EPA/ROD/R09-92/082
1992
EPA Superfund
Record of Decision:
HASSAYAMPA LANDFILL
EPA ID: AZD980735666
OU01
HASSAYAMPA, AZ
08/06/1992
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TABIiE OF CONTENTS
I. DECLARATION
A. SITE NAME AND LOCATION
B. STATEMENT OF BASIS AND PURPOSE
C. ASSESSMENT OF THE SITE
D. DESCRIPTION OF THE SELECTED REMEDY
E. STATUTORY DETERMINATIONS
II. DECISION SUMMARY
A. SITE NAME, LOCATION AND DESCRIPTION
B. SITE HISTORY AND ENFORCEMENT ACTIVITIES
C. HIGHLIGHTS OF COMMUNITY PARTICIPATION
D. SCOPE AND ROLE OF THIS DECISION DOCUMENT WITHIN THE SITE STRATEGY
E. SUMMARY OF SITE CHARACTERISTICS
F. SUMMARY OF SITE RISKS
G. DESCRIPTION OF ALTERNATIVES
H. SUMMARY OF THE COMPARATIVE ANALYSIS OF ALTERNATIVES
I. THE SELECTED REMEDY
J. STATUTORY DETERMINATIONS
K. SIGNIFICANT CHANGES
LIST OF FIGURES
1. Hassayampa Landfill Site Location Map
2. Hydrogeologic Cross-Section Landfill Vicinity
3. Water Level Contours for Unit A
4. Potentiometric Contours for Unit B
5. Map of the Hazardous Waste Area and Sampling Locations
6. Soil Vapor Contamination Map
7. Target Area for Groundwater Remediation
LIST OF TABLES
1. Waste Types Disposed at the Hassayampa Landfill Site
2. Summary of Enforcement Activities
3. Summary of Community Relations Activities
4. Comparison of Waste and Soil Contaminant Concentrations from Pit 1 to Health Based Guidance
Levels
5. Air Contaminant Concentrations
6. Chemicals of Potential Concern
7. Risk Assessment Summary
8. Comparison of Costs of the Remedial Alternatives
9. Estimated Cost of the Selected Remedy
LIST OF APPENDICES
A. Applicable or Relevant and Appropriate Reguirements (ARARs)
B. Responsiveness Summary
C. Index of the Administrative Record
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RECORD OF DECISION
HASSAYAMPA LANDFILL
SUPERFUND SITE
July 1992
Record of Decision
Hassayampa Landfill Superfund Site
Concurrence — Assistant Regional Administrator
Record of Decision
Hassayampa Landfill Superfund Site
Concurrence — Water Management Division
Record of Decision
Hassayampa Landfill Superfund Site
Concurrence — Office of Regional Counsel
Record of Decision
Hassayampa Landfill Superfund Site
Concurrence — Waste Programs
Record of Decision
Hassayampa Landfill Superfund Site
Concurrence — Hazardous Waste Management Division
I. DECLARATION
A. SITE NAME AND LOCATION
This Record of Decision (ROD) is written for the Hassayampa Landfill Superfund Site (the
Hassayampa Landfill Site, the Site), which is located in Maricopa County, Arizona, approximately
40 miles west of Phoenix, Arizona. For purposes of this ROD, the Site shall be defined as the
10-acre area of the 47-acre municipal landfill where hazardous wastes are known to have been
disposed, as well as any areas where site-related contaminants have come to belocated.
B. STATEMENT OF BASIS AND PURPOSE
This decision document presents the selected remedial action for contaminated soil and
groundwater at the Hassayampa Landfill Site, chosen in accordance with the Comprehensive
Environmental Response, Compensation, and Liability Act (CERCLA) as amended by the Superfund
Amendments and Reauthorization Act (SARA), and, to the extent practicable, the National Oil and
Hazardous Substances Contingency Plan (NCP). This decision document is based on the
Administrative Record for the Site, the index of which is attached as Appendix C.
C. ASSESSMENT OF THE SITE
Actual or threatened releases of hazardous substances from this Site, if not addressed by
implementing the response action selected in this ROD, may present an imminent and substantial
endangerment to public health, welfare, or the environment.
D. DESCRIPTION OF THE SELECTED REMEDY
The selected remedy for the Hassayampa Landfill Site includes remediation of groundwater and
vadose zone (including soil and soil vapor above the water table) contamination. The
groundwater component of the remedy includes extraction of contaminated groundwater, treatment
of the water using air stripping technology (vapor phase carbon adsorption will be performed as
necessary to meet Federal, State, and County regulations pertaining to air emissions),
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reinjection of the treated water, and continued groundwater monitoring to measure the
effectiveness of the remedy. Federal Maximum Contaminant Levels (MCLs) have been chosen as
groundwater cleanup standards. For those contaminants detected on Site for which no MCLs exist,
Health-Based Guidance Levels proposed by the State of Arizona have been selected as groundwater
cleanup standards. The groundwater cleanup standards shall be met at all points within the
contaminated aguifer.
The vadoze zone component of the remedy includes capping the 10 acre Hazardous Waste Area of the
landfill using a cap that complies with the substantive capping and maintenance reguirements for
Resource Conservation and Recovery Act (RCRA) Interim Status facilities as described in 40 CFR
Parts 265.310 and 265.117, and as described in the "EPA Technical Guidance Document: Final
Covers on Hazardous Waste Landfills and Surface Impoundments." In addition, the vadose zone
component of the selected remedy includes performing soil vapor extraction at all locations at
the Site where soil vapor levels exceed cleanup standards, treating the soil vapor using vapor
phase carbon adsorption or catalytic oxidation technology (to be determined during remedial
design), and implementing access and deed restrictions. The soil vapor cleanup standards shall
be levels that are protective of groundwater guality (meaning that the migration of contaminants
from the vadoze zone to groundwater will not result in groundwater contamination that exceeds
the groundwater cleanup standards). The soil vapor cleanup standards will be determined through
site-specific analytical modeling conducted during the remedial design stage. Additional
investigation will also be performed during the remedial design stage in order to determine the
extent of groundwater and soil vapor contamination.
E. STATUTORY DETERMINATIONS
The selected remedy is protective of human health and the environment, complies with Federal and
State reguirements that are legally applicable or relevant and appropriate to the remedial
action, and is cost-effective. This remedy uses permanent solutions and alternative treatment
technologies to the maximum extent practicable, and satisfies the statutory preference for
remedies that employ treatment that reduces toxicity, mobility, or volume as a principal
element.
Because the selected remedial action allows contaminated soil to remain onsite in excess of
health-based levels, a review will be conducted within five years of commencement of remedial
actions to ensure that the remedy continues to provide adeguate protection of human health and
the environment.
II. DECISION SUMMARY
A. SITE NAME, LOCATION AND DESCRIPTION
1. LOCATION
The Hassayampa Landfill Site is located in a rural desert area approximately 40 miles west of
Phoenix, Arizona. The Site is approximately three-fourths of a mile west of the Hassayampa
River, one and a half miles northwest of the town of Hassayampa, three miles north of the town
of Arlington, and five miles east of the Palo Verde Nuclear Generating Station. Figure 1
depicts the location of the Hassayampa Landfill Site.
The Hassayampa Landfill occupies a fenced 47-acre area located on a 77-acre parcel owned by
Maricopa County. The hazardous waste area (HWA) of the landfill occupies a 10-acre area within
the northeast section of the landfill. For purposes of this ROD, the Site shall be defined as
the 10-acre area of the landfill where hazardous wastes are known to have been disposed, as well
as any areas where site-related contaminants have come to be located.
2. LAND USE
The non-hazardous portion of the Hassayampa Landfill is still operated as a municipal landfill.
Maricopa County personnel have indicated that the expected life of the non-hazardous portion of
the landfill at the current rate of use is an additional ten years. The HWA is fenced and is no
longer being used for landfill purposes. Approximately one-sixth of the land surrounding the
landfill is cultivated, while the remaining areas are desert. Most of the cultivated land is
located east of the Hassayampa River and south of the Arlington Mesa. The immediate vicinity of
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the landfill is sparsely vegetated. Vegetation consists mainly of creosote bush and salt bush.
3. POPULATION
Presently, the nearest residents live approximately 1,000 meters south of the HWA. Communities
located within a three mile radius of the landfill include Hassayampa and Arlington. The
combined 1985 census population for these two communities was 1,100 people. A growth rate of
one to two percent was used to calculate a current population of 1,120 people. According to the
Maricopa County Human Resources Department, a population growth of 10 to 15 percent is expected
to occur over the next 20 years within a five mile radius of the Site. Several workers are
employed at the non-hazardous portion of the Hassayampa Landfill.
4. CLIMATE
The Site is characterized by a dry desert climate. The average precipitation at the Buckeye
meteorological station (about nine miles to the east) was 7.08 inches per year, most of which
occurred during a few days each year. Precipitation of 0.10 inches or more occurs on an average
of 20 days per year. Records from the Buckeye station indicate the average daily maximum
temperature is approximately 87 F, and the average daily minimum temperature is approximately 52
F. The average pan evaporation measured at the Salt River Valley station in Mesa (about 54
miles to the east) was about 106 inches per year.
5. TOPOGRAPHY
The Site is located on the broad southward-sloping alluvial plain of the Hassayampa River basin.
The basin is bounded on the east by the White Tank Mountains, on the south by the Buckeye Hills,
and on the west by the Palo Verde Hills. The surface of the alluvial plain occupied by the Site
is generally flat; however, approximately one half mile south of the Site, the plain is broken
by the Arlington Mesa. The HWA is currently overlain by a graded soil cover. The altitude of
the land surface at the HWA is approximately 910 to 915 feet above mean sea level.
6. SURFACE TSATER
The Hassayampa Landfill Site lies within the Hassayampa River drainage area, but outside of the
100-year floodplain of the river. The Site is located about three-guarters of a mile west of
the Hassayampa River, which flows to the south. The Site is near a north-trending surface water
drainage divide between the Hassayampa River and an unnamed wash to the west, which is a
tributary of the Luke Wash. The Hassayampa River and the Luke Wash are ephemeral desert washes
that are tributaries of the westward flowing Gila River. Presently the Gila River is perennial
at its confluence with the Hassayampa River.
7. GROUNDWATER
Regional hydrogeologic units in the area of the Site include in order of increasing depth:
Recent alluvial deposits, basin-fill deposits, and the bedrock complex. Groundwater levels in
the vicinity of the Site generally lie below the base of the Recent alluvial deposits. However,
where saturated, the Recent alluvial deposits may yield moderate guantities of groundwater to
wells. The thickness of the basin-fill deposits appears to exceed 1,200 feet in the vicinity of
the landfill. The basin-fill deposits comprise the principal source of groundwater to wells in
the area of the Site, and are generally referred to as the regional aguifer. Within a three
mile radius of the Site, 349 groundwater wells have been identified, 172 of which potentially
service individual residences. These wells yield groundwater from the regional basin-fill
deposits aguifer. The reported depths range from 5 feet below land surface to 250 below land
surface. The nearest downgradient domestic well is about 2,500 feet south of the Site.
The basin-fill deposits have been classified in order of increasing depth into the Upper,
Middle, and Lower Alluvium units. The Upper Alluvium unit beneath the Site was the target of
the hydrogeologic investigations conducted at the Site. For purposes of the Remedial
Investigation (RI), the Upper Alluvium unit was subdivided in order of increasing depth into the
upper alluvial deposits unit, basaltic lava-flow unit, Unit A, and Unit B (Figure 2).
The upper alluvial deposits unit consists of a coarse-grained part and a fine-grained part. The
average depth to the base of the coarse-grained part is about 34 feet; while the average depth
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to the base of the fine-grained part is about 58 feet. The basaltic lava-flow unit consists of
vesicular, basaltic rock and is part of the Arlington Mesa basalt flows. This unit appears to
thin and dip towards the north. The presence of contaminated groundwater in Unit A indicates
that the basaltic lava-flow unit is not an impermeable unit.
The part of the Upper Alluvium unit from the base of the basaltic lava-flow unit to the top of
the Middle Alluvium unit is the uppermost water-bearing part of the regional aguifer, and has
been subdivided into Units A and B. There is no confining unit separating Units A and B, and
Units A and B are considered to be water-bearing zones within the same aguifer. Unit A
comprises the uppermost fine-grained water-bearing unit, while Unit B is the uppermost
coarse-grained water bearing unit. Unit B is underlain by a silty clay. This clay has
tentatively been classified as the Palo Verde Clay, and appears to comprise the basal confining
unit for Unit B.
The direction of groundwater flow in Units A and B is generally to the south, although local
variations in the flow direction may occur. The average depth to the water table beneath the
Site is 73 feet. Water level contours and potentiometric contours for Units A and B are
presented in FiguresS and 4.
B. SITE HISTORY AND ENFORCEMENT ACTIVITIES
1. HISTORICAL ACTIVITIES
The Hassayampa Landfill is presently owned by Maricopa County and is operated by the Maricopa
County Landfill Department. Maricopa County had signed a 20-year lease on the 77-acre parcel
from the U.S. Federal Aviation Agency, and after the lease expired in 1963 the parcel was
transferred to Maricopa County by guitclaim deed.
Disposal of municipal and domestic waste began at the landfill in 1961 and has continued to the
present. According to a 1977 report prepared for the Arizona Department of Health Services
(ADHS), the types of waste disposed at the landfill were unrestricted but consisted chiefly of
garbage, rubbish, tree trimmings, and other plant refuse. In that report, it was stated that
the Hassayampa Landfill was not suitable for the disposal of hazardous waste. Based on this
report, Maricopa County prohibited the disposal of hazardous waste at the landfill.
On February 15, 1979, ADHS prohibited disposal of industrial waste at the City of Phoenix's
landfills. Because no alternate waste disposal sites were available in Arizona, ADHS
characterized the situation as an "extreme emergency." Conseguently, ADHS reguested that
Maricopa County accept hazardous waste at the Hassayampa Landfill for a 30-day period beginning
on April 20, 1979. After the initial 30-day period, several time extensions for hazardous waste
disposal at the landfill were granted. On October 28, 1980, the disposal of hazardous waste at
the Hassayampa Landfill was prohibited.
During the 18-month period from April 20, 1979 to October 28, 1980, disposal of hazardous waste
at the landfill was conducted under a manifest program operated by ADHS. An inventory performed
by ADHS indicated that a wide range of hazardous wastes consisting of up to 3.28 million gallons
of liguid waste and up to 4,150 tons of solid waste were approved by ADHS for disposal at the
landfill. However, an inventory conducted by consultants for the potentially responsible parties
(PRPs), indicated that the amount of hazardous waste approved by ADHS for disposal consisted of
up to 3.44 million gallons of liguid waste and up to 3,710 tons of solid waste.
The hazardous waste area was composed of several unlined pits that were designated for disposal
of hazardous or nonhazardous wastes. Pits 1, 2, 3 (including 3a, 3b, and 3c), 4 (including 4a,
4b, and 4c), and the Special Pits were designated for disposal of hazardous waste (Figure 5).
The waste types varied greatly and included heavy metals, solvents, petroleum distillates, oil,
pesticides, acids, and bases. Specific pits were designated to receive certain types of waste,
but it is not clear that this practice was always followed. The designated waste types, the
actual received waste types, and the guantities for each pit, as reported in the RI report, are
presented in Table 1.
Pits A and B were designated for the disposal of non-hazardous waste. Although Pit A was
intended for cesspool and septic tank wastes, other substances (whitish grey sludge, black oily
liguid, and pesticide containers) were also disposed (Ecology and Environment, 1981). The
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contents of Pit B were not well defined. It should be noted that the wastes disposed in Pits A
and B were not recorded under the manifest system.
2. SITE DISCOVERY
In 1981, under the Resource Conservation and Recovery Act (RCRA) Open Dump Inventory Program,
ADHS installed three groundwater monitoring wells at the Hassayampa Landfill. Groundwater
samples collected from one of these wells was found to be contaminated with volatile organic
compounds (VOCs). Also in 1981, Ecology and Environment prepared a site inspection report for
the U.S. Environmental Protection Agency (EPA). In 1984, ADHS conducted site inspections of the
landfill. The Site was added to EPA National Priorities List in July 1987.
3. SITE INVESTIGATIONS
The major preliminary investigation reports prepared for the Site are summarized below:
• Hydrogeologic Conditions and Waste Disposal at the Hassayampa, Casa Grande, and
Somerton Landfills, Arizona (Schmidt and Scott, 1977);
• The Hassayampa Landfill Hazardous Waste Disposal Site: Disposal Analysis (April 20,
1979 - October 28, 1980) (ADHS, 1980);
• Site Inspection Report on Hassayampa Landfill, Hassayampa, Arizona (Ecology and
Environment, 1981) ;
• Geotechnical Evaluation of the Influence of Hassayampa Landfill Hazardous Wastes on
the PVNGS Conveyance Pipeline (Ertec Western, 1982);
• Open Dump Inventory of Hassayampa Landfill, Groundwater Criterion (ADHS, 1982);
• Hassayampa Landfill Site Inspection Report (ADHS, 1985);
• Results of Preliminary Hydrogeological Investigations, Hassayampa Landfill, Maricopa
County, Arizona (Montgomery and Associates, 1987).
The Remedial Investigation for the Site was conducted by the PRPs, with oversight provided by
EPA and the Arizona Department of Environmental Quality (ADEQ). The Remedial Investigation was
initiated in 1988, and the Remedial Investigation report was approved by EPA on April 4, 1991.
A Risk Assessment report was completed by EPA on September 12, 1991. The Feasibility Study
report, which was completed by the PRPs, was approved by EPA on May 20, 1992.
4. ENFORCEMENT ACTIVITIES
Significant enforcement activities conducted at the Site are summarized in Table 2.
C. HIGHLIGHTS OF COMMUNITY PARTICIPATION
As described below, EPA has satisfied the public participation reguirements of CERCLA Section
113(k)(2)(B) and 117. EPA currently maintains Hassayampa Landfill Site information repositories
at the Buckeye Library in Buckeye, Arizona and at the EPA Region 9 office in San Francisco. The
EPA Region 9 office and the Buckeye Library maintain copies of the entire Administrative Record
File. EPA also maintains a computerized Hassayampa Landfill Site mailing list, currently with
over 500 addresses. Furthermore, EPA conducted a public meeting and accepted comments on the
Proposed Plan and RI/FS. EPA has prepared a Responsiveness Summary (Appendix B) which
summarizes EPA's responses to public comments received on the RI/FS and Proposed Plan.
A chronological list of community relations activities conducted by EPA for the Hassayampa
Landfill Site is provided in Table 3.
D. SCOPE AND ROLE OF THIS DECISION DOCUMENT WITHIN THE SITE STRATEGY
This ROD selects remedial measures for vadose zone contamination (including soil and soil vapor
above the water table) and groundwater contamination at the Hassayampa Landfill Site. The
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remedial measures selected under this ROD constitute a final remedy for the Site.
Sufficient information currently exists to select a remedy for the Site. However, additional
investigation will be conducted during the remedial design phase in order to define the extent
of groundwater and soil vapor contamination. This additional investigation is not expected to
affect the remedy selected for the Site. As necessary, the remedial design will be modified to
reflect the additional data collected.
E. SUMMARY OF SITE CHARACTERISTICS
1. CONTAMINANTS OF CONCERN
Waste and Soil Contamination
Site-related contaminants have been detected in soil, soil vapor, groundwater, and air at the
Site.
Soil borings drilled through the disposal pits indicate that the base of these pits (which have
since been filled) range in depth from 6 to 20 feet below land surface. Consolidated, moist,
colored material encountered within the pits is referred to herein as waste material. Waste
samples were collected from Pits 1, 2, 3a, 3c, 4b, and 4c. Soil samples were also collected from
beneath Pits 1, 2, 3b, 3c, 4b, 4c. No waste or soil samples were collected from the Special
Pits area due to the scattered nature of these pits. Instead soil vapor sampling was performed
in the Special Pits area. Vadose zone monitoring borings were also installed at several
locations and soil vapor samples were obtained. Figure 5 shows the location of soil borings,
vadose zone monitoring borings, and soil vapor samples taken at the Site.
Volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) have been detected
in waste and soil within the hazardous waste area. The concentrations of contaminants in waste
and soil were compared with Health-Based Guidelines Levels (HBGLs) for surface soil developed by
ADHS. The HBGLs are derived from calculations based on ingestion of soil. The HBGLs have not
been promulgated. The only pit which contains waste contaminants at concentrations in excess of
their HBGLs is Pit 1, which contains tetrachloroethane and trichloroethene at levels in excess
of their respective HBGLs (Table 4). Similarly, the only pit which is underlain by soil
contaminants at concentrations in excess of their HBGLs is Pit 1, which has 1,Idichloroethene,
dichloromethane, 1,2-dichloropropane, tetrachloroethene, 1,1,Itrichloroethane, and
trichloroethene present at levels in excess of their HBGLs (Table 4). It should be noted that
the highest level of soil contamination was detected in the deepest sample taken beneath Pit
1(about 60 feet). This sample was taken immediately above the basaltic lava-flow unit.
Waste and soil contaminant concentrations were also compared to Toxicity Characteristic Leaching
Procedure (TCLP) levels and Extraction Procedure Toxicity (EP Tox) levels. The TCLP test was
designed to determine the mobility of organic and inorganic analytes, and is one of the criteria
used to determine whether a material is a hazardous waste. The EP Tox test preceded the TCLP
test and has since been replaced by the TCLP test. The TCLP levels for organics were exceeded
only by waste from Pit 1, where levels of 1,Idichloroethene, trichloroethene, and
tetrachloroethene exceeded the TCLP levels. All inorganic waste and soil concentrations were
below the TCLP and EP Tox levels with the exception of two compounds. Chromium was detected in
waste from Pit 2 at a concentration of 9.9 mg/1 (compared to EP Tox level of 5 mg/1) and lead
was detected in waste from Pit 3c at a concentration of 11.5 mg/1 (compared to EP Tox level of 5
mg/1).
Soil Vapor Contamination
Based on the results of soil vapor surveys, several areas of soil vapor contamination have been
identified (Figure 6). The soil vapor contaminants consist of volatile organic compounds
(VOCs)including 1,1-dichlorethene, tetrachlorethene, 1,1,1-trichloroethane, trichloroethene, and
trichlorotrifluoroethane. The area in the vicinity of Pit 1 contains the highest levels of soil
vapor contamination. Soil vapor contamination also exists in an area north of Pit 1, extending
beyond the boundaries of the HWA. Investigation of the extent of soil vapor contamination north
of Pit 1 is ongoing and will continue during the remedial design phase. Elevated levels of soil
vapor contamination have also been identified in the central and southwest portions of the
Special Pits area.
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Groundwater
As mentioned previously, two water-bearing units beneath the Site were identified and
investigated. The direction of groundwater flow in both units is generally to the south,
although local variations in the flow direction may occur. Water level contours and
potentiometric contours for Units A and B are presented in Figures 3 and 4), while hydraulic
parameters for both units are identified below.
Analytical results for routine constituents indicate that the chemical guality of groundwater in
Unit A is consistent with chemical guality of groundwater in shallow aguifers in the landfill
area, and that chemical guality of groundwater in Unit B is generally better than that of Unit
A.
Volatile organic compounds were detected and confirmed in groundwater samples obtained from Unit
A monitor wells MW-1UA, MW-4UA, MW-5UA, MW-6UA, MW-7UA, and from abandoned ADHs well HS-1 (see
Figure 3 for well locations). The compounds detected in groundwater from Unit A are presented in
Table A-l. Eight of these chemicals have been detected at levels in excess of the selected
cleanup standards (see Section I - The Selected Remedy for a discussion of cleanup standards).
The approximate target zone for groundwater remedial action is presented in Figure 7. It must
be stressed that this target zone does not correspond to a groundwater plume, but merely
represents a contiguous area within which are located the monitoring wells that have yielded
contaminated groundwater from Unit A. The boundaries of the contaminant plume will be further
defined during the remedial design phase. To date, no significant contamination has been
detected in groundwater from Unit B.
Air
Air sampling using Tenax tubes was conducted to determine the impact of Site conditions on air
guality. The results of this sampling event are presented in Table 5. Generally, only
relatively low levels of VOCs were detected in the air samples. Exposure by workers to VOCs in
air is regulated under the Permissible Exposure Levels (PELs) established by the Occupational
Safety and Health Administration (OSHA). The levels of VOCs detected in air at the Site are
well below the PELs. Caution should be used in interpreting the sampling results as being
representative of annual average conditions, because these results may vary with different
meteorological conditions.
Soil cover in the HWA consists of a reddish-brown to brown silty sand which ranges from two to
eight feet in thickness. The soil cover appears to effectively retard the release of gas from
buried waste materials in the pits.
Surface Sediment
Surface sediment samples were collected from drainage channels in the vicinity of the Site. Low
levels of pesticides were detected in several samples; however, pesticides were also detected in
a background sample at similar concentrations suggesting that the Site is not the source of this
contamination. The presence of these pesticides may be the residual effect of past agricultural
activities.
F. SUMMARY OF SITE RISKS
1. HUMAN HEALTH RISKS
The human health assessment consists of several steps including identification of Contaminants
of Potential Concern (COPCs), exposure assessment, toxicity assessment, and risk
characterization.
a. Chemicals of Potential Concern
For the most part, all chemicals found to be present at the Site during the RI were identified
as COPCs in the Risk Assessment report. However, the list of COPCs was narrowed down based on
the following criteria:
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• Common laboratory contaminants were removed from further evaluation if the Site
sample concentrations were less than ten times the maximum amount detected in any
blank. For all other chemicals, if the Site contaminant concentrations were less
than five times the maximum amount detected in any blank, the chemicals were removed
from further evaluation;
• Chemicals that were judged to be present at background concentrations were
eliminated from further evaluation; and
• With the exception of trichlorofluoroethane (Freon 113), tentatively identified
compounds (TICs) were not considered COPCs. Freon 113 was retained due to the large
volumes (approximately 10,384 gallons) thought to have been disposed at the Site.
COPCs were identified by environmental medium - subsurface soil (including waste material),
groundwater, and air. Onsite surface soil is not considered a medium of concern because the HWA
has been covered with clean soil. No COPCs were identified in surface sediments in the vicinity
of the landfill.
The specific COPCs identified for subsurface soil, groundwater, and air are presented in Table
6. Vinyl chloride was identified as a COPC even though it was not detected in groundwater at
the Site. This decision was based on the fact that vinyl chloride is a potent carcinogen, and
is a potential breakdown product of VOCs that were identified at the Site.
b. Exposure Assessment
The objective of exposure assessment is to estimate the types and magnitudes of exposure to
COPCs associated with the Site. As part of this process, pathways of current and future
exposure are identified. There are several pathways by which individuals could be exposed to
contaminants disposed in the HWA. These pathways were evaluated under current land-use and
future land-use scenarios.
Under the current land-use scenario, the nearest offsite residence is about 1,000 meters south
of the HWA. If contaminated groundwater is allowed to continue to migrate, residents at this
location could be exposed to site-related contaminants through the use of domestic wells. Since
the prevailing wind direction is from the northeast about 50 percent of the time, the residents
at this location could also be exposed to site-related contaminants via inhalation. Exposure of
workers to VOCs at the landfill was not evaluated by the Risk Assessment. However, the
concentrations of VOCs to which landfill workers are expected to be exposed are well below
Permissible Exposure Levels (PELs) established by the Occupational Safety and Health
Administration (OSHA). The following exposure routes were evaluated under the current-use
scenario:
• Ingestion of VOCs in contaminated groundwater migrating offsite;
• Inhalation of VOCs in contaminated groundwater migrating offsite; and
• Inhalation of VOCs released from the Site to air.
Under the future-use scenario, exposed populations are assumed to be present onsite and domestic
wells are assumed to be installed onsite. Potentially exposed populations evaluated included
both residential and industrial users. Although residential and industrial use of the landfill
seems unlikely in the near future, it is not unrealistic to assume that such use could occur in
the more distant future. The following exposure routes were evaluated under the future use
scenario for both onsite residential and onsite industrial populations:
• Ingestion of contaminated soil;
• Ingestion of VOCs in groundwater;
• Inhalation of VOCs in groundwater, particularly via showering (residential only);
and - Inhalation of VOCs released from the Site to air.
Exposure intake parameter values were based on standard assumptions and best professional
judgement. It should be noted that under all scenarios, it was assumed that the exposed
individuals were adults. The only scenario under which children would demonstrate significantly
different behavioral patterns which would affect their exposure was onsite residential
(ingestion of soil). However, as explained later, this exposure pathway was not evaluated
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quantitatively.
c. Toxicity Assessment
Both carcinogenic and non-carcinogenic chemicals have been identified in soil and groundwater at
the Hassayampa Landfill Site. Reference doses (RfDs) have been developed by EPA for indicating
the potential for adverse health effects from exposure to chemicals exhibiting non-carcinogenic
effects. The RfD is an estimate, with an uncertainty of approximately an order of magnitude, of
a lifetime daily exposure for the entire population (including sensitive individuals) that is
expected to be without appreciable risk of deleterious effects. Estimated intake of chemicals
from environmental media (e.g. the amount of a chemical ingested from contaminated drinking
water) can be compared to RfDs. RfDs are derived from human epidemiological studies or animal
studies to which uncertainty factors have been applied (e.g. to account for the use of animal
data to predict effects on humans). These uncertainty factors help ensure that the RfDs will
not underestimate the potential for adverse non-carcinogenic effects to occur.
For chemicals classified by EPA as proven or probable human carcinogens, risk was evaluated
using cancer potency factors (CPFs) which have been developed by EPA's Carcinogenic Assessment
Group for estimating excess lifetime cancer risks associated with exposure to potentially
carcinogenic chemicals. CPFs were multiplied by the estimated intake of the potential carcinogen
to provide an upper-bound estimate of the excess lifetime cancer risk associated with exposure
at that intake level. The term upper-bound reflects the conservative estimate of the risks
calculated from the CPF. Use of this approach makes underestimation of the actual cancer risks
highly unlikely.
EPA's Region 9 office has generated guidance for calculating toxicity values for chemicals
considered to be "possible human carcinogens," such as 1,1-dichlorothene (1,1-DCE). EPA Region
9 has proposed developing a modified RfD for 1,1-DCE rather than using its CPF. The modified
RfD is calculated by dividing its oral RfD by a safety factor of 10.
d. Risk Characterization
The risk characterization step of the risk assessment process combines the information from the
previous steps to determine if an excess health risk is present at the Site. Excess lifetime
cancer risks are determined by multiplying the intake levels by the CPFs. These risks are
probabilities that are generally expressed in scientific notation (e.g. 1 X 10[-6]). An excess
lifetime cancer risk of 1 X 10[-6] indicates that, as a plausible upper-bound, an individual has
a one in one million chance of developing cancer as a result of a site exposure to a carcinogen
over a seventy year lifetime under the specific exposure conditions at a site. As is stated in
the National Contingency Plan (NCP) (40 C.F.R. Section 300.430 (e)), "For known or suspected
carcinogens, acceptable exposure levels are generally concentration levels that represent an
excess upper-bound lifetime cancer risk to an individual of between 10[-4] and 10[-6]."
Potential concern for the non-carcinogenic effect of a single contaminant in a single medium is
expressed as a hazard quotient (HQ), which is the ratio of the estimated intake derived from the
contaminant concentrations in a given medium to the contaminant's reference dose. By adding the
HQs for all contaminants within a medium or across all media to which a given population is
exposed, the hazard index (HI) can be generated. The HI provides a useful reference point for
gauging the potential significance of multiple contaminant exposures within a single medium or
across media. An HI in excess of one is generally regarded by EPA as representing an
unacceptable lifetime, non-carcinogenic human health risk.
As discussed previously, 1,1-DCE is classified as a "possible human carcinogen," reflecting the
fact that there is only limited evidence available suggesting that this substance is a human
carcinogen. Thus, in accordance with EPA Region 9 guidance, carcinogenic risk for 1,1-DCE was
evaluated differently than for other carcinogens. The evaluation of 1,1-DCE's carcinogenicity
is analogous to the calculation for the non-carcinogenic contaminants described above. A cancer
hazard index (CHI) in excess of one is regarded by EPA Region 9 as representing an unacceptable
lifetime human health risk.
The results of the risk characterization step are summarized in Table 7. This table presents
both typical and reasonable maximum exposure (RME) risks calculated for the current offsite
residential, future onsite residential, and future onsite commercial or industrial scenarios.
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The typical (or average) exposure risk is based on exposure to mean contaminant levels and mean
values for contact and intake variables, including exposure freguency and duration. The RME risk
is based on exposure to a concentration defined as the95 percent upper confidence limit of the
arithmetic mean concentration and 90 to 95 percent percentile values for contact and intake
variables.
For a current offsite receptor located at a distance of a thousand meters downwind and
downgradient from the site, the risk associated with VOCs in air does not appear significant (HI
and CHI are less than one and carcinogenic risk is less than 10[-6]). For the groundwater
pathways, the carcinogenic and non-carcinogenic risk levels are below the benchmarks of 10[-6]
and one, suggesting there is no significant health threat. However, the CHI for 1,1-DCE is
nearly four times the acceptable level of one (under both average and RME conditions),
suggesting that continued migration of contaminated groundwater could result in unacceptable
health risks.[*]
* If carcinogenic risk for 1,1-DCE had been evaluated using the traditional
approach, the RME risk due to ingestion of groundwater and inhalation of VOCs in
groundwater under the current offsite residential scenario would have been 1X10[-3] excess
cancers. Similarly, under the future onsite residential scenario, the RME risk would have
been 2X10[-3] excess cancers. Thus, carcinogenic risk under both of these scenarios
exceeds the acceptable risk range of 10 [-4] to 10[-6] excess cancers, suggesting that
continued migration of contaminated groundwater could result in unacceptable health
risks.
Under the future onsite residential scenario, the risk associated with ingestion and contact
with onsite waste and soil was not evaluated guantitatively and was not summed with the other
pathways evaluated, since only limited data from the pits was available at the time of writing
the Risk Assessment. However, due to the presence of chromium, lead, and copper and high levels
of VOCs and SVOCs in several of the pits, it was assumed that exposure to waste and soil would
result in unacceptable health risks for onsite residents (termed significant risk in Table 7).
Risk associated with inhalation of ambient air exceeded the acceptable benchmarks of 10[-6]
(average and RME conditions) and 1 (RME conditions only) for carcinogenic risk and CHI,
suggesting unacceptable health risks for onsite residents. Finally, the CHI associated with
ingestion of groundwater and inhalation of VOCs in groundwater also exceeded 1 (average and RME
conditions), again suggesting unacceptable health risks for onsite residents.[*] *
If carcinogenic risk for 1,1-DCE had been evaluated using the traditional approach, the RME risk
due to ingestion of groundwater and inhalation of VOCs in groundwater under the current offsite
residential scenario would have been 1X10[-3] excess cancers. Similarly, under the future
onsite residential scenario, the RME risk would have been 2X10[-3] excess cancers. Thus,
carcinogenic risk under both of these scenarios exceeds the acceptable risk range of 10 [-4] to
10[-6] excess cancers, suggesting that continued migration of contaminated groundwater could
result in unacceptable health risks. Since the total risk calculated for the future
onsite residential scenario does not include exposure to waste and soil within the pits (for
reasons described above), the total risk values presented in Table 7 for this scenario represent
minimum values and are expected to be significantly higher. Still, the total risk exceeded the
10[-6] benchmark (average and RME), CHI of 1 (average and RME), and HI of 1 (RME).
Similarly, under the future onsite commercial or industrial scenario the risk associated with
exposure to waste and soil was not evaluated guantitatively, but was assumed to be significant
and indicative of unacceptable health risks for future workers in the HWA. The carcinogenic
risk associated with inhalation of ambient air (average and RME) also exceeded the benchmark of
10[6], indicating unacceptable health risks for future workers in the HWA. Again, as described
above, the total risk calculated for the future onsite commercial/industrial scenario does not
include exposure to waste and soil within the pits, and the total risk values presented in Table
7 for this scenario represent a minimum value and are expected to be significantly higher.
Still, the total risk exceeded the 10[-6] benchmark (average and RME) and CHI of 1 (average and
RME) .
Due to the threat of exposure to groundwater contaminants as a result of future offsite
migration of contaminated groundwater, and the threat of exposure to contaminated waste and soil
under the residential and commercial/industrial scenarios; actual or threatened releases of
hazardous substances from this Site may present an imminent and substantial endangerment to
public health or welfare.
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2. ENVIRONMENTAL EVALUATION
The ephemeral Hassayampa river (which drains to the south) and associated riparian habitat, is
located about 3/4 mile east of the landfill. Although the Hassayampa Landfill is located within
the drainage area of this river, the landfill is located outside of the projected 100-year
floodplain of the river.
The Arizona Game and Fish Department (AGFD) identified the Gambel's Quail, Mourning Dove, and
Jack Rabbit as the most likely game species in the area and noted that interspersed stands of
larger trees may be used by migratory birds. The U.S. Fish and Wildlife Service (USFWS)
indicated that no listed or proposed threatened or endangered species or biological resources
would likely be affected by contamination at the Site. USFWS did indicate that a candidate
category 1 species, the Lowland Leopard Frog, may be found in the vicinity of the Site.
Under current Site conditions, there is no information to suggest that ecological receptors may
presently be exposed to Site contamination. The HWA is covered by clean soil and the perimeter
is bermed to prevent erosion and offsite drainage. Although contaminated groundwater appears to
be migrating south, the nearest perennial surface water body where groundwater might discharge
is the Gila River, which is more than 2 miles from the Site.
With the understanding that the HWA is covered with soil, AGFD concludes that the likelihood of
exposure to wildlife seems low. AGFD did identify wetland and riparian habitat and associated
species along the Gila River that might be affected if groundwater contamination were to migrate
that distance. Groundwater modeling performed in the Risk Assessment indicates that this
scenario is unlikely. There are no wetlands or riparian habitat within the boundaries of the
Site.
G. DESCRIPTION OF ALTERNATIVES
EPA initially considered a wide range of technologies and alternatives for remediation of the
vadose zone (including soil and soil vapor above the water table) and for remediation of
groundwater. The alternatives which survived the screening process and were evaluated in the
detailed analysis are described below. For all of the alternatives except for the No Action
Alternative, two groundwater options were evaluated. Since these two groundwater options are
common to all of the alternatives except No Action, the groundwater options will be discussed
first.
The cost of each of the alternatives evaluated is presented in Table 8.
1. GROUNDWATER
EPA evaluated two groundwater options for the Site. These two options were identical with the
exception that the treatment systems differed. Both options consisted of groundwater extraction,
groundwater treatment, reinjection of the treated water, and continued groundwater monitoring.
The two treatment options considered were air stripping and ultra-violet (UV) oxidation. Under
these options, groundwater would be extracted from Unit A using several extraction wells.
Calculations performed in the Feasibility Study suggest that four to five extraction wells
operating at five gallons per minute would achieve ARARs in Unit A within a maximum of 20 to 30
years. However, the exact number of extraction wells, well locations, and pumping rates would be
determined during remedial design.
The extracted groundwater would be treated through air stripping or UV oxidation. Air stripping
involves the transfer of VOCs dissolved in water to a stream of air flowing counter-current to a
stream of water over a bed of packing material.
Contaminants which have been transferred to the air stream, can be discharged directly to the
atmosphere or treated prior to discharge. Calculations performed in the Feasibility Study
suggest that uncontrolled VOC air emissions from the air stripper would be 1.3 Ibs/day, which is
substantially below the Maricopa County guideline of 3 Ibs/day and the EPA guideline of 15
Ibs/day. Nevertheless, vapor phase carbon adsorption would be reguired to treat air emissions
from the air stripper if total VOC emissions at the Site exceed the Maricopa County guideline.
UV oxidation uses ultraviolet light and an oxidant (typically hydrogen peroxide or ozone) to
destroy organic contaminants. Water and a small amount of chloride salts and carbon dioxide are
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produced as by-products, but there are no substantial air emissions from the process.
The treated groundwater would be reinjected, either onsite or in the immediate vicinity of the
Site. The Feasibility Study indicated that one injection well screened in Unit B and located to
the west of the hazardous waste area would be the most advantageous scenario. However, the
number of injection wells, the location of the injection wells, depth of the injection wells,
andinjection rates would be determined during remedial design.
Continued groundwater monitoring would be performed to monitor and ensure the effectiveness of
the remedy. The number of monitoring wells and freguency of sampling would have to be
sufficient to monitor the effectiveness of the remedy. Additional investigation would be
performed during remedial design to characterize the extent of groundwater and soil vapor
contamination.
2. VADOSE ZONE
The following alternatives were evaluated for remediation of the vadose zone (including soil and
soil vapor above the water table).
Alternative 1 - No Action.
Under this alternative no additional action would be taken at the Site following the RI/FS.
Continued monitoring would be reguired at the Site, although the cost estimate for this
alternative does not reflect the cost of performing such monitoring. EPA is reguired to carry a
No Action alternative through the final detailed analyses.
Alternative 2 - Access & Deed Restrictions, Cap, Groundwater
Extraction/Treatment/Reinjection/Monitoring.
Under this alternative the perimeter fence would be upgraded and maintained to restrict
unauthorized access to the Site. Long-term deed restrictions would also be imposed, thereby
restricting future use of the Site. These restrictions would include (1) access limitations
(including a reguirement that a fence be maintained around the Site) and (2) use limitations
restricting future use of the Site and restricting use of groundwater beneath the Site.
This alternative would also include the construction of a cap over the hazardous waste area.
The purpose of this cap would be to prevent direct contact with contaminated waste and soil left
in place, to reduce infiltration of water, and to reduce the release of VOC vapors to the
atmosphere. At a minimum, this cap would have to meet the substantive reguirements of a RCRA
cap for Interim Status facilities as described in 40 CFR Parts 265.310 and 265.117 and as
described in the "EPA Technical Guidance Document: Final Covers on Hazardous Waste Landfills
and Surface Impoundments" (EPA/530-SW-89-047). The construction details and design reguirements
of this cap would be determined during remedial design.
As described previously, this alternative would also include groundwater extraction, groundwater
treatment, reinjection of treated water, and continued groundwater monitoring to ensure the
effectiveness of the remedy.
Alternative 3 - Access & Deed Restrictions, Cap, Soil Vapor Extraction/Treatment, Groundwater
Extraction/Treatment/Reinjection/Monitoring.
This alternative is identical to Alternative 2 with the exception that it also includes soil
vapor extraction and treatment of the extracted soil vapors. Soil vapor extraction would
involve the installation of extraction vents in order to remove VOCs and SVOCs from the vadose
zone. These vents would be installed within waste and soil in areas where waste and soil
contamination has been demonstrated to be a threat to groundwater and where soil vapor has been
identified as being present in excess of the soil vapor cleanup standards (see Section I - The
Selected Remedy for a discussion of soil vapor cleanup standards). A vacuum system would be
applied to the vents in order to induce air flow through the soil, causing the VOCs and SVOCs
present in the waste and soil to volatilize into the air stream. Water in the air stream would
be condensed, separated from the air stream, and transferred to a water treatment system. The
contaminated air stream would then flow through an air and vapor treatment system consisting of
either a vapor phase carbon adsorption unit or a catalytic oxidation system (catalytic oxidation
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is essentially a thermal incinerator which uses a catalyst to promote the oxidation of VOCs).
The specific soil vapor treatment system would be selected during remedial design.
Alternative 4 - Access & Deed Restrictions, Cap, Soil Vapor Extraction/Treatment,
Excavation/Soil Washing, Groundwater Extraction/Treatment/Reinjection/Monitoring.
This alternative is identical to Alternative 3, except that it also includes excavation of
approximately 1,400 cubic yards of waste from Pit 1, soil washing, and replacement of the
treated material. Waste that is present at levels in excess of the Arizona Health-Based
Guidance Levels for surface soil would be excavated using standard excavation eguipment. The
excavated waste would then be treated using a soil washing process. Soil washing involves
contacting the waste with water to partition the contaminants from the solid phase to the liquid
phase. Excavated wastes would be slurried with water to remove contaminants from the wastes and
pumped through a filter press to separate the solids from the wastes. The contaminated water
would then be collected for treatment, while the decontaminated soils would be backfilled into
Pit 1.
H. SUMMARY OF THE COMPARATIVE ANALYSIS OF ALTERNATIVES
Each of the alternatives described in the preceding section was evaluated according to the nine
criteria defined below. Each criterion is discussed in detail on the pages that follow this
list.
Threshold Criteria
Overall protection of human health and the environment.
Addresses whether the alternative can adequately protect human health and the environment, in
both the short and long-term, from contaminants present at the Site.
Compliance with ARARs.
Addresses whether the alternative will meet all Federal and State environmental laws that are
applicable or relevant and appropriate requirements (ARARs) or provide grounds for invoking a
waiver of the ARAR.
Primary Balancing Criteria
Long-term effectiveness and permanence.
Refers to the long-term effectiveness and permanence afforded by the alternative along with the
degree of certainty that the alternative will prove successful.
Reduction of toxicity, mobility, or volume through treatment.
Refers to the degree to which the alternative reduces toxicity, mobility, or volume of the
Site contaminants through treatment and reduces inherent hazards posed by the Site.
Short-term effectiveness.
Refers to the short-term risks posed to the community, the potential impact on workers, and the
potential environmental impact during implementation of the alternative.
Implementability.
Refers to the ease or difficulty of implementing the alternative by considering technical
feasibility, administrative feasibility, and availability of materials and services.
Cost.
Includes capital costs, annual operating and maintenance costs (0 & M costs), and net present
value of 0 & M costs.
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Modifying Criteria
State acceptance.
Indicates whether the State concurs with, opposes, or has no comment on the preferred
alternative.
Community acceptance.
Indicates whether the community agrees with, opposes, or has no comment on the preferred
alternative.
COMPARATIVE ANALYSIS
Overall Protection of Human Health and the Environment
Alternative 1 is not protective of human health and the environment since no action is taken to
prevent future exposure to contaminated groundwater. In addition, future land use could result
in direct exposure to waste material and contaminated soil.
Alternatives 2, 3, and 4 attain similar levels of protection of human health and the environment
by preventing exposure to contaminated groundwater through groundwater extraction and treatment.
In addition, these alternatives prevent contact with waste material and contaminated soil
through the use of a cap and access and deed restrictions.
Alternatives 3 and 4 attain a slightly greater level of protection as compared to Alternative 2,
since they use soil vapor extraction to reduce soil vapor contamination to levels that are
protective of groundwater guality. This reduces the chances of exposure to the soil vapor
contaminants through exposure to groundwater. Similarly, Alternative 4 attains a slightly
greater level of protection as compared to Alternative 3, since contaminated waste from Pit 1
would be excavated and treated. This provides additional protection in the unlikely event that
deed and access restrictions and the cap fail to prevent direct contact with the waste material.
The two groundwater treatment options considered, air stripping and UV oxidation, attain similar
levels of protection of human health and the environment.
Compliance with ARARs
Alternative 1 does not comply with ARARs since it would not meet the groundwater cleanup
standards. Alternatives 2, 3, and 4 all meet ARARs. Under these alternatives, it is estimated
that groundwater cleanup standards would be met in a maximum of 20-30 years. However, since
Alternatives 3 and 4 use soil vapor extraction to prevent vadose zone contaminants from
continuing to contaminate groundwater, it is possible that these two alternatives could attain
the groundwater cleanup standards more guickly than Alternative 2.
The two groundwater treatment options considered would both meet the groundwater cleanup
standards. It is expected that emissions from the air stripper and the soil vapor extraction
system would meet Federal and County guidelines. In the event that these guidelines are
exceeded, vapor-phase carbon will be reguired in order to comply with these standards.
ADEQ Health-Based Guidance Levels for surface soil have been identified as TBCs for Alternative
4, which involves excavation and treatment of contaminated waste and soil. Under this
alternative, contaminated waste and soil would be excavated and treated to the ADEQ HBGLS.
Alternatives 2 and 3 meet the ADEQ HBGLS for surface soil indirectly by preventing exposure to
contaminated waste and soil through the use of access and deed restrictions and a cap.
Long-Term Effectiveness and Permanence
Since Alternative 1 does not involve remediation at the Site, it does not provide long-term
protection.
Alternatives 2, 3, and 4 provide similar long-term effectiveness with respect to groundwater by
extracting and treating contaminated groundwater. However, Alternatives 3 and 4 provide greater
long-term effectiveness with respect to groundwater as compared to Alternative 2, because
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Alternatives 3 and 4 use soil vapor extraction to prevent vadose zone contamination from being a
continuing source of groundwater contamination. Both of the groundwater treatment options, air
stripping and UV oxidation, are considered permanent remedies.
Alternatives 2, 3, and 4 use a cap and access and deed restrictions to attain long-term
effectiveness and permanence with respect to soil contamination. Through the use of soil vapor
extraction, Alternative 3 attains a greater level of long-term effectiveness than Alternative 2.
Alternative 4 provides a slightly greater level of long-term effectiveness since it also
includes excavation and soil washing. However, since the volume of soil to be excavated and
treated is relatively small (1,400 cubic yards), the added long-term effectiveness is limited.
Reduction of Toxicity, Mobility, or Volume Through Treatment
Alternative 1 does not involve any treatment and would not result in a reduction of toxicity,
mobility, or volume.
Alternatives 2, 3, and 4 all attain a significant reduction in mobility and volume of
groundwater contaminants through the use of groundwater extraction and treatment. Alternatives
2, 3, and 4 would also result in a reduction in mobility of vadose zone contamination through
the use of a cap. The cap would limit the amount of infiltration, and would thereby reduce
migration of vadose contamination to groundwater. Of the two groundwater treatment options
considered, UV oxidation attains a greater reduction of toxicity, mobility and volume as
compared to air stripping.
Alternatives 3 and 4 attain a greater reduction in mobility and volume of vadose zone
contamination as compared to Alternative 2, since Alternatives 3 and 4 include the use of soil
vapor extraction to treat vadose zone contamination. Alternative 4 attains a slightly greater
reduction in mobility and volume as compared to Alternative 3, since Alternative 4 includes soil
washing of waste material in Pit 1.
Short-Term Effectiveness
Since water supply wells in the vicinity of the Site have not yet been impacted by site-related
chemicals and since access to the Site is currently restricted, there are few short-term risks
associated with the Site. Alternative 4, which includes removal of contaminated waste, could
potentially pose some short-term risk to remedial workers during implementation; however, this
risk could be eliminated through proper engineering, safety, and management practices.
Implementability
All of the alternatives are readily implementable. Alternative 1 is the most readily
implementable since it involves no action. Alternatives 2, 3, and 4 rely on demonstrated
technologies and proven and effective methods and eguipment. Of the groundwater treatment
technologies evaluated (which are identical for Alternatives 2, 3, and 4), air stripping would
be easier to implement than UV oxidation, since UV oxidation would reguire a treatability
study prior to implementation.
Cost
Table 8 presents a cost comparison of the four alternatives. Alternative 1 has no additional
costs since there would be no action taken at the Site. The costs of Alternatives 2, 3, and 4
increase progressively. A cost sensitivity analysis performed in the feasibility study
indicated that the net present worth of Alternative 4 remains significantly higher than the
other alternatives irrespective of operating life. Although the groundwater component of the
remedy is identical for Alternatives 2, 3, and 4, the cost of the two groundwater treatment
technologies considered for these alternatives differs substantially. The cost of UV oxidation
is significantly more expensive than the cost of air stripping.
State Acceptance
The State of Arizona, through both the Department of Environmental Quality and the Department of
Water Resources, has participated in the RI/FSprocess. Both agencies have assisted in the
development of ARARs and the remedy selection process. Since Alternative 1 is not protective of
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human health and the environment, this alternative would not be acceptable to either agency.
Since Alternative 2 does not include soil vapor extraction and there is potential for continuing
contamination of groundwater by soil vapor, this alternative would not be acceptable to either
agency. Both Alternatives 3 and 4 would be acceptable to the two agencies.
Community Acceptance
Since Alternative 1 is not protective of human health and the environment, this alternative
would not be acceptable to the community. Several community members have expressed a preference
for treatment of contaminated soil gas, and as a result it is unlikely that Alternative 2 would
be acceptable to the community. Alternatives 3 and 4 generally appear acceptable to the
community; although several community members have expressed a preference for Alternative 4
since this alternative includes excavation and treatment of contaminated soil. Finally, several
community members expressed a concern over the time reguired to reach the groundwater cleanup
standards under Alternatives 2, 3, and 4.
I. THE SEIiECTED REMEDY
Alternative 3 is the selected remedy for the Hassayampa Landfill Superfund Site. The selected
remedy includes vadose zone (including soil and soil vapor above the water table) remediation
and groundwater remediation. Table 9 provides an estimate of the cost of the selected remedy
with respect to the vadose zone and groundwater components.
GROUNDWATER
The groundwater component of the remedy includes extraction of contaminated groundwater,
treatment of the water using air stripping, reinjection of the treated water, and continued
groundwater monitoring to measure the effectiveness of the remedy. The number, location, and
pumping rates of the extraction wells will be determined during the remedial design stage. To
date, groundwater contamination has been restricted to Unit A, so it is anticipated that
contaminated groundwater will only be extracted from this unit. In the event that groundwater
contamination is identified in Unit B, then groundwater will also be extracted from Unit B.
Air stripping, rather than UV oxidation, was selected as the groundwater treatment technology.
Both technologies are capable of attaining the selected cleanup standards; however, air
stripping is significantly less expensive. It is anticipated that combined air emissions from
the air stripper and SVE system at the Site will meet the Federal VOC guideline of 15 pounds per
day and the Maricopa County VOC guideline of 3 pounds per day. In the event that these
guidelines are exceeded, vapor phase carbon adsorption will be added to the air stripper (the
selected remedy already calls for emissions controls to be placed on the SVE system). The
treated water meeting the groundwater cleanup standards will be reinjected onsite or in the
immediate vicinity of the Site. The number, location, depth, and injection rates of the
reinjection well(s) will be determined during remedial design.
Continued groundwater monitoring will be performed to ensure the effectiveness of the remedy.
The number of monitoring wells and freguency of sampling will have to be sufficient to measure
the effectiveness of the remedy.
Federal MCLs have been selected as groundwater cleanup standards for the Site (Appendix A). The
groundwater cleanup standards shall be met at all points within the contaminated aguifer. For
the chemicals detected at the Site, the ADEQ MCLs and non-zero MCLGs are identical to the
Federal MCLs, and, therefore, were not selected as cleanup standards. For those chemicals for
which MCLs do not exist, ADEQ HBGLs have been selected as cleanup standards. There was one
chemical, 1,1-dichloroethane, for which no ARARs or TBCs exist; however, this chemical is
present at concentrations below risk-based levels. As a result, no groundwater cleanup standard
was selected for this chemical.
VADOSE ZONE
The vadose zone component of the remedy includes installation of a cap over the 10-acre
Hazardous Waste Area, soil vapor extraction and treatment, and access and deed restrictions.
The purpose of the cap is to prevent direct contact with contaminated waste and soil left in
place, to reduce infiltration of water, to reduce the release of VOC vapors to the atmosphere,
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and to improve the efficiency of the soil vapor extraction system. The design and construction
details of the cap will be determined during remedial design; however, at a minimum the cap must
meet the substantive capping and maintenance requirements for Resource Conservation and Recovery
Act (RCRA) interim status facilities as described in 40 CFR Parts 265.310 and 265.117 and as
described in the "EPA Technical Guidance Document: Final Covers on Hazardous Waste Landfills
and Surface Impoundments" (EPA/530-SW-89-047).
The vadose zone component of the remedy also includes performing soil vapor extraction at all
locations at the Site where soil vapor levels exceed cleanup standards, and where waste and soil
contamination has been demonstrated to be a threat to groundwater quality. While the specific
areas of the Site which require soil vapor extraction will be determined by EPA during the
remedial design, EPA presently expects these areas to include Pit 1, the area of soil vapor
contamination north of Pit 1, and several portions of the Special Pits area. The location,
number, and construction details of the soil vapor extraction vents will be determined during
remedial design. The soil vapors will be treated using vapor phase carbon adsorption or
catalytic oxidation, as determined during remedial design. The soil vapor cleanup standards will
be levels, established by EPA, that are protective of groundwater quality (meaning that the
migration of contaminants from the vadose zone to groundwater will not result in groundwater
contamination that exceeds the groundwater cleanup standards), as determined by site-specific
analytical modeling.
The selected remedy also includes implementation of access and deed restrictions at the Site.
The perimeter fence will be upgraded and maintained to restrict unauthorized access to the Site.
Long-term deed restrictions will also be imposed, thereby restricting future use of the Site.
These restrictions will include (1) access limitations (including a requirement that a fence be
maintained around the Site) and (2) use limitations (restricting future use of the Site and
restricting use of groundwater beneath the Site).
Additional investigation will be performed during remedial design to define the extent of
groundwater and soil vapor contamination at and in the vicinity of the Site.
The selected remedy for the Site allows contaminated waste and soil to remain onsite. As
described in Section II-E of this ROD, "Summary of Site Characteristics," Pit 1 was the only
location where contaminants in waste or soil exceeded ADEQs proposed HBGLs or EPA' s TCLP or EP
Tox levels for organic chemicals. There were two pits which had minor exceedences of EP Tox
levels for inorganic chemicals. It should be noted that the HBGLs have not been promulgated and
that the TCLP levels were not necessarily intended to be used as cleanup standards. Through the
use of access and deed restrictions and a cap, the selected remedy will prevent direct contact
with contaminated waste and soil. Through the use of soil vapor extraction, the selected remedy
will limit the migration of vadose zone contaminants to groundwater.
EPA believes that the selected remedy provides the best balance of tradeoffs with respect to the
nine criteria. While Alternative 4 may provide a slight increase in protection of human health
and the environment and reduction of toxicity, mobility or volume through treatment; EPA does
not believe that these marginal benefits are necessary or justify the additional costs.
J. STATUTORY DETERMINATIONS
Under its legal authorities, EPA's primary responsibility at Superfund sites is to undertake
remedial actions that achieve adequate protection of human health and the environment. In
addition, Section 121 of CERCLA establishes several other statutory requirements and preferences
that EPA must consider when evaluating remedial alternatives for a Superfund site. Section 121
of CERCLA specifies that when complete, a selected remedial action must comply with ARARs
established under Federal and State environmental laws unless a statutory waiver is justified.
The selected remedy also must be cost effective and utilize permanent solutions and alternative
treatment technologies or resource recovery technologies to the maximum extent practicable.
Finally, Section 121 of CERCLA includes a preference for remedies that employ treatment that
permanently and significantly reduces the volume, toxicity, or mobility of hazardous wastes as
their principal element. The following sections discuss how the selected remedy meets these
statutory requirements.
1. PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT
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Threats to human health and the environment posed by the Site include ingestion of contaminated
groundwater, inhalation of VOCs in groundwater, and ingestion and contact with contaminated
waste and soil. The selected remedy addresses the threat of exposure to contaminated groundwater
through the extraction of contaminated groundwater and treatment to Federal and State regulatory
levels. The selected remedy reguires that these levels be met throughout the contaminated
aguifer. The implementation of deed restrictions will provide further protection by ensuring
that drinking water wells are not installed onsite.
By reguiring soil vapor extraction to levels that are protective of groundwater guality, the
selected remedy ensures that vadose zone contaminants (soil and soil vapor) will not migrate to
groundwater. The selected remedy addresses the threat of ingestion and contact with
contaminated waste and soil through the use of access and deed restrictions and a cap. The cap
will also minimize infiltration and limit the migration of vadose zone contaminationto
groundwater.
2. COMPLIANCE WITH ARARS
The selected remedy will comply with all Federal and more stringent State ARARs identified in
Appendix A. In addition, the selected remedy will comply with TBCs identified in Appendix A.
3. COST-EFFECTIVENESS
The selected remedy is cost-effective in addressing the risks posed by the Site. Section
300.430(f)(ii)(D) of the NCP states that once a remedial action satisfies the threshold criteria
(overall protection of human health and the environment and compliance with ARARs),
cost-effectiveness is determined by evaluating the following three balancing criteria:
long-term effectiveness and permanence; reduction of toxicity, mobility or volume through
treatment; and short-term effectiveness.
The selected remedy provides the best overall effectiveness at the lowest cost. Alternatives 3
and 4 attain a similarly high level of overall protection of human health and the environment;
compliance with ARARs; long-term effectiveness and permanence; and short-term effectiveness.
Alternative 4 would provide a slightly greater reduction of toxicity, mobility or volume through
treatment; however, EPA does not believe this slight reduction merits the significant increase
in cost.
The groundwater treatment technology selected for the Site also provides the best overall
effectiveness at the lowest cost. Two groundwater treatment technologies, air stripping and UV
oxidation, were evaluated as part of Alternatives 2, 3 and 4. Air stripping (which is a
component of the selected remedy) provides a similar level of protection and treatment at
substantially less cost than UV oxidation.
4. UTILIZATION OF PERMANENT SOLUTIONS AND ALTERNATIVE TREATMENT TECHNOLOGIES OR RESOURCE
RECOVERY TECHNOLOGIES TO THE MAXIMUM EXTENT PRACTICABLE
EPA has determined that the selected remedy represents the maximum extent to which permanent
solutions and treatment technologies can be used at the Site in a practicable manner. The
selected remedy provides the best balance of trade-offs in terms of long-term effectiveness and
permanence, reduction in toxicity, mobility or volume through treatment, short-term
effectiveness, implementability, and cost, while also considering State and community
acceptance.
The selected remedy will result in a reduction in the volume and mobility of groundwater
contaminants through groundwater extraction, treatment, and reinjection. Continued groundwater
monitoring will be performed to ensure that the remedy is protective of human health and the
environment. The selected remedy uses soil vapor extraction and treatment to prevent vadose
zone contamination from continuing to contaminate groundwater. Additionally, a cap will be used
to prevent contact with contaminated waste and soil and to further limit the migration of vadose
zone contamination to groundwater.
5. PREFERENCE FOR TREATMENT AS A PRINCIPAL ELEMENT
The selected remedy satisfies the statutory preference for remedies that employ treatment as a
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principal element. By treating the contaminated groundwater using air stripping, the treated
water can be returned to its beneficial use through reinjection. By performing soil vapor
extraction and treatment, vadose zone contamination will be prevented from continuing to
contaminate groundwater.
The selected remedy does allow a relatively small volume of contaminated soil (1,400 cubic
yards) which exceeds ADEQ Health-Based Guidance Levels to remain onsite. By reguiring access
and deed restrictions and a cap, the selected remedy will prevent exposure to these
contaminants. EPA does not believe that treatment of this contaminated soil is necessary or
worth the additional cost.
K. SIGNIFICANT CHANGES
There are no significant differences between the remedy identified in the Proposed Plan and the
remedy selected in the Record of Decision.
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APPENDIX A
ARARs AND OTHER CRITERIA FOR THE SELECTED REMEDY AT THE HASSAYAMPA LANDFILL SITE
This appendix identifies ARARs and other criteria to be considered (TBCs) for the selected
remedy for the Hassayampa Landfill Site. The selected remedy shall meet the requirements of the
ARARs identified below. Furthermore, unless otherwise indicated, the selected remedy shall also
meet the requirements of the TBCs identified below.
CHEMICAL-SPECIFIC ARARs AND TBCs
Table A-l presents chemical-specific ARARs and TBCs for water arranqed by chemical compound.
The Safe Drinkinq Water Act (SDWA) Maximum Contaminant Levels (MCLs) are based on human
consumption of water for drinkinq, cookinq, bathinq, etc. Economic considerations and technical
feasibility of treatment processes are included in the justification for these levels. MCLs are
applicable to drinkinq water at the tap pursuant to the SDWA, and are ARAR for treated water
when the end use is drinkinq water. Pursuant to 40 C.F.R. Section 300.430 (e) (2) (i) (B), MCLs and
non-zero Maximum Contaminant Level Goals (MCLGs) are relevant and appropriate as in-situ aquifer
standards for qroundwater that is or may be used as drinkinq water.
ADEQ Aquifer Water Quality Standards (ADEQ MCLs), established pursuant to A.R.S. Section 49-223
are identical to SDWA MCLs for the compounds detected in qroundwater at the Hassayampa Landfill
Site. Since ADEQ MCLs are not more strinqent than the SDWA MCLs, these ADEQ standards are not
ARARs and are not included in Table A-l. ADEQ HBGLs for qroundwater are TBCs for the Site. The
HBGLs are derived from calculations based on inqestion of qroundwater. The HBGLs have not been
promulqated. ADEQ HGBLs were selected as cleanup standards only for chemicals for which no SDWA
MCL or MCLGs existed.
Federal Health Advisories, which are criteria developed by either EPA's Office of Drinkinq Water
Health Advisory Proqram or the National Academy of Sciences (NAS), were considered at the Site.
The Federal Health Advisories are based on NAS-suqqested Non-Adverse Response Levels (SNARLs) at
which no known or anticipated adverse human health effects would occur, qiven an adequate marqin
of safety. These Federal Health Advisories were not selected as cleanup standards, since they
were less strinqent than the SDWA MCLs and ADEQ Health-Based Guidance Levels (HBGLs).
LOCATION-SPECIFIC ARARs
Table A-2 identifies location-specific ARARs and TBCs for the Hassayampa Landfill Site.
Location-specific ARARs are concerned with the area in which the Site is located. Actions may
be required to preserve or protect aspects of the environment or cultural resources of the area
that may be threatened by the existence of the Site, or by remedial actions to be undertaken at
the Site.
ACTION-SPECIFIG ARARs
Table A-3 identifies action-specific ARARs for the Hassayampa Landfill Site. The actions
included in Table A-3 are components of the selected remedy.
ADDITIONAL STATE ARARs and TBCs
Arizona Revised Statute Section 49-224 is applicable or relevant and appropriate at the
Hassayampa Landfill Site. A.R.S. Section 49-224 classifies all Arizona aquifers as drinkinq
water aquifers. Section 45-454.01 of the Arizona Groundwater Manaqement Act (GMA) (A.R.S.
Sections 45-454.01), is also applicable or relevant and appropriate to the Site. All offsite
uses of treated qroundwater are subject to state law outside the context of the Superfund
action. However, for activities conducted onsite, the substantive portions of the provisions
referenced within Section 45-454.01 of the GMA shall be applicable or relevant and appropriate.
While the State of Arizona has cited 49 A.R.S. Section 282(D)(2) as an ARAR, EPA has not
identified this Arizona law as an ARAR since it does not establish qroundwater cleanup standards
that are more strinqent than the federal cleanup standards selected for the Hassayampa Landfill
Site. Like Section 300.430(a) (iii) of the National Continqency Plan, 49 A.R.S. Section
282(D)(2) evinces an intent that remedial actions shall, to the extent practicable, provide for
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the control, management, or cleanup of hazardous substances so as to allow the maximum
beneficial use of the waters of the State. The maximum beneficial use of groundwater in Arizona
appears to be "drinking water protected use," which is defined as the protection and maintenance
of aquifer quality for human consumption. See Ariz. Admin. Comp. R. 18-11-501; 49 A.R.S.
Section 224 (which classifies all aquifers in Arizona as drinking water aquifers). Under 49
A.R.S. Section 223, aquifer water quality standards are established as primary maximum
contaminant levels, which are the groundwater cleanup standards selected in this ROD in
accordance with CERCLA Section 121(d).
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