PB98-963125
EPA 541-R98-116
December 1998
EPA Superfund
Explanation of Significant Difference
for the Record of Decision:
Silver Bow Creek/Butte Area
Silver Bow & Deer Lodge, MT
8/31/1998
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EXPLANATION OF SIGNIFICANT DIFFERENCES
STREAMSIDE TAILINGS OPERABLE UNIT
SILVER BOW CREEK/BUTTE AREA
(Original Portion)
NATIONAL PRIORITIES LIST SITE
SILVER BOW AND DEER LODGE COUNTIES
MONTANA
Montana Department of Environmental Quality
Remediation Division
2209 Phoenix Avenue
Helena, Montana 59620-0901
(Lead Agency)
United States Environmental Protection Agency
Region 8 Montana Operations
301 South Park, Drawer 10096
Helena, Montana 59626-0096
(Support Agency)
August 1998
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Explanation of Significant Differences - SSTOU August 1998
INTRODUCTION
This document presents an explanation of significant differences from the Record of Decision (ROD) for the
Streamside Tailings Operable Unit (SSTOU) of the Silver Bow Creek/Butte Area National Priorities List (NPL)
Site. The ROD for this site was prepared in 1995 by the Montana Department of Environmental Quality (DEQ)
and the U. S. Environmental Protection Agency (EPA)(DEQ & EPA, 1995). A cooperative agreement between
EPA and DEQ designates DEQ as the lead agency for Remedial Design.
Since the ROD was issued in November 1995, the principal potentially responsible party, the Atlantic Richfield
Company (ARCO) prepared a work plan for remedial design (RD) (ARCO, 1997a) and submitted preliminary
and intermediate design documents to the agencies (ARCO, 1997b and 1997c). Following ARCO's April 1997
refusal to continue work on the SSTOU RD, DEQ prepared the preliminary final design report, which will guide
construction for Reach A, the first mile of the operable unit (Maxim ef a/, 1998). In the course of preparing the
SSTOU design, DEQ and EPA reevaluated certain elements of the remedy as described in the ROD in light
of new site information developed in the design process. For example, the estimated volume of materials that
would be remediated, the cost of the remedy, and some aspects of the technical approach to remediation were
reevaluated during design. These modifications identified during design represent changes in the scope and
cost of the SSTOU remedy, but they do not change the fundamental approach to remediation of this operable
unit. The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). as amended,
provides for public disclosure of the reasons for significant differences through this document. The pertinent
section of CERCLA, §117(c), requires that the lead agency address post-ROD significant changes in the
following instances:
After adoption of a final remedial action plan (1) if any remedial action is taken [under sections
104 or 120], (2) if any enforcement action under section 106 is taken, or (3) if any settlement
or consent decree under section 106 or section 122 is entered into, and if such action,
settlement or decree differs in any significant respects from the final plan [ROD] the [lead
agency] shall publish an explanation of the significant differences and the reasons such
changes were made.
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Explanation of Significant Differences - SSTOU August 1998
Interim Final Guidance on Preparing Superfund Decision Documents (EPA. 1989) clarifies the definition of
"significant differences." Changes that significantly alter the scope, performance, or cost of a component of
the remedy, without fundamentally changing the overall approach of the remedy as presented in the ROD,
should be addressed in an explanation of significant differences (ESD). Certain of the differences below, such
as the volume and cost estimates, clearly represent changes requiring an ESD, although none of them
fundamentally changes the selected remedy described in the ROD. Some of the differences described below
could be viewed as development of the detailed design of the selected remedy rather than changes. However,
the agencies include them in this ESD to clarify the remedy as designed and to explain the basis for these
important design elements.
DEQ and EPA have identified nine significant differences from the remedy described in the ROD. These
differences, developed during detailed design of Reach A (the uppermost mile of the operable unit), also apply
to the design and implementation of the remedy in the remainder of the SSTOU. The significant differences
discussed in this ESD are the following:
1. An increase in the volume of tailings/impacted soil in the operable unit;
2. Modifications to the alignment of Silver Bow Creek and the channel profile (i.e., elevation profile);
3. Use of a temporary stream diversion during and after construction to facilitate dewatering and
excavation of near-stream tailings and to enhance floodplain and streambank revegetation efforts.
4. Changes in the criteria for in-stream sediment removal as a result of other design changes;
5. Modifications to the mine waste relocation repository (MWRR) design;
6. The inclusion of sediment basins to contain contaminated overland flow run-on from off-site mine
waste sources;
7. Elimination of treatment wetlands as the end land use in Subarea 1;
8. Changes in the estimated schedule to implement the SSTOU remedy; and
9. An increase in the estimated cost of the SSTOU remedy.
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Explanation of Significant Differences - SSTOU August 1998
This ESD will be placed in the administrative record for the SSTOU The administrative record for the SSTOU
is maintained at the U. S. EPA Montana State Office. Federal Building, 301 South Park, Helena. Montana.
Office hours are 8:00 to 5:00 on federal business days. The ESD will also be placed in all information
repositories for the SSTOU.
SITE DESCRIPTION
The SSTOU is one of several operable units that make up the Silver Bow Creek/Butte Area NPL Site. The
SSTOU comprises the geographic area of contamination along and in Silver Bow Creek between the western
end of the Colorado Tailings area and the point at which Silver Bow Creek enters the Warm Springs Ponds.
extending for approximately 24 river miles. As defined in the ROD, it includes the extent of fluvially deposited
tailings along Silver Bow Creek, the adjacent railroad beds, which are contaminated with mine waste, and all
areas in close proximity which are necessary for remedy implementation. It expressly excludes the Rocker OU.
SITE HISTORY
The principal contaminants of concern at the SSTOU are arsenic, cadmium, copper, lead, mercury, and zinc.
These contaminants are present in five major media at the site: tailings/impacted soils, in-stream sediments,
railroad materials, groundwater, and surface water. Tailings and other mining wastes were deposited in and
along Silver Bow Creek by historic mining and milling operations and redistributed in the floodplain by
occasional flooding, precipitation, snow melt, and ice jam events that have occurred since the 1870s when
mining and milling commenced in the Butte/Anaconda area. Entrainment of tailings in runoff and metal loads
present in groundwater and surface water further contributed to contamination of in-stream sediments. Portions
of the three railroad embankments within the operable unrt were constructed with mine wastes and other
contaminated materials which impact the stream and the floodplain. In addition, concentrate shipped in rail cars
has spilled and further contaminated the railroad beds.
EPA listed the Silver Bow Creek/Butte Area Site (original portion) on the NPL in 1983. Site investigations began
in 1984. The Phase I Remedial Investigation (Rl) for the entire site was conducted by DEQ, with supplemental
investigations by EPA. That work was followed by a Phase II Rl investigation of the SSTOU conducted by
ARCO. The draft SSTOU Rl report (ARCO, 1995a) defined the nature and extent of contamination to the extent
necessary to identify remedial alternatives and provide information to complete the baseline human health and
ecological risk assessments. The SSTOU feasibility study (FS). published by ARCO in June 1995, described
the development, screening and evaluation of potential remedial alternatives (ARCO, 1995b).
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Explanation of Significant Differences - SSTOU . August 1998
EPA and DEQ issued a Proposed Plan identifying the preferred remedy for the SSTOU in June 1995. Later
in 1995, the agencies modified the preferred remedy in response to public comment and issued a Record of
Decision (DEQ and EPA, November 1995) identifying the selected remedy for the SSTOU. In March 1996, EPA
issued a Unilateral Administrative Order (UAO) to ARCO for remedial design and remedial action, including
operation and maintenance. Under the UAO, ARCO submitted drafts of a Preliminary Design Report (ARCO,
1997b) and an Intermediate Design Report (ARCO, 1997c) for Subarea 1. In April 1997, ARCO stopped work
on the remedial design at the site. The agencies are now completing the remedial design for the SSTOU.
SUMMARY OF SSTOU SITE RISKS
Human health and ecological risks at the SSTOU are evaluated and presented in the Draft Baseline Risk
Assessment (COM, 1994).
Human Health Risks
The SSTOU Baseline Human Health Risk Assessment evaluated three exposure scenarios to determine the
health risks related to OU use by residents, workers (occupational), and recreationists. Both existing and
reasonably anticipated future exposure scenarios were evaluated. Risks were divided into those that may cause
cancer and those that cause adverse health effects other than cancer (non-carcinogenic risks). The primary
carcinogenic risk to people living in or near the SSTOU comes entirely from potential exposure to arsenic in soil
and groundwater. Elevated concentrations of arsenic can be found in tailings areas such as the Ramsay Flats
and in near-stream, upper alluvial (less than 20-feet below ground surface) groundwater. Noncarcinogenic risks
exceeded acceptable levels for arsenic in soils under the residential exposure scenario. As with the
carcinogenic risks, the noncarcinogenic risks vary depending on the amount of contamination a person
contacts. Noncarcinogenic risks related to arsenic, cadmium, copper and zinc in groundwater we- -; ,::nd only
in near-stream, upper alluvial groundwater within and directly adjacent to the floodplain. The risks posed by
lead contamination in soil are generally within the acceptable range based on the risk model used in Butte.
Ecological Risks
In Silver Bow Creek, the presence of mine waste contamination is the primary factor limiting the health of the
aquatic environment. Those contaminants affect both the water quality and in-stream sediments in Silver Bow
Creek and create a toxic environment for fish and most benthic macroinvertebrates. The creek is devoid of fish
and has severely impacted populations of most other aquatic life forms. Concentrations of metals in surface
water and sediments are well in excess of ecological effects concentrations for those parameters. The risk
assessment also evaluated other physical and chemical conditions that may adversely affect the health of Silver
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Explanation of Significant Differences - SSTOU August 1998
Bow Creek, including siltation of the stream bottom, channelization, disturbance of adjacent land and
streamside (riparian) habitat, nutrient loading, dissolved oxygen concentrations, and organic contamination from
the Montana Pole and Treating Plant NPL Site. Although they may have some impact on Silver Bow Creek,
these factors are considered to be much less significant than the mining waste contamination risk to the SSTOU
environment.
SUMMARY OF ROD
The ROD describes the final remedial action for the five media of concern at the SSTOU. Much of the treated
material will remain in the operable unit Consequently, long-term management and monitoring of the operable
unit are required. This section summarizes the basic elements of the remedy as presented in the ROD.
Tailings/Impacted Soils: The ROD requires removal of contaminated tailings and impacted soils from the
present 100-year floodplain of Silver Bow Creek unless: (1) the particular tailings/impacted soils are not
continuously or seasonally in contact with groundwater, (2) treatment of those tailings/impacted soils with
Streambank Tailings and Revegetation Study (STARS) treatment can be used reliably to immobilize the
contaminants, and (3) the tailings/impacted soils will not be subject to erosion and reentrainment into the
stream. The volume of tailings/impacted soils, as defined by the order-of-magnitude criteria presented in the
Rl, was estimated to be approximately 2,500,000 cubic yards (cy). Of that amount, about 1,550,000 cy would
be excavated and relocated and about 950,000 cy would be treated in place.
Excavated tailings/impacted soils are to be relocated to safe, local repositories clearly outside the 100-year
floodplain as defined by CH2M Hill (1989). The excavated materials will be fully treated with lime amendments
in 2-foot lifts and the local repositories are to be revegetated in accordance with the STARS technology. If
appropriate repository locations cannot be found or an appropriate institutional controls/monitoring and
maintenance program cannot be implemented, excavated tailings/impacted soils and other wastes would be
removed to centralized, dry repositories and appropriately handled and disposed. Replacement fill is required
in most locations where tailings/impacted soils are removed. Replacement fill and reconstructed streambanks
will require suitable growth media having appropriate texture and particle distribution. A key to long-term bank
stabilization will be establishment of mature riparian vegetation. The overall topography of the replacement
fill material will be appropriately sloped toward the stream channel with the goal of creating geomorphic
stability.
Because numerous repositories, containing contaminated tailings/soils treated with the STARS technology, will
be located near the flood plain in several areas along the length of the stream, and because in Subarea 2 and
Subarea 4 a substantial amount of tailings will be treated with the STARS technology on the edges or just
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Explanation of Significant Differences - SSTOU August 1998
outside of the flood plain, a permanent monitoring, management, and maintenance program is an integral part
of the remedy.
In-Stream Sediments: The ROD requires that fine-grained sediments (defined as less than or equal to one
millimeter in size) located in depositional areas be removed and placed in repositories along with the excavated
tailings/impacted soil and railroad materials. The in-stream sediment volume was estimated at 73.000 cubic
yards in the RI/FS and ROD. After removal of contaminated sediments, the channel bed and streambank is
to be reconstructed to an appropriate slope and other critical dimensions with materials of appropriate size,
shape and composition. This reconfigured bed will contain suitable bedform morphology (riffles, runs, and
pools) for aquatic habitat. Stream banks will require adequate growth media to allow for immediate
establishment of a healthy riparian vegetative system to protect the remedy from high flows. In-stream
sediment monitoring will be performed during and after the response action to ensure that contaminated in-
stream sediments have been adequately remediated.
Railroad Materials: The ROD requires excavation, treatment and/or cover of all contaminated railroad bed
materials that pose a risk to human health or the environment. All concentrate spills, which are the primary
human health concern for the railroad beds, will be removed and disposed in an appropnate and secure
disposal facility in accordance with any applicable RCRA requirements. Railroad materials that directly impact
the stream either at bridge abutments or along the streambank will be excavated and disposed in repositories
along with the tailings/impacted soils and in-stream sediments. The estimated volume of excavated railroad
materials in the ROD was 71,000 cubic yards. In-situ STARS technology or soil capping are expected to be
appropriate for all other areas of the inactive grade presenting environmental risk. Monitoring and maintenance
of the remediated railroad materials will be required to ensure that contaminant sources are not exposed as
a result of erosion and do not cause future contaminant loading to the stream.
Groundwater and Surface Water: While Silver Bow Creek groundwater and surface water are primary
receptors of SSTOU contamination, no separate remedial action is prescribed for these media. Remedial
activities for other SSTOU media under the ROD and for sources of contaminants upstream and off-site under
other cleanup actions will limit further releases to groundwater and surface water, with the goal of ultimately
attaining groundwater and surface water standards within the operable unit.
Coordination and Schedule: An institutional controls, monitoring, and maintenance program is required under
the ROD. Construction of the proposed remedy is to be coordinated with other cleanup or natural resource
damage restoration activities along Silver Bow Creek. Releases of contaminated in-stream sediments and
surface waters prior to. during, and following remedial action, which might recontaminate Silver Bow Creek,
must be suitably controlled. The design and schedule of the operable unit remedy is to be coordinated with
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Explanation of Significant Differences - SSTOU August 1998
the design and installation of upstream sediment control basins. If adequate upstream control facilities are not
in service at the time of initiation of construction of this remedy, then additional sediment control and treatment
facilities may be provided as a part of the SSTOU remedy or other scheduling adjustments may be made.
At the time the ROD was issued, Butte-Silver Bow County and ARCO had initiated research on constructed
wetlands as a potential treatment technology for municipal waste water nutrient discharge and stormwater
metals contamination. In light of that research, the ROD delineated the anticipated future land use in Subarea
1 as treatment wetlands.
DESCRIPTION OF SIGNIFICANT DIFFERENCES
As described in the introductory text of this ESD, DEO and EPA have identified nine significant differences in
remedy implementation plans relative to the remedy described in the ROD. These differences evolved during
the design of the upper reaches of the SSTOU. In this section each significant difference is described and the
basis for the change is explained.
1. INCREASE IN TAILINGS/IMPACTED SOIL VOLUME
The estimate of tailings/impacted soil volumes stated in the ROD was based on limited Rl data. Because these
data were inadequate for remedial design and construction, detailed test pit sampling was conducted in
Subarea 1 of the SSTOU by ARCO in 1996 and 1997. The intent of this detailed sampling was to provide a
more precise identification of the volume and location of the tailings/impacted soils to be remediated. About
400 test pits on a 150-foot grid were excavated in Subarea 1, with tailings and soil materials sampled in four-
inch vertical intervals. The data from this sampling were used to determine the vertical depth at which metals
decreased by an approximate order of magnitude (the performance standard identified in the ROD). Using this
approach, an additional 256,000 cubic yards (cy) of tailings/impacted soils were identified in Subarea 1.
The design process also confirmed that additional removal of soils would be necessary to account for the
variability of the elevation of the base of the tailings/impacted soils to be removed. Remedial design test pit data
from Subarea 1 show that the base of tailings surface varies in elevation to such extent that the 150-foot test
pit grid was insufficient to provide the necessary confidence that the base of tailings/impacted soil was
accurately mapped and that excavation to the mapped depth would remove the contaminated material from
the floodplain as required by the ROD. Therefore, the designed depth of excavation was increased by 0.5 feet
to ensure that at least 90 percent of the tailings/impacted soils would be removed. The increase in excavation
depth to attain this confidence, required by the ROD and UAO/SOW. results in removal of an additional 121,000
cubic yards of material (Maxim, ef. a /.. 1998). Therefore, the new volume information developed during design
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Explanation of Significant Differences - SSTOU August 1998
increased the estimated total excavation volume for Subarea 1 from the 285,000 cy estimated in the ROD to
662.000 cy.
Design activities have not been initiated for the remaining three subareas, so detailed information about actual
volumes of tailings in those subareas is not available. However, based on knowledge gained during the design
of Subarea 1, the agencies anticipate that some increase in volumes of tailings over that estimated in the ROD
will be found. The increase in the downstream subareas is not expected to be as great as in Subarea 1.
Because of different soil chemistry in the downstream areas, the tailings/impacted soils visually are more
distinct from the underlying natural soils, and the agencies believe that there has been less migration of
contaminants below the tailings. The agencies' current projection is that, in each of the other subareas, an
additional 30 percent to 80 percent over the ROD-estimated tailings/impacted soils will have to be excavated
and placed in repositories and an additional 10 percent to .40 percent over the ROD-estimated tailings/impacted
soils will have to be treated in-srtu with the STARS technology.
The increase in the volume of tailings/impacted soils affects other design elements as well. An increase in the
amount of excavation is required, resulting in an increase in the amount of storage capacity required for the
MWRRs. In addition, the volume of backfill material needed to reconstruct the floodplain increases in order
to meet the lines and grades of a geomorphically stable configuration. All of these items directly impact the
overall cost of the remedy.
At the time of ROD issuance, DEQ and EPA recognized that there was considerable uncertainty associated
with the estimate of the volume of tailings/impacted soils in the SSTOU. The initial efforts of the RD were
directed to reducing that uncertainty. Even with the significant increase in volumes now defined, the agencies
have determined that the remedial approach selected in the ROD, i.e., primarily excavation and placement of
floodplain tailings/impacted soils into controlled local repositories, with limited use of in-srtu treatment of tailings,
remains the most cost-effective alternative that provides acceptable overall protection of human health and the
environment and that complies with applicable or relevant and appropriate requirements (ARARs), except
where waived.
2. MODIFICA TIONS TO STREAM CHANNEL GRADE AND ALIGNMENT
The ROD requires that, after excavation and backfilling of the floodplain area, the channel bed and
streambanks be reconstructed to provide a geomorphically stable system. It did not explicitly provide for
changes in channel slope and channel location that might be needed to establish a geomorphically stable
channel. At the direction of DEQ, Mussetter Engineering Inc. (MEI) undertook a study of the upper Silver Bow
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Explanation of Significant Differences - SSTOU August 1998
Creek drainage to evaluate design criteria for creating a geomorphically stable channel. In its study. MEI
identified and evaluated:
1. The man-made and natural controls, the bed and overbank sediment gradations, and other
geomorphic characteristics of Silver Bow Creek;
2. The hydrologic conditions for which the stream channel is to be designed, including the frequency,
magnitude and duration of flood and non-flood flows;
3. The stream's hydraulic characteristics (i.e.. velocities, flow depths, shear stresses) for the expected
range of flows;
4. The estimated composition and amount of upstream and lateral sediment supply under existing and
anticipated future conditions; and
5. The sediment-transport dynamics of the stream, including the potential for significant short-term and
long-term aggradation (stream bed deposition) and degradation (stream bed erosion).
MEI's Channel Stability Analysis report (MEI & Inter-Fluve, 1997) synthesized the above information to provide
guidelines for selecting appropriate criteria for stream channel design in Subarea 1. The MEI report
recommended that the channel grade (elevation, or steepness of slope) of Silver Bow Creek be :;.anged to
eliminate the more severe aggradational and degradational reaches. Included in these changes were bridge
modifications that would establish new vertical control and eliminate some aggradational potential. This report
also recommended changes in planform (lateral position) of the stream in order to provide appropriate
sediment transport capacity or to protect infrastructure.
fnter-Fluve, Inc. developed a conceptual design for Subarea 1 of the SSTOU that made more specific
recommendations for channel grade and alignment changes consistent with the MEI recommendations (Inter-
Fluve & MEI, 1998). Inter-Fluve developed potential alternative grade and alignment changes to Silver Bow
Creek that would provide varying degrees of improved channel stability. Often the most satisfactory grade and
alignment changes from a stability viewpoint provide additional cost benefits. The proposed grade changes
generally require a decrease in elevation of the channel bed and an attendant decrease in floodplain elevations
and reduced requirement for floodplain backfill. In addition, the clean soils obtained during excavation of the
new channel can be used for floodplain backfill. This design therefore results in some cost savings by reducing
the amount of fill material that must be purchased and imported for floodplain reconstruction. For example,
the excavation requirement for Reach A of Subarea 1 is 166.400 cubic yards of tailings/impacted soils and
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Explanation of Significant Differences -SSTOU August 1998
20.500 cubic yards of clean material in order to obtain the most satisfactory channel grade. However, only
75,000 cubic yards of imported backfill will be required to reconstruct the floodplain because the channel and
floodplain will generally be designed to lower elevations.
DEQ and EPA have adopted the design recommendation for channel grade and alignment changes for the
reasons identified above. This approach provides the most cost-effective way to comply with the ROD
requirement that the the reconstructed channel be designed as a geomorphically stable, naturally meandering
alluvial system to the degree possible. Other approaches, such as using extensive reaches of riprap to control
the stream's reaction to unstable gradients, would have required considerable additional long-term
maintenance.
3. UTILIZA TION OF TEMPORARY STREAM DIVERSION
The ROD requires that contaminated in-stream sediments and saturated contaminated tailings/impacted soils
be excavated and relocated to MWRRs. Although the ROD anticipated that various approaches to dewatering
the excavation area would be considered and potentially utilized, it did not explicitly provide for the construction
of a temporary stream diversion to support and enhance dewatering and other remediation elements. During
the Subarea 1 remedial design, DEQ's technical consultants conducted additional evaluations of dewatering
approaches, particularly during the pilot test of dewatering and streambank reconstruction techniques. As a
result of those evaluations, it became clear that utilizing a temporary diversion of the stream channel during
and after construction would greatly simplify near-stream excavation and backfill work and would enhance the
ability of the floodplain and streambank revegetation to establish successfully. Stream diversion techniques
evaluated during RD included use of fabric- or rock-lined channels and steel or plastic conduits to safely handle
various possible flow conditions.
During excavation and backfilling of the stream channel and near-stream saturated areas, some method of
localized dewatering of the excavation area is required. The dewatering activities can be greatly simplified and
enhanced by keeping existing streamflows out of the excavation area. This can be accomplished by diverting
the existing stream flow and drying out the existing stream channel prior to and during excavation. If relatively
long reaches of the stream channel construction area can be dried out, general excavation and haul equipment
mobility and access is improved and simplified by reducing stream crossings and the need for built-up access
roads in wet areas. Risks associated with potential washout of the exposed excavation area also can be
reduced by routing high-streamflow precipitation or runoff events through the diversion channel during the
construction period. After construction is complete, base flows can be routed to the newly constructed channel,
while high flows continue to be diverted around the reconstructed floodplain.
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Explanation of Significant Differences - SSTOU August 1998
After the floodplain and stream channel are recontoured, the streambanks and floodplain will be seeded and
planted to establish appropriate vegetation consistent with the requirements of the ROD. In the arid Montana
climate, it often takes several growing seasons for reseeded areas to establish healthy vegetation that can
withstand erosive forces from rainfall runoff and stream flows. With the reconstruction of the streambank of
an active stream, such as Silver Bow Creek, that experiences a wide range of flow conditions, including erosive
high flows during spring runoff, establishing durable streambank vegetation can be difficult. If the flow through
the reconstructed channel can be regulated to prevent high flows from occurring during the period of vegetation
establishment, the success of the revegetation can be greatly improved and the risk of streambank failure
substantially reduced. The temporary diversion of the stream to accommodate high flow events can accomplish
that regulation of flow.
For the reasons identified above, the agencies adopt the use of temporary stream diversions, where
appropriate, as an element of the SSTOU remedy. Decisions on the use and design of diversions for each
stream reach will be based on design and construction needs for that particular reach. Stream diversion will
not be appropriate for all reaches. For example, in some reaches the floodplain is too narrow to accommodate
a diversion. The need for the diversion to accommodate construction work or revegetation, the design flow rate
for the diversions, the sizing of riprap or other erosion-resistant material, the location and configuration of the
channel, the duration of the use of the diversion, and other key design elements will be decided on a reach-
specific basis.
4. CHANGES IN STREAM SEDIMENT REMOVAL CRITERIA
The ROD required that fine-grained (less than one millimeter) in-stream sediments in depositional areas be
excavated and placed in MWRRs. This criterion, based on assumptions that the source of sediments of this
size fraction in the depositional areas would primarily be near-stream tailings materials that eroded into the
stream and that the sediment contaminant concentrations would be well correlated with tailings contaminant
concentrations, was specified as an alternative to a performance standard based on contaminant concentration.
To identify the depositional areas and verify their contamination characteristics, ARCO undertook an in-stream
sediment sampling program for Subarea 1 in 1996. These data were presented in the Intermediate Design
Report (ARCO, 1997c). ARCO's analysis of the data found no significant correlation between metals
contamination and either the type of the depositional feature (e.g., channel bar. side bar, point bar) or the grain
size distribution of the material. Since the agencies were unable to define an acceptable procedure for
identifying and removing contaminated in-stream sediments, the ROD requirement must be modified and new
design criteria developed to address the stream sediments.
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Explanation of Significant Differences - SSTOU
August 1998
The agencies subsequently evaluated the possibility of defining a simple depth of excavation, utilizing an
approach similar to that used for floodplain tailings/impacted soils. Additional data collected by DEQ's
contractors (Maxim, 1998a) are summarized in Table 1. These data indicate that in-stream metals
contamination, although distributed throughout differing depositional forms, is confined to relatively shallow
depths. Based on data from four boreholes drilled within the existing stream channel to a depth of 10 feet below
the stream bed surface, relatively elevated concentrations were observed in certain metals in the shallow
depths in some borings, with a marked decrease in arsenic and metals concentrations below the three foot
depth.
TABLE 1
STREAMBED SEDIMENT SAMPLING RESULTS4"
Subarea 1 • SSTOU Remedial Design
Lower
Depth (in)
12
30
36
42
Soil Class
12)
GP
ML
ML
GP
PH
(S.U.)
4.8
5.9
5.2
5.4
Total Concentration
Arsenic
<2
11
<2
<2
Cadmium
21
19
14
<2
Copper
767
3150
787
671
18
30
48
54
66
SM
SP
SW
SP-SM
SP-SM
6.6
70
5.6
5.5
6.2
<2
<2
<2
4
6
2
6
<2
<2
<2
30
9
30
30
46
milliqrams/kiloqrain)
Lead
70
200
150
66
Mercury
<0.2
1.5
<0.2
<0.2
Zinc
1330
2570
1040
86O
50
50
20
<20
53
30
36
42
60
36
42
48
54
60
SM
SM
SM
SP-SM
SP
SP
SP
SP
SP
6.8
6.8
7.2
7.0
6.4
6.5
6.5
6.2
52
<2
<2
<2
<2
36
3
4
12
12
17
14
<2
<2
91.8
24
4
2
2
677
1460
58
51
836
45
230
57
13
56
93
30
40
240
30
40
30
40
<02
<0.2
<0?
2340
573
424
220
•o.iij 270
<0.2
<0.2
-=0.2
<02
<0.2
<0.2
<0.2
<02
<02
662
827
75
100
1480
230
376
250
70
(1) - Data from Maxim. 1998a
(2) - Classification according to the Unified Soil/Classification System
GP = poorly graded gravel; ML = silt or sin with sand: SM = silty sand: SP = poorly graded sand:
SW = well graded sand; SM = poorry graded sand with sift.
< = less than the detection limit
Page 12 of 31
-------
Explanation of Significant Differences - SSTOU August 1998
This limited data set is assumed to be representative of the whole channel, since the data from shallow intervals
correlate well with the larger data set collected by ARCO in 1996. In all samples submitted for analysis from
these borings, including those from the shallowest depths, arsenic and metals concentrations were less than
the order-of-magnitude tailings criteria used to define the tailings/impacted soils that must be removed from
the floodplain.
The agencies' efforts to develop revised criteria for addressing in-stream sediments at the SSTOU was
conducted concurrently with the evaluation of potential modifications to the stream channel grade and
alignment discussed in ESD Item 2 above. The two evaluations were combined and the following new set of
criteria for addressing contaminated in-stream sediments was proposed.
1. Due to the relocation of the stream channel in portions of the operable unit, much of the existing
stream channel will be abandoned. In reaches where the old channel is to be abandoned, the existing
contaminated sediments within the old channel will be treated as all other floodplain tailings/impacted
soils. If these materials are identified as tailings/impacted soils under the order-of-magnitude removal
criteria, then they will have to be excavated and placed into MWRRs. In general, in Subarea 1 these
materials meet the requirements for being below the order-of-magnitude decrease in contaminant
concentrations and will not be removed as tailings/impacted soils, but will remain in place. As part of
the reconstructed floodplain, they will no longer be in direct contact with Silver Bow Creek surface
water and will not impact the aquatic environment.
2. In areas where the existing stream channel will be reconstructed in the same location, the direct
contact of the surface water and the aquatic receptors with the stream bed materials necessitates that
the contaminated stream sediments be removed. Existing in-stream sediments will be excavated to
a minimum depth of one foot and placed in a MWRR. The new channel bed will be constructed with
clean fill material. If channel construction requires additional excavation to meet new channel grade
requirements, excavated material from deeper depths that is determined to have metals concentrations
below the order-of-magnitude removal criterion will not be placed in a MWRR, but rather will be used
for floodplain backfill. In Subarea 1. all materials to be excavated at depth to meet channel grade
requirements are below the order-of-magnitude criterion and will be used for general backfill.
The design of excavation approaches to meet these in-stream sediment removal requirements will depend in
part on the streambed characteristics in specific reaches. For example, in parts of Subareas 2. 3, and 4, the
stream channel is composed of significant reaches of alluvial cobbles in addition to depositional pools and point
bars containing fine-grained sediments. Additional sampling of subsurface conditions and characteristics in the
Page 13 of 31
-------
Explanation of Significant Differences - SSTOU August 1998
stream channel will be necessary to design detailed removal specifications for the downstream subareas
consistent with the removal criteria above.
The agencies adopt the new criteria to replace the in-stream sediments removal criteria defined in the ROD.
The new criteria provide an acceptable approach addressing contaminated in-stream sediments that is cost-
effective and consist with other elements of the SSTOU remedial design. It particularly complements the
stream gradient and alignment changes identified in this ESD. The agencies have determined that the revised
criteria are more protective than the prior ROD criteria because (1) the new criteria address the entire Silver
Bow Creek channel in the SSTOU, rather than just depositional areas, (2) the original criteria were found not
to define adequately those contaminated sediments requiring removal, and (3) the stream bed of the new Silver
Bow Creek channel will be constructed of clean, imported materials and the stream will be more stable
geomorphically, reducing potential reentrainment of and exposure to contaminated materials in the stream.
5. MODIFICATIONS TO MINE WASTE RELOCATION REPOSITORY (MWRR) DESIGN
Addition of Soil Cover to MWRR
The ROD specified that the MWRRs would be revegetated in accordance with STARS technology which uses
lime application to neutralize acidity, minimize metal migration, and enhance plant growth. As part of DEQ's
decision process for determining the location and construction of MWRRs, Maxim (1998b) produced an
Alternatives Analysis for Mine Waste Relocation Repositories report which analyzed various repository -ettings
and designs. The primary purpose of this effort was to determine which designs would minimize potential
contaminant loading to groundwater and prevent violations of groundwater quality standards and other ARARs
identified in the ROD. In the analysis, Maxim determined that addition of a 1.5-foot to 2.0-foot thick soil cover
is necessary to improve protectiveness of groundwater in comparison with utilization of only a simple STARS
treatment approach with no cover soil. The soil cover would lessen the amount of infiltration into the waste and
thereby reduce the production of metals-enriched leachate. The reduction in leachate would occur because:
1. It was more certain that a good vegetative cover could be developed if the vegetation was planted in
uncontaminated, suitable backfill material. The improved vegetation increases evapotranspiration,
which directly reduces infiltration; and
2. The backfill material is generally finer textured than the tailings/impacted soil and therefore transmits
water less easily to the waste.
Page 14 of 31
-------
Explanation of Significant Differences - SSTOU August 1998
These conclusions are consistent with conclusions in the earlier STARS studies, which found that there was
better success in establishing vegetative cover and metals immobilization wrth the use of a topsoil cover.
To make this determination on the infiltration properties of the different MWRR designs, Maxim used the
Hydrologic Evaluation of Landfill Performance (HELP) model which calculates water migration based on
properties of the soils, vegetation, and climate data. A simple STARS design, without cover soil, was found
to have an average leachate percolation rate of 2.2 inches per year. Use of an imported soil cover reduced
the percolation rate to 0.6 inches per year. Because of concerns that near-stream repositories would have
potential for affecting groundwater quality, OEQ and EPA chose to minimize the potential impact by selection
of the soil cover design.
DEQ and EPA have adopted the modified MWRR design described above because the utilization of soil cover
on MWRRs is necessary to assure that rainfall and snowmelt infiltration into the MWRRs and potential
contaminant migration are minimized. This is accomplished by covering the MWRR with less permeable soils
that will also enhance establishment of more intensive vegetative cover.
Investigation of Potential Use of Additional Amendments Below the MWRR to Attenuate Arsenic
Prior studies of methods to neutralize and immobilize metals in tailings have suggested that arsenic mobility
may not be controlled satisfactorily by lime amendment approaches. In its evaluation of potential repository
designs (Maxim, 1998b), Maxim conducted a series of laboratory tests to investigate the potential for leachate
containing elevated concentrations of arsenic to be generated as water passed through the lime-amended
tailings/impacted soils in the unlined MWRRs. Varying rates of water were introduced to amended tailings
samples in the laboratory, and the resulting concentrations of arsenic and metals in the leachate were
determined. This information was used to predict potential changes that may occur to the quality of
groundwater beneath an MWRR based on certain assumptions about the physical setting of the repository, the
distance to groundwater, and the physical and chemical characteristics of the intervening materials between
the base of the lime-amended wastes in the repository and the water table. These simulated laboratory tests
indicated that arsenic, but not other contaminants, has a potential to impact the quality of groundwater beneath
the repository. The study left some uncertainty as to whether arsenic concentrations above the Montana
Circular WQB-7 water quality standard might result in certain locations.
In an effort to address the potential impacts of arsenic on groundwater. OEQ is investigating the potential for
incorporating an arsenic attenuating layer in the subgrade of SSTOU MWRRs. An iron-based arsenic
attenuating process was used to adsorb arsenic dissolved in groundwater at the Rocker Operable Unit. DEQ
has contracted with Montana State University to conduct preliminary bench scale trials to determine the
Page 15 of 31
-------
Explanation of Significant Differences - SSTOU August 1998
adsorption potential of certain commercially available products. DEQ intends to continue this bench scale
research and. if the results of these tests are promising, incorporate this mechanism into the design of MWRRs.
DEQ and EPA are modifying the ROD to allow for the addition of an arsenic-attenuating material at the base
of specific MWRRs because use of the attenuating material may be necessary in certain locations to attain the
ROD ARARs for protection of groundwater.
Potential Use of Consolidated Tailings Repositories
An additional option that will be considered and may be adopted in the design process for tailings/impacted soils
removed from certain areas will be consolidation of the removed tailings with existing off-site tailings deposits
which will remain in place. The agencies may determine during detailed design for the lower reaches that some
tailings/impacted soils can be efficiently consolidated with existing tailings deposits in areas near the SSTOU,
such as the Opportunity Ponds. The decision to consolidate such wastes rather than construct local waste
repositories in those areas may be based on benefits such as eliminating the need to acquire additional lands
for new repositories, reducing costs of repository construction, reducing lime requirements, reducing future
monitoring and maintenance costs, and reducing the amount of currently uncontaminated land and
groundwater impacted by repositories.
6. ADDITION OF SEDIMENT BASINS TO CONTROL RUN-ON OF OFF-SITE CONTAMINATED SEDIMENTS
The ROD anticipated that sediment loads from upstream source areas would have potential to recontaminate
the remediated SSTOU and proposed coordination with other operable unit remedial activities to ensure that
recontamination would not occur. It was assumed in the ROD that sedimentation basins would be constructed
in Butte to control those sources. Construction of facilities to control runoff from the major sources on the Butte
Hill is currently underway under the Priority Soils Operable Unit of the Silver Bow Creek/Butte Area NPL Site.
However, the issue of potential recontamination of the floodplain from upland areas adjacent to the SSTOU
was not addressed in the ROD. Ultimately, the upland areas will be addressed as part of the Butte Non-Priority
Soils Operable Unit, but work on that site is not yet underway. Contaminated run-on, particularly from the
Neversweat-Washoe railroad line immediately north of the Silver Bow Creek floodplain in Subarea 1, will need
to be controlled through construction and maintenance of sediment basins. Because this effort was not
identified as a potential remedial measure in the ROD, the construction of these sediment basins constitutes
a significant difference.
During remedial design of Subarea 1, seven locations were identified where run-on of contaminated materials
from the Neversweat-Washoe line would impact the remediated floodplain. Mapping of waste materials in the
Page 16 of 31
-------
Explanation of Significant Differences - SSTOU August 1998
railroad line embankment determined which tributary drainages could be affected, and Maxim designed
sediment basins in each of these drainages to trap sediment. The basins were sized to settle material from the
10-year, 24-hour rainfall event using the methods in the Montana Sediment and Erosion Control Manual (DEQ,
1996). Larger flow events will be passed through rock-lined spillways. The structures are intended to remain
in place until the Butte Non-Priority Soils Operable Unit is remediated.
Although design investigations of the downstream subareas of the SSTOU have not been conducted, it is
possible that the SSTOU remedy also may need to control similar run-on from other contaminated areas
adjacent to the remainder of the OU. DEQ and EPA therefore adopt the utilization of run-on control sediment
basins as potential necessary components of the SSTOU remedy for all subareas in order to provide protection
of the remediated floodplain until off-site contaminant source areas are addressed under other cleanup actions.
7. ELIMINA TION OF TREA TMENT WETLANDS AS DESIGN A TED END LAND USE FOR SUBAREA 1
The ROD specified that the end land use for Subarea 1 would be treatment wetlands. This designation was
in response to public comments on the Proposed Plan. ARCO and Butte-Silver Bow County were initiating
research on the use of treatment wetlands to control stormwater metals contamination and municipal waste
water nutrient discharge. If such treatment could be developed, ARCO and Butte-Silver Bow County indicated
a desire to use at least a portion of Subarea 1 as treatment wetlands to treat contamination from upstream, off-
site sources. The ROD allowed for that end land use after removal of contaminated tailings/impacted soils.
At the time of the remedial design for Subarea 1, plans for implementation of treatment wetlands had not been
developed, and it is uncertain whether any portion of Subarea 1 would be needed for wetlands treatment
systems. Therefore, the design for Reach A of Subarea 1 does not incorporate treatment wetlands. Any
implementation of future wetlands treatment systems would have to be constructed separately from the
remedial action for Subarea 1. Therefore, the end land use of Subarea 1 is not designated as treatment
wetlands, but can be any land use consistent with the requirements of the ROD and the goals of the remedial.
design.
8. CHANGES IN THE ESTIMATED SCHEDULE TO IMPLEMENT THE REMEDY
The ROD estimated that the SSTOU remedy could be implemented in four to six years. During the design
process, the agencies reevaluated the approach to scheduling the construction of the remedy and have revised
the schedule to provide for a 12-year construction period. This has been done for a number of reasons. The
primary reason is to avoid the significant risk of having large reaches of reconstructed streambank and
floodplain exposed to potential erosion during high flow conditions. By limiting the length of stream
Page 17 of 31
-------
Explanation of Significant Differences - SSTOU August 1998
reconstructed in each year, the agencies will limit the amount of unvegetated banks and floodplain exposed
at any one time. If approximately two miles of stream are reconstructed each year, a maximum of two miles
will be exposed with no vegetation during any one year. If construction were to be compressed into a 4-year
schedule, approximately six miles of newly constructed, unvegetated streambank and floodplain would be
exposed each year. Additional reasons to approach the project with a more conservative schedule include the
following.
to allow upstream sediment and water treatment controls to be placed into service before
implementing the remedy on Silver Bow Creek to minimize the risk of recontamination from upstream
sources;
to evaluate the stability of the new streambanks in the upper reaches of Silver Bow Creek in response
to high flow conditions and implement design modifications and improvements as appropriate in the
lower reaches later in the project;
to break the construction work into more easily managed units of two to three miles of stream at a time
to improve quality control rather than attempt to have very large major construction projects over a
short period; and
to reduce the impact of construction on local communities by reducing the size of the construction
operation at any one time.
For the reasons identified above, the agencies have adopted the revised schedule for construction of the
SSTOU selected remedy. Operation and maintenance are expected to continue in perpetuity.
9. INCREASE IN ESTIMATED COST TO IMPLEMENT THE REMEDY
While preparing the Preliminary Final Design Report for Reach A of Subarea 1, DEQ determined that the cost
estimates utilized in the FS and the ROD seriously understated the overall costs of the Streamside Tailings OU
remedy. The level of cost underestimation became apparent as OEQ's design engineers prepared detailed
cost estimates for the various components of the Reach A design. To correct the deficiencies in the earlier
estimates, DEQ carefully constructed a new cost estimate for the entire SSTOU, building on and updating
earlier cost estimates, and utilizing new information regarding the volumes of tailings/impacted soils and the
final design concepts and approaches developed during the Reach A design. DEQ's revised cost analysis has
been reviewed and approved by EPA. The revised cost estimates are presented in the in Tables 2 through 5.
Table 2 presents the summary calculation of total present worth for the SSTOU remedy. DEQ currently
estimates that the remedy will require approximately $98.14 million to construct and that construction will occur
over a 12-year period. The present worth of construction is estimated at about $76.09 million in 1998 dollars,
Page 18 of 31
-------
TABLE 2
CONSTRUCTION AND O&M SCHEDULE AND PRESENT WORTH CALCULATION
STRE AMSIOE TAILINGS OPERABLE UNIT
SILVER BOW CREEK/SUITE AREA NPl SITE
AUGUST 1998
Inveumenl Rale el Return = 7%
InrTalion Rate « 3%
Nel Ducount Rale • 4%
Yew
1 (1998)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1?
18
19
20
21
22
23
24
25
26
2?
28
29
30
Perpetuity
Toll) Eip«nt«s
Subar«« SuOror.au
1M8Pi*>*nlWortfl
Subarea Sutxolali
Subarea 1
Tailings Railroad O&M
4.000,000 100.000
8.400000 360,000 74.000
8.500,000 360.000 94,000
94000
89.000
69.000
69.000
69,000
69.000
82.000
82.000
57.000
47.000
42.000
42000
42000
42000
42000
42000
42.000
42.000
42.000
42.000
35.000
35.000
35.000
35.000
35.000
35.000
35.000
875.000
20.900.000 820.000 2.435.000
24.155,000
1S.tM.MS 741.013 S1.2)1,OC4
21.150.991
Subarea 2
Ta*no» Railroad O&M
9.000.000 220.000
9.100.000 220.000 74.000
9,100.000 220,000 94.000
94.000
89.000
69.000
69.000
69.000
69.000
82.000
82.000
57.000
47,000
42.000
42.000
42,000
42.000
42.000
42.000
42,000
42.000
42.000
42.000
35.000
35.000
35.000
35.000
675.000
27.200.000 660.000 2.330.000
30.190.000
22.M4.I17 542,760 S1.0SS.037
24.002.424
Subarea 3
Tailings Railroad O&M
-
6.000.000 600.000
6,100.000 600,000 77,000
60.000
60,000
70.000
70.000
72,000
72.000
72.000
65.000
60.000
40.000
40.000
37,000
37,000
37,000
37.000
37.000
37.000
37,000
37.000
37,000
37.000
25.000
625.000
12.100,000 1.200.000 1.818.000
15.118,000
S.OIi.717 8S4.US $820,181
10.731.263
Sutwrea4
Ta*no» Rerlroad O&M
.
.
.
.
8,700.000
8.800.000 60.000 74.000
8.800.000 60.000 94.000
8.800,000 94.000
89.000
69.000
69.000
69,000
69.000
82.000
82.000
57.000
47.000
42.000
42.000
42.000
42.000
42.000
42.000
42.000
42.000
42.000
875.000
35100.000 120.000 2.14B.OOU
37.368.000
21.270.23C 7»,60» S8M.118
24.245862
Total
Tailings Railroad O&M Total
4,000.000 100.000 • 4.100.000
6.400.000 360.000 74000 8.834000
8.500.000 360.000 94.000 8.954.000
9.000.000 220.000 94.000 9.314.000
9.100.000 220.000 163.000 9.483.000
9.100.000 220.000 163.000 9.483.000
6.000.000 600,000 163.000 6.763.000
6.100.000 600.000 235.000 6.935.000
8.700.000 • 218.000 8.916.000
8.800. 000 60.000 305.000 9.165.000
8.800.000 60.000 315.000 9.175.000
8,800000 290.000 9.090.000
290.000 290,000
265.000 265.000
240.000 240.000
223.000 223.000
213000 213.000
206.000 206.000
206 QUO 206.000
178 000 178000
168 000 166.000
16J.OOO 163000
163.000 163.000
156.000 156.000
156.000 156.000
156.000 156.000
149.000 149.000
149.000 149000
14900U 149000
137.000 137.000
3 250 000 3 250 000
95.300.000 2.BOO 000 » /31 000 106.831 000
checksum 106831000
71.820,484 2,2t6,C6C 4,044.1}* 80,110.640
checksum 80 1 30 540
Half All exf*ndi(ui
-------
TABLE 3
TAILINGS/IMPACTED SOILS/SEDIMENTS REMEDY COST ESTIMATE
STREAMSICE TAILINGS OPERABLE UNIT
SH.VER BOW CREEK/BUTTE (WEA NP
AUGUST I9M
0««cnMm LM I** Coo)
S«con«»rY *044*io»Eav 4cco*«i •«•• I 2OOOOOO
«~«.r» *o»*i 4« ««• > '000000
S«C*«HV *w t 'ODOOOO
'onang « t 100
«»*oMCrora»i«l-O«»lonl «« t 2.90000
•Mood Cra»»*i«« • EJvcf »*» M 1 Z9 000 00
TtMecom r > 'ooooo
Bftoo* • ••4K4*Morc • 4o«d «o 1 10 OOO 00
CUvMOn* M t 10000
04WWVI* front*** • 1 400
ji»imn««o»» •* > 300000
EravonCaM M 1 30000
UcovMOX CT 9 2.00
SIM*** 4n 1 (0000
r*4n**»«B*l iMO •*•«•» «T t 1.29
STMS AMOCO*** CT > 1.10
MSjroiooMCuma CT 1 200
Flora*** If 1 79000
Cluiwol CM CoMuokon » 1 3900
fin: »•!.«• ijimn. K S 1)0000
OoomOrwoM CT I 20.00
&AMIOI neoOPBM
Ejuvooon M 9*«»**o cr 1 100
Tlon* nor*m*rt CT t 12.00
Or*«r*ofi ComBVCoon *• S SO 000 00
O~rma***mo.* CT I 2.00
T>««c» 0*0*1 >M Cn«»noii«i CT > ' 00
• ••••MM «OC*J*O 4C S 1.00000
ItAloi*! Str*aM &»*I»OM
MM* W*M* «»p*o«e>101
IMAC4M04C« 4C 1 100000
O*a»fl*G«*> »c 9 WOOD
an* »«» an cioMi CT I oao
UnlOIOJ^IBI* 4C
Ser*4*w* MM Win** CT S 0*4
L.»* *I*»*MI*U CT 1 2-30
C. •LIN CT > 012
«»c» M va •« 1 10000
S»* M* ftodwto cy 1 O.*0
t«ci»«ioo CT 1 190
•««J t«fov(«1 mtml cy t 1 TO
*%tfl»ilin <>1 «m«) cy S 1.2S
o»e«OK» Sol Coxw CT I 1 00
«i i»n«i»i M 1 '90000
SiootM rm< CLBI^J' •••! Catf*
OvwnMtf ir«*cw« »f fc^iai^ *l>«0 Ciniiirwuon C««*l
3 .
;<«>»« «n»g 0««g» t 19%
CmmcaanOvwii0« i« 19%
Cw*«n4CbOM CemMMO** W 2O%
OAHM«V C««
10 t 200.000
'i roooo
<• t 1WOOO
UOOO 1 110000
10 1 29.000
2 > 90000
441 < *M.OOO
I S 10000
irj 9 140000
44000 9 I'lOOO
10 1 00000
171 9 92.900
•7S.OOO 9 VUO.OOD
4«* 9 17*400
400.000 9 •0.000
179.000 9 MS.790
0 9
900.000 9 1.000.000
171 9 111.290
27500 1 M1.900
179 9 20.900
2.900 9 90.000
9 • «3I 300
77.000 9 211000
23.000 1 2»4000
2 9 (O.OOO
1.760 9 1.920
99.000 9 99000
a 9 n.ooo
9 01520
40 9 40.000
23 9 20.000
M.OOO S 34400
9
aM.OOO 9 439200
MOOOO 9 1.700000
MO 000 9 217 MO
MOOD 9 (4010
29 9 17.900
10.000 9 W.OOO
9 2.912.700
90 9 40.000
40.000 9 24.000
iro.ooa 9 9*>.on
1*1000 9 114.900
119.000 9 «01290
40000 9 40000
90 9 79000
9 2.011790
9 14 IMT70
9
9 22341*1
9 2.*7t «M
Ojuamv COM
10 1 200000
'1 70000
11 t '40000
77000 1 1X000
7 1 17900
1 t 21000
149 1 4MOOO
0 1
170 9 2*0000
W.OOO 9 1(2.000
31 1 M.OOO
170 9 111.000
7*4000 9 1 Mo OOO
4*9 9 174.400
1*7,000 9 «74.*00
1*7.000 9 1JMJ30
1M.009 9 V 041.400
9t«.OOB 9 1032 ODD
2*9 9 1M.790
W.OOO 9 10U.OOO
170 9 911.000
2.900 9 90.000
9 11 704*00
7*000 9 234.000
U.OOO 9 IH.OOO
2 9 80 000
1.7*0 9 1120
60.000 9 flO.OOO
a 1 22.000
9 SM9JC
•4 9 44.OOO
40 9 12.000
100.000 9 00000
9
7** 000 9 911140
71*000 9 19*7100
7*4000 9 2994(0
100.000 9 100000
40 9 n.ooo
10.000 9 9*000
9 117*140
ro 9 94.000
M.OOO 9 U490
1*7.000 9 5*0X30
294.000 9 44O1OO
111.000 9 414000
MOOD 9 MOOD
70 9 101000
9 2 074 400
9 17 991 200
1 2 912.***
9 :f}3M*
9 3JI023I
**«r«« J
rttmn, Co«
9 f laoooo
7 1 '0000
12 1 120000
97000 1 -MOOO
10 I 29000
2 I 90.000
112 9 71.000
0 9
110 1 9*000
44.000 9 171000
9 9 19.000
110 9 13.001
240.000 9 4*0.000
.III 9 244*00
90.000 9 U.OOO
1*0.000 9 417.900
a 9
1MOOO 9 112.000
110 9 (2.100
27.900 9 e*2JOO
110 9 141.000
1.000 1 20.000
9 9M4.70O
K.OOO 1 97000
2.000 9 24000
2 9 00.000
1.7*0 9 1.920
17.000 9 17.000
• 9 (.000
9 K7 920
1* 1 1*000
10 < 1000
24000 9 19.400
9
2*1.000 9 14704O
2*1.000 9 012.900
2*1.000 9 13110
21.000 9 2*000
21 9 17900
1000 9 10.000
9 1 034.1*9
• 2O 9 14.000
II 000 9 »400
117 000 9 179.100
21.000 9 41.100
(2.000 9 29*000
14.000 9 1*000
20 9 10000
9 7O1400
t 7 7*1 4*0
9 3** 7(*
9 19000
9 i inw7
1 1 1441(7
1 1 914.1*9
1 4 3O77((
9 170447O
(^•••4
Ouomi coot
19 t 300000
19 1 190000
24 S 340 OOO
90000 t 1*0.000
1 9 7900
2 9 90000
•74 1 074000
0 1
719 9 404.000
97000 9 221.000
3* 9 117.000
79* 9 224.100
4*4.000 9 1TO.OOO
«24 9 40JOO
441.000 9 7*1.700
403400 9 1301.710
494.000 9 2.(*9.*aO
1*2.000 1 1.124.000
XB 1 221.790
34.000 9 1 .2*0.000
79* 9 1.132.900
9.000 9 100.000
9 1*044000
M.OOO 9 2*2.000
22JBD 1 2*4000
2 9 40.000
1.7SO 9 1.920
72.000 9 72.000
27 9 27 OOO
9 70*120
*• 1 W.OOO
90 9 40.000
110.000 9 41 OOO
9
(44.00O 9 1UKO
•44000 9 2.1*0.000
•44.000 9 274440
110000 9 110.000
90 1 79.000
17.000 9 101.000
9 1.410440
71 1 40.000
00.000 9 MOOO
422.000 9 903000
1*7.000 9 2*3(00
2S5000 9 «2»7»
40000 9 40.000
71 1 112.900
9 2414190
9 22. n* 110
9 1 131 (94
1 19000
9 ) 3H M7
9 3 3*1 147
9 4 927 121
9 1244*911
9 19.149.N1
T«»
rtmm, Co«
4* 9 ••*.*••
71 9 7Mo«*
2n.«*4 9 971.044
14 9 71.4M
7 9 179.44*
i.in 9 i.a*.***
1 9 M.04*
1.410 9 1.12*.***
1CLOM 9 771.***
144 9 31«.*«4
Ml* 9 I1I.H*
ZJ7V404 t 9.W4.M*
1jn 1 1^*7.(*»
1.I9*.*** 1 1TT1***
1JH.99X 1 4.111J**
I.7X.I4* 1 X714J4*
1.7X«4* 9 1.1**. 4*1
04 9 M1J9*
12t.**4 9 4.2UJ4*
Ml* • 2.11U4*
11.04* 9 22*.**4
10.404 9 Mo.***
M.4M 9 •<*.•*•
II ,Jtt*
7.444 9 4.4X
2*>.(l* 9 14 1.4**
n 1 77.(4»
9 2.107.11*
•71 • 1*0**
MOLOOO -;004
J
M44.0a* 9 1.1(1.149
2.400.H* 9 «.9».***
2.W4.0** 9 •SU9»>
344.44* 9 344.4(4
U4 9 >1*.*4*
44X4M 9 2**.*4*
9 14J71.M*
1 1J*V*** 1 •**• (44
I-!* 9 177.044
172.4(4 9 14X24*
U**.*** 9 t.M*.***
ni.441 9 1.**1J*4
•ML*** 9 2.141.***
in,*** 9 172,0**
211 9 12U44
9 7.2U.44*
9 H.m.12*
1 3.144.2*1
9 (4.04*
9 7.I*7.*U
9 (.41Z.W3
9 12.977.12.
9 12.412.»1
9 *1.2*7.(21
MOiOT
• mtmMnmt
urro •** CM • iziami
1 »04» »»»«*»•.
910Mo*|2*4>*l>»
Tp-jinn ^>*»«
B*MMonS4>v..caoilO " *•
jr-ir*- —
SST-i «*M«P cm**
Page 20 of 31
-------
TABLE 4
RAILROAD MATERIALS REMEDY COST ESTIMATE
T-AIUNGS o«>6R*Bi.e UNIT
SILVER BOW CREEK/BUTTE *R&» NPt. srTE
AUGUST 1908
Oaacription Unit Unit Coal
'
Soil Ramoval •* CmoankiMnt .Slraafn Mtarfaca
£tcavalon cy 2.00
-UU -o Raoo*K»v cy 3 30
. P'ltafFjonc O 020
OravaO/Sanu cy 1000
1'Cvtp cy 20 00
Su)fmn E/rearwam-Sva'arti mawfaea
Soil Rwnoval «l Brio?* AbMtmaflta
Rtmov, C«OTl/Matanal M S '390000
3iOrap cy S 20 00
Sunn* Brag* Anutiwm
Railroad RafOutMRaalacamafll
Cjosang Bndga RvnovH aa> 1 5.000 00
ConmclNwenog* •• 190.00000
fraen Crottcwi/SMIOm « 2O.OOO 00
eaott cy 300
Cdmpaclon cy 2.00
!%c« ranvorary T™« cy 5 00
Ramon TampcrarY Track aa 20.00000
Skjotnw Ra*n*d RarouMMapMnniOT
f loodpltin Protection v«)GnA cy 130
ucavtton cy 1 73
CofflMcton cy }00
OuMtsncwn <• t. ooo oo
'rax? f 800
Kt£iow C«Knram B«»n*
|RMKX»I ol CpBc>mv< It 5.00000
EzcawMn cy 20 00
Dfl-KM Owoul cy 100 00
SKKM cy 900
Cormcten cy 3 00
S<£toui Cancinrat* Rtmovn
SuitoM F«tM Constucton Cc«v
O»«KMa (PncMil al SuMoul FliX Conduction Co*.)
uoaw«O«n«Mn • 3«
EngnMnntOrapt It 13%
CmtucMn Ovmyi • 15%
Connuctai Corantmon • 20%
Steam Owtma
SuMr.4 1
QtianlKy COM
3800 t 7600
1SOO S 13.300
60000 t 12000
370 S 3.700
3200 $ MOOD
t 100.600
3 t 10.300
300 t 6.000
f «6.300
1 t 19.000
1 f 130.000
1 ( 20.000
0 «
0 t
0 S
0 S
I 1U.OOO
8300 S 102.000
1.000 t 1.730
I.OOO 1 3.500
t 107250
2.400 S 3600
6.500 S 11375
6500 S 19.900
9 S 9.000
5000 S 30000
S 69873
3 S 15.000
50 S 1.000
30 S 5.000
50 S 150
50 S 250
f 21. MO
< 530625
1 2«.531
S 71994
S 7I5S4
S 108.125
I 291.M4
1172.461
SuMrM 2
Outnlity COM
1500 t 7600
3.MO S 13300
60000 S 12.000
370 I 3700
3200 S «4000
1 100600
3 t 40500
300 t 6000
t 4«500
0 *
1 S 150.000
0 S
0 S
0 S
0 <
0 <
( 150.000
8.500 S 102.000
1.000 f 1.750
1000 t 3.500
t 107250
0 t
0 t
0 t
0 S
0 1
f
3 S 15000
50 1 1.000
50 t 5.000
50 » 150
50 I 230
S 21400
t 425.750
1 21 2U
S 638O
f 63.813
I M.I SO
S 234163
1 •It.'ll
SubflrM 1
OiuMHy Con
20.0OO ( *0 000
20000 t 70000
!SOOOO S 30000
8 OOO S 80.000
'2.000 S 240000
S 460000
3 t 40500
300 S 6000
< 46300
1 t 15000
1 « 130000
0 t
0 f
0 S
0 $
0 S
f 165000
8.500 S 102.000
I.OOO S 1.750
1000 S 3.900
t 107250
0 S
0 t
0 S
0 S
0 S
I
3 S 15000
50 S 1000
50 S 5.000
50 I ISO
SO > 290
t 21400
f 800.150
S 4000*
t 120.021
I 120.023
t 160030
S 440083
I 1.ZM.2U
3uB*r**4
Quanuty Co*i
0 1
0 S
0 S
0 I
0 I
1
1 1 13900
!OO S 2.000
< 15900
0 S
0 S
0 <
0 S
0 S
a s
0 <
<
5 OOO S 60000
80O S 1.400
800 S 2.800
t 84.200
0 S
0 S
0 f
0 S
0 S
I
0 S
0 S
0 S
0 S
0 S
s
S 79700
t 3 MS
* 11.994
* 11.909
S 15940
S 438J3
i in sis
c
Tolll
Quantify Co«f
17*00 US.200
27.800 IH.800
270.000 SMOOO
8.740 U7.400
11.400 I3U.OOO
SMI. 200
10 1113.000
1.000 110.000
1115.000
2 (30.000
1 U90.000
1 120.000
0 10
0 10
0 U
o to
1500.000
10.SOO I3U.OOO
J.8OO K,(90
X800 119.100
I3M.680
1400 UMO
8.500 111 ;•»
«.joo m.soo
* n.000
9.000 OO.OOO
ta.879
1 -' MO
190 U.OOO
190 115.000
ISO U90
190 (750
1*4.300
1 I.838.Z79
.1- ill
U79.4J4
1779.434
(MT.Z49
n.on.tu
% 2.644) 14*
XKUtjm ( 2846 149
Hal*m
DISOOUI atiocafUWIR
OatxaM u txm WWRR
OUROI4I tl «C R> C ••Cl-y
Page 21 of 31
-------
TABLE 5
ANNUAL OPERATIONS AND MAINTENANCE COST ESTIMATE
STREAMSIDE TAILINGS OPERABLE UNIT
SILVER BOW CREEK/BUTTE AREA NPL SITE
AUGUST 1998
Year
Stream
Diversion
Per Subarea, Subareas
1
2
3
4
5
6
7
8
9
10
11
12
13
T4 thru 23
24 and after
Subarea 3
1
2
3
4
5
6
7
8
9
10
11
12
13
14 thru 23
24 and after
$0
$5,000
$10.000
$10.000
$10,000
$10,000
$10.000
$10.000
$10.000
$0
$0
$0
$0
$0
$0
Flood plain
Remedy
1.2,4
SO
$7,000
$17.000
$17.000
$17.000
$2.000
$2.000
$2.000
$2.000
$2,000
$2.000
$2.000
$2.000
$2,000
$5.000
Streambanks
$0
$3,000
$3,000
$3.000
$3,000
$3,000
$3.000
$3,000
$3,000
$18,000
$18.000
$18.000
$8.000
$8,000
$10.000
Borrow
Areas
Repositories
Railroad
Berm
Weed
Control
$0
$5.000
$5,000
$5,000
$0
$0
$0
$0
$0
$0
$0
$0
$0
$0
$0
$0
$2.000
$7.000
$7,000
$7,000
$2.000
$2.000
$2,000
$2,000
$2,000
$2.000
$2,000
$2,000
$2,000
$5.000
$0
$2.000
$2.000
$2.000
$2,000
$2.000
$2.000
$2.000
$2.000
$10.000
$10.OOO
$5,000
$5.000
$0
$0
$0
$15.000
$15.000
$15.000
$15,000
$15,000
$15,000
$15.000
$15.000
$15,000
$15.000
$15.000
$15,000
$15.000
$15.000
$0
$2,000
$5,000
$5.000
$5.000
$5,000
$0
$0
$0
$0
$0
$0
$0
$0
$0
SO
$7.000
$7.000
$7,000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$0
$8,000
$8.000
$8,000
$8,000
$8.000
$13.000
$13.000
$13.000
$8.000
$3,000
$3.000
$3.000
$3,000
$5.000
$0
$5.000
$5.000
$5,000
$0
$0
$0
$0
SO
SO
$0
$0
$0
$0
$0
$0
$2,000
$2,000
$2,000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$2.000
$3.000
$0
$3.000
$3.000
$3,000
$3.000
$3.000
$5,000
$5.000
$5,000
$3,000
$3.000
$3.000
$3.000
$0
$0
$0
$15.000
$15.000
$15.000
$15.000
$15.000
$15.000
$15,000
$15.000
$15,000
$15,OOO
$15,000
$15.000
$15.000
$15,000
Long-term
Monitoring
Total
$0
$35.000
$35,000
$35,000
$35.000
$35.000
$35.000
$35,000
$35.000
$35.000
$35,000
$15,000
$15.000
$15.000
$0
$0
$74.000
$94,000
$94,000
$89,000
$69.000
$69,000
$69.000
$69,000
$82,000
$82,000
$57,000
$47,000
$42,000
$35,000
$0
$35.000
$35.000
$35,000
$35.000
$35.000
$35.000
$35.000
$35.000
$35.000
$35,000
$15,000
$15.000
$15.000
$0
SO
$77.000
$80.000
$80,000
$70,000
$70,000
$72,OOO
$72,000
$72,000
$65,000
$60,000
$40,000
$40,000
$37,000
$25,000
esd cost.jds
Page 22 of 31
-------
Explanation of Significant Differences -SSTOU August 1998
assuming a net discount rate of 4% (investment rate of return of 7% less inflation rate of 3%). Total operation
and maintenance (O&M) costs, including funds reserved to cover O&M in perpetuity, are estimated at
approximately $8.73 million, with a present worth of about $4.04 million in 1998 dollars. Total present worth
for construction and O&M is estimated at approximately $80.13 million in 1998 dollars.
Table 3 presents the detailed construction cost estimate for the combined tailings/impacted soils and in-stream
sediments remedies, since these two media are addressed together in the current RD/RA approach. Table
4 presents the detailed railroad remedy cost estimate. Table 5 presents the detailed estimate for annual O&M
costs for the SSTOU. These three tables represent DEQ's best estimate of the cost to implement the SSTOU
remedy as currently designed. Additional costs necessary to meet protectiveness requirements, such as
improved MWRR design elements, are incorporated into the revised estimate. Cost savings achieved through
design, such as less expensive approaches for addressing in-stream sediments, channel stability, and backfill
needs, are also incorporated into the revised cost estimate.
There are two key reasons that the SSTOU remedy cost estimate has been substantially increased during the
remedial design process. First, the SSTOU FS and ROD underestimated the volume of tailings/impacted soils
within the OU. This issue is discussed in detail in ESD Item 1 above. In Subarea 1 alone, the vc;ume of tailings
delineated during detailed RD is 130 percent larger than that estimated in the FS and ROD. OEQ anticipates
that the degree of disparity between the RO and FS/ROD volume estimates will decrease in the downstream
reaches of Silver Bow Creek. Without detailed design information for downstream subareas, the agencies now
anticipate an approximate 30 to 80 percent increase in the volumes of tailings/impacted soils over the estimates
used in the FS/ROD. For the purposes of the cost estimate, an approximate 50 percent increase is assumed.
Second, to better estimate the full cost of the remedy, the current cost estimate includes a number of additional
cost items that DEQ expects will be incurred during construction. Such items include utility relocation, fencing,
security, flagging and traffic control, railroad crossings, surveying, construction dewatering, and purchase and
import of fill material. In addition, a construction contingency, to account for potential costs undiscovered until
construction is underway, is also included. Other cost items underestimated in the FS/ROD include repository
construction, engineering design, construction oversight, and revegetation.
A detailed comparison of the 1995 SSTOU ROD cost estimate and the current RD/RA cost estimate is included
as Table 6. The table presents only nondiscounted construction costs (not present worth). The ROD estimates
are the maximum cost scenario for the selected remedy and they have been increased by 3 percent per year
for three years to adjust them to 1998 dollars for comparison purposes. Overall, the difference between the
current RD/RA cost estimate and the ROD cost estimate is approximately $49.21 million, in 1998 dollars.
Page 23 of 31
-------
TABLE 6
COMPARISON OF CONSTRUCTION COST ESTIMATES (1995 ROD vs. 1998 RD/RA)
STREAMSIOE TAILINGS OPERABLE UNIT
SILVER BOW CREEK,BUTTE AREA NPL SITE
AUGUST 1998
Construction Cost Estimate Line Item
Cost Elements Not Included in ROD
Utility Relocation/Protection
Security
Fencing
Traffic Control
Construction Railroad Crossings
Bridge Replacement
Construction Dewatenng
Surveying
Stream Channel Bed Construction
Oebns Disposal
Stream Diversion
Purchase/Excavate/Haul Borrow Material
Construction Contingency
Potential Historic Preservation Mitigation
Subtotal
Cost Elements Underestimated in ROD
Erosion Control
Tailings Hauling
Till Placement/Grading
Streambank Reconstruction
Floodplain Revegetatkm
Repository Construction
Mobilization/Demobilization
Design/Construction Oversight
Railroad Remediation
Subtotal
Cost Elements Overestimated in ROD
Site Clearing and Grubbing
Sediment Control Basins
Tailings/Sediments Excavation
STARS
Construction Overhead (c|
institutional Controls
Road Construction
Subtotal
Total Construction Costs
ROD Cost Estimate
(1995$)
SO
SO
SO
SO
SO
SO
SO
so
so
$0
so
so
so
so
so
$72.146
$1.800.864
S2.480.61 2
$4363.520
$1 467.095
S3. 252. 053
$1.828.221
$5.190.461
$1 969 764
$22.924 736
$2.594.480
S368.456
$6966.120
$5.443 355
$4.570 551
$750.000
$1 161 640
$21 854 502
S44.779.338
(1998$([al
$0
SO
so
so
so
so
so
so
$0
so
so
so
so
so
so
$78,836
$1.967.853
$2.710.632
$5.314.500
$1.503.134
$3.553,606
$1.997,746
$5.671 .757
$2.152.414
$25.050.478
$2.835.058
$402.672
$7.612.067
S5.948.t01
$4.994.364
$819.545
$1.269.355
S23881.112
S48.931.S90
RD/RA Cost
Estimate (1998S)
$200.000
$720.000
$578.000
$1.638.000
$250.000
$80.000
$1.333.600
$1.497,600
S4.235.000
$220,000
$2.107.080
$7.235,900
$12.577,124
$60.000
$32.732,304
$423.000
S6.334.850
$4. 109.250
$8.045.000
$2.115.000
$10.275.840
$3.144281
$16.630.796
$2.846.149
$53.924.166
$1.128.000
$318,000
$5.146.000
$3.735.500
SO
SO
SI. 160.000
$11.487.500
S98. 143.970
Cost Estimate
OifferencefbJ
5200.000
$720000
$578000
$1 638000
$250.000
$80.000
$1 333.600
$1 497.600
$4.235000
$220000
$2.107.080
$7.235.900
$12.577 124
$60.000
$32.732 304
$344 t64
$4366997
$1 398613
$2.73C.500
$511 366
$6.722.234
$1 146.535
$10559.039
$693735
$28 373 688
I
($1 707.058)
($84622)
($2.466067)
($2.212.601)
($4994 364)
;$81954S)
($•09355)
f$l2 393 512)
S49.212.380
esa cost xis
Notes [a] 1998$ calculated by compounding ROD 1995S by 3% per year for 3 years
(b| Cost estimate difference = RD/RA estimate (1998$) - ROD estimate . 1998$)
(cj RD/RA estimate includes construction overhead in line item unit ccsts
Page 2-* of 31
-------
Explanation of Significant Differences - SSTOU August 1998
Cost Consequences of Changes in Volume Estimate
The FS/ROD estimated the volume of tailings/contaminated soils in the OU at approximately 2.55 million cubic
yards (cy). The present estimate of tailings/contaminated soils is approximately 3.81 million cy. This represents
approximately a 50 percent increase in contaminated materials that must be addressed at the OU. In
developing its revised cost estimate, OEQ reviewed in detail all line item cost elements and estimates that
approximately $17 million of the additional $49.21 million of SSTOU remedy costs are necessary to address
the increased volume of tailings now identified or assumed at the OU. This figure was developed by comparing
the difference in total construction cost using current volume estimates with that using FS/ROD volume
estimates, with the calculation based on DEQ's current estimates of unit costs and ancillary costs, as well as
current design criteria.
Costs Not Included or Underestimated in FS/ROD Documents
The original cost estimates in the FS/ROO were primarily based on the ARCO analysis of costs presented in
the FS Appendix F-3 (ARCO, Draft Cost Estimate Methodology for Streambank Tailings Removal and In-Situ
Treatment, May 23,1994, with appended technical memoranda). In finalizing the FS and ROD, the agencies
made minor modifications to the ARCO analysis, primarily to eliminate duplication of costs that occurred when
media-specific components were assembled into site-wide alternatives for comparison purposes. The agencies
did not comment extensively on or require modifications to the ARCO cost analysis. As long as the costs were
applied consistently across the various alternatives, the agencies believed that the analysis we..- . . - ;ccurate
enough to adequately compare relative cost differences among the various alternatives.
Cost line items included in the current SSTOU cost estimate are shown in Table 6. The line items have been
grouped into three categories: (1) those that were not specifically included in the ROD cost estimate. (2) those
that generally were underestimated in the ROD compared to the current estimate, and (3) those that appear
to have been overestimated in the ROD compared to the current estimate. All construction costs identified in
the ROD and the current cost estimate are included. Operation and maintenance costs are not included.
The first group of line items in Table 6 were not found in the cost estimate spreadsheets in the FS/ROD,
although some of the backup analysis by ARCO indicates that at least a portion of these costs were considered
in the "construction support" category of ARCO's estimate. However, DEQ has concluded that since the
construction support category as a whole was underestimated in the FS/ROD analysis, all costs in the first
group of line item costs are considered by DEQ to be "new costs" not included in the original ROD estimate.
A total of $32.73 million in new costs are included.
Page 25 of 31
-------
Explanation of Significant Differences • SSTOU August 1998
The FS/ROD also significantly underestimated certain construction, engineering design, and construction
oversight costs. These are shown as the second group of line item costs in Table 6. DEQ believes that the
tailings/impacted soils construction line items were underestimated primarily because they were based on
inaccurate volume estimates. Design and oversight costs were underestimated because they are calculated
as a percentage of total construction costs, which was underestimated. The FS/ROD also underestimated the
cost for remediation of contaminated railroad berm materials. A remedy element not anticipated in the FS/ROD
is the construction and maintenance of sedimentation basins in Subarea 1 to control contaminated run-on from
Neversweat - Washoe railroad berms located outside of the SSTOU. The need for these basins is discussed
in ESD Item 6 above. Cost increases also result from the use of more realistic estimates for design and
construction oversight for the railroad materials remedy. As shown in Table 6, the revised cost estimate
includes an additional $28.87 million in previously underestimated costs from the FS/ROD estimate.
The FS/ROD also overestimated certain line item costs, in comparison with the current estimate. They appear
as overestimated line items simply because they account for costs that are included in other line items in the
current DEQ estimate. To account accurately for the overall difference in the current cost estimate from the
estimate in the FS/ROD, the overestimated costs in the third group of line items in Table 6 are deducted from
the cost increases presented in the other two groups of line items. The overestimated amount is $12.39 million.
Operation and Maintenance Costs
The ROD estimated the present worth of long-term operation and maintenance (O&M) at $2.71 in 1995 dollars.
The present worth calculation was based on a 7 percent discount rate over 30 years as required by EPA
guidance on preparing RODs. Adjusting for inflation, the ROD estimate is equivalent to approximately $2.96
million in 1998 dollars. DEQ currently estimates the present worth of long-term O&M at approximately $4.04
million. The current estimate utilizes a more reasonable net discount rate of 4 percent and also includes
estimated costs beyond the typical 30-year period to account for anticipated perpetual O&M at the SSTOU.
The increase in present worth for O&M over that presented in the ROD is therefore approximately $1.09 million.
REVIEW OF REMEDY SELECTION IN LIGHT OF NEW INFORMATION
DEQ and EPA have also reviewed the alternative selection process in the ROD to determine if one of the other
evaluated alternatives should be selected in light of new information. Seven OU-wide alternatives, comprised
of various combinations of media-specific remedial approaches, were considered in the ROD. Alternative 1
(no action) and Alternative 2 (primarily in-situ treatment of contaminated materials in the floodplain) were found
to be totally inadequate in terms of meeting threshold protectiveness and applicable or relevant and appropriate
requirements (ARARs). Alternative 3 (partial relocation and partial in-situ treatment) and Alternative 4 (partial
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Explanation of Significant Differences - SSTOU August 1998
removal and partial in-srtu treatment) were considered to be more protective than Alternatives 1 and 2, but also
did not adequately comply with protectiveness and ARARs requirements. Alternatives 3 and 4 were equally
protective, with Alternative 4 being the more expensive of the two. Modified Alternative 5 (more extensive partial
relocation with limited in-srtu treatment) was the selected alternative. Alternative 6 (more extensive partial
removal with limited in-situ treatment) was similar to Alternative 5 in protectiveness, but was more expensive.
Alternative 7 (total removal) met the threshold criteria, but was considerably more expensive than the selected
remedy.
The only ESO differences that directly affect the evaluation of the alternatives are the revised volume and cost
estimates. After consideration of these new estimates, the agencies have determined that the selected
alternative, as modified by the other changes in this ESD, remains the appropriate remedy for the SSTOU.
Generally, the relative cost comparison between the selected remedy and the other alternatives does not
change greatly as a result of the revised cost estimates. For example, there is no significant change in the
relative cost difference between the selected remedy and the next most promising, lower-cost alternative,
Alternative 3. Under the selected remedy, approximately two-thirds of the OU's tailings/impacted soils would
be excavated and placed into repositories and one-third treated in situ by STARS technology. Under Alternative
3. approximately two-thirds would be treated in situ by STARS and one-third relocated into repositories. Based
on cost estimates in the ROD, Alternative 3 was expected to be 13 percent less costly than modified Alternative
5. the selected remedy ($40 million vs. $46 million). Utilizing DEQ's current estimated volumes and unit costs,
the present worth for Alternative 3 is now projected to be approximately $67.8 million. When .;• --oared to the
estimated present worth of $80.1 million for the selected remedy, the relative cost difference between the two
alternatives remains about the same (Alternative 3 is 15 percent less costly).
All other analyses in the ROD concerning relative overall effectiveness and compliance with ARARs of all of
the alternatives remain unchanged. Alternatives 1 and 2 were totally inadequate in meeting protectiveness
requirements. Alternatives 3 and 4 would not adequately comply with floodplain and solid waste ARARs
because of their heavy reliance on in-situ STARS treatment near the stream. Likewise, the in-situ STARS
treatment of tailings in near-stream locations would not be consistent with the criteria identified in the ROD for
the application of the STARS technology and the necessary ARAR waiver. Alternatives 6 and 7 have equal or
better protectiveness compared to the selected remedy, but are more expensive. While there may be areas
where removal rather than relocation may be cost effective and may be included in the remedy (see discussion
of consolidation with off-site tailings in ESO Item 5 above), the increased costs generally weigh against cost-
effectiveness of these more expensive alternatives. DEQ and EPA have determined that, even with the new
information developed during design and with the additional cost noted in this ESD. the selected alternative,
as modified by this ESD, is the appropriate remedy under CERCLA and the National Contingency Plan.
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Explanation of Significant. Differences - SSTOU
August 1998
SUPPORT AGENCY COMMENTS
EPA concurs in and adopts the changes and decisions identified in this document for the reasons explained
above.
PUBLIC PARTICIPATION ACTIVITIES
DEQ and EPA strive to have full involvement by the public in all SSTOU activities. During the two-year remedial
design process, design meetings were open to the public and representatives of local government and local
interest groups routinely participated in those meetings.
This ESD and the information utilized to develop the ESD have been placed in the SSTOU administrative
record. The administrative record is located at the U.S. EPA Montana Operations Office, Federal Building. 301
South Park, Helena, Montana. Office hours are 8:00 to 5:00 on federal business days. In addition, the ESD
is placed in the SSTOU information repositories at the following locations:
Anaconda
Bozeman
Butte
Hearst Free Library
MSU Renne Library
Silver Bow Library
Montana Tech Library
EPA Office
CTEC Office
Deer Lodge Grant-Kohrs Ranch
Deer Lodge Public Library
Helena DEQ Office, 2209 Phoenix Ave.
State Library
Missoula Missoula Public Library
UM Mansfield Library
Clark Fork Pend Oreille Coalition
AFFIRMATION OF THE STATUTORY DETERMINATIONS
Considering the new information that has been developed and the changes that have been made to the
selected remedy, DEQ and EPA believe that the selected remedy, as modified by this Explanation of
Signification Differences, remains protective of human health and the environment, complies with Federal and
State requirements that are applicable or relevant and appropriate to this remedial action or involves
appropriate waivers of these requirements, and is cost-effective. In addition, the revised remedy utilizes
permanent solutions and alternative treatment technologies to the maximum extent practicable for this site.
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Explanation of Significant Differences - SSTOU August 1998
APPROVAL
Mart/A. Simonich, Director Date
Montana Department of Environmental Quality
Max H. Dodson, ARA Date
Office o1 Ecosystems Protection and Remediation
U.S. Environmental Protection Agency
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Explanation of Significant Differences • SSTOU August 1998
REFERENCES
ARCO, 1995a. Draft Remedial Investigation Report. Streamside Tailings Operable Unit. Silver Bow
Creek/Butte Area NPL Site. January 1995.
ARCO, 1995b. Draft Feasibility Study Report. Streamside Tailings Operable Unit. Silver Bow Creek/Butte Area
NPL Site. June 1995.
ARCO, 1997a. Final Comprehensive Remedial Design Work Plan. Streamside Tailings Operable Unit, Silver
Bow Creek/Butte Area NPL Site. Prepared by Titan Environmental Corp., January 1997.
ARCO, 1997b. Preliminary Design Report, Subarea 1, Streamside Tailings Operable Unit, Silver Bow
Creek/Butte Area NPL Site. Prepared by Titan Environmental Corp., January 1997.
ARCO, 1997c. Intermediate Design Report, Subarea 1. Streamside Tailings Operable Unit, Silver Bow
Creek/Butte Area NPL Site. Prepared by Titan Environmental Corp., March 1997.
COM, 1994. Draft Baseline Risk Assessment, Streamside Tailings Operable Unit, Silver Bow Creek/Butte Area
NPL Site, December 29, 1994.
CH2M Hill, 1989. Silver Bow Creek Flood Modeling Study. Prepared for Montana Department of Health and
Environmental Sciences. Helena, Montana. November 1989.
Inter-Fluve & MEI, 1998. Final Conceptual Design Report for Silver Bow Creek, SSTOU, Subarea One,
Remedial Channel Design. Prepared for the Montana Department of Environmental Quality, February
1998.
Maxim Technologies, Inc., 1998a. Streambed Sampling Data. Streamside Tailings Operable Unit - Remedial
Design. Memorandum to Andy Young. Mine Waste Cleanup Bureau, Montana Department of
Environmental Quality dated January 14,1998
Maxim Technologies, Inc., 1998b. Draft Final Alternatives Analysis for Mine Waste Relocation Repositories.
Streamside Tailings Operable Unit. Butte Silver Bow NPL Site. Report prepared for Montana
Department of Environmental Quality. August 1998.
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explanation of Significant Differences - SSTOU August 1998
Maxim Technologies, Inc., Inter-Fluve, Inc., Reclamation Research Unit, Bighorn Environmental, 1998.
Preliminary Final Design Report, Remedial Action, Subarea 1. Reach A, Streamside Tailings Operable
Unit. Silver Bow Creek/Butte Area NPL Site. Prepared for the Montana Department of Environmental
Quality, August 1998.
DEQ, 1996. Montana Sediment and Erosion Control Manual. Montana Department of Environmental Quality.
Helena. Montana.
DEQ and U.S. EPA, 1995. Record of Decision, Streamside Tailings Operable Unit. Silver Bow Creek/Butte
Area NPL Site. November 1995.
ME) & Inter-Fluve, 1997. Channel Stability Analysis, Silver Bow Creek SSTOU, Subarea 1. Prepared for the
Montana Department of Environmental Quality, November 1997.
U. S. EPA, 1989. Interim Final Guidance on Preparing Superfund Decision Documents: The Proposed Plan.
the Record of Decision, Explanation of Significant Differences, the Record of Decision Amendment.
Office of Emergency and Remedial Response, Environmental Protection Agency. Document No.
EPA/540/G-89/007.
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