REMEDIAL INVESTIGATION (RI): The draft report presents the findings of the Remedial Investigation (RI) that was performed at the AMCO Chemical Superfund Site (the Site) from September 2004 to November 2006. Field activities associated with the RI are ongoing, and this report will be updated to include data collected between November 2006 and the completion date of the feasibility study. The RI report will be finalized once the feasibility study is completed.
The purpose of the RI is to characterize the nature and extent of contamination at the Site, and to gather the data necessary for a baseline human health risk assessment (HHRA). The results of the RI and the HHRA will be used to support the feasibility study for the Site and, ultimately, the selection of any remedial actions.
The hydrogeology beneath the Site was studied as part of the RI to help evaluate the fate and transport of the site-related contaminants. The Site is underlain by two unconsolidated aquifers that are separated by a regional aquitard. The results of the RI indicate that groundwater contamination is limited to the upper aquifer. The upper aquifer is composed of fill and native sand and silty sand deposits with thin layers of silt and clay (Merritt Sand), and is approximately 55 to 70 feet thick. The Merritt Sand is underlain by a competent confining layer consisting primarily of lean clay (Older Bay Mud.) The Older Bay Mud is generally 50 to 90 feet thick in the vicinity of the Site. The lower aquifer (Alameda Formation) consists of variable fluvial and marine sediments. Its thickness below the site is unknown.
During the RI, the shallow water table fluctuated from approximately 2.5 to 6.5 feet below ground surface (bgs). In the dry season (May through October), flow generally appears to be toward the southwest; in the wet season (November through April), flow is generally to the south. Gradients are relatively flat in the vicinity of the Site, but are typically steeper north of 3rd Street, ranging from 0.006 to 0.008 feet per foot (ft/ft), and shallower south of 3rd Street, ranging from 0.001 to 0.003 ft/ft. Total dissolved solids (TDS) in groundwater south of 3rd Street generally exceed the Regional Water Quality Control Board’s Basin Plan criterion of 3,000 milligrams per liter (mg/L) for drinking water as a beneficial use.
To achieve the objectives of the RI, samples of light non-aqueous-phase liquid (LNAPL), groundwater, soil, soil gas, air, and produce were collected and analyzed in accordance with the 2004 Sampling and Analysis Plan (SAP) and subsequent modifications. Based on the history of the site and surrounding areas, samples of various media were collected from temporary and/or permanent locations both on and off the former AMCO facility, and were analyzed for some or all of the following: volatile organic compounds (VOCs), semivolatile organic compounds (SVOCs), metals, pesticides, polychlorinated biphenyls (PCBs), dioxins/furans, cyanide, general chemistry, and physical parameters.
Over 200 individual contaminants were detected in groundwater, soil, soil gas, and air during the RI, with 98 of those identified as contaminants of concern (COCs). The COCs include VOCs (chlorinated and petroleum hydrocarbons), SVOCs, organochlorine pesticides, PCBs, metals, and dioxins/furans.
Key findings of the RI are as follows:
· Several feet of LNAPL were observed floating on groundwater beneath the central area of the former AMCO facility. The LNAPL consists primarily of VOCs, including tetrachloroethene (PCE) and trichloroethene (TCE), but also contains SVOCs, pesticides, and dioxins/furans. The LNAPL is serving as the primary continuing source of contamination to groundwater, soil, and soil gas.
· The highest concentrations of contaminants in groundwater and soil gas (primarily VOCs) were generally observed in the central and south-central areas of the former AMCO facility, corresponding with the known locations of former chemical storage units and buried distribution piping. However, other distinct areas of elevated contamination concentrations in groundwater and soil gas were observed beneath the large vacant lot on Center Street and beneath the UPRR/Amtrak yard south of the facility, suggesting that separate releases of contaminants have occurred in these areas.
· Groundwater contaminant concentrations beneath the central and south-central portions of the former facility decrease rapidly with depth. The concentrations in the deepest monitoring wells at the site are low or below detection levels, indicating that dense non-aqueous-phase liquid (DNAPL) has not migrated below approximately 20 to 30 feet bgs at the site.
· The VOCs identified as key contaminants (chlorinated solvents and petroleum hydrocarbons) are undergoing significant biodegradation in groundwater. However, data suggests that the down gradient edge of the VOC plume is expanding despite the naturally occurring biodegradation.
· 1,4-Dioxane, a highly mobile and recalcitrant contaminant, has widely migrated in groundwater from the site, and it is expected to continue migrating. Other contaminants mobilized in groundwater are soluble arsenic, iron, and manganese. Other metals, organochlorine pesticides, PCBs, and dioxins/furans generally have limited mobility in the environment, and the extents of these compounds are limited to the immediate vicinities of their historic suspected source areas.
· The lateral extent of groundwater contamination in the mid and deeper zones has not been fully delineated to the current screening levels.
· Several contaminants in groundwater currently exceed risk criteria for the ingestion pathway; however, groundwater is not currently used nor is it likely to be used in the future as a source of drinking water.
· The distributions of contaminants in soil are less centralized and more widespread than in groundwater, suggesting multiple industrial, non-industrial, and non-point sources. Many contaminants in soil, particularly lead, exceed risk criteria for industrial and residential receptors.
· Elevated lead concentrations were detected at several residential properties adjacent to or near the former AMCO facility. The concentrations of lead detected in the soil posed an immediate risk to residents, particularly children. A soil removal action to address the lead contamination was performed at all residential parcels occupying the same block as the former AMCO facility.
· Several VOCs were detected above screening levels, but within the acceptable risk range in residential soil gas, crawlspace air, and ambient air. No VOC detections exceeded acute reference concentrations, indicating that there is no immediate health threat to residents. The primary source of the VOCs in residential soil gas and air is groundwater, not soil.
· At the former AMCO facility and off-facility locations, the concentrations of several contaminants in soil, soil gas, and groundwater would pose an unacceptable risk to site workers. However, the current concrete pavement at the former AMCO facility and off-facility locations provides a protective layer that isolates workers from the contaminated soil, soil gas, and groundwater underneath.
· Several VOCs were detected above screening levels, but within the acceptable risk range in crawlspace air samples collected from the facility office. No VOC detections exceeded acute reference concentrations, indicating that there is no immediate health threat to workers.
· Concentrations of metals and VOCs in sampled homegrown produce are below levels of concern for ingestion.
Investigación Correctiva (RI, según sus siglas en inglés):