PB95-963140
EPA/ESD/R09-89/132
May 1995
EPA Superfund
Explanation of Significant Difference
for the Record of Decision:
Del Norte County Pesticide Storage
Area Site, Crescent City, CA
9/21/1989
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
REGION IX
215 Fremont Street
San Francisco. Ca. 94105
2 1 SEP 1989
Explanation of Significant Difference
for
Remedial Action
at the
Del Norte County Pesticide Storage Area
Superfund Site
Introduction
The Del Norte Pesticide Storage Area Site, located ap-
proximately one mile northwest of Crescent City, California,
consists of less than one acre of rural land. Both the site and
the land surrounding it are owned by Del Norte County. The
entire county-owned parcel (including the pesticide storage area)
is approximately 480 acres in size. The County property is
bounded by: McNamara Field, the airport which serves Del Norte
County, to the north; Pebble Beach, a public area on the Pacific
Ocean, 2,000 feet to the west; privately owned farmland and
Washington boulevard to the south; and approximately seven
residences and Riverside Drive to the east. The site is about 45
feet above sea level, with a very gently sloping surface. Cer-
tain areas surrounding the pesticide disposal area are seasonal
wetlands.
The Environmental Protection Agency (EPA) has been the lead
agency for this site since it was established on the National
Priorities List (NPL) in 1983. Support agencies include the
California Department of Health Services (DOHS), the North Coast
Regional Water Quality Control Board (RWQCB), and the California
Coastal Commission (CCC).
This Explanation of Significant Difference (BSD) fulfills
the requirements of the Comprehensive Environmental Resource Con-
servation and Liability Act (CERCLA) of 1980 Section 117(c). The
objective of this BSD is to justify and document the change in
the groundwater treatment method that was selected in the Record
of Decision (ROD) signed on September 30, 1985. This selected
remedy, carbon filtration, coagulation and sand filtration, has
been changed to aeration. The reason for this change is that
site contaminant characteristics are different than originally
described.
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Reductions in concentrations of two of the compounds targeted for
treatment and the natural occurrence of the other, indicate aera-
tion as a more appropriate treatment technology as well as a
shorter duration of the treatment period.
Summary of Site History
The storage area operated from 1970-1981 as a county-wide
collection point for the storage of pesticide and herbicide con-
tainers generated by local agricultural and forestry-related
industries. In the fall of 1981, soil and groundwater
contamination was discovered by RWQCB and DOHS. This discovery
indicated that the pesticide containers had been rinsed on-site,
and that the residues and rinseates were improperly disposed of
in a bermed, unlined sump area. Preliminary investigations from
1981-1983, by RWQCB and DOHS, identified soil and groundwater
contamination with herbicides, pesticides and volatile and semi-
volatile compounds. The County's inability to fund further site .
investigations initiated the process of incorporating the site on
the NPL in the fall of 1983. The extent of contamination was
determined in the Remedial Investigation conducted by EPA in
1985.
The remedy selected in the ROD required excavation and
removal of contaminated soils and extraction and treatment of
contaminated groundwater. The groundwater treatment remedy
consisted of carbon adsorption for removal of organics and
pesticides and coagulation and sand filtration for chromium
treatment. The spent carbon filters and chromium-rich waste
brine were to have been disposed of at a RCRA approved, offsite,
Class I, hazardous waste disposal facility. The treated
groundwater was to be disposed of through a pipeline to the
County sewer main. The two organic and pesticide compounds
specifically targeted for treatment with carbon adsorption were
2,4-Dichlorophenoxyacetic Acid (2,4-D) and 1,2-Dichloropropane
(DCP). DCP and 2,4-D are the primary contaminants of concern be-
cause their ingestion, at levels above the relevant drinking
water criteria, has been linked to an increased cancer risk. The
ROD clean-up criteria are 100 ppb for 2,4-D and 10 ppb for DCP.
In a 1987 EPA Removal Action, 250 yards of contaminated
soils were excavated and disposed of at an approved hazardous
waste facility. This action removed highly contaminated soils
from the site, thereby eliminating the source of additional in-
cremental groundwater contamination. This has been confirmed by
subsequent reductions in measured groundwater contaminant levels.
Additional soil and groundwater characterization was conducted
by EPA's Removal and Remedial programs throughout 1987 and in
1989.
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Basis for Significant Differences
The basis for proposing aeration as the ground water
treatment method focuses on the three contaminants specifically
targeted for treatment in the carbon absorption, coagulation and
sand filtration system selected in the ROD. The contaminants,
chromium, 2,4-D and DCP are discussed in detail below. Ground-
water data for organic compounds was compiled from the Remedial
Investigation (1985), Report of the Pump Test Program (1987) and
Removal Preliminary Assessments (1987 & 1989), in a report
titled, Del Norte Groundwater Data Analysis July 28, 1989 by
EPA's Technical Assistance Team (TAT). Chromium results can be
found in the RI/FS, Removal PA report and the Chromium and PAH
Groundwater Sampling Technical Memorandum (1986).
Chromium
Cleanup of chromium in soils or ground water was not ex-
plicitly included as an objective in the ROD because the nature
and extent of the problem was not sufficiently understood. Sub-
sequent investigations have determined that chromium in the
groundwater near the Del Norte Site is in the particulate form,
as evidenced by low or not detectable concentrations of dissolved
chromium and high concentrations of total chromium; hexavalent
chromium concentrations were below detection; and dissolved
total chromium concentrations were well below the 50 ppb Maximum
contaminant Level (MCL). Research of past land uses in the area
by the U.S. military, Del Norte County and milling operations at
Dead Lake did not reveal actual or potential sources of chromium
contamination. Soil sampling conducted by EPA's Technical Assis-
tance Team in May, 1987 compiled chromium results very similar to
those of the RI/FS. These results show that chromium, consis-
tently detected in concentrations near 200 ppm, is present in
surface and subsurface soils on and off site.
This homogenous distribution indicates that chromium is
naturally occurring and no point source will be identifiable. In
fact, chromite (chromium ore) is present in ultramafic rocks of
the western Klamath Mountains in concentrations which have al-
lowed commercial exploitation. Since some of these deposits are
within 10 miles of the site, it is likely that elevated con-
centrations of chromium in the ground water are the result of
natural weathering processes of the nearby ultramafic rocks.
Under Section 104 (a)(3)(A) of CERCLA as amended by SARA,
response to a release or threat of a release of a naturally
occuring substance from a location where it is naturally, found is
not permitted. Based on this information, treatment of
groundwater by coagulation and sand filtration for chromium was
previously deleted from the Remedial Action Scope of Work.
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2,4-Dichlorophenoxyacetic Acid
2,4-D was detected at or above the 100 ppb Maximum Con-
taminant Level (MCL) in two samples. These two samples and
duplicates were collected from Monitoring Well #1 on 3/5/85 and
3/25/85. The associated levels detected were 100 ppb and 150 ppb
and the duplicates were respectively 82 ppb and 50 ppb. MCLs are
intended to protect public healthufrom contaminants in drinking
water that may present an imminent and substantial hazard to
exposed individuals. These criteria are designed to protect a 70
kg adult ingesting 2 liters of water per day for a 70-year
lifetime.
Other than the two samples noted above, no 2,4-D values
exceeded 100 ppb in all the groundwater sampling results reviewed
from the RI/FS, Remedial Design and Removal phases of this
project. The next highest value was 34 ppb from MW #105 on
9/27/87 and in 5 subsequent samples collected from this well
2,4-D was not detected. 2,4-D was not consistently detected in
samples from any well except MW #1. Additional samples and
duplicates were collected from MW #1 by TAT on 5/19/87 and
2/28/89. The respective results were 20 ppb (duplicate 28 ppb)
and <2 ppb (duplicate 2.7 ppb), the detection limit being 2 ppb.
These results support the conclusion that the clean-up criteria
of 100 ppb for 2,4-D has been met prior to treatment. The over-
all decrease in 2,4-D concentrations to below the detection limit
can be attributed to its documented rapid biodegradation, source
removal and specific site parameters such as abnormally high
rainfall.
1,2-Dichloropropane
The most recent sample results indicate that DCP concentra-
tions have decreased from approximately 2100 ppb to 600, likely
because of volatilization and dilution. This is still sig-
nificantly higher than the applicable drinking water advisory
level of 10 ppb, which is the basis for the ROD treatment
specification of 10 ppb. Plume projections predict that con-
taminated groundwater could migrate into private wells utilizing
the same aquifer down gradient of the site. This modeling is
documented in the RI/FS and the Preliminary Risk Assessment for
the Del Norte Site.
To obtain information regarding treatment of DCP in ground
water, a database developed by the EPA Risk Reduction Engineering
Laboratory, Cincinnati, Ohio was searched. This search iden-
tified activated sludge and aeration as effective treatment tech-
nologies for DCP. A higher efficiency, 99%, was evidenced when
aeration was used to degrade the highly volatile contaminant in
wastewater.
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Description of Significant Difference
The difference between the remedy presented in the ROD and
the action now proposed is groundwater treatment by aeration in
stead of carbon filtration, coagulation and sand filtration.
Aeration was considered in the original ROD alternatives but was
not chosen due to its ineffective removal of 2,4-D. Many aera-
tion systems are available on the market, the particular system
proposed is currently in use at the American Thermostat NPL Site
in Region II. It can be easily assembled from components nor-
mally available at local hardware stores and has a demonstrated
volatile organic removal of 99.9999%. A reduced scope of work
and a decrease in cost of over 1 million dollars will result from
this change. The less complex technology of the aeration system
requires less site preparation and environmental destruction,
resulting in less site restoration activities as required at the
end of the project.
Summary of Support Agency Comments
An interagency meeting was held on August 18, 1989 to dis-
cuss the change in groundwater treatment remedy and the basis for
the change as described above. A draft BSD was distributed for
support agency review in late August. Their comments, briefly
summarized below, are attached. EPA maintains its commitment to
comply with all concerns and requests of the support agencies.
California Department of Health Services
Prior to termination of the previous Remedial Action con-
tract, DOHS was consulted and concurred with the decision based
on the contaminant factors described above. The change in
groundwater treatment remedy and scope of work may require amend-
ments to the Superfund State Contract. The State project manager
is currently investigating requirements for amending the Super-
fund State contract. DOHS supports the proposed groundwater
treatment method of aeration and maintains its cost share commit-
ments .
North Coast Regional Water Quality Control Board
RWQCB agrees that the proposed treatment method of aeration
is suitable for treatment of DCP contaminated groundwater. Sup-
porting data, drawings, spill contingency plans and an O & M
manual will be provided for review as requested. EPA acknow-
ledges that the apparent decrease in DCP concentrations may be
attributed to many factors such as source removal, volatiliza-
tion, dilution, biodegradation and plume migration.
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The existing pumping well has demonstrated adequate aquifer
drawdown in the pump tests. Therefore EPA intends to initially
extract groundwater only from this well. Subsequent sample
results will indicate the necessity and most efficient placement
of an additional extraction well. RWQCB will be consulted to as-
sess the appropriateness of an additional well in the future.
Treated discharge was not discussed in the ESD because no
change was made to the Discharge method described in the ROD.
EPA acknowledges that pretreatment requirements will be met prior
to any discharge to the Crescent City municipal waste treatment
plant. During startup of the treatment plant, treated water will
be contained in holding tanks and analyzed for contaminant levels
before being discharged to the sewer main.
California Coastal Commission
As requested by the CCC, a negative determination will be
submitted for the proposed project. EPA reiterates its commit-
ment to minimize impacts to adjacent habitat and to restore any
impacts that cannot be avoided. EPA will seek assistance from
CCC in determining the specific elements and adequacy of site
restoration activities when that phase of the project is reached.
Affirmation of Statutory Determinations
Considering the new information that has developed and the
changes that have been made to the selected remedy, the U.S. En-
vironmental Protection Agency and the California Department of
Health Services believe that the remedy remains protective of
human health and the environment, complies with Federal and State
requirements that are applicable or relevant and appropriate to
this remedial action, and is cost-effective. In addition, the
revised remedy utilizes solutions and alternative treatment tech-
nologies to the maximum extent practicable.
Public Participation Activities
A fact sheet, distributed in early August 1989, notified the
public that EPA was evaluating alternative groundwater treatment
methods due to changing site conditions. It indicated that when
EPA had decided on the proposed action the ESD will be made
available to the public. At the same time EPA will prepare and
distribute a fact sheet describing the ESD, publish a notice of
availability and a brief description in the local newspaper and
make the ESD available for review as part of the Administrative
Record, on file at the local library. In addition, the ap-
propriate documents utilized in this explanation will be placed
in the Administrative Record, filed at the local library in Cres-
cent City California, if they are not already there.
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