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IRP Site 1 – Waste Blowdown Area
IRP Site 1, the Waste Blowdown Area, occupies about 750 square feet near the west-central portion of NRTF Dixon, adjacent to the backup generator facility. Potential contaminants from generator exhaust discharged over a 20-year period consisted of waste oil and a small amount of carbon. The blowdown area ground surface is currently mostly uncovered asphalt pavement and native grassland vegetation.
The Waste Blowdown Area was first identified as an IRP site in a 1988 preliminary assessment report. Analyses of soil samples and a groundwater sample collected during a 1990 site inspection detected total petroleum hydrocarbons below reportable limits. An unidentified hydrocarbon mixture that could be waste oil or heavy fuel oil was also found in the groundwater sample.
Groundwater samples were collected during a 1993 supplemental site investigation and analyzed for aromatic hydrocarbons, total petroleum hydrocarbons, oil and grease, and metals. Constituents analyzed in groundwater were below reporting limits except for metals which are addressed under an IR Site 5 basewide study.
The two underground storage tanks (UST) installed in 1971 to supply fuel for backup power generators were removed from the site in 1995. Although annual pressure tests conducted on the tanks while they were in use indicated that they did not leak, one well contained groundwater with a low concentration of total petroleum hydrocarbons as diesel.
Additional soil borings and a groundwater monitoring well were installed at IRP Site 1 during a limited investigation. Analyses of the groundwater sample collected from 1997 to 1998 found some volatile organic compounds, a pesticide compound, and total petroleum hydrocarbons. Metals were detected in soil and groundwater. No contaminants were reportedly detected at levels that present a health risk.
A 2003 record of findings presented the results from previous investigations, demonstrating that IRP Sites 1 through 4 were adequately characterized and supporting the Navy’s conclusion that no further action was warranted for IRP Site 1 and the other three IRP sites. A 2004 water quality site assessment addendum concluded that elevated levels of metals in groundwater were unlikely the result of IRP Site 1 landfill activities.
Analyses of groundwater samples collected at IRP Site 1 in May and November 2010 did not detect total petroleum hydrocarbons or other volatile organic compounds related to petroleum products. The Navy proposed no further action at IRP Site 1 as recommended in the 2011 draft closure report. The Central Valley Regional Water Quality Control Board concurred in an October 5, 2011, correspondence letter with the no further action determination for the waste blowdown area and the two USTs.
IRP Site 2 – Landfill Area A
IRP Site 2, historically referred to as Landfill Area A, is a former waste disposal site near the southeast corner of NRTF Dixon. Trench and fill operations occurred at the approximately two-acre site from the late 1940s to the early 1970s, consisting of the disposal of household and municipal wastes. IRP Site 2 is currently being used for the cultivation of hay and alfalfa.
Landfill Area A was first identified as an IRP site in a 1988 preliminary assessment report. A 1989 geophysical survey of the area detected a centrally located, east-west trending anomaly but ground-penetrating radar found no indications of trench disposal. A site inspection was recommended because of the reported disposal of hazardous materials in the landfills.
Analyses of soil samples collected during a 1990 site inspection resulted in a recommendation of no further soil investigation at the IRP Site 2. All analytes detected in samples collected from the groundwater monitoring wells were below screening levels except for metals. Periodic sampling of the groundwater monitoring wells was recommended as well as an investigation of nearby wells to determine if background levels of chromium and selenium in groundwater are naturally elevated.
A 1996 supplemental site investigation was conducted to verify the presence and assess the extent of contaminants in soil and groundwater found in 1990, and to collect soil and groundwater background metals data. The investigation report concluded that no further action was necessary at IRP Site 2
Analyses of samples collected at IRP Site 2 during a limited investigation from 1997 to 1998 found some volatile organic compounds, a pesticide compound, and total petroleum hydrocarbons in groundwater, and metals in soil and groundwater samples. No contaminants were reportedly detected at levels that present a health risk, if the landfill cell was not disturbed.
A 2003 record of findings presented the results from previous investigations, demonstrating that IRP Sites 1 through 4 were adequately characterized and supporting the Navy’s conclusion that no further action was warranted for IRP Site 2 and the other three IRP sites. A 2004 water quality site assessment addendum concluded that elevated levels of metals in groundwater were unlikely the result of IRP Site 2 landfill activities.
Analyses of groundwater samples collected from five monitoring wells in 2010 did not detect total petroleum hydrocarbons or other volatile organic compounds above screening levels. Metals were detected in groundwater but did not appear to be related to releases at IRP Site 2. IRP Site 5 was created to further investigate and address metals in basewide groundwater study.
The 2012 record of decision selected waste sampling, characterization, excavation of landfill material, backfilling, and off-site disposal of stockpiled debris as the remedy at IRP Site 2. A 2021 report documented the remedial action as completed and the Central Valley Regional Water Quality Control Board concurred in a September 20, 2021, correspondence letter with the Navy’s no further action recommendation.
IRP Site 3 – Landfill Area B
IRP Site 3, historically referred to as Landfill Area B, is a former waste disposal site occupying a 1.6-acre area near the south-central portion of NRTF Dixon and is comprised of six solid waste disposal trenches covered with native soil. Trench and fill operations occurred from the late 1940s to the early 1970s, consisting of the disposal of household and municipal wastes. IRP Site 3 is covered with native grassland vegetation and is traversed from east to west by a gravel and asphalt road.
The Navy’s preliminary assessment of NRTF Dixon conducted in 1987 included the Landfill B area. A geophysical survey evaluated three former landfill areas for buried drums, but detected none at Landfill Area B. A site inspection was recommended.
Analyses of soil samples collected at the site during a 1990 site inspection found volatile organic compounds, pesticide compounds, polychlorinated biphenyls, total petroleum hydrocarbons, and metals. Several metals were detected above screening criteria in groundwater samples, as well as the volatile organic compound toluene. A 1996 supplemental site investigation recommended further evaluation of the potential impact to groundwater of the leaching of copper, lead, mercury, and nickel in soil at IRP Site 3.
Analytical results from IRP Site 3 samples collected during a 1997 to 1998 limited investigation were used to evaluate human health risks from potential exposure to soil and groundwater at the site. Antimony and pesticide compounds were identified as contaminants of potential concern in soil. Total petroleum hydrocarbons as motor oil were detected equal to the water quality objective level and tetrachloroethane was detected above the regional screening level but below the maximum contaminant level for groundwater. Chromium was also detected in groundwater and is being addressed separately as part of an IR Site 5 basewide study.
A 2003 record of findings presented the results from previous investigations, demonstrating that IRP Site 3 was adequately characterized. A 2004 water quality site assessment addendum concluded that elevated levels of metals in groundwater were unlikely the result of IRP Site 3 landfill activities.
Updated groundwater analysis was conducted in November 2010 with detections of volatile organic compounds below screening criteria and total petroleum hydrocarbons as oil and grease above screening criteria but in a background well. Metals were also detected and are addressed as IRP Site 5.
A 2021 record of decision addressing unacceptable risks to human health from polychlorinated biphenyls, lead, and cadmium in soil at IRP Site 3 is being prepared for Navy and regulatory agency signature. The selected remedy for IRP Site 3 is land use controls maintained at IR Site 3 in perpetuity or at least 30 years, as long as waste remains at the site. Land use controls include site access controls and deed restrictions to prevent changes in land use. IRP Site 3, Landfill Area B, remains open under the IRP.
IRP Site 4 – Automobile Landfill
IRP Site 4, a former landfill for automobile debris, occupies 0.44 acres in the designated natural resources management area near the south-eastern corner of NRTF Dixon. Facility personnel reported that automobiles and parts were buried at Site 4 at an unknown time in the past. In addition to automobile bodies and parts, the landfill contained concrete, wood, and metal debris. The site is currently covered by native grassland vegetation.
The Navy’s preliminary assessment of NRTF Dixon conducted in 1987 included the general area of the Automobile Landfill. A geophysical survey evaluated three former landfill areas for buried drums. None were detected in the area that would become IRP Site 4 but buried automobile parts were found. In 1989 excavation of impacted soil, auto parts and debris was conducted.
Adjoining piles of soil and metal debris and automobile scrap were observed during a 1996 supplemental site investigation. No soil or groundwater samples were collected at what was newly identified as IRP Site 4.
Analyses of samples collected at IRP Site 4 during a 1997 to 1998 limited investigation found low concentrations of volatile organic compounds including tetrachloroethene in soil and total petroleum hydrocarbons as motor oil in soil and groundwater, but not at levels that present a health risk. Metals were also detected in groundwater and are being addressed separately as part of an IR Site 5 basewide study.
The Navy removed automobile parts and surrounding debris at IRP Site 4 in 1999 and stockpiled the materials for analytical testing. Results from confirmation samples showed that the naturally occurring metals and total petroleum hydrocarbons in soil did not pose a threat to human health or the environment. The excavation was backfilled with stockpiled soils.
A 2003 record of findings presented a compilation of the previous investigations, a human health risk evaluation, and a screening level ecological risk assessment (SLERA) demonstrating that IRP Site 4 was adequately characterized. A 2004 water quality site assessment addendum reported that only one previously collected soil sample contained metals exceeding background concentrations, although these levels did not impact groundwater quality. Chromium concentrations in groundwater exceeding water quality limits were consistently detected and are also being specifically addressed as part of the IR Site 5 basewide study.
The SLERA found no significant potential risk to terrestrial receptors at IRP Site 4 from contaminants in soil. Groundwater was not evaluated because it did not discharge to surface water in the immediate area and was considered inaccessible to ecological receptors.
No volatile organic compounds or chlorofluorocarbons, semi-volatile organic compounds, polynuclear aromatic hydrocarbons, pesticides, polychlorinated biphenyls, oil and grease, or total petroleum hydrocarbon compounds were detected in any groundwater samples collected from the four monitoring wells at Site 4 in November 2010. The technical memorandum documenting the event recommended no further action for soils or groundwater, except for metals in groundwater. The Navy created IRP Site 5 to investigate and address metals including chromium in groundwater at the entire facility.
The remedy selected for IRP Site 4 in the 2012 record of decision is no further action for soils or groundwater, except for metals in basewide groundwater. The Central Valley Regional Water Quality Control Board concurred in a January 13, 2012, correspondence letter with the Navy’s recommendation for no further action.
IRP Site 5 – Basewide Groundwater (Facility-Wide)
IRP Site 5 was established as a basewide groundwater investigation of hexavalent chromium and other metals detected in shallow groundwater beneath NRTF Dixon. IRP Sites 1 through 4 are all within the boundaries of IRP Site 5.
The Navy conducted a 1987 preliminary assessment at NRTF Dixon. IRP Site 1, the Waste Blowdown Area; IRP Site 2, Landfill Area A; and IRP Site 3, Landfill Area B were identified in the 1988 report, and investigations were initiated. IRP Site 4, the Automobile Landfill, was identified in 1989 during a geophysical survey conducted to locate potential buried drums at the landfill sites. A 1990 site inspection, 1993 and 1996 supplemental site investigations, and a 1997 to 1998 limited investigation were subsequently conducted at the NRTF Dixon IRP sites.
A 2003 record of findings presented the results from previous investigations, demonstrating that IRP Sites 1 through 4 were adequately characterized and supporting the Navy’s conclusion that no further action was warranted for the four IRP sites. Elevated levels of metals were reported in soils at most sites and in groundwater at all sites. Concentrations of hexavalent chromium detected in groundwater across the facility consistently exceeded water quality limits. A 2004 water quality site assessment addendum to the record of findings concluded that elevated levels of metals in groundwater were unlikely the result of activities at the IRP sites.
Groundwater monitoring intended to provide additional assessment and closure information at all four sites was initiated at IRP Site 1 in May 2010. Hexavalent chromium concentrations in groundwater exceeding screening criteria were detected. The subsequent November 2010 sampling event at all four IRP sites detected hexavalent chromium in 14 of 15 monitoring wells. The results were included in a closure report for IRP Site 1, a technical memorandum for IRP Sites 2 and 3, and a technical memorandum for IRP Site 4. The Navy’s agreement with the regulatory agencies to address hexavalent chromium and other metals in a basewide groundwater investigation was included in these documents. IRP Site 5 was established as a basewide groundwater investigation of hexavalent chromium and other metals detected in shallow groundwater beneath NRTF Dixon.
A 2013 IRP Site 5 draft preliminary assessment evaluated metals in groundwater and the historical use or disposal of hexavalent chromium at the facility and assessed hexavalent chromium in groundwater for plume‐like characteristics suggesting a release. The presence of conditions responsible for the generation and persistence of hexavalent chromium in groundwater was also assessed, whether natural and anthropogenic, considering that hexavalent chromium is pervasive in soil and groundwater throughout the region. The draft report concluded that past waste burial practices at NTRF Dixon did not contribute to the presence of hexavalent chromium in groundwater at the facility. The preliminary assessment was updated in 2016 with collection of additional soil samples. However, groundwater was not sampled due to drought conditions.
An additional round of groundwater sampling from all basewide wells in 2019 conducted at the request of regulatory agencies showed that chromium concentrations were generally within the range of historical data. The 2021 final preliminary assessment report incorporated this data, concluding that the presence of hexavalent chromium in groundwater at NRTF Dixon is not the result of a release at the facility, but is more likely the result of natural processes common in soil in the area, possibly accelerated by anthropogenic processes such as agricultural activity and possibly municipal wastewater treatment facility activities upgradient from but not related to NRTF Dixon operations. No further action for metals in groundwater was recommended and the Central Valley Regional Water Quality Control Board concurred in a May 3, 2021, correspondence letter.
MRP Site UXO1 – Former Limited Firing Range Area
Munitions Response Program (MRP) Site UXO1, the former Limited Firing Range Area, is within the designated natural resource management area at NRTF Dixon. The range area was used for solely recreational pistol firing, skeet shooting, and pheasant hunting during Navy operation of the facility from 1941 to 1979. The munitions used at the range included .38-caliber pistols and shotguns. The boundaries are defined by the traditional layout of a skeet range, estimated at 30 acres, and include the berm and range fan. Interviews, records research, and site visit observations indicate that the area was not used for military training, no munitions were stored in the area or at the installation, and the range was constructed for recreational purposes only. The berm may have been used both as a target and a firing line for pole-mounted targets and hand-thrown skeet targets. The site is undeveloped and includes an intermittent stream and some wetland areas.
The Navy conducted a 2007 preliminary assessment to evaluate NRTF Dixon for the presence of munitions and explosives of concern (MEC) and munitions constituents (MC). Expended rounds were observed on the berm at the range, indicating that small arms were fired at the site. Small arms ammunition is mainly composed of lead, the primary munition constituent of concern. No evidence was found indicating the presence of MEC on the site. Because the range was used for small arms, the range is not suspected to contain MEC. The potential for MC, specifically lead, exists at the site. Expended rounds consistent with small arms were observed on the soil berm. Based on the data collected and presented in the PA, a site inspection was recommended for MC at NRTF Dixon.
According to the 2010 site inspection report, 150 soil samples were collected and either screened onsite for lead or sent for off-site analysis. Expended .38- and .45-caliber slugs and one unexpended round were observed in the front of the berm. No lead or other munitions constituents were detected above human health, ecological, or background screening criteria in the range fan and the site inspection report recommended no further action for the area. A removal action was recommended to address lead contamination in soil in the northwest face of the berm and reduce potential risk to both human and ecological receptors.
A 2019 draft action memorandum documents the Navy’s decision to undertake a removal action at MRP Site UXO1 addressing lead in surface soil in the berm area by excavation and off-site disposal of contaminated soil. The Navy is preparing a final action memorandum for regulatory agency review as well as planning documents for the proposed removal action at the site.
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