FIELD DOCUMENTS AND WORK PLANS 2004-2014

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State of California—California Environmental Protection Agency Department of Toxic Substances Control system (low levels) are used to remove chromium from water extracted at approximately 450 gallons per minute from a series of 16 wells screened in the shallow(60-85'), 100-foot sand (90-110') intermediate(130-170'), and deep zones(180- 220'). Treated groundwater is discharged to the Stockton Diverting Canal, north of the Site in accordance with National Pollutant Discharge Elimination System (NPDES) permit number CA0081787 and WDR permit number R5.2003-0100. Compliance monitoring of groundwater and reporting of effluent discharges to the Canal are on a scheduled frequency. Environmental restrictions are recorded with the County on the North and South Yard to protect the remedial systems and prevent certain uses. Enforceable Agreement HAS-A 01/02-140 is in place to continually operate and maintain the remedial systems with periodic reporting requirements. In 2003, an in-situ reductant solution of calcium polysulfide (3%) and ethanol (1-2%) in water was successfully tested through a phased pilot study approach under authorization granted by a 2003 RWQCB Waste Discharge Requirements permit. Monitoring data collected during the first pilot study Indicated that a single point of injection could affect a reactive treatment zone with a radius of approximately 22 feet. This reactive treatment zone was shown to remain effective for the entire duration of the Phase I Pilot Study such that chromium impacted groundwater moving through the treated zone would also be reduced. Phase 11 involved the creation of a linear reactive barrier zone using injection points in a spaced grid approach. The calcium polysulfide solution generates reduced conditions resulting in dissolution of ferric hydroxide mineral coatings from aquifer grains. This dissolution in tum liberated arsenic in groundwater, previously present as a ferric arsenate co-precipitate in the ferric hydroxide coatings. The calcium polysuiflrde solution converts the ferric iron to ferrous iron, followed by the reaction of the ferrous iron with the hexavalent chromium which is removed from water when it forms the insoluble chromium hydroxide. Arsenic concentrations liberated as a result of this process were shown to decrease over time. The ethanol was used as a substrate for indigenous microorganisms to control sulfate generation. HEALTH RiSK DISCUSSION: A baseline risk assessment was performed as part of the remedial investigation in 1990 to address the potential human health and environmental impacts associated with the site in the absence of any further remedial action. Risks to human health and ecological resources were identified and several remedial measures were implemented over the last 16 years since the 1990RAP. The primary chemicals of concern addressed in the 1990 RAP are arsenic and copper in shallow soil and chromium in deep soil and groundwater.The arsenic cleanup level used for soil was 30 mg/kg commercial, 14 mg/kg residential. Arsenic and copper which was not excavated and disposed off off-site has been encapsulated under a Resource Conservation and Recovery Act (RCRA) cap, Risks to potential ecological receptors have been addressed by the removal of impacted surface soil drainage ditches and piping and containment of Site run-off water. Chromium in groundwater is present at levels greater than 5,000 ugtL. The current groundwater remediation system continues to operate to remove this contaminant. Left unabated, chromium may reach the public water system because groundwater represents a complete exposure pathway. The 1990 RAP estimates a health based concentration as high as 0.33 mg/L can be present in groundwater without endangering public health assuming an individual ingests 75 percent of their daily intake of 1.4 liters of water at home every day of the year for 9 years, and weighs 70 kilograms. The cleanup goal established for chromium in groundwater in the 1990 RAP is 0:050 mg/L based on the California MCL for total chromium. The RAP Amendment proposes to maintain this goal for cleanup. A major toxicological study is underway to determine if ingested hexavalent chromium from water causes cancer. One result may be the establishment of a Public Health Goal which is a step towards an MCL value. An ecological assessment was completed in 1990(Ecological Assessment by Clement Associates in May 1990 as part of the Remedial Investigation/Feasibility Study (RI/FS)Volume IV, dated May 1990). This report concluded that no adverse effects are expected in terrestrial wildlife consuming water or fish from the Stockton Diverting Canal assuming chemical concentrations in the canal do not exceed the NPDES permit limits. Aquatic life in the Stockton Diverting Canal is not expected to be adversely affected by the aquatic concentration limits of arsenic, copper, and chromium set by the NPDES permit because these levels are based on United States Environmental Protection Agency (U.S. EPA) Ambient Water Quality Criteria, or more restrictive health-based criteria. In addition, aquatic toxicity studies with the treatment plant effluent have shown no significant adverse effects. No adverse effects are expected to the Giant garter snake, a state- listed threatened species that may inhabit the Stockton Diverting Canal. Snakes living in the Stockton Diverting Canal could potentially be exposed to the chemicals of concern through ingestion of water and fish in the Stockton Diverting Canal; however, snakes are not expected to experience adverse effects because the margins of safety for birds and mammals are so high. The site is completely paved and surrounded by a residential and light industrial area. There are no natural habitats that will be affected by the proposed action and, due to the highly disturbed nature of the project site; no sensitive species are expected to inhabit the area or surrounding lands. Pumping and treating extracted groundwater is currently on-going at the site. The treated groundwater is discharged to the Stockton Drainage Canal under an existing NPDES permit. As a result of no significant biological change in the immediate area of the site, information provided within the 1990 Ecological Assessment has been determined relevant today. Additional ecological assessment is not suggested at this time based on the proposed remedial action for the site and surrounding areas. DTSC 1327(1/Mo8)