                                                                                     ARCO FS Disclaimer 10

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RESPONSE:
Initially, EPA has consistently identified RCRA ARARs as applicable to site contamination throughout
the RI/FS process.  Nevertheless, EPA has reconsidered the need for a RCRA Subtitle C repository for
the Rocker OU.  The rationale for heating the waste as solid waste rather than hazardous waste, and
thereby disposing of Rocker wastes on site, is contained in responses to ARCO's comments on the
Proposed Plan.

COMMENT:
On-site Repository
Siting of an on-site repository should be considered for a location on or adjacent to the Rocker OU to minimize short-term
risk to exposure from transport of materials on public roads and through Rocker. Soils not passing a TCLP test would be
stabilized and placed, within mis repository.  Soils passing me TCLP test would be considered hazardous and replaced on
site.  This would eliminate short-term exposures from loading to haul trucks or rail cars which expose the community and
environment to contaminated soils.  This repository may need to be located in the 100 year flood plain but would be
designed and constructed to withstand flood events in accordance with 40 CFR Part 257 requirements.

RESPONSE:
See prior response and responses to ARCO's comments regarding the Proposed Plan for the Rocker
OU.  The waste will be treated to meet TCLP requirements and disposed of on site, outside of the
100-year floodplain in accordance with applicable requirements.

COMMENT:
ARCO had proposed a "contingent" water supply to be activated should the tertiary aquifer be threatened. Currently the
tertiary water supply is not threatened or contaminated and monitoring will provide adequate safety for triggering design of
a new alternate water supply system.

In other words, a new water supply system would be installed once monitoring of the tertiary aquifer indicated a potential
migration of arsenic from the shallow alluvial aquifer.  This water supply system would upgrade the  current connection to
the Butte Water System to enable all current and foreseeable future community water needs without relying on the uncertain
productivity of the local groundwater system. In addition, an alternative water supply is shown for all alternatives except
the No Action alternative.  The seven alternatives are listed with respect to the rigor of treatment.  This compounding effect
provides no additional risk reduction but does add redundancy. For instance, Alternative 5 has plume remediation with iron
sulfate in addition to an alternative water supply.

RESPONSE:
EPA has concluded that the alternate water supply is a vital part of any remedy and is essential to
achieve the objectives of preventing continued contamination of two valuable  groundwater resources
(deep allirAum and the tertiary alluvium aquifers). ARCO's comment is responded to in more detail
in the section dealing with ARCO's comments on the Proposed Plan.

COMMENT:
Institutional Controls
ARCO made a substantial effort to define detailed institutional controls (ICs) that were realistic for each alternative.  ICs
play a very important role in alternative development by supporting the technical aspects of the alternative, and cannot be
assumed to be the same for each alternative.  Specific ICs relating to the Rocker OU are attached.

RESPONSE:
EPA has utilized ARCO's 1C work to  describe the general ICs necessary for the selected remedy.
More specific ICs will be identified during remedial design and implemented  in the  remedial action.
                                                                                  ARCO FS Disclaimer 11

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 COMMENT:
 Hot Spots. Hot Spots are defined by EPA as areas posing unacceptable risk due to direct contact with surface soils
 containing arsenic. ARCO does not agree with EPA's broad and unsupported characterization of hot spots.  ARCO's basis
 is presented in the attached risk assessment comments.

 Figure 1 represents the short-term relative effectiveness for surface soils or "Hot Spots" alternatives (HS-2). All
 alternatives, except for the No Action alternative, will provide soil cover for hot spot areas, basically providing the same
 measure of risk protection. Since short-term relative effectiveness is increased by exposure of workers to heavy
 construction equipment and contaminated materials, the risk is greater man the risk associated with the no action alternative.
 Conversely, the long-term risk associated with hot spots shown in Figure 2 is less than the long-term risk associated with the
 no action alternative.

 RESPONSE:
 Specific responses to concerns regarding the risk assessment are included after each of ARCO's
 Baseline Human Health Risk Assessment comments.  However, in the comment, there are no
 conclusions reached as to how one compares the short-term increases with the benefits of the long-
 term risks being reduced by the implementation of the action alternative. EPA believes the short-term
 risks during the remediation are quite easily managed based upon using properly trained  workers with
 equipment and techniques that have proven to be practical and easily implementable.  This risk level
 is preferable to the unsuspecting, long-term problems from repeated exposure to the  site over time
from occupational or trespass exposure.

 Figures 1 and 2 indicate qualitative short-term and long-term risks for each alternative.  There is no
 indication as to how these results were derived or how they relate to acceptable or unacceptable
 short-term or long-term risks for each of the alternatives.   The comment seems to indicate during the
 remedial actions for the hot spots the short-term risks are greater than the No Action alternative.
 This is consistent with the feasibility study. Also, the commentor indicates that "action" alternatives
pose a lower long-term risk than the No Action alternative.  This is also consistent with the feasibility
 study.

 EPA cannot respond further to this comment or the associated figures without further information.

 COMMENT:
 Groundwater.  Figures 3 and 4 evaluate the short and long-term risks associated with groundwater at the Rocker site. The
 short-term risks increase with the rigor of each alternative in numerical order in Figure 4.  This is reflected by the rough
 number of man hours associated with each alternative which is reflective of the short-term risk associated with worker
 exposure.  Figure 5 shows the long-term risks associated with groundwater alternatives.  The no action alternative shows a
 greater risk potential man the other alternative. Even this risk is minimal unless the unlikely use of this poorly conductive
 and separated zone occurs by drilling of future well units into this zone. However, since the remaining alternatives all have
institutional controls and an alternate water supply, they provide the same measure of risk protection.

RESPONSE:
Again, the development and interpretation of figures 3 and 4 are similar to those mentioned earlier
for figures 1 and 2 (see response to the above comments).  They are non quantified,  are quite
 subjective, and are therefore open to wide interpretation.  There is no attempt to measure differences
 in short-term versus longer-term risks and they again ignore the risk reduction based upon using
properly trained workers implementing straightforward technologies.  Figure 5 does not show long-
term risk associated with groundwater alternatives.  It is a "Cartoon of Conceptual Models " non
quantitatively demonstrating the theoretical decrease in arsenic concentration in the alluvial and
tertiary aquifers should certain assumptions hold true. The Agencies are concerned that hydraulic
 connection between the contaminated shallow alluvial aquifer can be exacerbated based upon future

                                                                                ARCO FS Disclaimer 12

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development of area groundwaters and therefore recognized the need for institutional controls and
alternative water supplies in the FS comments.  But this cannot be the only remedy for the site, as
such measures are not permanent.   The NCP states that institutional controls are to be supplemental
to active cleanup, not a replacement for active cleanup.  The selected remedy therefore contains both
components and presents a reasonable, cost-effective cleanup for the site.

COMMENT:
Arsenic Dilution/Attenuation. EPA's FS incorrectly considers and severely underestimates the real and supportable limited
arsenic fate and transport of the sites'  groundwater. This is translated to an inappropriate comparative analysis of the
alternatives.  By incorporating the information from Figures 1 through 5, it appears that Alternative 2 would minimize
short-term and long-term risk with regard to overall risk at the site. To further evaluate these alternatives, mobility of
arsenic will be assessed. Relative arsenic mobility of alternatives that depend on natural attenuation processes are shown in
Figure 5.  This graph represents a conceptual model showing individual and combined components of arsenic dilution and
attenuation, as arsenic from the source migrates through alluvial sediments to the tertiary zone.  The graph is not to scale
but shows a conceptual relationship of arsenic concentration versus aquifer medium with both attenuation and dilution which
is supported by data and knowledge available from the remedial investigation.  Attenuation alone and a combination of both
attenuation and dilution decrease the concentration of arsenic dramatically at the alluvium/tertiary interface in response to
strong geochemical gradients and probable geological separation. Even if attenuation stops and dilution continues the
arsenic concentrations will continue to decrease.

RESPONSE:
ARCO again attempts to use these  same subjective figures to attempt to make their point (see
responses to the two comments preceding this one).  The agencies do agree that attenuation and
dilution are mechanisms that will reduce arsenic concentrations in the  shallow alluvial aquifer as
stated in the FS comments.  The problem, however, is the rate at  which these concentrations can be
reduced which  is influenced by the degree of aggressiveness of various groundwater remediation
technologies considered in the FS comments.  EPA believes that without active remediation of the
"source materials," achieving cleanup  standards would not be achieved in a reasonable time frame
and preventing migration would be jeopardized by the lack of permanency associated with institutional
controls alone.  The remedy provides an appropriate balance of the need for short-term institutional
controls and long-term, effective remediation.

COMMENT:
Additional Alternative. As discussed previously, it appears that Alternative 2 offers the best short and long-term risk
protection.  Considering the effects of natural attenuation and dilution, it further demonstrates the limited probability for
migration of low arsenic concentrations into the tertiary zones. However, to augment this process of existing iron
attenuation, an additional in-situ alternative would provide greater potential for immobilization with very  little impact on
short-term risk.

The EPA screened out in-situ remedies because it did not seem feasible to implement. ARCO is continuing to conduct
treatability studies to evaluate an in-situ iron flood.  In-situ treatment of an iron flood, or a combination of in-situ and
ex-situ treatment would require very little  exposure to workers while reducing short-term risks. Long-term risks would also
be reduced because of the increased effect of attenuation. To examine implementation and effectiveness concerns, ARCO
has  conducted batch tests of this technology with promising results. ARCO will continue with column tests and a field study
to further demonstrate mis technology. Current EPA guidance requires the agency to seriously consider  mis innovative
treatment technology. Specifically the NCP (Federal Register, 1990) states EPA has a preference for treatment alternative
over a simple removal.  Also, recently EPA (USEPA, 1995) stated source control through treatment can be considered when
groundwater is determined to be impractical to restore below a given standard.

RESPONSE:
Again,  ARCO utilizes their interpretation of Figures 1 through 5, the limitations that have been
discussed in responses to the previous three comments, to attempt to make their points to influence


                                                                                   ARCO FS Disclaimer 13

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remedy selection.  The agencies believe that, because of the demonstrated hydraulic connection of the
shallow alluvial aquifer with the deeper units and the potential future development of future
groundwater resources, controlling plume migration is important.  EPA continues to believe that
dilution and natural attenuation are factors that should be considered with remedy selection and that
if iron salts could be added in such a way that the iron could be widely distributed in the shallow
alluvial aquifer,  the rate of adsorption would rapidly increase thus reducing significantly the time
frame to diminish arsenic concentrations in the shallow aquifer. The problem is, that use of trenches
to distribute the iron as ARCO proposed, will not lead to uniform distribution.  The  concern is not
with the chemistry but rather the way to implement its required extensive distribution in the aquifer so
it can do its job.  That is why  the agencies considered such thing technologies as well injection and
excavation, mixing and backfill, and direct addition to the exposed groundwater system as part of
their suggested alternatives.  EPA's more detailed response to ARCO's proposal and the NCP issues
is presented elsewhere.

COMMENT:
Costs developed for the detailed alternatives  were calculated for both rail and truck haul. Assumptions for rail haul are not
clearly defined but a substantial cost savings by rail is shown.

RESPONSE:
This is  correct.  Cost estimates for rail haul were based on costs provided to Mr. Jim Ford, Montana
Department of Health and Environmental Sciences for the Streamside Tattings Operable Unit in a
letter dated April 12,1995.

COMMENT:
Costs developed by the Agency and their contractors are stated to be within +50% to - 30%. In the draft FS, ARCO
developed cost ranges based on past project experience in the local area, vendor information, established cost estimating
databases (i.e., Means, MDT), and engineering judgment, and provided a detailed explanation of cost  sources and
assumptions. A list of cost items that seem questionable is presented below:

Institutional Controls costs have not been developed for any of the alternatives.

RESPONSE:
Institutional controls costs were included in the cost estimates for each of the alternatives.  These
costs were included as annual  operation and maintenance costs.

COMMENT:
Costs developed by the EPA for a RCRA Subtitle C repository seemed very unrealistic at $6/cubic yard. Costs for a RCRA
Subtitle C repository could be 2 to 5 times this amount.

RESPONSE:
Cost estimates for development of a Subtitle C repository were based on a similar project in
Colorado. Actual costs will vary.  Costs were used consistently in all alternatives to facilitate
comparison.

COMMENT:
Water Supply costs were developed by EPA  using Means Cost database. Based on preliminary design, costs developed by
ARCO are 1/2 to 1 $M dollars more.

RESPONSE:
Costs presented in the feasibility study were not based on preliminary design level calculations, thus

                                                                              ARCO FS Disclaimer 14

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the requirement for the +50% to -30% cost spread.  Actual costs will vary.  Costs were prepared on
a consistent basis to allow comparison of alternatives.
                                                                           ARCO FS Disclaimer 15

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COMMENT:
Stabilization costs were not added to excavation material before going to a RCRA Subtitle C repository.

RESPONSE:
This is correct.  Stabilization will increase the cost of off-site disposal slightly and can likely be
completed within the cost range of the estimate.

COMMENT:
Transport (truck and rail) costs were compared in the sensitivity analysis by the EPA with the result of the rail being more
cost effective.  This seems questionable when considering such cost elements as land acquisition, railroad
design/construction, and coordination with existing rail lines.

RESPONSE:
Cost estimates for rail haul were based on costs provided to Mr. Jim Ford, Montana Department of
Health and Environmental Sciences for the Streamside Tailings  Operable Unit in a letter dated April
12,1995. Contingencies were added to each of the alternatives to realize differing site conditions
than anticipated.

COMMENT:
Capital costs were not calculated for pilot/field studies for pump and treat and solidification/stabilization alternatives.

RESPONSE:
This is correct.  Nevertheless, the cost estimates remain within the acceptable range and are
consistent among alternatives. Pilot/field studies  would not add significant costs to relevant
alternatives.

COMMENT:
Sludge disposal volumes prepared by the EPA do not appear to assume multiple treatment trains operating simultaneously.

RESPONSE:
Sludge disposal volumes were estimated based operation of similar treatment systems and a
groundwater treatment rate of 1,000 gallons per day. More detailed design considerations such as
the number of treatment trains is not appropriate for feasibility study level cost estimates.

COMMENT:
Recognition and Use of Current EPA Guidance
In summary, major revisions made by the EPA to the PS reflect a bias toward a removal-based alternative.  Most of EPA's
experience in the past has been with removal-based remedies, therefore, there is often a presumptive need for removal in
EPA's analysis. Experience by industry and agencies alike has shown significant problems associated with both removal and
pump and treat based remedies. This has been reflected nationally and has been recognized in recent changes in EPA
guidance. Major points and referenced guidance are indicated below:

o       Preference for treatment (NCP)

o       Source Control includes treatment (Luftig)

o       Phased approach (i.e. contingency ROD); Allows use of innovative technologies through testing (Luftig, NRC, TI
        Guidance)

o       Up front TI analysis and innovative technologies (Luftig, NRC)

o       Pump  and treat performance limited (Luftig, NRC, TI Guidance)

                                                                                  ARCO FS Disclaimer 16

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 RESPONSE:
 The feasibility was completed in accordance with EPA guidelines and policy.  There were treatment
 alternatives considered and a phased approach to the implementation of the remedial actions is part
 of the Proposed Plan and the selected remedy for the site.  Because removal of source area soil,
followed by innovative treatment was selected does not indicate a bias away from treatment
 technologies; it indicates that removal was the best alternative based on the nine criteria required to
 be considered.  EPA also notes that the cited NRC guidance is not EPA guidance, but is nevertheless
 lawful and was fully considered  in selecting the Rocker OU remedy.

 COMMENT:
 Appendices
 Appendix D/E—Geochemical and Hydrogeological Factors/Pump & Treat
 Both these Appendices give descriptions of potential limiting factors for a pump and treat of groundwater at the Rocker OU.
 Estimates for pore volumes required for pump and treat of arsenic (3 to 10 pore volumes) were obtained by EPA contractors
 through a review of organic contaminant literature.  ARCO prepared a treatability study (Chatham, 1995) to evaluate iron
 flooding as a potential remedial alternative.  From this study, pore volumes between 40 to 100 were estimated. These pore
 volume estimates are consistent with other available off site information on arsenic clean ups.  The treatability study and
 more technically defensible pore volume estimates were part of ARCO's Draft FS but were removed by the Agencies. This
 estimate of pore volumes seems more reasonable since it was based on Rocker source materials and soils/alluvium.
 In addition, ARCO U preparing a Technical Impracticability (TI) Memorandum, which will illustrate the inability of pump and treat lyitenu to achieve
 remediation goals at the Rocker Site.

 RESPONSE:
 The 3 to 10 pore  volumes that were used in the groundwater pump and treat calculations as the lower
 limit of the required volumes of water to be pulled through the system.  Text accompanying the pump
 and treat calculations clearly discuss the uncertainties of pore flushing and the effect on preferred
flow paths and short-circuiting.  EPA has reviewed ARCO's 77 Memorandum  and has responded
 specifically to that document.
References

1.      Chatham, William.  1995. Treatability Study: Iron Flood Method for In Situ Remediation of
        Arsenic at the Rocker Timber Framing and Treating Plant

2.      Groundwater Technology, Inc.  1995.  Case Study: In situ Iron Reduction Shows Results
        After Two Weeks

3.      Federal Register.  Volume 55, No.  46, March 8, 1990.  National Oil and Hazardous
        Substances Pollution Contingency Plan: Final Rule

4.      USEPA.  1995.  Steve Luftig Memorandum:  Consistent Implementation of the FY 1993
        Guidance on Technical Impracticability of Ground-Water Restoration at Superfund Sites

5.      USEPA.  1993.  Guidance for Evaluating the Technical Impracticability of GroundWater
        Restoration
                                                                               ARCO FS Disclaimer 17

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6.     National Resource Council.  1994. Alternatives for Groundwater Treatment
                                                                          ARCO FS Disclaimer 18

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                  COMMENTS OF THE ATLANTIC RICHFIELD COMPANY
              ON PROPOSED PLAN FOR THE ROCKER TIMBER FRAMING &
                     TREATING PLANT OPERABLE UNIT ("Rocker OU")
COMMENT:
ARCO strongly opposes the Proposed Remedy identified in the Proposed Plan (the "Proposed Remedy"). Among other things,
the Proposed Remedy: 1) ignores CERCLA and EPA directives encouraging the use of innovative technologies; 2) fails to
consider the propriety, availability and additional cost of offsite disposal; 3) arbitrarily imposes RCRA Subtitle C requirements
on the disposal of excavated materials; 4) may have serious adverse effects on the local shallow aquifer groundwater system;
5) identifies an action of level of 380 ppm for arsenic in soils which does not reflect risks posed by exposure to arsenic at the
Rocker OU and is inconsistent with action levels determined for similar sites in the UCFRB; 6) is not supported by the local
community; 7) unlawfully requires an alternative water supply system as a component of the remedy under CERCLA; 8) is not
cost effective and costs have not been accurately estimated; 9) arbitrarily and without authority under CERCLA proposes to use
an 18 micrograms per liter (ug/1) arsenic standard for groundwater when the federal, protective drinking water standard (MCL)
for arsenic is SO ug/1 and attainment of the 18 ug/1 is technically impracticable through remedial action; and 10) identifies a
contingency remedy which is contrary to EPA policy and directives. A decision to select the Proposed Remedy as the remedy
for the Rocker OU would be arbitrary and capricious, not in accordance with law, inconsistent with the National Contingency
Plan (the "NCP") and contrary to CERCLA.

RESPONSE:
ARCO raises several issues regarding the proposed remedy that are addressed briefly in the sequence they
were presented.  In some  circumstances, more detailed responses  follow, in reply to ARCO's more
detailed comments:
        1.      EPA appropriately rejected ARCO's innovative technology proposal, which was submitted
               by ARCO late in the RIF process without adequate testing and which was not effective
               or implementable as presented.  The EPA Final Remedy for the Rocker OU includes the
               technologies that ARCO  was considering to  be part of their proposal which they
               considered innovative.  Specifically, ARCO had proposed mixing of iron with arsenic
               contaminated  groundwater  that  would  allow  adsorption  of  arsenic  with  iron
               oxyhydroxides.   The  selected remedy includes this innovative treatment, except that, in
               addition to ARCO's proposal, complete mixing with groundwater in the source area is
               provided.  Iron treatment of arsenic contaminated soils is also considered innovative by
               EPA.  EPA believes  that with this selection, the preference for innovative technologies
               has been satisfied.

       2.      Offsite disposal is no longer a part of the Remedy.   The remedy contained in the ROD
               includes excavation, treatment and onsite disposal above the water table.

       3.      The Final remedy does not invoke RCRA subtitle C requirements.

       4.      The Final  remedy provides for the most  complete mixing of iron  with the arsenic
               contaminated groundwater possible.   This approach will provide the  greatest potential
              for arsenic removal from the  source area.  In addition, iron additions to contaminated
               soils should provide for  arsenic  removal  from  soil solutions  and placement  of
               contaminated soils out of the water table.   All of this  will provide for  significant
               reductions in arsenic concentrations in groundwater.  Should conditions develop that are
               not anticipated, the remedy also provides contingencies for containing the plume.

       5.      Action  levels for exposure to arsenic developed for this remedy are consistent  with

                                                                                 ARCO Comments 1

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            guidance that has been developed for the Clark Fork Superfund sites and other EPA risk
            Assessment guidance.  The form  of arsenic at the Rocker  Wood Treating Plant (six
            percent arsenic trioxide by weight, dissolved in a heated caustic solution) is believed to
            have a higher bioavailability than at other sites within the Upper Clark Fork River Basin,
            and ARCO provided no site specific data to refute this.

     6.      The remedy proposed by ARCO was not fully supported by the community. In comments
            provided during the public comment period,  the Rocker Water and Sewer District
            conveyed an  expectation  that the  arsenic groundwater treatment (considered  risky by
            EPA) would meet the drinking water standard in the groundwater within a 3-5 year time
            frame.  When ARCO clarified their position on their proposed remedy that extended the
            time frame to 7+ years for implementation, with no intent of meeting ARARs in the area
            of the plume,  and that its remedy was plume containment rather than cleanup, community
            support for ARCO's plan, as expressed in verbal conversations from community leaders
            to EPA, diminished significantly.

            When EPA visited with the major stake holders involved with the Rocker OU, following
            the public comment period,  the  community provided their support for  the remedy
            contained in this ROD.  The proposed remedy is consistent with EPA's position regarding
            iron flood technologies that was  conveyed  originally during the development of the
           feasibility study.

     7.      Comments follow that respond to the  issue of the legality of requiring an alternate water
            supply under  CERCLA.

     8.      The Rocker  remedy has been costed  using  standard engineering  cost estimating
           procedures and is considered accurate within the  +50 to -30% range required for a
           feasibility study.   The documentation for  the cost  estimate is available in EPA's
            administrative record.

     9.      CERCLA  section 121  provides  that for any hazardous  substance, pollutant, or
            contaminant that will remain on-site, remedial actions must satisfy any applicable or
            relevant and appropriate promulgated State standard, requirement, criterion or limitation
            under State environmental or facility siting law that is identified in a timefy manner and
            is more stringent than any federal requirement. *  The State's WQB- 7 standard for arsenic
            of 18 ug/l meets these criteria. In response to the legislative directive, the State standard
            was duly promulgated by  the Board of Environmental Quality on August 3, 1995. The
           forthcoming modification of the standard was timefy identified to EPA by letter on May
            23, 1995 and the State standard of 18 ug/l is  more stringent than any federal water
           quality standard such as the federal drinking water standard for arsenic of 50 ug/l.

           ARCO  incorrectly reads the  water quality statute as requiring site-specific risk-based
           levels. The Montana legislature clearly mandated that the board promulgate one standard
           for each contaminant to apply on a state-wide basis. The board has determined, after
           proper notice and comment, that 18  ug/l represented a 1 x 103 excess cancer  risk for
1 The NCP also contains this requirement at 40 CFR 300.400(g)(4).

                                                                          ARCO Comments 2

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                arsenic.2 As set forth above, it is that arsenic standard that is the State groundwater
                ARARfor arsenic.

                The  background concentrations for the site  are well below  the  18 ug/l for arsenic
                promulgated by the State, Site data from uncontaminated portions of all three aquifers
                identified within or near the OU indicate that arsenic concentrations are below 10 ug/l.

                EPA has responded to ARCO regarding their request for a Technical Impracticability
                waiver and has found (in part) that consideration of a waiver on the basis of technical
                impracticability is not appropriate at this site until a well documented effort to clean up
                the contamination present within the OU has been conducted.

        10.     The contingency remedy identified in the ROD to further contain the arsenic plume if it
                spreads in an unacceptable manner is sensible, consistent with EPA guidance, and lawful.

                In summary, the selected remedy is fully consistent with CERCLA and the NCP.

COMMENT:
By these comments, ARCO also formally presents the "ARCO/ Rocker Community Proposed Remedy" to EPA and MDEQ (the
"ARCO/Rocker Proposal"). The ARCO/Rocker Proposal has previously been presented to the community,  was favorably
received in public meetings, and principle components of the remedy were adopted and approved by resolution of the Country
Water and Sewer District of Rocker. The ARCO/ Rocker Proposal is fiilly protective of human health and the environment,
consistent with the NCP and better satisfies the statutory requirements for remedial action set forth in CERCLA.

RESPONSE:
EPA disagrees and demonstrates throughout the responsiveness summary that the  ARCO Rocker
Community proposed remedy is not protective regarding the field evaluation of arsenic source stabilization
using iron flood technologies.  Basically, ARCO's propose in-situ remedy would not result in uniform and
adequate delivery of iron to the contamination source and plume.  EPA has consistently  maintained this
position during the preparation of the feasibility study and the issues which  EPA raised questioning
ARCO's approach have not been addressed by ARCO to date. Those issues focus largely  on the inability
    2 Section 75-5-301 was amended to read:

        Consistent with the provisions of 80-15-201 and this chapter, the board shall:
        (2)(A.) formulate and adopt standards of water quality, giving consideration to the economics of waste
        treatment and prevention.

        (B)  Standards adopted by the Board must meet the following requirements:

        (I) For carcinogens, the water quality standard for protection of human health must be the value
        associated with an excess lifetime cancer risk level, assuming continuous lifetime exposure, not to exceed
        1 x 10"3 in the case of arsenic and 1 x 10~5 for other carcinogens. However, if a standard established at a
        risk level of 1 x 10"* for arsenic or 1 x 10"5 for other carcinogens violates the maximum contaminant
        level obtained from 40 CFR, Part 141, men the maximum contaminant level must be adopted as the
        standard for that carcinogen.

        Chapter 497, Section 5, 1995 Legislature, (emphasis added).
                                                                                    ARCO Comments 3

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 to force iron solutions into fine grained portions of the saturated zone  where high concentrations of
 arsenic will continue to release to the groundwater.  The extreme uncertainty of the technology, unproven
 in any field setting, the short time involved in the studies, and the uncertainty regarding the measure of
 success for the proposed field investigation, lead EPA to select a remedy that was more reliable and that
 could be implemented in a shorter time frame.   The selected remedy does incorporate some aspects of
 ARCO's proposal.

 COMMENT:
 The Proposed Remedy Does Not Use Alternative Technologies to the Maximum Extent Practicable.

 CERCLA and the NCP require that the selected remedy use alternative treatment technologies to the maximum extent
 Practicable. See 42 U.S.C. 9621(b)(l); NCP, 40 C.F.R. 300.430(f)(l)(ii)(E). The Proposed Remedy ignores this requirement
 through unnecessary reliance on source area removal prior to application of iron salt to the excavation zone. This procedure is
 necessary to replace naturally occurring iron that is removed during excavation. In contrast to the Proposed Remedy, the
 ARCO/Rocker Proposal clearly meets the NCP requirement by appropriate use of alternative, innovative iron flooding treatment.
 Though EPA understands the potential for the iron flood treatment to effectively control arsenic migration in groundwater, the
 Proposed Remedy nonetheless rejects this innovative approach in favor of source removal. The Proposed Remedy is therefore
 inconsistent with the NCP which further provides with respect to use of innovative technologies:

         EPA  expects to  consider using innovative technology when such technology offers  the  potential for
         comparable or superior treatment performance or implementability, fewer or lesser adverse impacts than
         other available approaches, or lower costs for similar levels of performance than demonstrated technologies.

 40 C.F.R. 300.430(a)(l)(iii)(E).   The ARCO/Rocker Proposal fulfills  EPA's expectation that use of innovative technologies
 be maximized. In contrast to the ARCO/Rocker Proposal, selection of the Proposed Plan as the remedial action for the Rocker
 OU would also be inconsistent with EPA's encouragement of the use of innovative technologies under recent Superfund
 administration reform initiatives.  See e.g., Superfund Administrative Improvements, Initiative 9  (February 13,  1995)
 (encouraging the use of innovative technologies and risk sharing between PRPs and EPA on innovative technologies.

 RESPONSE:
 As noted, the final remedy selected by EPA for the Rocker site has been modified substantially compared
 to the remedy denoted in  the Proposed  Plan and to which these comments are directed.  EPA, in
 considering the final remedy, has considered all written comments and oral  testimony, including that
from ARCO.  The selected remedy does utilize alternative treatment technologies (chemical fixation and
 solidification) of the removed source materials.  Further, the final remedy also utilizes natural and
 enhanced attenuation to enhance the arsenic attenuation rate in the plume itself, which is thefiirther use
 of this innovative treatment technology.

As to the application of CERCLA and the NCP to ARCO's proposed remedy, the provisions of CERCLA
 cited by ARCO do  encourage  EPA to  select innovative treatment technologies under appropriate
 conditions, but not in gtt conditions.  First, the NCP requires innovative  technologies to be developed
during and as part of the RI/FS evaluation (40 CFR 300.430(e)(3)(5).  The Preamble to the Final NCP
explains "[bjecause innovative technologies may not have been as thoroughly demonstrated, treatability
studies during the RI/FS may be necessary to provide information sufficient for an appropriate evaluation
of these technologies.   The goal of treatability studies is to establish through the use of good science and
engineering the probable effectiveness of innovative technologies" 55 FR 8714 (March 8,1990). Here,
the commentor and proponent of the rejected  innovative technology, ARCO, performed the RI/FS over
a five year period and had every opportunity to perform the  appropriate studies and analyses in a timefy
manner to demonstrate their technology.  The limited bench scale testing that  was done was conducted
very late in the RI/FS process (approximately one month before the Feasibility Study was to be released),
provided little opportunity for EPA to oversee the investigation (ARCO did not seek and did not get EPA

                                                                                   ARCO Comments 4

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 input or approval of the project), and had limited application to the field setting.  This did not allow
 sufficient time for good science and evaluation of the proposal.  Second, remedies, even innovative ones,
 are to be selected if they meet the nine criteria for selection of a remedy only.  One of those criteria is
 "long-term effectiveness and performance*.  40 CFR  300.430 (e)(a)(iii)(c).  The Preamble to the Final
 NCP again states that innovative technologies are appropriate where they can reliably, logically, and
feasibly be protective and attain ARARs, and that the burden of presenting adequate information on such
 technologies would be on the promoting PRP - here ARCO. 55 FR 8748 (Mar. 8,1990). After EPA was
 informed of ARCO's limited  study on the innovative technology,  it repeatedly stated that it had no
 information demonstrating that the delivery of iron salts into soil and groundwater without excavation
 could be accomplished in a reliable and effective manner so that protectiveness and ARAR compliance
 could be achieved. ARCO's response, delivered to EPA after the proposed plan was released, involved
 a proposal for additional laboratory (column and field studies) which were projected to take an additional
 two years to conduct.  EPA  concluded that the technology of introducing iron into  the fine  grained
 shallow alluvial aquifer was not promising and that the time involved in the proposed investigations would
 simply mean a delay in implementing a remedy.

 EPA's recent Superfund Administrative Improvements, Initiative 9, does not change the need for adequate
 study and demonstration, and effectiveness for innovative technologies.  Rather, the selection of the
 remedy found  here,  which uses innovative technology in a reliable manner, is consistent with the
 Administrative Improvements initiative.

 EPA continues to believe that the remedy ARCO proposed, which is the addition of iron compounds to
 the groundwater system without excavation, would fail to work because of the technical difficulties in
 delivering the iron solutions into the fine textured portion of the aquifer.  EPA believes that ARCO has
failed to demonstrate how they would successfully address this concern.   EPA clearly recognizes the
 benefits of iron oxide attenuation as an innovative treatment technology but believes that the  introduction
 and complete mixing of these chemicals with the source materials and/or contaminated plume cannot be
 effectively implemented without excavation.

 COMMENT:
 The Proposed Remedy May Have Serious Adverse Effects on the Local Shallow Groundwater System.

 Unlike the ARCO/Rocker Proposal, the excavation component of the Proposed Remedy has the potential to adversely affect the
 local shallow groundwater system by increasing arsenic concentrations due to arsenic mobilization from soil to groundwater.
 Broad excavation of soils will also impair the naturally occurring attenuation capacity of the system.

By disrupting the  soils, arsenic that the natural system has already stabilized through adsorption could desorb  to the point of
causing significant increases in arsenic concentrations within the surrounding groundwater. The ARCO/Rocker Plan minimizes
this potential by first attempting to immobilize the arsenic through stabilization, or simply enhancing the existing adsorption
processes. If this primary remedy is not successful,  then the contingency allows for removal to be considered.

RESPONSE:
EPA recognizes that during the excavation of source materials from  below the water table, there will be
a short-term increase in arsenic  concentration in  the shallow groundwater system due to  the physical
disturbance. After removal of the source materials,  excess iron will be added to the exposed groundwater
so that  the arsenic concentrations will begin to attenuate on the iron oxyhydroxides as they form in the
system.   The excess iron will also begin to migrate down gradient below the excavated area and further
reduce arsenic  concentrations in the down gradient portion of the arsenic plume, and will retard arsenic
migration.  The  concentrations  in the plume itself will decrease as a result of enhanced attenuation


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 associated with the extra availability of iron.  EPA believes that this is effective management of possible
 short term effects.  EPA  recognizes that as iron  rich groundwater from the source area treatment
 penetrates the down  gradient undisturbed aquifer, iron precipitation  will gradually  coat the aquifer
 matrix, thereby decreasing aquifer permeability.  Therefore, the most significant reductions in arsenic
 groundwater concentrations should occur within the first several years following remedy implementation.

 As stated previously, the ARCOplan is not implementable because the iron cannot be reliably distributed
 throughout the affected fine grained portions of the aquifer.
 COMMENT:
 Source Removal is Not Necessary to Protect Human Health and the Environment and is Not Supported by the Community

 ARCO believes the results of the RI/FS for the Rocker OU demonstrate that source removal is not required to ensure protection
 of human health and the environment. In fact, as mentioned immediately above, the source removal advocated by EPA is likely
 to exacerbate conditions and might cause a threat to human health not posed by current conditions or by the ARCO/Rocker
 Proposal. The results of the treatability studies which have been completed examining the iron flood in-situ treatment technique
 are favorable and support selection of mis technology for remedial action. In-situ treatment immobilizes arsenic in the
 subsurface, greatly  enhancing the capacity of the native soils to adsorb arsenic and limit transport of arsenic through the
 groundwater system. Because no current water supplies are threatened by contamination, immobilization and natural attenuation
 of arsenic source material eliminates any need for an alternative water supply. If this approach were not successful, limited
 source removal to a local repository would be an available contingent remedy, again eliminating the need for provision of an
 alternative water supply system.

 RESPONSE:
 The source material is an ongoing, almost infinite supply of contamination to the groundwater system.
 EPA believes that without removal of the source materials,  there is virtually no hope in the next few
 hundred years of cleaning up the residual groundwater plume, which would continue to pose a risk to
 surrounding aquifers that are the preferred source of water for the community. As explained above, the
 ARCO proposed in-situ remedy remains flawed owing to delivery and implementability problems.  The
 EPA remedy utilizes the addition  of excess  iron to deal with short  term increases  in groundwater
 contamination due to disturbance of the source materials, and will enhance the removal of arsenic from
 the existing plume.  When the iron  is well mixed with the arsenic residuals in an oxidizing environment
 at the appropriate pH, there is no question that the iron will adsorb arsenic.  This is well born out by
 the ARCO treatability tests.  Unfortunately, ARCO's plan cannot distribute the iron effectively throughout
 the fine grained contaminated aquifer.  Also, the Town Pump well, which is in the tertiary aquifer, is
 hydraulicalfy connected to the arsenic plume at well RH-6, therefore groundwater supplies are threatened
 by the plume.  The  current remedy  will require a well ban during implementation of the remedy,  which
 in  turn, justifies the need for an alternative water supply.  Further justification of the alternate water
 supply is described in subsequent responses to comment.

 COMMENT:
 In discussions with the community regarding the technical basis for ARCO's position, ARCO was informed of the potential
 limitations of the current water system to provide adequate service to the community.  These infrastructure limitations are
 unrelated to the presence of hazardous substances in groundwater at the Rocker OU. In forming a consensus opinion with the
 community on an appropriate approach to remedial action for the Rocker OU, ARCO has volunteered to fund  certain
 infrastructure improvements to foster future economic development of the Rocker  community.

 RESPONSE:
EPA disagrees that the alternate  water  supply can be  characterized as a volunteer  infrastructure
 improvement.   Consistent with the preceding  response to  comment  and more detailed  subsequent

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 responses, EPA maintains that an alternate water supply is justified based on the ongoing threat to
 aquifers hydraulicalfy connected to the arsenic plume.  There has been a demonstrated, although limited,
 excursion of the arsenic plume into the deep alluvial and tertiary sediments aquifers.   Development is
 occurring at an increased pace in the Rocker area.  Future development of deeper aquifers could alter
 hydraulic relationships that would accelerate the migration of the arsenic plume into these high quality
 aquifers. An alternate water supply therefore is an important pan of this remedy to offset further reliance
 on groundwater resources, until  the remedy is determined to be successful and the contamination is
 cleaned up, a process  that could take many years.

 COMMENT:
 At the public meeting held July 27, 1995, testimony presented by residents of Rocker and the surrounding area overwhelmingly
 rejected the approach to remedial action recommended by EPA and the State under the Proposed Plan. A description of the
 ARCO/Rocker Proposal supported by ARCO and the local community, as well as the additional infrastructure improvements
 offered by ARCO, are discussed in Section III of these comments.

 RESPONSE:
 The community did not reject the approach to remedial action recommended by EPA and the State in the
proposed plan.  It appears that the  community used their  negotiation  skills to obtain  additional
 concessions from ARCO in exchange for their support ofARCO's innovative technology investigations.
 The community kept the EPA/State preferred remedy as a contingency in the event thatARCO's treatments
 were unsuccessful. As explained above, EPA has determined that ARCO's proposed plan is not effective
 or implementable, but has modified the proposed plan to include some aspects of ARCO's proposals which
 are workable.  This approach is supported by the community.

EPA was disillusioned with the ARCO proposal when ARCO stated that the research time frame necessary
 to evaluate the  technology (clarified to be a seven-year period) instead of the time period for cleanup (3
to 5 years) established by the community, and when ARCO stated its lack of intent to clean  up  the
 contamination in the shallow alluvium at all, in contrast to community expectations. ARCO's success
 criteria for their proposed remedy was whether the plume migrated or not during the term of their
investigations, realizing that the evaluation of plume migration would occur during a period when a well
ban would be in place that would eliminate additional pressure on the adjoining aquifers that would
influence plume migration.

 When EPA visited with the major stake holders involved in the Rocker remedy after the close of public
comment and conveyed their continued concern over the inability of the ARCO proposal to effectively
contain arsenic release from the fine grained materials  within the shallow alluvial aquifer,  there was
apparent support from the affected communities for EPA's revised remedy contained in this ROD.

COMMENT:
Health Risks as Identified in the Proposed Remedy are Overstated and Action Levels Inconsistent with EPA Decisions at Other
UCFRB Sites.

By requiring remedial action for soils containing more than 380 ppm arsenic, EPA's Proposed Remedy overstates the health risks
posed by exposure to arsenic in soils under a reasonable maximum exposure scenario. Furthermore, an action level of 380 ppm
arsenic for the Rocker OU is  inconsistent with remedial action  requirements identified for other sites having  similar
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 characteristics and potential for human exposure within the Upper Clark Fork River Basin ("UCFRB").3 As discussed in the
 detailed comments which follow and in ARCO's comments upon the EPA Baseline Risk Assessment for the Rocker OU, ARCO
 believes that action levels used for soils should be consistent with the action levels determined to be appropriate by EPA for
 other UCFRB sites. Furthermore, the Proposed Plan has inadequately justified EPA's selection of the remedy which would
 require soil removal versus placement of a soil cover on hot spots with appropriate revegetation. Soil cover and revegetation
 of hot spots, in combination with the institutional controls which EPA is recommending, is equally protective when compared
 to removal, and poses more limited potential short term risks during implementation.

 RESPONSE:
 See EPA responses to ARCO comments on the Baseline Human Health Risk Assessment.  The Rocker site
 is different from the other sites in the area contaminated with mining wastes in that wood treating fluids
 were used at Rocker that involved dissolving  arsenic trioxide in a heated solution together with other
 caustic  compounds.    The  Clark Fork Position Paper on Bioavailability of arsenic states  that   an
 assumption of 100% bioavailability will be applied at sites where arsenic contamination is associated with
 the application of pesticides/herbicides, wood treatment processes and/or fossil fuel combustion.  Unlike
 many other UCFRB sites, ARCO has not provided site specific information to EPA for the Rocker OU that
 indicates that the arsenic compounds present are any less bioavailable than the assumptions used in the
 Baseline Human Health  Risk Assessment.   Given these assumptions, EPA's remedy that addresses soil
 contamination greater than  380 ppm arsenic is justified.

 The EPA remedy also employs removal of arsenic contaminated soils greater than 1000 ppm (which can
 also be considered source materials) and combining and treating them with the other excavated soils form
 the source area.

 COMMENT:
 No Legal Authority Exists Under CERCLA for Requiring Installation of an Alternate Water Supply Under the Circumstances
 of the Rocker Operable Unit

 Six of the seven proposed remedial alternatives evaluated in the Proposed Plan,  with me exception of the "No Action"
 Alternative, identify the installation of an expanded capacity alternate water supply as a necessary component of the selected
 remedy (pg. 2, Proposed Plan).  While ARCO is willing to voluntarily provide an alternate water supply  as  part of the
 ARCO/Rocker Proposal, EPA is  without authority under CERCLA to require an alternative water supply as a component of
 the  CERCLA remedy. A decision to include an alternate water supply at the Rocker Site as part of me selected remedy is
 inconsistent with  CERCLA, the  NCP and the EPA "Guidance Document for  Providing Alternate Water Supplies," EPA
 540/G-87/006, OSWER Directive 9355.3-03, February 1988 (the "Guidance Document"). The Guidance Document addresses
 the process and criteria for selection of alternate water supply remedies consistent with CERCLA and the NCP. These criteria
 are  not met at the Rocker OU.

 RESPONSE:
EPA's response to this comment is provided below (see pages 26-29 of this response).

 COMMENT:
The Proposed Remedy is Not  a Cost Effective Alternative.

Section 121 of CERCLA mandates the selection of cost effective remedies. A  remedy is "cost effective" if its "costs are
proportional to its overall effectiveness." NCP, 40 C.F.R. 300.430(f) (l)(ii)(D). The construction, excavation, transportation,
and disposal costs of the Proposed Remedy are excessive in proportion of its effectiveness. In ARCO's opinion, the Proposed
          3See e.g., Old Works Record of Decision  identifying an action level of 1,000
ppm for	.


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 Plan substantially underestimates the cost of the Proposed Remedy. The ARCO/Rocker Proposal, in comparison: (1) meets
 CERCLA's requirements of protection of public health and the environment and attainment of applicable or relevant and
 appropriate requirements ("ARARs"); (2) out performs the Proposed Remedy under the NCP's "balancing criteria" of reduction
 of toxicity, mobility or volume through treatment, implementability, short term effectiveness and cost; and (3) satisfies
 CERCLA's and the NCP's requirements for selection of cost effective remedies.

 Under the ARCO/Rocker Proposal, limited source removal is an available contingency remedy if the iron flood technology is
 not implemented successfully. Disposal of excavated materials in a local repository is far more cost effective than transport to
 Smelter Hill for disposal.


 RESPONSE:
 EPA'sfinal remedy is cost effective.  Consideration was given to ARCO's and other's comments regarding
 costs and the need to classify the excavated waste as hazardous substances and transport to a suitable
 RCRA  C facility at Smelter Hill was removed from the selected remedy at considerable financial savings
 compared to the Proposed Plan costs.  The remedy  considers excavation,  chemical fixation,and onsite
 disposal for the source materials. As was stated earlier, ARCO's proposed remedy was not considered
protective or  effective  due to the difficulties of implementability.   Further, as proposed, it did not
 recognize the significance of the arsenic source to being a never ending  source of contamination to the
 shallow alluvial aquifer. Therefore, to compare ARCO 's remedy 10 either the earlier EPA Proposed Plan
 or to the current final remedy is not appropriate since the ARCO remedy is not protective.

 In summary, EPA considered cost effectiveness carefully in selecting the remedy described in this ROD.
 It reduced costs where  sensible and determined that  the expected costs are proportional to the benefits
 of actual source and plume cleanup and protection of surrounding aquifers and potential industrial site
 users.

 COMMENT:
 The 18  d) State  Ground-water Standard for Arsenic Should Not be an ARAR. Attainment of mis Concentration Level is
 Technically Impracticable from an Engineering Perspective and is Unnecessary to Protect Public Health.

The 18 d) groundwater arsenic concentration level does not meet the legal requirements for applicable or relevant and appropriate
requirements ("ARARs") under section 121 of CERCLA or the NCP. The current MCL for public drinking water systems across
the country and in Montana is SO ug/1. It is absurd to require cleanup of groundwater not utilized for water supply to levels that
are more stringent than EPA and the State have determined are appropriate to serve as protective standards for drinking water.
Given that background concentrations of arsenic in groundwater in the Rocker area may occur as high as 30 ug/1 there is no
justification for EPA and the State to require treatment of groundwater to a condition better than existed under natural conditions.
For these reasons and in light of technical information developed during the FS which demonstrates that attainment of 18 d is
and will be technically impracticable from an engineering perspective using pump and treat technology, the 18 ug/1 State standard
should be waived pursuant to Section 121(d) of CERCLA.

RESPONSE:
 The response to this comment was provided in the previous introductory comments/responses.  The 18 ug/l
standard is a legally mandated standard and is above background. There is nothing absurd in requiring
this remedy to comply with legitimate state environmental laws.

COMMENT:
The Proposed Remedy Fails to Meet the NCP's Requirement of  Implementability.

Implementability involves the "ease or difficulty of implementing alternatives." NCP, 40 C.F.R.  300.430(e)(9)(iii)(F).  As
discussed above, the Proposed Plan identifies 18 ug/1 arsenic to define the source area subject to excavation. First, the Proposed
Plan fails to take into account the fact that local background concentrations of arsenic range up to 30 ug/1. Therefore, it is both

                                                                                      ARCO Comments 9

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 unreasonable and technically impracticable to impose 18 d as * performance standard for remedial action. Second, the Proposed
 Plan assumes the availability of a repository at Smelter Hill. While ARCO has agreed to provide a repository on its property
 on Smelter Hill in certain instances (such as for Flue Dust or Arbiter/Beryllium wastes), EPA does not have the authority to
 require as a component of the Proposed Remedy the placement of a "Subtitle C" or other repository on Smelter Hill.

 RESPONSE:
 EPA recognizes that previous studies associated with the Rocker OU showed arsenic concentrations in
 shallow groundwater can range up to 30 ug/L.  However, the  most recent testing of all three aquifers
 identified during the Rocker RI showed arsenic concentrations  less than 8 parts per billion.  The issue
 of background concentrations of arsenic was not a specific aspect of studies conducted during the RI;
 however,  it is EPA's policy to not set remedial action goals that are below natural background
 concentrations. Based on the most recent information available, it appears that natural background is
 below the State standard of 18 parts per billion arsenic.

 While the State standard remains the standard for all aquifers  within the Rocker OU, the feasibility of
 attaining this standard in all aquifers cannot be fully determined until the remedy has been implemented
 and monitoring is conducted over an  extended period of time.  As trends  in  groundwater quality
 improvement are developed, a determination can be made whether groundwater standard in the shallow
 aquifer can be met.  It  is clearly recognized that there was significant technical difficulty in attaining
 ARARS over an extended period of time for any groundwater treatment technology  as noted in the FS
 which was why EPA considered contingency actions for the groundwater cleanup following monitoring
for an extended period of time.

 EPA, with the final remedy, does not require the construction of a repository at Smelter Hill for technical,
 cost, and community acceptance reasons.  It is unclear why ARCO believes that EPA does not have the
 authority to require this in appropriate circumstances.

 COMMENT:
 The Proposed Plan Identifies a Contingent Remedy which is Contrary to EPA Policy and Guidance.

 Under limited circumstances described in EPA guidance, a contingency remedy may be identified as part of a remedial action.
 In short, a contingency remedy may be identified where an innovative technology is selected for remedial action. EPA has not
 followed that approach in the Proposed Plan. In contrast to the Proposed Plan, the identification of a contingent remedy (limited
 removal to a local repository) as part of the ARCO/Rocker Proposal is consistent with EPA guidance.

 RESPONSE:
 The  contingent remedy described in the Proposed Plan has been altered to better reflect EPA's level of
 concern with respect to  the need for additional measures to be required to  contain the arsenic plume
 and/or to  meet remedial action goals within the current area of contamination.  This groundwater
 contamination problem and the technologies available to abate  the arsenic concentrations is considered
by EPA to have moderate uncertainty. Under these circumstances EPA guidance encourages the inclusion
of contingency measures should the remedy  not  meet performance  criteria.   EPA  considers  this
particularly important at the Rocker OU to prevent contamination of a  valuable groundwater resource.

COMMENT:
THE ARCO PROPOSED PLAN FOR REMEDIAL ACTION
On August 2nd, the County Water and Sewer District of Rocker ("CWSDR") held a special meeting to consider taking a position
on the proposed cleanup plan at the Rocker OU. The meeting was attended by local residents, property owners, and other
interested parties  including the EPA and ARCO. Proposed plans  by the EPA and  ARCO were discussed and evaluated.
Afterwards, ARCO and the CWSDR agreed on a common remedy that was passed  by the  Board and local residents. The


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ARCO/Rocker Proposal discussed below encompasses the concerns and specific objectives of the CWSDR.

RESPONSE:
Comment Noted - No Further Response Required

COMMENT:
The following discussion describes the problem, objectives, and ARCO/Rocker Proposal for groundwater and surface hot spots.

GROUNDWATER

Problem:                                •       Elevated concentrations in very shallow (20-40 ft.) groundwater

Objective of Remedy:                     •       Ensure human health is protected by not allowing arsenic from
                                                Rocker site to spread into drinking water aquifer
                                        •       Do not allow horizontal  movement to affect Silver Bow Creek
Proposed Remedy:
                                                 Innovative In-Sibi Stabilization
                                                 Natural Attenuation
                                                 Groundwater Monitoring
                                                 Limited Groundwater Ban
                                                 Contingency Excavation/Removal only if Innovative Technology is
                                                 Unsuccessful

 Remedy Description.  The groundwater area of concern would be defined by the arsenic concentration of > 50 ug/1 as
 presented in the FS. An arsenic level of 18 ug/1 would be very difficult to define and is below background levels of arsenic,
 according to Mike Bishop (Rocker EPA Project Manager).

 The arsenic source will be immobilized by in-situ treatment stabilization. This approach to the source reduces long term risk
 by not allowing groundwater arsenic concentrations to spread and by reducing current levels in the arsenic plume. Short term
 risk due to worker and community exposure is minimized through an in-situ approach. Since this process is an innovative
 technology, a demonstration is being performed to measure the effectiveness and implementability of the technology. This
 evaluation will consist of batch tests, column tests, and field demonstrations. Results from batch tests performed by Dr. Bill
 Chatham at Montana Tech demonstrate that the addition of iron salt is effective in immobilizing arsenic. Column tests and field
 demonstrations will help determine site-specific effectiveness and implementation methods.

 If, after a five year period, this innovative technology is deemed not to be protective, ARCO is willing to implement a
 contingency remedy where source materials will be removed to a secure repository. This repository would be located on or near
 the Rocker OU site if technically practicable.

 Furthermore, natural attenuation will continue to immobilize arsenic. The alluvial aquifer has an abundant supply of iron which
 is the primary element contributing to arsenic attenuation. Removing the naturally occurring iron through excavation (as required
 by the Proposed Plan) would likely upset the equilibrium of the natural system. Groundwater monitoring of the shallow and deep
 aquifer will occur during and after remediation to monitor conditions of the plume.

 RESPONSE:
 EPA agrees  that reaching  18 ug/L arsenic in the shallow groundwater will be difficult.  As mentioned
 earlier, EPA believes ARCO's proposed remedy is not implementable.  There is no  detail or appropriate
 backup studies presented by ARCO that will assure that the iron additions will be placed in a manner to
permit reaction with the contaminant source materials or the plume itself.  The test work conducted at
 Montana Tech by Dr.  Chatham utilized excavated soils and pore waters intimately mixed with the added
 iron compound in a physically mixed system.  EPA, with its final remedy, utilizes intimate mixing of iron
 compounds and solidification agents to bind up the arsenic and reduce permeability of the treated mixture
 so  that arsenic cannot be  released in the future.   The remedy also  utilizes  natural and  enhanced
 attenuation  to accelerate the rate of plume cleanup once the source materials  have  been removed.

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 Previous responses addressed the issue of background in relation to the 18ppb State standard and the
 issue of exacerbation from excavation.

 COMMENT:
 SURFACE SOIL HOT SPOTS

 Problem:                               •       Surface Soil "Hot Spots' are potential human health concern due. to
                                              direct contact with soil

 Objective of Remedy:                    •       Limit potential for human exposure which may cause health concern

 Proposed Remedy:                       •       Soil Cover
                                       •       Revegetation
                                       •       Institutional Controls

 Remedy Description. Surface soils with arsenic concentrations above 1000 ug/1 would define "Hot Spots." This concentration
 is protective and consistent with other UCFRB Superftmd sites in Anaconda. Soil cover areas would be revegetated. Institutional
 controls will include private land ownership with restrictions due to the active rail lines and the 100 year floodplain. In addition,
 land use will be primarily industrial with limited recreational use. Current floodplain regulations will also apply.

 RESPONSE:
 As mentioned earlier, EPA believes that the risks to workers and trespassers are significant enough to
 warrant removal of hot spots in excess oflOOOppm arsenic and subsequent treatment with other source
 materials. It also believes 380ppm arsenic in surface soils is an appropriate standard.  This is consistent
 with general risk assessment guidance and with the Clark Fork Guidance.  The basis for this is fully
 explained in the Baseline Human Health Risk Assessment, and EPA's response to ARCO comments on
 the Baseline Human Health Risk Assessment.  The contaminated materials will be treated and solidified
 (as necessary to pass TCLP testing during treatment/disposal) with the source materials and disposed
 ofon-site.

 COMMENT:
 VOLUNTARY INFRASTRUCTURE IMPROVEMENTS

 Infrastructure improvements are summarized below followed:

        •      Alternative Water Supply

        •      Allow for the drilling of Community Well

        •      As a contingency ARCO will  compensate Rocker Community for use of BSB water compared with a
               community well.

 ARCO proposes to provide the community with an alternative water supply.4 The community is currently supplied water via
a six-inch line from the Butte Silver Bow ("BSB") Big Hole Water Treatment Plant. The community wants to be less dependent
on BSB water supply and construct their own supply well. The Rocker Water & Sewer District could drill a well as long as it
is not within a limited well ban extending a quarter mile from the  site. Use of existing wells for water supply would continue.
ARCO would increase the size of the current six-inch BSB line to twelve inches and provide a 300,000 gallon storage tank. Both
the  community groundwater supply and the new twelve-inch line would  be connected to the storage  tank. Groundwater
          4As discussed in more detail in these comments, EPA is not authorized to
require development of an alternative water supply under CERCLA.
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 monitoring wells would provide an early warning system for water quality of the deeper tertiary aquifer which provides the
 current water supply for several local wells and will be the source for the community well. The risk of arsenic movement into
 the deeper tertiary aquifer is remote. However, if arsenic contamination occurs, the enhanced BSB water system provided by
 ARCO would eliminate any potential risk to the Rocker community. As a contingency, ARCO will offset the yearly costs
 associated with using BSB water instead of a community well water if arsenic is detected and poses a threat to the tertiary
 aquifer.

 RESPONSE:
 EPA believes that once the bleeding arsenic rich source materials to the groundwater are removed and
 treated in conjunction with groundwater treatment in the  same area, the remaining groundwater plume
 will begin to attenuate at a much faster rate. Even with the accelerated natural and enhanced attenuation
 rates, it may still take many years to meet groundwater standards in the area of the current plume.  Given
 these facts, EPA believes it necessary to restrict additional development of the identified aquifers in the
 area within a 1/4-mile radius of the arsenic plume to prevent major changes in groundwater gradients
 that could exacerbate plume migration.  An alternative water supply for the residents of Rocker will be
 implemented to off-set future demands for potable water and further reduce the potential for migration
 of contaminants.  While EPA appreciates ARCO's willingness to provide an alternative water supply,
 there is disagreement whether this aspect of the remedy can be considered voluntary.  EPA also is pleased
 that ARCO understands and support the need of a well ban to temporarily prevent additional groundwater
 development.

 COMMENT:
 The Selection of the Proposed Remedy Would be Arbitrary and Capricious and Inconsistent with CERCLA and the NCP.

 1.      The Proposed Remedy Does  Not Satisfy the NCP Remedy  Evaluation and Selection Criteria  as  Well as the
 ARCO/Rocker Proposal.

 The Proposed Remedy presented in the Proposed Plan does not satisfy the remedy  evaluation and selection criteria set forth in
 the NCP.  Specifically, the NCP requires an assessment of remedial alternatives that identifies the key tradeoffs (relative
 advantages and disadvantages) among alternatives with respect to the criteria. See 40 C.F.R. 300.430(e)(9) and (f); Preamble
 to Final NCP, 55 Fed. Reg. 8719. Under such an evaluation, the ARCO/Rocker Proposal is superior to the Proposed Remedy
 identified in the Proposed Plan. Specific NCP criteria are discussed below.

 RESPONSE:
 EPA  has responded to the specific issues that ARCO raises with regard to the remedy selection criteria
 below. Furthermore, EPA feels that the Feasibility Study,  the Proposed Plan, and the ROD did evaluate
 the trade-offs between different technologies and alternatives before  the final remedy was selected.  The
 issue that EPA has consistently conveyed to ARCO with respect to their proposed remedy is the inability
 of the technology to deliver the iron solutions, that would enhance arsenic attenuation, to the fine grained
portion of the shallow alluvial aquifer.

 COMMENT:
 EPA's and MDEQ's Cursory Consideration of the NCP's Implementability  Criterion in Identifying the Proposed Remedy is
 Inadequate.

 Implementability assesses the technical and administrative feasibility of a remedy. NCP, 40 C.F.R.  300.430(e)(9)(iii)(F). The
 Proposed Remedy exhibits several significant implementability problems which  are glossed over in the Proposed Plan. First,
 EPA has not provided a defensible basis for delineating the extent of die source area defined in the Proposed Plan as the 10,000
 mg/1 concentration line in soil down to a depth of 5 feet below  the groundwater  surface. This definition of the "source" of
 arsenic which may impact groundwater can not be technically supported. It is likely that only a small percentage of the soil
 media within the source area defined by EPA truly contributes to the elevated arsenic concentrations found in groundwater at
 the Rocker OU. If EPA persists in its present approach to define a "source," the Record of Decision should, at a minimum,

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 recognize that the resulting area and depth is a conservative upper bound estimate which must be further refined during remedial
 design.

 RESPONSE:
 EPA and ARCO, working together during the development of the FS, mutually recognized that further
 source term material identification would be necessary during RD/RA should source materials be removed
 or treated. The 10,000 ug/L isoconcentration line was mutually accepted as the estimated boundary for
 purposes of the FS so that comparative calculations of quantities and costs could be made to provide
 comparative values for analysis as the guidance  requires.  This volume was limited by the data available
 from the RI, but the estimate  was considered within the accuracy range of +507-50% required for a
 feasibility study.  EPA can not agree that the current definition represents an upper bound.  This will be
 determined during the RD/RA phases of the Rocker  OU.

 COMMENT:
 In contrast, the ARCO/Rocker Proposal avoids the problem of defining a discrete source by utilizing the 50 ug/1 arsenic MCL
 to define the extent of the source area to be targeted for in-situ treatment. Thus, the ARCO/Rocker Proposal would immobilize
 arsenic within the "source" and "plume" by treating all affected areas with in-situ stabilization.

 RESPONSE:
 EPA has provided  comments as to  why ARCO's  proposed remedy is not protective,  effective, or
 implementable in response to Summary Comments earlier.

 COMMENT:
 Additionally, the Proposed Plan identifies several contingencies in the event that the Proposed Remedy fails to attain the State's
 18 ug/1 standard within a reasonable time frame. Among the contingencies listed in the Proposed Plan is the utilization of pump
 and treat technology to address arsenic contamination at the Rocker OU. ARCO believes that this contingency fails to satisfy
 the implementability criterion specified in the NCP and should not be included in the final remedy selected in the Rocker OU
 Record of Decision. In the Proposed Plan, EPA explicitly recognizes the limits of pump and treat systems to attain required
 ground-water standards. Specifically, the Proposed Plan states that:

        The US EPA Office of Solid  Waste and Emergency Response is aware of the difficulty of restoring some
        aquifers to health based plant criteria as a result of a study conducted to evaluate the effectiveness of
        groundwater extraction systems in achieving specified goals.  The findings indicate groundwater extraction
        systems were generally effective in containing the contaminant plume and the systems guaranteed significant
        contaminate mass removal. However, although the contaminant concentration decreased significantly after
        initiation of extraction, it tended to level off at  concentrations above EPA's cleanup goals (i.e., MCLs).
        Therefore, EPA recommends identifying contingency measures for remedial actions that may not  meet
        chemical specific standards. This information is reflected in a remedy with contingencies proposed here. (p.
        6, Proposed Plan.)

 RESPONSE:
As mentioned previously, EPA guidance supports the inclusion of contingencies for groundwater remedies
 that have a moderate level of uncertainty associated with them.  In addition, EPA has moved away from
 the concept of a pump and treat contingency in  the final remedy, because: 1.) the Agency projects  that
 there will be a low potential need for a contingency remedy; and 2.) while a pump and treat component
to the remedy might speed the early stages of arsenic concentration declines, over the long term, pump
and treat would have a declining effect on arsenic concentrations as costs would continue to mount.

 COMMENT:
ARCO evaluated the viability of using pump and treat technologies at the Rocker OU in its Technical Impracticability Report,
submitted to EPA and MDHES on June 30, 1995. The Rocker TI Report demonstrated die limitations of pump and  treat
technologies due to specific site hydrogeology, arsenic release rates, and technological barriers. See ARCO's Technical

                                                                                   ARCO Comments 14

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 Impracticability Report (TI Report), June 1995.

 RESPONSE:
 In the FS comments, EPA clearly recognized the technical difficulties and limitations of pump and treat
 technologies  as applied to the Rocker  Site  and this position  is restated in the response provided
 previously.  EPA  is providing comments  regarding ARCO's  Technical Impracticability Report under
 separate cover.  However, EPA's  brief response is that ARCO's 77 demonstration did not provide a
 convincing case that a 77 waiver is appropriate at this time.  If after implementation of the remedy,
 monitoring data clearly demonstrate that it will be technically impracticable to achieve and maintain
 remediation  standards in the  arsenic plume in  the shallow alluvial aquifer, then  a  Technical
 Impracticability Waiver could be granted by the Agency for the smallest possible affected area.

 COMMENT:
 Dr. Bill Chatham of Montana Tech conducted a treatability study indicating that it would take arsenic 1000 to 50,000 years to
 release from the site. See TI Report, Appendix C. This slow release is a result of natural attenuation or the chemistry of existing
 iron hydroxides at the Site. Any pump and treat technology implemented at the site would depend upon arsenic release and not
 be effective in reducing arsenic levels in the groundwater to 18 ug/1, as specified by EPA. See TI Report, pp. 2-23 to 24; 2-28
 (finding that the presence of iron oxyhydroxides at the site inhibit the migration of arsenic by groundwater).

 RESPONSE:
 EPA agrees that arsenic attenuation to iron oxyhydroxides, given favorable oxidizing andpH conditions,
 is very stable.  EPA has also recognized in the FS that pumping and treatment of contaminated site
 groundwater  is not without its problems.   In the FS, EPA also acknowledges the potentially long  time
frames  required  with  any  technologies  to  meet  groundwater  ARARs;  although  with  source
 removal/treatment  this time frame is shortened considerably.   Data developed following  remedy
 implementation will allow a refined conclusion to be drawn regarding time frames required to meet the
 State 18 ppb  standard. However,  the remedy proposed is expected  to protect the two most valuable
 aquifers in  the area that are largely uncontaminated.

 COMMENT:
 The TI Report also found that a primary reason why pump and treat technology will  not achieve ARARs at the Rocker OU is
 the chemical nature of arsenic. Elements such as arsenic tend to strongly adsorb to soil. TI Report, p. 2-4; p. 2-28. This process
 decreases the rate at which the metals migrate. Id. In summary, the TI study found that it will take a minimum of 1,000 years
 to attain groundwater ARARs. TI Report, p. 2-33.

 RESPONSE:
 See previous comments.  EPA does not select pump and treat technology in this ROD.

 COMMENT:
 Dr. Chatham's study and the TI Report, however, do support the ARCO/Rocker Proposal for the Rocker OU, which would
utilize iron to immobilize arsenic in the site soils.

RESPONSE:
See earlier  comments concerning EPA's concerns regarding ARCO's proposed remedy.

COMMENT:
Finally, EPA cannot assume that a repository is available at Smelter Hill as part of the Rocker remedy. While ARCO has agreed
to build repositories on its property at Smelter Hill for flue dust and Arbiter/Beryllium waste materials in the past, EPA is
without authority to require a repository at Smelter Hill for Rocker waste materials. The ARCO/Rocker Proposal includes limited
source removal if the iron flood treatment  is, following an adequate period  of monitoring, not protective of human health by
immobilizing  arsenic in subsurface soils. If mis contingency would arise,  a local repository at or  near the Rocker OU is

                                                                                  ARCO Comments 15

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 supported by ARCO and the community, and would better satisfy the NCP implementability as well as the cost-effectiveness
 criteria.

 RESPONSE:
 The final  remedy does not require that an off-site repository be constructed.

 COMMENT:
 The Proposed Remedy Presents Serious Short-Term Effectiveness Problems Particularly with Respect to Local Groundwater
 System Impacts.

 Short-term effectiveness addresses the period of time necessary to complete a remedy and any adverse impacts on human health
 in the environment that may be posed during the construction and implementation period. NCP, 40 C.F.R. 300.430(e)(9)(iii)(e)
 The Proposed Remedy raises two short-term effectiveness concerns that are not raised by die ARCO/Rocker Proposal. First,
 the removal of source materials contemplated for the Proposed Remedy may increase arsenic groundwater concentrations by
 mobilizing arsenic from soils to groundwater during excavation. Further, excavation of source area materials including adjacent
 native soils, as defined in the Proposed Plan, will remove natural soils rich in iron and thereby significantly reduce the capacity
 of Site soils to adsorb arsenic at the Rocker OU. The capacity of Site soils  to attenuate arsenic mobility naturally is apparent
 from the fact that limited groundwater contamination is present more than 70 years after wood treating operations were initiated.
 Thus, the Proposed Plan approach to  remedial action will  substantially impair the ongoing natural attenuation of arsenic
 concentrations.

 RESPONSE:
 EPA has previously explained how, during and after excavation, the groundwater will be treated with
 additional iron which will further enhance the  rate of attenuation  of arsenic in the groundwater,  and
 otherwise address the issues raised here.

 COMMENT:
 Second, the Proposed Remedy creates greater short-term risk of exposure to workers during excavation and transport of materials
 from the Rocker OU to a RCRA Subtitle C repository on Smelter Hill. In particular, the Proposed Remedy contemplates several
 loading and unloading steps, i.e.,  excavation and loading to rail car,  transfer from rail car to truck, and unloading from truck
 to repository,  that increase risk of exposure and injury to workers. In addition, there are short term risks posed by exposure
 to arsenic bearing soils and groundwater during excavation, dewatering and backfill operations. In summary, the Agencies have
 not taken into account "the potential threat to human health and the environment associated with excavation, transportation and
 redisposal or containment" as required by section 121(b)(l)(G) of CERCLA.

 RESPONSE:
 Excavation of soils with subsequent  treatment and on-site disposal will require properly trained workers
familiar with conventional excavation and material handling equipment  operating practices.  This type
 of operation utilizes standard practices and, with proper caution, should pose no unusual added risk to
 workers, or to nearby residents.  In addition, ARCO's incomplete proposed remedy does not explain how
 their remedy reduces such risk compared to standard excavation because they do not explain how they
 intend to intimately incorporate the  added iron compounds in the source and affected plume areas.

 COMMENT:
 In contrast, the ARCO/Rocker Proposal creates none of these short-term effectiveness concerns. Furthermore, the ARCO/Rocker
 Proposal iron flooding component will enhance rather man eliminate,  the natural attenuation capacity of Site soils. Because the
 ARCO/Rocker Proposal contemplates an in-situ remedy, risks associated with excavation, transport and redisposal of materials
 in an off-site repository are eliminated.

 RESPONSE:
 See earlier responses.
                                                                                       ARCO Comments 16

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 COMMENT:
 The Cost of the Proposed Remedy is Underestimated in the Proposed Plan.

 Cost evaluates the estimated capital costs and operation and maintenance costs for 30 years. NCP, 40 C.F.R.  300.430(e)(9)
 (iii)(G). ARCO's estimate of EPA's Proposed Plan costs range between 8.5  and 10.5 million dollars. ARCO believes mat the
 costs estimated by EPA are inaccurate because the Proposed Plan does not include costs for stabilization of excavated soils or
 costs associated with contingency actions described in the Proposed Plan.3 ARCO estimates the present worth costs for the pump
 and treat contingency remedy range from $17.9 to $28.3 million dollars.6 In addition, the Proposed Plan costs for a RCRA
 Subtitle C repository are based on $6.00/cu. yd. Previous ARCO experience suggests a range between $30-$50/cu. yd. is more
 accurate is RCRA Subtitle C requirements are met. In contrast, the ARCO/Rocker Proposal would cost between 3.5-5 million
 dollars without contingencies and up to 15 million with the source removal contingency.


 RESPONSE:
 EPA did consider the cost of construction of a RCRA C facility  and the cost of treatment utilizing barium
 sulfate.   EPA  did not  cost out the pump and  treatment  contingency  because it  was in fact, just a
 contingency.  The final selected remedy  was developed based  upon the  public comment process and is
 considerably less costly due to the elimination of off-site  treatment and disposal.


 COMMENT:
 EPA and MDEO's Position on ARARi is Arbitrary and Capricious and Inconsistent with CERCLA and the NCP.

 The ARARs criterion assesses whether a remedy is capable of meeting ARARi identified for the operable unit. NCP 40 C.F.R.
 300.430(e)(9)(ui)(C). ARCO'i position on Rocker OU's ARARs issues has been set forth in several letters to EPA and MDEQ, including ARCO's May, 1993
 ARARs Scoping Document for the Rocker OU, ARCO's June 30,1995 letter to D. Henry Elsen regarding Rocker OU RCRA Subtitle C Issues, ARCO's March
 14, 1995 Rocker OU RI Report disclaimer letter, and ARCO'i June 26, 1995 Rocker OU FS Report disclaimer letter, all of which are incorporated herein by
 reference.

 In particular, ARCO believes that EPA and MDEQ have identified certain requirements which do not satisfy the statutory standards for ARARs specified in
 CERCLA and the NCP.

 EPA has identified die State of Montana standard of 18 ppb as an ARAR for arsenic in groundwater. This standard is drawn from Circular WQB-7 which was
 recently revised to include (his and other changes mandated by recent legislation.  Adoption of  the 18 ug/1 standard for arsenic was intended to comply with
 amendments to Section 75-5-301, Moot Code Ann. as part of Senate Bill 331 passed during the last legislative session.
 The revised statute requires, in pertinent part, that:

 (B)       Standards adopted by the Board must meet the following requirements:

 (1) For carcinogens, the water quality standard for protection of human health must be  the value associated with an excess lifetime cancer risk kvel, assuming
 continuous lifetime exposure, not to exceed 1 x 10' in Ac case of arseaic and 1 x Iff' for other  carcinogens. However, if a standard established at a risk level
 of 1 x 10' for arsenic or 1 x 10' for other carcinogens violates the mMJmum contaminant level obtained from 40 CFR, Part 141, ben the nmrmum contaminant
 level must be adopted as the standard for that carcinogen.

 To be an ARAR for this Site, EPA must demonstrate that 18 ug/1 represents a risk level no more stringent than 1 x 10'. In making mis demonstration, EPA
 must consider risk factors and assumptions which equate to a reasonable m«Timiim exposure scenario in light of the policy and directives provided in the recently
           5A number of requirements are prescribed in the Proposed Plan which were not
fully analyzed in the RI/FS, and their rationale for use, benefits, and costs are clearly
not understood by EPA.  This would include the  need for and cost of treatment to meet
"universal treatment standards" for excavated soils  and the cost and efficacy of  a grout
curtain as a hydraulic  control.


           "The pump and treat system includes pumping system, injection/Kix system,
chemical treatment system,  sludge drying beds, arid operation and  maintenance
associated with treatment.


                                                                                          ARCO Comments 17

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 published land me guidance.1 Given the location of the Site within the 100-year floodplain and EPA'i acknowledgment that current water luppliei are unaffected
 by Site cootaminanta, there ii no baiii upon which EPA may conclude that 1S ug/1 rcpreienta an appropriate risk-based standard for arsenic in groundwater within
 the Site.' Under the current condition and a reasonable maximum future exposure scenario, IS ug/1 must represent a risk level which is more stringent than
 1x10' because consumption of groundwater oolite will not occur and can be effectively precluded through implementation of appropriate institutional controls.
 Accordingly, State law requires that EPA adopt the MCL as the State standard for water quality at this Site.
 At best, EPA could justify use of an 18 ug/1 for off-site migration of arsenic where residential development may occur. However, the documented background
 concentration of arsenic for the Site area is 30 ug/1. Moreover, the TI Report concludes that remediation to the 18 ug/1 standard is not practicable. Thus, if EPA
 concludes mat 18 ug/1 is an ARAR for this Site, this standard is appropriately waived pursuant to Section 121(d)(4) on the basis that compliance is technically
 impracticable from an engineering perspective. In place of the 18 ug/1 standard, EPA should adopt the MCL or other, appropriate risk-based standard for arsenic
 to be met at a point of compliance established at the edge of the waste management unit Within the designated waste management unit, in-irai treatment using
 iron flooding to immobilize arsenic would occur.

 RESPONSE:
 Prior responses address several elements of this comment. Justification for the State standard of 18 ug/l
 is provided in response to ARCO's introductory comments to the proposed plan.  ARARsfor cleanup are
 determined independent of risk.  Background and risk issues are also addressed in response to previous
 comments.  The risks calculated by EPA  are based on  reasonable future exposure  where groundwater
 resources connected to  the arsenic plume could become contaminated, particularly with the projected
future demand for these  resources, and on the assumption that an aquifer classified as potential drinking
 water by the State could also be used.  In this light, EPA views the groundwater remedy as "pollution
prevention", which is an important theme to the Agency.  The point of compliance for the 18 ug/l State
 standard in groundwater applies throughout the affected aquifers.   The point of  compliance for this
 standard is addressed in Appendix 1  of the ROD.  As previously stated, EPA does not believe that a 77
 waiver is appropriate for the  site at this time, but may consider it for the shallow aquifer in the future.

 COMMENT:
 No basis is provided for requiring disposal of materials exceeding 1000 ug/l arsenic in a RCRA Subtitle C repository. RCRA
 subtitle C requirements clearly are not applicable to excavated waste materials exceeding 1000 ug/l arsenic unless such materials
 exceed 5.0 mg/1 as measured by the  toxicity characteristic leaching procedure (TCLP).9 Based upon the  results of the treatability
 studies which have been completed, it is highly unlikely that the S.O mg/1 standards would be exceeded for excavated materials
unless the materials  exceed 5000 ug/l arsenic. ARARs only  apply to onsite response actions; they do not apply to off-site
disposal. See 42 U.S.C. 121(d)(2).  Thus, the Agencies have no authority to determine that RCRA Subtitle C requirements are
 "relevant and appropriate" to an off-site disposal facility.

RESPONSE:
EPA identified RCRA Subtitle C as applicable, not relevant and appropriate, to the Rocker OU waste.
           7See Land Use in the CERCLA Remedy Selection Process, OSWER Directive
No. 9355.7-04.
           8A more stringent standard based upon protection of potential environmental
receptors can not be justified either in light of the fact that the results of the Rl did not
document an impact to the sediments or surface waters of Silver Bow Creek from  the
Site. (p. 10, Proposed Plan)

           9The Proposed Plan does not identify the requirements for management of any
listed hazardous waste as ARARs, and no basis exists for determining that excavated
materials would be listed hazardous wastes under 40 C.F.R. Part 261, Subpart D.

                                                                                     ARCO Comments 18

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 However, the remedy selected by EPA does not include disposal off site in a RCRA Subtitle C facility.
 The rationale for this change from the proposed plan has been provided previously.  However, EPA does
 consider materials containing high levels of arsenic contamination ft.e.,  > l.OOOppm, arsenic) to be a
 threat to human health, ground-water, and surface water owing to the near stream environment and the
 shallow groundwater.

 COMMENT:
 The Proposed Plan Disregards the Long-Term Effectiveness of Iron Flooding in Favor of Removal/Extraction Technology.

 The long-term effectiveness criterion assesses the ability of a remedy to maintain protection of human health and the environment
 overtime. NCP, 40C.F.R. 300.430(e)(9)fiii)(C). In the Proposed Plan, EPA and MDEQ recognize that the natural attenuation
 capacity of the Rocker OU has limited the lateral and vertical extent of arsenic migration at die Site. As noted above, the
 ARCO/Rocker Proposal contemplates in-situ treatment with iron salt mat will greatly enhance attenuation of arsenic in the natural
 soil media. The Proposed Remedy actually inhibits the effectiveness of natural attenuation through excavation of soils rich in
 iron along with source area materials for subsequent placement in a repository.

 RESPONSE:
 EPA has previously explained how the remedy in the proposed plan and in the final remedy properly
 consider the enhanced attenuation of arsenic both in the affected portions of the aquifer.  In addition,
 EPA has explained why ARCO's proposed remedy is inadequate and ineffective,  and has selected  a
 remedy which appropriately addresses all criteria, including long-term effectiveness.

 COMMENT:
 The ARCO/Rocker Proposal Provides Overall Protection of Human Health on the Environment.

 The overall protection of human health and the environment criterion assesses how a remedy, as a whole, provides and maintains
 protection of human health and the environment. This threshold criterion, which uses the evaluations from other criteria, must
 be met by a selected remedy. The ARCO/Rocker Proposal clearly meets this threshold criterion.

 The ARCO/Rocker Proposal will directly address and reduce human and environmental risks posed by arsenic at the Rocker
 OU through the application of the iron flooding in-situ treatment technology described above. Immobilization of arsenic within
 the shallow alluvium will protect die deeper tertiary aquifer used for water supply. Additionally, die institutional controls to be
 established for lands within and adjacent to die Rocker OU by ARCO in concert with the local government will diminatr.
 residential exposures and maintain appropriate land  uses necessary for maintenance and preservation of the remedy. These
 measures will result in die protection of both human health and the environment.

 RESPONSE:
 EPA has previously explained the shortcomings of the ARCO proposed remedy.  It is not implementable
 as described and therefore cannot be as effective in protection of human  health and the environment as
 the EPA Proposed Plan nor the final remedy.

 COMMENT:
 The Proposed Remedy Does Not Reduce the Toxicity. Mobility  and Volume of Rocker OU Materials  as Well  as the
 ARCO/Rocker Proposal.

 The reduction of toxicity, mobility and volume criterion assesses die degree to which a remedy reduces die toxicity, mobility
 and volume of contamination.  40 C.F.R.  300.430(e)(9) (iii)(D). Through liberal introduction of iron salts using flooding
 techniques, source materials, and groundwater in contact with source materials, will be treated and stabilized. In contrast to the
 ARCO/Rocker in-situ approach, excavation, transport and partial treatment of soils media as described in the Proposed Plan will
 be less effective in reducing die toxicity, mobility and, in  fact, will increase the volume of contaminated media. Moreover,
 draining of pore waters during  excavation of source materials may increase die area! extent of contamination at die site. This
possibility is explicitly recognized in die Proposed Plan. Accordingly, application of in-situ methods better satisfies this criterion
man the Proposed Remedy.

                                                                                       ARCO Comments 19

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 RESPONSE:
 See  the previous  responses.   The  EPA remedy clearly does reduce mobility of the arsenic through
 treatment, and will do so in a more effective manner than ARCO's proposed remedy.

 COMMENT:
 The Proposed Remedy Fails to Fulfill the NCP Remedy Selection Criteria.

 As noted above, the NCP requires that each remedial action selected must be cost effective, provided that it satisfies threshold
 criteria (protective of human health and the environment in compliance with ARARs). Cost effectiveness is determined by
 evaluating three balancing criteria: long-term effectiveness and permanence, reduction of toxicity, mobility, or volume through
 treatment, and short-term effectiveness. This evaluation allows for an assessment of the overall effectiveness of the Proposed
 Remedy. Overall effectiveness is then compared to cost to insure that a remedy is cost effective. Under the NCP, a remedy shall
 be cost effective if its costs are proportional to its overall effectiveness. 40 C.F.R.  300.430(f)(l)(ii)(D).

 ARCO believes that the Proposed Remedy provides significantly less overall effectiveness than the ARCO/Rocker Proposal. The
 ARCO/Rocker Proposal is superior in its ability to reduce the toxicity and mobility of arsenic present at the Rocker OU through
 treatment and because the short-term risks proposed by the ARCO/Rocker Proposal are far less than associated with the Proposed
 Plan. As noted above, the Proposed Remedy may potentially increase the arsenic concentrations in groundwater due to the
 mobilization of arsenic during excavation and will also impair the natural attenuation capacity present in soils at the Rocker OU.
 Moreover, the Proposed Remedy will create greater short-term risks to workers in local communities due to excavation, transport
 and placement activities associated  with the Proposed Remedy. Thus, ARCO believes that the overall effectiveness  of the
 Proposed Remedy is less man that of the ARCO/Rocker Proposal. Given the difference in effectiveness of the Proposed Remedy
 and the ARCO/Rocker Proposal and the fact that the Proposed Remedy is significantly more expensive than the ARCO/Rocker
 Proposal, the Proposed Remedy cannot be selected as the cost effective remedy.

 RESPONSE:
 In previous comments EPA has clearly responded to ARCO concerns regarding cost effectiveness, long-
 term effectiveness and permanence, reduction of toxicity, mobility and  volume through treatment, and
 short term effectiveness.  EPA's selected final remedy clearly meets the mandate of the NCP.

 COMMENT:
 Alternate Water Supply Issue.

 Six of the seven proposed remedial alternatives evaluated in the  Proposed Plan,  with the exception of the  "No  Action"
 Alternative, identify the installation  of an expanded capacity alternate water supply as a necessary component of the selected
 remedy (pg.  2, Proposed Plan). EPA's decision to include an alternate water supply at the Rocker Site as part of the selected
 remedy is inconsistent with the  EPA "Guidance Document for Providing Alternate Water Supplied," EPA 540/G-87/006,
 OSWER Directive 9355.3-03, February 1988 (the "Guidance Document"), and inconsistent with the NCP and CERCLA. The
 Guidance Document addresses the process and criteria for selection of alternate water supply remedies consistent with CERCLA
 and the NCP.

 EPA acknowledges in the Proposed Plan (pps. 10-11), that there are no individuals exposed to contaminated groundwater at the
 Rocker Site.  The Guidance Document expressly provides that, under CERCLA remedial authority, no action relative to the
provision of  an alternate water supply should be taken if it can be documented that exposures to pollutants, as a result of
 consuming contaminated water, does not present a threat to public health (pg. 3-11, Guidance Document). EPA acknowledges
in the Proposed Plan that the existing water supply adjacent to the Rocker Site is uncontaminated and does not present a threat
to human health. Thus, the proposed alternate water supply remedy is inconsistent with the Guidance document and is not
 supportable as a remedial action requirement.

The primary  basis stated in the Proposed Plan for the alternate water supply remedy is EPA's concern for "fiiture potential
residents who may use the groundwater (adjacent to the Site)  such mat an imminent and substantial  endangerment may be
present" (pg. 11 Proposed Plan).  The Guidance Document does state mat remedial actions may be taken in cases where mere
is a threat of future contamination, if MCL's or other ARARs are not currently exceeded. However, the Guidance document
clearly provides that, in such instances, EPA must first make a determination mat the water supply is actually threatened with


                                                                                         ARCO Comments 20

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 contamination before the final remedy addressing an alternate water supply can be implemented, (pg 3-6, Guidance Document).
 According to the Guidance Document, EPA is required to make this determination by calculating the rate of plume movement
 using a form of Darcy's Law and quantifying the "threat"" to the aquifer based on the measured site characteristics. As an
 alternative, the Guidance Document identifies several other methods which EPA could use to quantify the actual threat of future
 contamination to support an alternate water supply remedy. At the Rocker Site, EPA has  failed to use any of the prescribed
 methods of making this assessment to support its remedy that an alternate water supply remedy is appropriate.

 The Proposed Plan justifies selection of an alternate water supply for the Site as a means to achieve "Plume Control" (pps. 1214,
 Proposed Plan). EPA presumably believes there may be a connection between a theoretical future increase in demand on the
 deeper aquifer for water supply and modification of the present vertical flow gradient in the aquifer system. EPA's proposed
 use of an alternate water supply remedy as a  means to  achieve "plume control" is totally inconsistent with the Guidance
 Document and with the NCP. There is no mention in the Guidance Document or any other EPA guidance document that supports
 selection an alternate water supply as * remedy component to achieve "plume control."

 Additionally, the Guidance Document provides that in cases where pollutants are only detected in wells which are isolated from
 the water supply aquifer, an alternate water supply is likely not justified (pg. 3-11, Guidance Document). In the Proposed Plan
 for the Rocker Site, EPA does not assert that there is a quantified actual threat of contamination to the existing public water
 supply based on the existing uses and demands. Neither does EPA assert that the quantity of water in the existing water supply
 is inadequate to meet the community's current drinking water needs. The only current potential quantity shortage of the existing
 public water supply identified in the Proposed Plan is to address alleged "fire suppression"  issues. In the Proposed Plan, EPA
 indicates it intends to address this shortage as part of the remedial action through the proposed construction of a new water
 storage tank. However, the Guidance Document does not identify potential shortages in public water supplies for fire suppression
 needs as a valid primary basis for selection of an alternate water supply remedy under CERCLA. Thus, EPA does not have
justification for an alternate water supply remedy at  the Rocker Site which is  consistent with this remedy selection criteria
 specified in the Guidance Document or the NCP.

 The Guidance Document also provides mat the selection process for an alternate  water  supply remedy should include an
 assessment of the quantity of water which will have to be supplied to the affected area and a determination whether the available
 usable water supply can serve community needs  without the  alternate supply remedy (pg. 3-7, Guidance Document). The
 Guidance Document states that this analysis is "critical" to the determination whether an alternate water supply is necessary and
 therefore must be performed. There is no indication in the Proposed Plan that  EPA has conducted any such analysis to support
 inclusion of an alternate water supply as a component of remedial action at the Rocker Site.

 To assess the appropriateness of an alternate water supply remedy, the Guidance Document also states that the demand estimates
 should not include projections for future growth. The Guidance Document confirms that the decision to implement an alternate
 supply remedy is contingent upon a showing that the existing water supply is insufficient to meet the community's current water
needs and which does not take into consideration any future development (pg. 3-11, Guidance Document). Accordingly, EPA
is directed in its own guidance not to consider the possibility of such future development in determining the need for or size of
an alternate water supply remedy. The rationale for this position is that the CERCLA program is  not to be used to provide for
the projected expansion of a community public water supply (pg. 3-7, Guidance Document). The Guidance Document states that
the  CERCLA remedial program is designed and should be used solely to correct problems with  existing public water supply
systems.

EPA's Proposed Plan at die Rocker Site includes an alternate water supply remedy at the Rocker Site which is predicated entirely
on a projection of future growth and demand. Since it is not based on current needs  to address existing contamination, or
quantified threats of contamination to the deep aquifer based on the existing demands and uses, the proposed alternate supply
remedy is unjustified  and inconsistent with the EPA Guidance Document governing the  selection of such remedies.

Finally, according to the Guidance Document, if an alternate water supply is demonstrated to be necessary, the feasibility study
should also focus solely on the provision of that water supply and not on the complete mitigation of the contaminant source (pg.
3-11, Guidance Document). EPA's attempt to characterize the provision of an alternate water supply as a component of the
remedy for "Plume Control"  fails to reconcile this directive with the Proposed Plan requirements. Accepting for purposes of
argument that EPA may require construction of an alternative water supply as a component of a remedy for the Rocker OU,
further actions described in the Proposed Plan to mitigate arsenic contamination and comply  with chemical-specific ARARs are
not only unnecessary to protect human health, but clearly contrary with mis aspect of the EPA Guidance Document. In this way
as well, selection of the remedy described in  the Proposed Plan would be arbitrary and capricious, and contrary to the


                                                                                             ARCO Comments 21

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 requirements of CERCLA and the NCP.

 RESPONSE:
 An alternative water supply is an appropriate and necessary component of this remedy.  Section 104(a)
 of CERCLA, 42 U.S. C. § 9604(a), authorizes EPA to provide for remedial action relating to the release
 or threatened release of hazardous substances "which (EPA) deems necessary to protect the public health
 or welfare or the environment."  Section 101(24) of CERCLA, 42 U.S.C.  §  9604(24), defines remedial
 actions to include specifically  "provision of alternative water supplies. *  Section. 106 of CERCLA,  42
 U.S. C. § 9606, gives EPA the authority to order actions as may be necessary to protect public health and
 welfare and the environment.  EPA's determinations in these matters are to be given deference. Section
 113(j)(2) of CERCLA,  42 U.S.C. § 9613(j)(2).  The NCP defines remedial  action as actions taken to
 prevent or minimize the release of hazardous substances, including the provision of alternative water
 supplies. 40 CFR § 500.5. Although the NCP discusses provision of alternative water supplies under
 removal authority generally as requiring actual exposure to contamination, 40 CFR § 300.415(d) (9),  no
 such limitation is discussed when alternative water supplies are provided as part  of a remedial action,
 Appendix D to Part 300, subpartff).  Appendix D  also identifies restrictions on use of groundwater to
 eliminate the potential exposure to the contamination as appropriate (emphasis added).

 At the Rocker OU, EPA has identified the following situation: a highly contaminated shallow alluvial
 aquifer; two highly productive surrounding aquifers - the tertiary and the lower alluvial - which are in
 current use by residents of the community of Rocker and local businesses; a hydraulic connection among
 these aquifers demonstrating that contamination of the two aquifers has occurred and that the situation
 will likely lead to more contamination if additional development occurs; and imminent plans by the
 community and other to install groundwater wells near the Rocker site.  As ARCO itself has pointed out,
 this  situation may be aggravated by  the implementation of source removal, which could temporarily
 increase the mobility of arsenic.

 Faced with  this situation, EPA's remedy will do two things - first, it will prevent further use of the
 contaminated aquifer and the surrounding, connected aquifers so that exposure does not occur and the
 contamination does not spread, and second, actively cleanup the source and contaminated groundwaters
 as described.  The first part of the response involves establishing institutional controls to prevent use
 within a 1/4 mile area of the arsenic plume and implementation of an alternative water supply for future
 users of groundwater within the 1/4 mile area.  Preventing exposure and use in this manner is necessary
 to protect public health and prevent the spread of contamination, so that cleanup can proceed. This is
 entirely sensible, directly related to the release or threatened release of hazardous substances at the
 Rocker OU, and fully consistent with CERCLA and the NCP.

 The Guidance Document for Providing Alternative Water Supplies, OWSERDir. 9355.3-03 (EPA 1988),
 cited extensively by the commentor, is explicitly not applicable to the Rocker OU remedy.  The Rocker
 OU remedy is a Final Remedial Action to address the long term health threats at the site.  The Guidance
 is explicitly applicable onfy to non-time critical removal actions, or interim remedial actions dealing with
 short term threats using an abbreviated RI/FS process.  See guidance at Hi ("This document provides
guidance for those sites that do not require a time-critical removal action but do require provision of an
alternative water supply  as either a non-time critical removal action or  a remedial action before
implementation of a final remedy can be achieved...  Items...four (final Remedial Actions) are outside the
scope of this guidance. *).  Thus,  the limitations from the guidance  cited by the commentor relating to
actual exposure or predicted exposure to contaminants, and exclusion of future growth,  do not apply  to
this action.  In, addition, the Guidance does not establish "requirements" to  be rigidly followed, but  is

                                                                             ARCO Comments 22

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general guidance for use as appropriate by EPA.

The alternative water supply described in the ROD is tailored to the site conditions, the plans and
preferences of the community, and makes good sense at the Rocker OU to prevent the spread of
contamination.  It is fatty consistent with CERCLA and the NCP,  and is necessary and appropriate for
the successful remediation of the Rocker OU.

COMMENT:
The Proposed Remedy'* Soil Excavation Component ii Not Justified Under CERCLA.

The Propoted Plan require! toil* containing 1000 ug/1 or greater anenic to be removed to a depth of 18 inches. The removed toili are to be placed in a RCRA
Subtitle C repository in the Smelter Hill Area." EPA has further proposed that barium sulfate should be added to certain removed soils to stabilize me anenic.

RESPONSE:
The final remedy will require that the excavated soils from the  hot spots will be combined  with the
excavated soils from the source area and will then be subsequently treated and disposed ofon-site.  The
remedy also provides a  contingency for adding solidification technologies (concrete) for materials that
may fail the TCLP characteristic test after initial treatment with iron.  There is no need for a RCRA C
disposal site with the final remedy.

COMMENT:
According to EPA's most recently published regulatory agenda, the Hazardous Waste Identification Rule (HWIR): Contaminated
Media reproposal is  scheduled for publication in September 1995, and final action is now anticipated to occur in September
1996. 60 Fed. Reg. 23, 981 (May 8, 1995). As such, there is currently no requirement governing the treatment or disposal of
contaminated media. As discussed above, RCRA Subtitle C requirements should not be identified as ARARs for off-site actions.
RCRA Subtitle C would apply to offsite disposal only if the material to be disposed is a hazardous waste. Excavated materials
should not exceed TCLP regulatory limits for arsenic unless arsenic exceeds at least 5000 ug/1 in the materials.

RESPONSE:
See previous responses.

COMMENT:
The Proposed Plan, page 15, states that "Hazardous wastes exceeding EPA's universal treatment standards (estimated to be 160
yards) will be stabilized with a barium sulfate compound prior to disposal." The Proposed Plan does not identify what the
"universal treatment standard" is, whether it has been finally promulgated for contaminated media, or why such a standard would
be applicable or "relevant and appropriate" to contaminated media at the Rocker Site. ARCO is unaware of any such requirement
under RCRA that is  applicable.11 As previously discussed, ARARs do not apply to offsite actions. Thus, unless the universal
treatment standards  are applicable to disposal  of waste  materials  off-site (i.e. all of the jurisdictional prerequisites for
applicability are met for the underlying RCRA requirements), compliance with such standards as ARARs under CERCLA cannot
          10As noted elsewhere in these comments, ARCO believes the costs for the
Proposed Plan are underestimated. The Proposed Plan costs for a RCRA C repository
are based on  $6.00/cu. yd.  Costs based on previous ARCO experience and  other
sources suggest a range between $30-$50/cubic yard to be more accurate.


          11 If treatment is  necessary as the Proposed Plan suggests, these costs have
not been included in the EPA analysis. Cost can vary from  $50 to $175/per cubic yard
based  on previous experience and information provided  by  vendors.


                                                                                  ARCO Comments 23

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 be required.

 RESPONSE:
 No response is required because the selected remedy does not include stabilization to meet universal
 treatment standards.   However, materials will be treated to below 'characteristic' levels using EPA's
 TCLP procedure.

 COMMENT:
 Finally, excavating and disposing the arsenic-contaminated soils off-site is contrary to CERCLA's mandates. Section 121(b)
 specifically requires EPA to consider, inter alia, "the potential threat to human health and the environment associated with
 excavation, transportation, and redisposal, or containment." 42 U.S.C.  9621(b)(l)(G). Physically removing soils from their
 current location to the Smelter Hill area (or a local repository) greatly increases the likelihood of airborne dispersion of the
 arsenic and wood-treating wastes, increasing the risk of airborne ingestion by workers and members of the surrounding
 community.  Disturbing and removing  arsenic-containing soils also increases the  potential for expansion of groundwater
 contamination on site as arsenic in soils may form a solution during excavation of the source area. Excavating the source area
 may also increase groundwater concentrations of arsenic by  destroying ongoing natural attenuation produced by  the
 naturally-occurring concentrations of iron in the site soils.

 RESPONSE:
 Previous   responses   have  already  addressed  ARCO's  concerns   about  increased  groundwater
 concentrations.  The selected remedy does not include  disposal off-site making comments relative to this
 issue moot.

 COMMENT:
 The ARCO/Rocker Proposal — i.e., treating the arsenic-containing soils on-site with an iron solution — on the other hand, meets
 CERCLA's mandate that "[t]he President shall select a remedial action mat is protective of human health and the environment,
 that is cost effective,  and that utilizes permanent solutions and alternative treatment technologies or resource recovery
 technologies to the maximum extent practicable." 42 U.S.C. 9621 (b)(l).

 RESPONSE:
 EPA has previously explained the. many shortcomings of ARCO's proposed remedy.

 COMMENT:
 Health Risks as Identified in the Proposed Remedy are
 Overstated and Action Levels Inconsistent with EPA Decisions at  Other UCFRB Sites.

 The Proposed Remedy for the Rocker Timber Framing and Treating Plant Operable Unit includes removing soil containing more
 than 1,000 ppm arsenic and covering remaining soil containing more than 380 ppm arsenic (U.S. EPA 1995). The Proposed
 Plan indicates that the 380 ppm arsenic cleanup level corresponds to a 1  in 10,000 excess cancer risk for site workers, based
 on the evaluation in the U.S. Environmental Protection Agency's ("EPA's") baseline human health risk assessment for the site
 (CH2M Hill 1995). ARCO has commented previously on the baseline risk assessment ("BRA") (Stilwell 1995, pers. comm.).
 Several issues were identified during ARCO's review  of the BRA that may have contributed to substantial overestimation of
 site-related risks.

 RESPONSE:
 Comment Noted - EPA has responded to these issues previously and in response to ARCO's comments regarding the Human
Health Risk Assessment.

 COMMENT:
One issue was that the way in which site data were aggregated to calculate exposure point concentrations. In the risk assessment,
the primary approach used to evaluate risks associated with site soils is the calculation of exposures and risks  for three sets of
exposure point concentrations: soil concentrations in samples from the soil cover placed over much of the site during a removal
action completed in 1989 (referred to as "on soil cover" in the risk assessment), soil concentrations in samples from site areas
that were not covered during the removal action (referred to as "outside soil cover"), and soil concentrations in original site soils

                                                                                         ARCO Comments 24

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 that are currently beneath the soil cover (referred to as "without soil cover" to reflect the hypothetical risk assessment scenario
 that these soils might be exposed at the site surface at some point in the future). However, the risk estimates derived in the risk
 assessment for areas outside the soil cover or exposures that might occur without the soil cover overstate risks that are likely
 associated with either current or future exposures. This is because an individual who currently has occupational or recreational
 exposures at the Site would be unlikely to have exposures only to covered areas or only to uncovered areas of the Site. Instead,
 exposures are likely to occur across the Site, and the exposure point concentration reflecting such activity patterns would be an
 area-weighted average of the concentrations reported in  soils that  are currently at the Site surface. Because a much greater
 proportion of the  sampled Site area is  currently covered, concentrations that better reflect likely activity patterns would closer
 to the "on soil  cover" exposure point concentrations presented  in the risk assessment  than  to  the  "outside soil cover"
 concentrations. Risk estimates would also be correspondingly lower.

 RESPONSE:
 This  comment  was a portion of the ARCO comment on  EPA's Baseline Human Health Risk Assessment
 (HHRA) and the response to the question can be found  in EPA's response on the  HHRA.

 COMMENT:
 Another source of overestimation of site risks arises from the fact that no quantitative adjustments were made to reflect the
 reduced bioavailability of metals in soil and dust can significantly influence risk estimates.  EPA's toxicity values for arsenic
 ingestion are based on exposure to arsenic dissolved in water. Because absorption of metals in soil and dust is generally less than
 that of metals in water or food, risk assessment calculations should account for these differences by  applying a bioavailability
 adjustment factor to either the toxicity factor or to the intake estimate. Studies of arsenic absorption from Anaconda soil samples
 indicate that fractional absorption of soil arsenic is between one-fifth and one-half of the fraction of dissolved arsenic that is
 absorbed (Freeman et al. 1993; Freeman et al., in press). The arsenic in soil from Anaconda is derived primarily from smelter
 emissions and, therefore, is expected to differ in mineralogic form from the arsenic present in the soil at Rocker. Recent studies
 by PTI Environmental Services suggest, however, that even soluble arsenic  salts mixed with soil are associated with reduced
 bioavailability after a period of weathering. PTI has developed an in vitro bioaccessibiliry test that may be used to compare the
 dissolution rates of arsenic from various soil samples in the gastrointestinal tract. PTI has found that arsenic in soil from several
 arsenical pesticide formulation facilities exhibits dissolution rates as slow or slower than those of arsenic from Anaconda soil.
 These data suggest that arsenic that has been in the soils at Rocker for 40 years or more is likely to be less than SO percent as
 bioavailable as arsenic dissolved in water. This suggests that a cleanup level twice as high as the one selected by EPA would
 still be associated with less than a 1 in 10,000 cancer risk.

 RESPONSE:
 This comment has been addressed previously in the responses to the Summary Portion of this document
 and in  EPA's responses to ARCO's comments on the HHRA.

 COMMENT:
 Finally, ARCO noted that the risk aiieismcnt doei not include * discussion of the uncertaintiei that surround EPA'i itandard toxicity factors for ingcitcd anenic.
 Numerous lines of evidence suggest that toxicity factors currently used in risk assessments by EPA to evaluate the toxicity of ingested arsenic overestimate toxic
 effects, particularly at the relatively low levels associated with exposures in the United States. The carcinogenic slope factor ("CSF") and reference does ("RfD")
 for ingested arsenic were derived by EPA from an ecological epidemiologies! study of the incidence of skin cancer and blackfoot disease in a Taiwanese
 population with elevated levels of arsenic in their drinking water (Tseng et al. 1968; Tseng 1977). EPA's Science Advisory Board, in commenting on EPA's
 draft Drinking Water Criteria Document on Inorganic Arsenic (Loehr and Ray 1993; U.S. EPA 1993), urged that the agency conduct an in-house quantitative
 risk assessment for caners other than skin cancer that accounts for potential nonlinearities in the  dose-response curve and me high background anenic
 concentrations in the Taiwanese populations compared to United States populations. New epidemiological analyses of the Taiwanese populations and new data
 on the dietary sources of arsenic in these populations provide further evidence that the present CSF a likely to overestimate risks for United States populations.
 Thus, actual risks associated with EPA's selected cleanup level are likely to be substantially leas than 1 in 10,000.

RESPONSE:
 These comments  were noted in ARCO's comments to the HHRA and have been  responded to in EPA's
 responses.

 COMMENT:
Finally, the identification of a 380 ppm action level for arsenic for the Rocker OU can not be reconciled with action levels determined by EPA to be protective
for the Old Works/East Anaconda Development  Area OU ("OW/EADA OU"). For the OW/EADA OU, engineered coven are required where arsenic levels
of 1000 ppm arc found. This requirement was deemed appropriate for recreational and potential commercial/industrial areas of the OW/EADA OU. Given the

                                                                                                 ARCO Comments 25

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potential similar land u»c iccoarioi for the Rocker OU, the lame requirement! would be dcfeniible. Certainly, the Proposed Plan provide! no baiii upon which
to conclude that more contervative remedial action requirement* are appropriate for the Rocker OU.

RESPONSE:
See previous responses regarding these same issues.

COMMENT:
The Identification of a Contingency Remedy in the Proposed Plan if not Consistent with EPA Guidance

The Proposed Plan improperly includes a contingency remedy under which EPA would require installation of a pump and treat system in five years if excavation
and removal of source materials fails to adequately reduce arsenic contamination in the groundwater. EPA guidance expressly provides that there are limited
situations in which contingency remedies should be employed, and  suggest! only two such situation!. Guidance on Preparing Superfund Decision Document!:
The Proposed Plan, The Record of Decision, Explanation of Significant Difference*, The Record of Decision Amendment, Interim Final, OSWER Directive
No. 9353.3-02, EPA/540/089/007, July 1989, p.9-13.

RESPONSE:
As responded previously, EPA guidance recommends  that contingencies be developed for groundwater
remedies that have a moderate level of uncertainty associated with them. In addition, the pump and treat
aspect of the proposed plan has been deleted from this remedy.  The contingencies provided in this remedy
address containment of the arsenic plume to prevent contamination of valuable aquifers in contact with
the contaminated zone.

COMMENT:
First, contingency remedies were developed to promote the use of innovative treatment technologies: "An innovative treatment
technology may appear to be the most appropriate remedy for a  site or operable unit during the RI/FS, but more testing is
needed during remedial design to verify the technology's expected performance potential." Id. This situation contemplates trying
an innovative technology, and providing a proven technology as a contingent back-up remedy. Instead, EPA has proposed a
traditional remedy for the Rocker OU, excavation and removal, with another traditional remedy identified  as a  contingency
remedy. In contrast to the Proposed Plan, the ARCO/Rocker Proposal employs an innovative remedy which, if selected by EPA,
would allow inclusion of a contingency remedy. However, as described elsewhere in these comments, the TI Report has
determined mat a pump and treat remedy would not be successful given the hydrogeologic conditions present at the Rocker
OU.12 Thus, the proposed contingency remedy under the ARCO/Rocker Proposal which EPA should select is limited source
removal to a local repository.

RESPONSE:
The EPA final remedy does not specifically include pump and treat technology, as a contingency. In the
EPA  comment  to ARCO's FS,  EPA  recognized the  technical  limitations  of pump  and  treatment
technologies as they relate to this site.

COMMENT:
The second situation in which a contingency remedy may be identified is where two different technologies under consideration
appear to offer comparable performance. If this is the case, a proposed plan may identify one of the two as the selected remedy
and the other as a contingency remedy. Because of the demonstrated limitation of a pump and treat remedy for this Site, the
remedy described in the Proposed Plan (which itself is flawed in many ways) is not comparable in terms of expected performance
           12To the extent EPA may believe that further testing may be necessary to
determine the potential effectiveness of  a  pump and treat system, ARCO's
Technological Impracticability Report thoroughly examined the technological
practicability of using a pump and treat system to attain EPA's groundwater
remediation goals  and  determined that such a system would be required to operate for
a minimum of 1000 years to achieve those goals.


                                                                                      ARCO Comments 26

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 with the contingency remedy EPA has identified. Id. Thus, neither of the situations described in applicable EPA Guidance
 Documents apply or support selection of the contingency remedy identified in the Proposed Plan.

 RESPONSE:
 EPA disagrees with this comment and has responded to this issue in previous responses

 COMMENT:
 Prior Comments.

 The following documents have been previously submitted to EPA and MDEQ and supplement these comments. These documents
 are part of the Rocker OU Administrative Record and are incorporated herein by reference:

 •       ARCO's "Institutional Controls Planning Document for the Rocker Timber Framing & Treating Plant Operable Unit".

 RESPONSE:
 EPA appreciates ARCO's work in this area and believes that certain proposals in this document should
 be used as a supplement to the remedy, as described in the ROD.

 COMMENT:
 ARCO's "Rocker Timber Framing & Treating Plant Operable Unit Remedial Investigation/ Feasibility Study Scoping Document;
 Applicable or Relevant and Appropriate Requirements under 121(d) of CERCLA (ARARs)," May, 1993.

 RESPONSE:
 The issues raised in this document  are addressed in Appendix 1 or in ARCO's specific  comments on
ARARs in this  document.

COMMENT:
Letter from Charles T. Stilwell to  Mr. Michael  Bishop submitting attached statement of disclaimer to  Final  Remedial
Investigation Report, March 14, 1995.

RESPONSE:
See the fall EPA response to ARCO's RI disclaimer.

COMMENT:
ARCO's "Rocker Timber Framing & Treating Plant Operable Unit Risk Assessment Scoping Document," March, 1992.

RESPONSE:
EPA's Baseline Human Health Risk Assessment and its response to ARCO's comments on this document
respond to the  issues presented in this document.

COMMENT:
Letter from Charles T. Stilwell to Mr. Michael Bishop referencing ARCO clarification of issues and prior correspondence
submitted by ARCO, April 21, 1995.

RESPONSE:
Responsive correspondence and this document address the issues raised in this letter.

COMMENT:
Letter from Charles T. Stilwell to Mr. Michael Bishop transmitting ARCO's Statement of Disclaimer concerning the Final
Rocker OU Feasibility Study Report, dated June 26, 1995.
                                                                                 ARCO Comments 27

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 RESPONSE:
 See EPA's response to ARCO's FS disclaimer statement.

 COMMENT:
 Letter from Richard O. Curley, Esq. to D. Henry Elsen, Esq. regarding Rocker Timber Framing & Treating Plant Operable
 Unit RCRA Subtitle C issues, June 30, 1995.

 RESPONSE:
 Appendix 1 and the ARAR specific issues addressed in this document respond to this letter.

 COMMENT:
 ARCO's "Comments on EPA's Draft Baseline Risk Assessment," April 10, 1995.

 RESPONSE:
 See EPA's response to ARCO's comments on the Baseline Human Health Risk Assessment and the Risk
 Assessment itself.

 COMMENT:
 ARCO's Rocker Timber Framing & Treating Plant Operable Unit Technical Impracticability Waiver Report, dated June 30,
 1995.

 RESPONSE:
 EPA has responded to this document separately.

 COMMENT:
 CONCLUSION
 The preceding discussion demonstrates that selection of the Proposed Remedy as the remedy for the Rocker OU would be arbitrary and capricious, not in
 accordance with law, inconsistent with the NCP and contrary to CERCLA. The Proposed Remedy would have serious impacts on me local shallow groundwater
 system at the Rocker OU. In contrast, the ARCO/Rocker Proposal is better suited as a remedy for the Rocker OU, satisfies the statutory and regulatory
 requirements for remedial action specified in CERCLA and the NCP better than the Proposed Remedy and is supported by the public. For these reasons, EPA
 and the State should withdraw the Proposed Remedy set forth in the Proposed Plan and select the ARCO/Rocker Proposal as me remedy for the Rocker OU.

 RESPONSE:
EPA's final remedy has been modified in part by comments received from ARCO and others during the
public comment period.  EPA believes that the remedy selected is protective of human health and the
 environment, is implementable, cost effective, and offers the best choice of technical components given
the extreme complexities of the site.   EPA  has  clearly  demonstrated why ARCO's plan  is not
 implementable hence  it cannot be protective or  effective.   It is our understanding that the affected
 communities support the final  remedy selected in the ROD.
                                                                                   ARCO Comments 28

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                                       REFERENCES

 CH2M Hill. 1995. Baseline human health evaluation for the Rocker Timber Framing and Treating Plant
        operable unit. Silver Bow Creek/Butte area (original portion) Superfund site, Rocker, Montana.
        Prepared for U.S. Environmental Protection Agency, Region Vm, Helena, Mt. CH2M Hill, Inc.,
        Helena, MT.

 Freeman, G.B., J.D. Johnson, J.M. Killinger, S.C. Liao, A.O. Davis, M.V. Ruby, R.L. Chancy, S.C.
        Lovre, and P.O. Bergstrom. 1993. Bioavailability or arsenic in soil impacted by smelter activities
        following oral administration in rabbits. Fund. Appl. Toxicol. 21:83-88.

 Freeman, G.B., R.A. Schoof, M.V. Ruby, A.O. Davis, S.C. Liao, and  P.O. Bergstrom. (In press.)
        Bioavailability or arsenic in soil  and house dust impacted by smelter  activities following oral
        administration in cynomolgus monkeys. Fund. Appl. Toxicol.

 Loehr,  R.C. (Executive Committee,  Science  Advisory  Board), and V.A. Ray (Drinking Water
        Committee,  Science  Advisory Board).  1993.  Letter to the Honorable Carol M. Browner,
        Administrator, U.S. Environmental Protection Agency, dated November 30, 1993, regarding
        SAB review of draft Drinking Water Criteria Document on Inorganic Arsenic. U.S.  Criteria
        Document on Inorganic Arsenic. U.S. Environmental Protection Agency, Washington,  D.C.

 Stilwell, C. 1995. Personal communication (tetter to  U.S. Environmental  Protection  Agency, dated
       March 1995, regarding comments on the Rocker baseline risk assessment). ARCO, Anaconda,
       MT.

 Tseng, W.P.  1977. Effects and dose-response relationships of skin cancer and Blackfoot disease with
       arsenic. Environ. Health Persp. 19:109-119.

 Tseng, W.P., H.M. Chu, S.W. How, J.M. Fong, C.S. Lin, and S. Yeh. 1968. Prevalence of skin cancer
       in an endemic area of chronic arsenicism  in Taiwan. J. Natl.  Cancer Inst. 40(3)453-463.

 U.S. EPA. 1993. Review of the draft drinking water  criteria document on inorganic arsenic. Draft.
       EPA-SAB-DWC-94-005.  U.S.  Environmental  Protection Agency,  Science Advisory  Board,
       Drinking Water Committee, Washington,  D.C.

 U.S. EPA. 1995. Proposed plan:  Rocker Timber Framing and  Treating Plant operable unit.  U.S.
       Environmental Protection Agency, Region VIE, Helena, MT.
No Response Required
                                                                            ARCO Comments 29

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                                ARCO AMENDED PROPOSED PLAN
 COMMENTS:
 The Environmental Protection Agency (EPA), in consultation with the Montana Department of Environmental Quality (MDEQ),
 has published a Proposed Plan for the Rocker Timber Framing and Treating Operable Unit (Rocker OU). There is a 30-day
 Public Comment period for comments on the EPA Proposed Plan which will end Friday, August llth. In response to the EPA's
 plan, ARCO is submitting an amended proposed plan which addresses the main elements of the EPA plan, is protective of human
 health and the environment, and encompasses the concerns of the Community of Rocker.

 ARCO AMENDED PROPOSED PLAN BACKGROUND
 On August 2nd, the County Water and Sewer District of Rocker (CWSDR) held a special meeting to consider taking a position
 on the proposed cleanup plan at the Rocker OU.  The meeting was attended by local residents, property owners, and other
 interested parties including the EPA and ARCO. Proposed plans by the EPA and ARCO  were discussed and evaluated.
 Afterwards, ARCO and die CWSDR agreed on a common remedy that was passed by the Board and local residents. The
 amended proposed plan discussed below encompasses the concerns and specific objectives of the CWSDR. This plan supersedes
 the previous ARCO plan.

 PROBLEM DEFINITION/OBJECTTVES/PROPOSED REMEDY
 The following discussion describes the problem, objectives, and proposed remedy for groundwater and surface hot spots.

 GROUNDWATER

 Problem:                                •       Elevated concentrations in very shallow (20-40 ft.) groundwater
                                         •       Contamination is  not currently affecting  any drinking water source
                                                 which is in the deeper aquifer
                                         •       Contamination is not affecting Silver Bow Creek

 Objective of Remedy:                     •       Ensure human health is protected  by not  allowing arsenic from
                                                 Rocker site to spread into drinking water aquifer
                                         •       Do not allow horizontal movement to affect  Silver Bow  Creek

 Proposed Remedy:                        •       Innovative In-Situ Stabilization
                                         •       Natural Attenuation
                                         •       Groundwater Monitoring
                                         •       Limited Groundwater Ban
                                         •       Contingency Excavation/Removal only if Innovative Technology is
                                                 Unsuccessful

Remedy Description.  The groundwater area of concern would be defined by the arsenic concentration of S50 ug/L as
presented in the FS. An arsenic level of IS ug/L would be very difficult to  define and is below background levels of arsenic,
according to Mike Bishop (Rocker EPA Project Manager).

The arsenic source will be immobilized by in-situ treatment stabilization. This  approach to the source reduces long term risk
by not allowing groundwater arsenic concentrations to spread and by reducing current levels in the arsenic plume. Short term
risk due to worker and community exposure is minimized though an in-situ approach. Since mis process is an innovative
technology, a  demonstration is being performed to measure the effectiveness  and implementability of the technology. This
evaluation will consist of batch tests, column tests, and field demonstrations. Results from batch tests performed by Dr. Bill
Chatham at Montana Tech indicate the effectiveness of iron salt addition in immobilizing arsenic. Column tests and field
demonstrations will help determine site effectiveness and implementation methods.

If, after  a five year period, this innovative technology is unsuccessful, ARCO  is willing to implement a contingency remedy
where source materials will be removed to a secure repository. This repository will be located on or near the Rocker OU site
if technically practicable.
                                                                              ARCO Amended Proposed Plan 1

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Natural attenuation will continue to immobilize arsenic. The alluvial aquifer has an abundant supply of iron which is the primary
source for arsenic attenuation. Removing the iron system through excavation may upset the equilibrium of the existing natural
system. Groundwater monitoring of the shallow and deep aquifer will occur during and after remediation to monitor conditions
of the plume.

SURFACE SOIL HOT SPOTS
Problem:                                 •       Surface Soil "Hot Spots" are potential human health concern due to
                                                  direct contact with soil

Objective of Remedy:                      •       Limit potential for human exposure which may cause health concern

Proposed Remedy:                         •       Soil Cover
                                          •       Revegetation
                                          •       Institutional Controls

Remedy  Description. Surface soils with arsenic concentrations above 1000 ppm would define "Hot Spots". This concentration
is similar to Superfund sites in Anaconda, which are a conservative level of potential concern. Soil cover areas would be
revegetated. Institutional controls will include private land ownership with restrictions due to the active rail lines and the 100
year flood plain. In addition, land use will be primarily industrial with limited recreational use. Current flood plain regulations
will also  apply.

INFRASTRUCTURE IMPROVEMENTS
Infrastructure improvements are summarized below followed:

                 •        Alternative Water Supply
                 •        Allow for the drilling of Community Well
                 •        As a contingency ARCO will compensate Rocker Community for use of BSB water compared with
                         a community well

ARCO proposes to provide the community with an alternative water supply. The community is currently supplied water via a
six-inch line from the Butte Silver Bow (BSB) Big Hole Water Treatment Plant. The community wants to be less dependent on
BSB water supply and construct their own supply well. The Rocker Water & Sewer District could drill a well as long as it is
not within a limited well ban a quarter mile of the site. Production of existing wells would continue. ARCO would increase the
size of the current six-inch line to twelve inches and provide a 300,000 gallon storage tank. Bom the community groundwater
supply and the new twelve-inch line would be connected to the storage tank. Groundwater monitoring  wells would provide an
early warning system for water quality of the deeper tertiary  aquifer which provides the current water supply for several local
wells and will be the source for the community well. The risk of arsenic movement into the deeper tertiary aquifer is remote.
However, if arsenic contamination occurs, the enhanced BSB water system provided by ARCO would eliminate any risk to the
Rocker community.  As a contingency, ARCO will offset the yearly costs associated with using BSB  water instead of a
community well water if arsenic is detected and poses a  threat to the tertiary aquifer.

RESPONSE:
All of the above "ARCO Proposed Remedy" was contained in ARCO's written formal  comments dated
August  11, 1995 as  Section III.   Responses  can be found in EPA's responses to Section III of the
referenced document.  They will not be repeated here.
                                                                              ARCO Amended Proposed Plan 2

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                                     ARCO PROPOSED REMEDY
COMMENT:
The basic components of the ARCO Proposed Plan are shown below:

Source:          Innovative In-Situ Stabilization; Contingency Remedy Only if Innovative Technology is Unsuccessful

Plume:           Contingent Water Supply; Groundwater Monitoring; Institutional Controls; Natural Attenuation

Hot Spots:        Clean Soil Cover; Revegetation (same as FS Alternatives 2-7); Institutional Controls

Hot spot remediation would be similar to the alternatives developed by the EPA in the Rocker FS. Clean soil material would
cover existing hot spots at a depth of 18 inches. These soils would also be revegetated.

The plume area would be defined by the arsenic concentration of 50 ug/L as presented in the FS. An arsenic level of 18 ug/L
would be very difficult to define and is below background levels of arsenic. Groundwater monitoring would provide an early
warning system to arsenic movement. Arsenic has been very immobile at the site for the past 40 years because of natural
attenuation. The risk of arsenic movement into the deeper tertiary aquifer is very remote. However, if mis occurs a contingency
water system will be provided to the Rocker community to eliminate any concerns. In addition, a groundwater ban will be
implemented within a half mile of the existing water supply system to prevent the use of the tertiary aquifer. Existing users of
the deeper aquifer will be connected to the contingency water supply.

Natural attenuation will continue to immobilize arsenic. The alluvial aquifer has an abundant supply of iron which is the primary
source for arsenic attenuation. Removing the iron system through excavation may upset the equilibrium of the existing natural
system.

The arsenic source will be immobilized by in-situ treatment stabilization.  This approach to the source reduces long term risk
by allowing groundwater arsenic concentrations to approach background levels.  Short term risk due to worker and community
exposure is minimized though an in-situ approach. Since this process is an innovative technology, a demonstration of this
technology is being performed to measure the effectiveness and implementability of this system. This evaluation will consist of
batch tests, column tests,  and a field demonstrations. Results from batch tests performed by Dr. Bill Chatham at Montana Tech
indicate the effectiveness of iron salt addition in immobilizing arsenic. Column tests and field demonstrations will help determine
site effectiveness and implementation methods. If this innovative technology is unsuccessful, ARCO is willing to implement a
contingency remedy.

                       ARCO'S ISSUES WITH THE EPA PROPOSED PLAN

18 ug/L Standard Cannot be Met.

         •        The EPA Proposed Plan defines the arsenic plume by the new Montana Department of Environmental Quality
                 (MDEQ) arsenic drinking standards of 18 ug/L. The current EPA maximum contaminant level (MCL) for
                 arsenic is 50 ug/L. It is impossible to meet an 18 ug/L concentration regardless of remedy with an arsenic
                 background of approximately 30 ug/L.

         •        Contingencies in the EPA proposed Plan are triggered if arsenic concentrations remain above 18 ug/L after
                 the five year review. Removal of source materials will not reduce arsenic groundwater levels below 18 ug/L
                 or even 50 ug/L within the 18 ug/L plume. In fact, removing the source will increase arsenic groundwater
                 concentrations by removing arsenic from soils to solution during excavation, especially in the saturated zone.

Pump and Treat is Technically Impracticable.

         •        The EPA  Proposed  Plan has pump and  treat as a  contingency plan. ARCO prepared an Technical
                 Impracticability (TI) evaluation of pump and treat technologies at the Rocker OU. The TI demonstrates the
                 limitations  of pump and treat technologies because of the specific site hydrogeology, arsenic release rates,


                                                                               ARCO Amended Proposed Plan 3

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                 and technology barriers. A treatability study completed by Dr. Bill Chatham of Montana Tech indicated mat
                 it would take arsenic 1000 to 50,000 years to release from the site. This slow release is a result of natural
                 attenuation or the chemistry of existing iron hydroxides at the site. As a result pump and treat would be
                 dependent on arsenic release and not be effective in reducing groundwater to 18 ug/L. Dr. Chatham's study
                 indicated the effectiveness of adding iron to stabilize or immobilize arsenic using Rocker OU soils.

EPA Does Not Fully Consider an Innovative In-Situ Remedy.

         •       The EPA Proposed Plan and the Rocker FS did not provide for an in-situ remedy at the site. Excavation of
                 a non-defined source increases the risk of exposure to workers  and the Rocker community. In addition,
                 excavation will increase arsenic groundwater concentrations by mobilizing arsenic from soils to groundwater.
                 To minimize this risk it seems more  appropriate  to  try a  in-situ approach for source soils  through
                 demonstration of an innovative technology - in-situ iron stabilization. The EPA Proposed Plan discusses the
                 validity of this natural occurring process. This process is currently being evaluated for effectiveness through
                 planned column testing and a field demonstration. More information regarding in-situ treatment of arsenic
                 is attached.

RESPONSE:
No response required because above plan is superseded by ARCO's Amended Proposed Plan dated 8-9-
95.
                                                                                ARCO Amended Proposed Plan 4

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