PB97-963115
EPA/541/R-97/039
November 1997
EPA Superfund
Explanation of Significant Difference
for the Record of Decision:
Apache Powder Co.,
St. David, AZ
4/16/1997
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SFUND RECORDS CTR
0048-01013
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
REGION IX
75 Hawthorne Street
San Francisco, CA 94105-3901 A R 0993
MEMORANDUM
DATE: April 16, 1997
FROM: John KemmerjjFr Acting Superfund Enforcement Branch Chief
TO: Keith Takata, Superfund Division Director
SUBJECT: Apache Powder Superfund Site - Explanation of Significant Differences
Attached for your signature is the Explanation of Significant Differences (ESD) for the
Apache Powder Superfund site final Record (ROD). The purpose of the ESD is to enhance
the selected groundwater remedy for the perched and shallow aquifer groundwater and to add
additional flexibility in cleaning up contaminated soils.
The first component of this ESD allows for the perched groundwater to be extracted and
treated by constructed wetlands (rather than by a brine concentrator) in conjunction with the
extraction and treatment of the nitrate-contaminated shallow aquifer groundwater hi the
southeast portion of the site.
The second component of this ESD allows for two locations (a northern and southern
location) for siting the constructed wetlands to treat the nitrate-contaminated shallow aquifer,
including the use of a pipeline or several pipelines to carry the nitrate-contaminated
groundwater from the extraction wells to the treatment areas.
The third component of this ESD allows for the recharge of the treated perched and shallow
aquifer groundwater by gravity-flow pipeline discharge to a shallow aquifer recharge location
in Wash 3 for the northern area wetlands and to a shallow aquifer recharge location in Wash
6 for the southern area wetlands.
The fourth component of this ESD allows for additional shallow aquifer extraction wells to be
located in areas of high concentrations of nitrate to expedite groundwater cleanup.
The fifth component of this ESD allows for the characterization, removal, treatment, and off-
site disposal of any newly discovered contaminated soils materials not previously identified in
the ROD pursuant to an EPA-approved Soils RD Workplan.
The need for these modifications became apparent during the remedial design activities at the
site. We believe it is appropriate to address these modifications through an ESD at this tune.
inltd on Recycled I'apcr
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EPA provided the State of Arizona with a fifteen day comment period on this ESD.
Comments received from the Arizona Department of Environmental Quality and the Arizona
Department of Water Resources are included in Section IV of this ESD. In addition, EPA
will publish a notice in the San Pedro Valley News-Sun and the Arizona Daily Star
newspapers which describes this ESD and its availability for review. This ESD and all
documents that support the changes and clarifications herein will be contained in the
Administrative Record for the Apache Powder Superfund site prior to the commencement of
the remedial actions affected by this ESD.
Please contact Andria Benner at 744-2361 or David Rabbino at 744-1336, if you would like
any additional information regarding this ESD.
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APACHE POWDER SUPERFUND SITE
EXPLANATION OF SIGNIFICANT DIFFERENCES
for the FINAL REMEDY RECORD OF DECISION
April 1997
I. INTRODUCTION
On September 30, 1994, the United States Environmental Protection Agency
(EPA) signed a Record of Decision (ROD) for the final remedy at the Apache Powder
Superfund site in St. David, Arizona. The State of Arizona concurred with the remedy
selected in the 1994 ROD. EPA now is modifying the ROD to explain differences
between the remedy selected in September 1994 and the remedy currently under
design and planned for implementation at the site. These changes are not
fundamental alterations of the remedy described in the 1994 ROD.
Under Section 117 of the Comprehensive Environmental, Response,
Compensation, and Liability Act of 1980 (CERCLA), as amended by the Superfund
Amendment and Reauthorization Act of 1986, and pursuant to 40 C.F.R Section
300.435(c)(2)(i)(55 Federal Register 8666, 8852 (March 8, 1990), EPA is required to
publish an Explanation of Significant Differences (ESD) when significant (but not
fundamental) changes are being considered to a final remedial action plan as
described in a ROD. EPA has made a few important changes that modify the ROD
requirements but did not alter the hazardous waste management approach that EPA
selected in the ROD. The changes will enhance the effectiveness of the remedy and
promote more effective cleanup of the site. The purpose for each of these changes is
described in detail in Section III of this document.
This document provides a brief background of the site, a summary of the
remedy selected in the 1994 ROD, a description of how this ESD affects the remedy
originally selected by EPA in the 1994 ROD, and an explanation of why EPA is
making these changes to the ROD. EPA is issuing this ESD #1 in order to take into
account information received by EPA during the on-going groundwater monitoring and
field investigative activities related to the final design for cleanup.
This ESD proposes to modify the previously selected remedy selected for the
site as follows:
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(1) allow for the perched groundwater to drain to the shallow aquifer to be
extracted and treated by constructed wetlands (rather than by a brine
concentrator) in conjunction with the extraction and treatment of the nitrate-
contaminated shallow aquifer groundwater in the southeast portion of the site;
(2) .allow for two locations, a northern and southern location, for siting the
constructed wetlands to treat the nitrate-contaminated shallow aquifer, including
the use of a pipeline or several pipelines to carry the nitrate-contaminated
groundwater from the extraction wells to the treatment areas;
(3) allow for the recharge of the treated perched and shallow aquifer
groundwater by gravity-flow pipeline discharge to a shallow aquifer recharge
location in Wash 3 for the northern area wetlands and to a shallow aquifer
recharge location in Wash 6 for the southern area wetlands;
(4) allow for additional shallow aquifer extraction wells to be located in areas of
high concentrations of nitrate to expedite groundwater cleanup; and
(5) allow for the characterization, removal, treatment, and disposal in a
hazardous waste or solid waste disposal facility of any newly discovered
contaminated soils materials not previously identified in the ROD.
Although this ESD modifies the remedy for the perched groundwater in the
southern portion of the site, the implementation of this portion of the remedy will be
delayed at least two years while EPA evaluates the effectiveness of the constructed
wetlands to treat the nitrate contamination in the northern portion of the site. Nitrate is
a contaminant of concern both in the northern and southern portions of the site. The
results of the first treatment system in the northern portion of the site will provide
valuable data for the southern area. Operational and performance monitoring data
from the northern area groundwater wetlands system may lead to adjustments in the
extraction and treatment strategy for the southern area. After two years of study, EPA
may recommend that rather than building a second wetlands, the nitrate contaminated
groundwater from the shallow aquifer adjacent to the perched zone in the southern
area be pumped to the already constructed wetlands in the northern area.
This ESD and supporting documentation will become part of the Apache
Powder Administrative Record. Copies of the Administrative Record for the Apache
Powder site (including this ESD) have been placed at the following locations:
Benson Library EPA Region 9 Superfund Records Center
302 South Huachuca 95 Hawthorne Street - 4th Floor
Benson, Arizona 95602 San Francisco, California 94105
(602) 586-9535 (415) 536-2000
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EPA provided a fifteen (15) working day comment period for the State of Arizona in
accordance with 40 C.F.R. Section 300.515(h)(3). The State of Arizona's comments
on this ESD are summarized in Section IV of this document and are also included in
the Apache Powder Administrative Record file. Pursuant to 40 C.F.R. Section
300.435(c)(2)(i), EPA will publish a notice summarizing this ESD in a major newspaper
of general circulation. A formal public comment period is not required for an ESD.
II. BACKGROUND
A. Site Background and Description
The Apache Powder Superfund site is located approximately 7 miles southeast
of Benson and 2.5 miles southwest of St. David, in Cochise County, Arizona. The
Apache Nitrogen Products, Inc. (ANP) property comprises 945 acres. The site study
area (approximately 9 square miles) includes areas of nitrate-contaminated
groundwater and surface water located outside ANP's boundary. The San Pedro
River bounds the eastern side of the site running from the southeast corner of the
property north towards the northwest. The predominant topography is "badlands",
characterized by eroded ridges and hummocks dissected by northeast trending
washes. Approximately eight residences are located immediately north of the facility,
in the vicinity of monitoring wells 17 and 18. The San Pedro River National
Conservation Area (SPRNCA), owned by the Bureau of Land Management, is located
approximately two miles south of the site along the San Pedro River (Figure 1).
ANP began operation in 1922 as a manufacturer of industrial chemicals and
explosives. Currently, ANP manufactures nitric acid, solid and liquid ammonium
nitrate, and nitrogenous fertilizer solutions. ANP also distributes explosives materials
to mining companies. ANP has an interim status permit under the Resource
Conservation and Recovery Act (RCRA) for treatment of explosive wastes in its Ash
and Burn Area. The Ash and Burn Area, also known as the Open Burn/Open
Detonation (OB/OD) Area, is currently undergoing closure review by the Arizona
Department of Environmental Quality (ADEQ) under its RCRA program authority.
Prior to 1971, facility wastewater composed of washdown and blowdown waters
from its power house cooling tower and nitric acid plant, and from the loading,
unloading, and storage of raw materials and products, was discharged on site into dry
washes which flow to the San Pedro River. From 1971 until March 1995, wastewater
was discharged into unlined evaporation ponds on site. The combination of these two
activities resulted in nitrate-contamination of a perched groundwater zone, the shallow
aquifer, and the surface water of the San Pedro River. The site was first identified as
an environmental problem in the early 1980s, proposed by EPA for listing on the
National Priorities List (NPL) in 1986, and placed on the list in 1990.
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B. Remedies Selected in the 1994 ROD
The remedies selected in the ROD for the perched groundwater, the shallow
aquifer groundwater and soils are shown in the second column of the attached tables
1, 2, 3, 4 and 5. The proposed changes to the ROD included in this ESD are shown
in the fourth column.
III. MODIFICATIONS TO THE ROD REMEDY
A. Treatment of the Perched Groundwater by Constructed Wetlands in
Conjunction with Treatment of the Shallow Aquifer in the Southeast
Portion of the Site (Rather Than by a Brine Concentrator)
The ROD identified two separate groundwater areas for treatment: the perched
groundwater zone, to be treated by a brine concentrator, and the shallow aquifer, to be
treated by constructed wetlands. Because recent groundwater monitoring data
indicate both that the perched zone is dewatering very rapidly and that the nitrate
concentrations have dropped to levels that could be effectively treated in a constructed
wetlands, EPA now proposes treating both the perched zone and shallow aquifer
together in constructed wetlands rather than by two separate treatment technologies.
Additionally, recent investigation activities indicate that the remaining perched and
shallow aquifer groundwater are very similar in water quality. Because the physical
distance between these two areas is only 150 feet and they are hydraulically
connected, it now appears more technically and economically feasible to choose
extraction locations that will provide for capture of perched groundwater after it has
entered the shallow aquifer.
The ROD identified the use of a separate extraction system for pumping the
perched groundwater into the brine concentrator. EPA now proposes to extract both
the contaminated perched and shallow aquifer groundwater from one point in the
southeast corner of the site. Due to ANP's process wastewaters no longer being
released to the evaporation ponds as of April 1995, individual wells are drying up in
the perched zone without any additional action being taken. New extraction wells in
the perched zone may go dry or only be an efficient pumping location for a limited
period of time. The cost of moving these extraction wells or reconfiguring piping would
be high. Rather than designing a perched zone extraction system which will need
constant changes, it should be more cost effective and technically feasible to extract
the perched groundwater at the point it enters the shallow aquifer, an estimated 150
feet from the currently defined eastern boundary of the perched groundwater zone.
Locating one extraction well and constructed wetlands treatment system in the
southeast corner of the ANP facility will accomplish both source control of the perched
groundwater zone and treatment of the geographic area of the plume with the second
highest levels of nitrate contamination (Figure 2).
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B. Two Locations (a Northern and Southern Location) For siting the
Constructed Wetlands to Treat the Shallow Aquifer, Including the Use of a
Pipeline or Several Pipelines to Carry the Nitrate-Contaminated
Groundwater from the Extraction Wells to the Treatment Area
ANP has presented several alternatives on proposed siting locations for the
constructed wetlands to EPA and the ADEQ. To support these alternatives, ANP has
updated the groundwater model and completed the analysis of a set of low-level aerial
photographs, in addition to the soil borings described above. (A detailed description of
these activities is summarized in the draft Remedial Design Workplan for Shallow
Aquifer Groundwater, Revision 4.0, dated September 30, 1996, which is available in
the Benson Library site repository.) After discussing the feasibility of each of these
alternatives in the context of this updated and new data, the agencies and ANP
reached consensus on two areas as strong candidates for siting the wetlands.
Northern Area Wetlands to be Located North of ANP Facility
EPA, ADEQ, and ANP agree that the first set of constructed wetlands for
treating the shallow aquifer should be located in a relatively "stagnant" portion of the
shallow aquifer north of the ANP facility near the area where the concentration of
nitrate in the shallow aquifer is the highest and where the groundwater circulation is
low. This area is in the vicinity of shallow aquifer monitor wells MW-17 and MW-18
and is referred to as the "northern area". Siting the first set of constructed wetlands in
this area will capture the nitrate-contaminated shallow aquifer plume as it migrates
north. It also will maximize treatment in the area with the highest levels of nitrate
where several residences are located and where there is the most potential for new
residential development (Figure 2). This first set of wetlands would be constructed
during the spring of 1997.
Southern Area Wetlands to be Located Near Southeast Boundary of ANP
Property
A second set of constructed wetlands will, if needed, be sited in the southeast
corner of the ANP property to treat both the shallow aquifer in the southeast corner of
the ANP property and water from the adjacent perched groundwater zone together
(Figure 2). This area is another "stagnant" portion of the shallow aquifer in the vicinity
of monitor wells MW-14 and MW-15 and is referred to as the "southern area". This
area is where the perched and shallow aquifer converge, the concentrations of nitrate
are very similar in both (unlike previously), and the perched zone was the original
source for the contamination now found in the shallow aquifer in the southern area.
Construction of the southern area treatment wetlands would begin approximately one
year after the full-scale operation of the northern area treatment wetlands.
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C. Recharge Treated Perched and Shallow Aquifer Groundwater by Gravity-
Flow Pipeline Discharge to a Shallow Aquifer Recharge Location in Wash
3 for the Northern Area Wetlands and to a Shallow Aquifer Recharge
Location in Wash 6 for the Southern Area Wetlands
The method of recharge of treated perched and shallow aquifer groundwater
was not specified in the ROD because additional groundwater modeling, investigation
and monitoring needed to be completed during remedial design. During 1995 and
1996, ANP completed several field studies and investigations. Based on the results of
this data, ANP completed an evaluation of various recharge alternatives. Various
concerns were raised by the State agencies, ADEQ and the Arizona Department of
Water Resources (ADWR) regarding the beneficial use of the treated effluent.
Because ANP could not quantify the amount (if any) of treated effluent that would
reach the shallow aquifer and or the San Pedro River if the treated effluent were
discharged into a wash adjacent to the wetlands treatment areas, EPA and the State
agencies concurred that ANP should recharge the treated groundwater via gravity-flow
pipeline discharge to shallow aquifer recharge locations along Wash 6 for the southern
area wetlands and Wash 3 for the northern area wetlands (see Figure 2). The
recharge would occur once the groundwater is treated to the federal and state drinking
water standard of 10 parts per million (ppm) for nitrate.
Other recharge options which were considered, included constructing pipelines
in Wash 3 and Wash 6 to transport treated effluent directly to the San Pedro River
(SPR) or the SPR floodplain. This would have required permits from the Army Corps
of Engineers if sited in a floodplain and may have impacted the habitats of
endangered species, and could have resulted in the disturbance of archeological sites
in the area.
The use of a pipeline routed outside of a wash until it reaches a recharge
location along the wash will be an efficient and effective method of recharge. The
treated groundwater will return quickly to the shallow aquifer system and will reduce
the level of nitrate in the underlying shallow aquifer groundwater in the vicinity of the
San Pedro River. Additionally, the selected point of discharge should provide the
ancillary benefit of enhancing the riparian and ecological quality of Wash 3, Wash 6,
and the San Pedro River in this area of discharge. The cost will be less than using
reinjection wells.
The treated groundwater will meet federal National Pollution Discharge
Elimination System (NPDES) permit requirements.
None of the other recharge alternatives studied, including agricultural irrigation
as a secondary use, are as cost-effective or efficient for recharging the treated
groundwater to the shallow aquifer. However, untreated shallow aquifer groundwater
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or treated effluent may be provided for agricultural use under certain conditions. This
could occur in areas where there is a high concentration of nitrate in shallow aquifer
groundwater or where the shallow aquifer is not present. An adequate monitoring well
network and proximity to the extraction or recharge pipelines would be necessary.
D. Locate Additional Shallow Aquifer Extraction Wells in Area of High
Concentrations of Nitrate to Expedite Groundwater Cleanup
After the extraction and treatment systems have been operational for several
years, groundwater monitoring data may indicate that the initial extraction wells are not
sufficient to capture the nitrate-contaminated groundwater plume. If necessary,
additional extraction wells will be installed.
Northern Area Wetlands
Remediation of this area involves treatment of the extracted groundwater from
the shallow aquifer in two phases. Contaminated shallow aquifer groundwater would
be extracted initially from one extraction well located in the vicinity of monitoring wells
MW-17 and MW-18 and transported via a pipeline routed along private and county
property until it crosses onto ANP property to the northern area wetlands treatment
area. After the northern area wetlands have been operating an estimated four years,
a review will be conducted to determine if an additional shallow aquifer extraction well
and the corresponding pipeline would be required to completely capture the
contamination in this area.
Southern Area Wetlands
Remediation of the southern area also involves extraction of groundwater from
the shallow aquifer in two phases. The first phase would extract groundwater in the
vicinity of monitoring well MW-14. The second phase would extract groundwater in
the vicinity of monitoring well MW-15. This would facilitate the overall cleanup of the
shallow aquifer while cleaning up any contaminated perched groundwater draining into
the shallow aquifer. During the first year or two of operation of the northern area
extraction and wetlands treatment systems, monitoring data obtained would be
collected to determine if additional monitoring wells or other design modifications are
needed for the southern area extraction and wetlands treatment systems.
E. Characterize, Remove, Treat, and Dispose Off-Site Any Newly Discovered
Contaminated Soils Materials Not Previously Identified in ROD
Due to the recent discovery of several drums which may contain dinitrotoluene-
contaminated soil in the vicinity of one of the inactive ponds, EPA proposes expanding
the soils remedy to include characterization, removal, treatment and off-site disposal of
any previously unidentified waste materials discovered in any of the soils on-site. The
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purpose of this proposed change is to incorporate other geographic areas of the site,
not previously identified in the September 30, 1994 ROD, to be included in the soils
cleanup remedies for the site. The cleanup of any other soils areas of the site will be
identified in the Soils Remedial Design (RD) Workplan to be approved by EPA. The
Soils RD Workplan will describe the additional areas to be characterized, the
contaminants of concern, the soils materials proposed to remain on the site, the soils
proposed for removal and, as appropriate, proposed treatment technologies, proposed
treatment facilities, and proposed disposal facilities. Additionally, the Soils RD
Workplan will identify any additional contaminants discovered during the RD
investigative phase, which may require new cleanup standards to be added to an
amended ROD.
IV. SUPPORT AGENCY COMMENTS
The Arizona Department of Environmental Quality (ADEQ) and the Arizona
Department of Water Resources (ADWR) reviewed this ESD. Both ADEQ and ADWR
support the proposed changes in this ESD. The ADEQ finds the ESD to be an
adequate and accurate representation of the approved changes to the remedial
design/remedial action. The ADWR additionally provided the following two comments:
1. The remedial action at the site, as modified by this ESD, is not anticipated to
negatively impact the base flow in the nearby perennial reach of the San Pedro
River, according to computer modeling.
2. The method of groundwater recharge included in this ESD was designed to
avoid waste of groundwater in the area in order to preserve groundwater for
future users. The method of recharge described in this ESD took into
consideration the fact that groundwater conservation is an important concern for
area residents, particularly those with domestic wells.
V. STATUTORY DETERMINATIONS
In light of the new information that has been developed, EPA believes it is
appropriate to modify the selected remedy as set forth in this ESD. EPA believes that
the remedy for the Apache Powder site will remain protective of human health and the
environment, will continue to comply with federal and state requirements that are
applicable and relevant and appropriate to this remedial action, and will continue to be
cost-effective. In addition, the revised remedy uses permanent solutions and
alternative treatment technologies to the maximum extent practicable for this site.
While the changes and clarifications contained in this ESD are significant, none of the
proposed changes fundamentally change the remedy. EPA believes these
modifications to the remedies will be cost effective, and accelerate the clean-up and
restoration of the groundwater and the soils at the Apache Powder Superfund site.
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VI. PUBLIC PARTICIPATION ACTIVITIES
EPA has presented these changes to the remedy in the form of an ESD
because the changes are of a significant but not fundamental nature. EPA provided
the State of Arizona with a fifteen (15) day comment period on this ESD. EPA also
held a community meeting in St. David, Arizona on November 14, 1996 to discuss the
recommended changes to the ROD contained in this ESD #1. In accordance with
Section 117(c) of CERCLA, 42 U.S.C. Section 9617, EPA will publish a notice in the
San Pedro Valley News-Sun and the Arizona Daily Star newspapers which describes
this ESD and its availability for review. In accordance with 40 C.F.R. Section
300.435(c)(2)(ii), this ESD and all documents that support the changes and
clarifications herein were contained in the Administrative Record for the Apache
Powder Superfund site prior to the commencement of the remedial actions affected by
this ESD.
Keith Takata Date
Director, Superfund Division
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Figure 1. Map of Apache Powder Superfund Site and Surrounding Area
10
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11
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Proposed Shallow Aquifer Extraction Well Locations
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TABLE 1
PROPOSED CHANGES TO RECORD OF DECISION (ROD)
PROPOSED PLAN
- JUNE 1994
ROD DECISION -
SEPTEMBER 1994
NEW DATA
COLLECTED
DURING RD /
EXPLANATION OF
SIGNIFICANT
DIFFERENCES
(ESD)
PROPOSED
CHANGES
TO ROD
ESD CHANGE
APRIL 1997
#1. TREATMENT OF PERCHED GROUNDWATER BY CONSTRUCTED WETLANDS
IN CONJUNCTION WITH TREATMENT OF THE SHALLOW AQUIFER IN THE
SOUTHEAST PORTION OF THE SITE (RATHER THAN BY BRINE CONCENTRATOR)
EPA's preferred alternative
in the Proposed Plan was to
use forced evaporation (a
brine concentrator) to clean
up the perched groundwater
to meet federal and state
drinking water standard of
10 parts per million (ppm)
for nitrate and the maximum
contaminant levels (MCLs)
for metals. Extraction was
to be done by 7 wells, with
evaporation of the
contaminated groundwater
in a brine concentrator,
condensation of distilled
water, and reuse of the
treated water in the ANP
plant processes. If the
treated water were not
recycled, it would be
recharged or reinjected to
the shallow aquifer after
meeting state aquifer and
surface water quality
standards.
The selected remedy in the
ROD for the perched
groundwater (GW) is the
same as the preferred
alternative in the Proposed
Plan, as described in the
first column of this Table #1.
(See ROD. Table 1 (page 2-
13) and page 2-27.)
During the period of 1995 to
1996. the water levels in the
perched GW monitoring
wells (MWs) & piezometers
dropped on an average of 7
feet Prior to ANP ceasing
discharge of process
wastewaters to the unlined
evaporation ponds in April
1995, the discharge was a
constant source of recharge
to the perched zone.
Several perched zone MWs
are now dry. The nitrate
concentrations in the
perched zone MWs and
piezometers currently range
from 50-500 ppm (approx.
average 180 ppm), as
compared to the 1980s and
the early 1990s when the
nitrate concentrations
ranged from 100-1,000 ppm.
The earlier, higher
concentrations were used as
a basis for the ROD
selecting the brine
concentrator to treat the
perched GW. Recent
investigations indicate that
the remaining perched and
shallow aquifer GW are very
similar in water quality.
Because these two areas
are hydraulically connected,
it now appears more
technical and economically
feasible to treat these two
areas together.
This ESD Change #1 was
proposed in a Nov 1996
Fact Sheet to document
EPA's decision to treat both
the perched GW zone and
the shallow aquifer together
in constructed wetlands
rather than by two separate
treatment technologies
(e.g., forced evaporation in
a brine concentrator and
constructed wetlands). It is
more sensible technically to
extract the perched GW at
the point it enters the
shallow aquifer (an estim.
150 feet from the eastern
boundary of the perched
GW zone) than to install
new extraction wells in the
perched zone which may
go dry or may only be an
efficient pumping location
for a limited period of time.
Also, the cost of moving
these extraction wells or
reconfiguring piping would
be high. Locating one
extraction well and a
constructed wetlands
treatment system in the
southeast comer of the
ANP facility will accomplish
both source control of the
perched GW zone and
treatment of the geographic
area of the plume with the
second highest level of
nitrate contamination.
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TABLE 2
PROPOSED CHANGES TO RECORD OF DECISION (ROD)
PROPOSED PLAN
-JUNE 1994
ROD DECISION -
SEPTEMBER 1994
NEW DATA
COLLECTED
DURING DESIGN/
EXPLANATION OF
SIGNIFICANT
DIFFERENCES
PROPOSED
CHANGES
TO ROD
ESD CHANGE #2
APRIL 1997
#2. TWO LOCATIONS (NORTHERN AND SOUTHERN) FOR SITING THE
CONSTRUCTED WETLANDS TO TREAT THE SHALLOW AQUIFER
EPA's preferred alternative
in the Proposed Plan was to
extract and dean up the
nitrate-contaminated
shallow aquifer groundwater
(GW) to meet federal and
state drinking water
standards for nitrate by
using a constructed
"treatment" wetlands.
Treated water would be
recharged to the shallow
aquifer by "leaky habitat"
wetlands, unless water
balance studies during the
RD investigative phase
recommended relnjection
into the shallow aquifer or
discharge to the San Pedro
River to maintain water
balance for downstream
users.
{The Proposed Plan did not
indicate the size, number or
location of either the
proposed "treatment"
wetlands or the "leaky
habitat" wetlands to be used
for recharging the GW.
These determinations were
to be made based on
studies completed during
the RD investigative phase.)
The selected remedy in the
ROD for the shallow aquifer
GW is the same as the
preferred alternative in the
Proposed Plan, as described
in the first column of this
Table #2, with one
exception. Based on
community comments on the
Proposed Plan, the ROD
expanded the alternatives to
be considered during the RD
investigative studies for GW
recharge to also include
agricultural irrigation. (See
Table #3 of this ESD for
additional details.)
(See ROD, Table 2 (page
2-13) and page 2-27.)
During the RD investigative
phase in 1995 and 1996,
Apache Nitrogen Products,
Inc. (ANP) proposed several
alternatives for constructed
wetlands siting locations to
EPA and the Arizona
Department of
Environmental Quality
(ADEQ). To support these
alternatives, ANP updated
the GW model and
completed the analysis of a
set of low-level aerial
photographs. ANP also
conducted additional soil
borings. Based on the
results of these studies,
EPA, ADEQ, and ANP
agreed that the most
efficient method of treating
the shallow aquifer is
through siting treatment
wetlands in two locations (a
northern and southern
location). The northern
location would capture the
plume to prevent it from
migrating further northwest.
A second set of wetlands in
the southern area of the site
would provide source control
of the remaining perched
GW and will capture the
remainder of the plume. By
siting two separate wetlands,
the number of years
required to reduce the
nitrate concentrations to
below 10 ppm (the MCL and
treatment standard in the
ROD) should be reduced.
This ESD Change #2 was
proposed in a Nov 1996
Fact Sheet to document
EPA's decision to treat the
contaminated shallow
aquifer GW in two locations
(a northern location and a
southern location). The first
set of wetlands should be
located In a relatively
"stagnant" portion of the
shallow aquifer north of the
ANP facility, near the area
where the concentration of
nitrate in the shallow
aquifer is the highest and
where the groundwater
circulation is low, in the
vicinity of monitoring wells
(MW)-17andMW-18, The
northern location also will
maximize treatment in an
area where several
residences are located and
where there is the most
potential for residential
development. -A second set
of constructed wetlands will
be constructed, if needed,
in the southeast comer of
the ANP property to treat
both the shallow aquifer in
this area and water from
the adjacent perched GW
zone. This area is another
"stagnant" portion of the
shallow aquifer in the
vicinity of MW-14 and MW-
15. (See Table 1 of this
ESD for additional details
on the southern area
constructed wetlands.)
14
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TABLE 3
PROPOSED CHANGES TO RECORD OF DECISION (ROD)
PROPOSED PLAN
- JUNE 1994
ROD DECISION -
SEPTEMBER 1994
NEW DATA
COLLECTED
DURING RD /
EXPLANATION OF
SIGNIFICANT
DIFFERENCES
(ESD)
PROPOSED
CHANGES
TO ROD
ESD CHANGE #3
APRIL 1997
#3. RECHARGE OF TREATED PERCHED AND SHALLOW AQUIFER
GROUNDWATER BY GRAVITY-FLOW PIPELINE DISCHARGE TO A SHALLOW
AQUIFER RECHARGE LOCATIONS IN WASH 3 (NORTHERN WETLANDS) AND
WASH 6 (SOUTHERN WETLANDS)
EPA's preferred alternative
in the Proposed Plan was to
recharge treated shallow
aquifer groundwater (GW)
by "leaky habitat" wetlands,
unless water balance
studies during the RD
investigative phase
recommended reinjection
into the shallow aquifer or
discharge to the San Pedro
River to maintain water
balance for downstream
users.
(See Table #2 of this ESD
for additional details.)
The selected remedy for
recharge of the treated
perched and shallow aquifer
GW was not specified in the
ROD because additional GW
modeling, investigation, and
monitoring needed to be
completed during the RD.
The ROD acknowledged the
comments received from the
community regarding the
need for additional studies,
including consideration of
agricultural irrigation, before
making a final determination.
(See ROD, Table 2 (page 2-
13); page 2-27; and pages
2-32 and 2-33.)
During 1995 and 1996, ANP
completed several field
studies and investigations.
16 borings were completed
in the Fall 1995 along the
San Pedro River, where the
recharge "leaky habitat"
wetlands were proposed to
be sited. Data indicated that
these areas were underlain
by an impermeable layer of
clay 6-10 feet below the
surface, which would
prevent treated GW from
effectively recharging into
the shallow aquifer.
Concerns were raised by the
Arizona Department of
Water Quality and the
Arizona Department of
Water Resources regarding
the beneficial use of the
treated effluent. ANP could
not quantify the amount (if
any) of treated effluent that
would reach the shallow
aquifer (or the San Pedro
River if the treated effluent
were discharged into a wash
adjacent to the wetland
treatment areas). None of
the other recharge
alternatives studied,
including agricultural
irrigation as a secondary
use, were as cost-effective
for recharging the treated
GW to the shallow aquifer.
This ESD Change #3 was
proposed in a Nov 1996
Fact Sheet to document
EPA's decision, in
coordination with the State
agencies, that ANP should
recharge the treated GW
via gravity-flow pipeline
discharge to shallow aquifer
recharge locations along
Wash 3 for the northern
area wetlands and Wash 6
for the southern area
wetlands. Other recharge
options which were
considered, including
constructing pipelines in
Wash 3 and Wash 6 to
transport treated effluent
directly to the San Pedro
river would have required
permits and could impact
the habitats of endangered
species, and could have
resulted in the disturbance
of archeological sites in the
area. The use of a pipeline
routed outside of a wash
until it reaches a recharge
location along the wash will
be an efficient and effective
method of recharge.
15
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TABLE 4
PROPOSED CHANGES TO RECORD OF DECISION (ROD)
PROPOSED PLAN
- JUNE 1994
ROD DECISION -
SEPTEMBER 1994
NEW DATA
COLLECTED
DURING RD
EXPLANATION OF
SIGNIFICANT
DIFFERENCES
(ESD)
PROPOSED
CHANGES
TO ROD
ESD CHANGE #4
APRIL 1997
#4. LOCATE ADDITIONAL SHALLOW AQUIFER EXTRACTION WELLS IN AREA OF
HIGH CONCENTRATIONS OF NITRATE TO EXPEDITE GROUNDWATER CLEANUP
EPA's preferred alternative
in the Proposed Plan was to
use four wells to extract the
shallow aquifer groundwater
(GW) for treatment in the
constructed wetlands. The
number of extraction wells
was based on an earlier
GW model developed by
Apache Nitrogen Products,
Inc. (ANP) for the Feasibility
Study (FS).
The selected remedy in the
ROD for the extraction and
treatment of shallow aquifer
GW is the same as the
preferred alternative in the
Proposed Plan, as described
in the first column of this
Table #4. However, based
on community and agency
comments regarding the
potential impact of pumping
on existing water resources,
the ROD states that during
the design process GW
analyses will be performed
to ensure that the extraction
and treatment of the
contaminated shallow
aquifer does not unduly
interfere or diminish the
existing water resources.
The ROD also provides that
the community will have an
opportunity to participate
during the selection of the
type and final siting of the
constructed wetlands and
the method of recharge for
the treated GW.
(See Rod, Table 2 (page 2-
13, page 2-26, and page
2-27.)
During 1996 and 1996. ANP
updated a previously
developed GW model for
several purposes, including:
(1) to determine the optimal
configuration and number of
GW extraction wells; (2) to
determine the optimal
locations and flow rates for
GW extraction wells; and
(3)0 to evaluate the impacts
of the northern area
extraction and treatment
wetlands system on the flow
of the San Pedro River and
the water levels in the
shallow aquifer. The initial
conclusions of this analysis
were that only one extraction
well would be needed for the
northern area wetlands in
the vicinity of monitoring
wells(MW)-17andMW-18
and only one extraction
wells would be needed in
the southern area. (See
Figure 2 of this ESD for a
map showing location of
extraction wells.)
This ESD Change #4 was
proposed in a Nov 1996
Fact Sheet to document the
record that after the
extraction and treatment
systems have been
operational for several
years, GW monitoring data
may indicate hat the initial
extraction wells are not
sufficient to capture the
nitrate-contaminated GW
plume. If necessary,
additional extraction wells
may be needed. After the
northern area wetlands
have been operating an
estimated 4 years, a review
would be conducted to
determine if an additional
shallow aquifer extraction
well and the corresponding
pipeline would be required
to expedite the cleanup of
this area. Also, during the
first year or two of operation
of the northern area
extraction and wetlands
treatment systems,
monitoring data would be
collected to determine if
additional MWs or other
design modifications are
needed for the souther area
extraction and wetlands
treatment systems.
16
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TABLE 5
PROPOSED CHANGES TO RECORD OF DECISION (ROD)
PROPOSED PLAN
- JUNE 1994
ROD DECISION -
SEPTEMBER 1994
NEW DATA
COLLECTED
DURING RD /
EXPLANATION OF
SIGNIFICANT
DIFFERENCES
(ESD)
PROPOSED
CHANGES
TO ROD -
ESD CHANGE #5
APRIL 1997
#5. CHARACTERIZE, REMOVE, TREAT, AND DISPOSE OFF SITE ANY NEWLY
DISCOVERED CONTAMINATED SOILS MATERIALS NOT PREVIOUSLY IDENTIFIED
IN RECORD OF DECISION (ROD)
The Proposed Plan
identified three media areas
were identified for soils
cleanup: (1) Inactive Pond
Soils and Sediments; (2)
White Waste Materials &
Drum Storage Area; and (3)
Wash 3 Area (Excluding the
Ash and Burn Area). EPA's
preferred alternatives for
these media areas were:
(1) On-srte containment
(backfill and clay cap) all
soils in the 10 inactive
ponds (no excavation or
disturbance to contaminated
soils); (2) Remove drums,
excavate & backfill of all
drummed wastes and
contaminated soils from the
white waste materials &
drum storage area,
including transport,
treatment (fixation), and
disposal at a RCRA
permitted treatment, storage
& disposal (TSD) facility;
and (3) Excavate & backfill
contaminated soils,
including transport,
treatment (fixation of lead-
contaminated soils;
incineration of DNT-
contaminated soils), and
disposal at a RCRA
permitted TSD facility.
The ROD identified the
same three media areas for
soils cleanup as the
Proposed Plan and selected
EPA's preferred alternatives,
as described in the first
column of this Table #5.
[Neither the Proposed Plan
nor the ROD included
language regarding how to
respond, if other areas of
soils contamination were
discovered on the site post
the ROD (e.g., during the
remedial design (RD)
investigative phase or during
the course of other future
field activities).]
(See ROD, Table 3 (page
2-16), Table 4 (page 2-17),
Table 5 (page 2-17), Table
12 (page 2-31); and page
2-29.)
Several drums (possibly
containing dinitrotoluene-
contaminated soil) were
discovered in inactive pond
8 during the RD investigative
phase in the Fall of 1995.
Apache Nitrogen Products
(ANP). the PRPs, notified
EPA of the finding and
requested permission to
sample the waste materials.
ANP also stated that there
was a potential for other
unidentified contaminated
soils materials to be
discovered given the size
and history of the
approximately 1,000 acre
explosives manufacturing
facility. In December 1995,
EPA instructed ANP to
include a proposal for
cleanup of these drums, and
any other newly discovered
areas of potential soil
contamination, in the Soils
RD Workplan for the site.
This ESD Change #5 was
proposed in a Nov 1996
Fact Sheet to document the
record that additional areas
of soils contamination not
previously identified In the
ROD may be identified on
the site and may require
response actions, including
characterization, removal,
treatment, and disposal. If
such areas of contaminated
soils are discovered, the
language in this ESD #5
allows any newly
discovered areas to be
incorporated into the Soils
RD Workplan and the
remedial action (RA), as
necessary. This ESD also
clarifies that any such
actions are subject to the
approval of EPA and may
require new cleanup
standards to be added to
an amended ROD.
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