ARCHIVED REPORTS_REMEDIAL INVESTIGATION REPORT 2000

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Treadwell&Rollo two borings to 40 feet bgs. The two deeper borings were converted to 2-inch diameter • monitoring wells (MW-1, MW-2). Soil samples were collected at 5.5 feet bgs in the shallow borings, and at 5-foot intervals in the deeper borings. According to the report, three soil samples were created by compositing samples from borings 1 through 7, 8 through 14, and 15 and 16. One soil sample collected at the "water table interface"from each deep well was also collected. The five samples were analyzed for"priority pollutant metals" and for volatile organic compounds (VOCs). Groundwater samples from the two monitoring wells were analyzed for the same suite. WaterWork noted that groundwater samples exhibited concentrations of cadmium, total chromium, lead, and selenium that equaled or exceeded the California maximum contaminant levels (MCLs), and that soil was contaminated by past land use practices. The City of Stockton retained RESNA in 1992 to conduct a Phase II Subsurface Investigation, including installing seven 4-inch diameter monitoring wells (MW-3 through MW-9) and analyzing soil and groundwater samples. Wells were installed to approximately 37 to 38 feet bgs. Soil samples collected at 5 feet bgs were analyzed for California Title 22 Metals and • volatile organics. Groundwater samples were collected from existing wells MW-1 and MW-2 through new wells MW-3 through MW-9, and from three wells located at Area 20. Groundwater samples were analyzed for Title 22 Metals and volatile organics. Groundwater flow was determined to be in a north to northwesterly direction. Results of the soil analyses indicated that no metals concentrations exceeded Total Threshold Limit Concentration(TTLC) values or Soluble Threshold Limit Concentration (STLC) values. No volatile organics were detected in soil. Groundwater analyses for the Area 10 wells detected acetone at 120 parts per billion (ppb) at well MW-4, and arsenic, cadmium, chromium and selenium at certain wells at levels exceeding California Maximum Contaminant Levels (MCLs). The Area 20 wells indicated the presence of selenium at concentrations exceeding the MCLs at MW-3. RESNA concluded that shallow soil at Area 10 was minimally impacted by metals, and that groundwater chemistry was consistent with that found throughout the Stockton area. RESNA could not find a correlation between groundwater chemistry and past site operations, and concluded that no further investigations were warranted. • 4 25970309.DGD 24 August 2000