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Limited Groundwater Investigation 2 ■ Stockpile samples contained total extractable Cr at concentrations ranging from 12 to 42 µg/L and extractable Cr VI concentrations ranging from 9.7 to 39.9 µg/L at the French Camp facility. The stockpile samples collected from the Lathrop facility contain total extractable Cr concentrations ranging from 12 to 49 µg/L and Cr VI concentrations ranging from 9.5 to 39.8 µg/L. The total Cr and Cr VI extracts were obtained using the DIWET method. ■ Storm water samples collected at the French Camp facility on February 23, 2001 and March 5, 2001 contained concentrations of dissolved Cr VI ranging between 63 µg/L and 95 µg/L, total Cr ranging between 64 µg/L to 100 µg/L, and dissolved Cr ranging between 59 µg/L and 91 µg/L. Storm water samples collected at the Lathrop facility during the same storm water sampling events contained concentrations of dissolved Cr VI ranging between 28 and 144 µg/L, total Cr ranging between 44 and 150 µg/L, and dissolved Cr ranging between 28 and 150 µg/L. ■ Cement used at the French Camp facility contained extractable total Cr (<5 to 15 µg/L) and Cr VI (19 to 69 µg/L). The cement from Lathrop also contained extractable total Cr (36 to 53 µg/L) and Cr VI (47 to 68 µg/L). These extracts were obtained using DIWET method. In some cases, the concentration of the Cr VI found in the cement was greater then the extractable levels detected in the stockpile samples. ■ Fly ash used only at the Lathrop Facility contained extractable levels PrNVET) of total Cr and Cr VI at concentrations of 200 µg/L and 184 µg/L,respectively. The concentrations of these metals found in the fly ash were greater then the stockpile extracts. . ■ Extractable concentrations (DIWET) of total Cr and Cr VI obtained from the stockpile samples and total Cr and Cr VI concentrations found in the storm water samples at these facilities were greater than their respective PHGs for drinking water. This report presents a discussion of the groundwater sampling activities,groundwater results and a comparison of the previous data and WQGs to the groundwater results. 2.0 GEOLOGY AND HYDROGEOLOGY This section describes the geologic and hydrogeologic conditions at the facilities and the surrounding area. The Lathrop facility is approximately 0.5 miles south of the French Camp facility. The following descriptions are based on published literature and the May 7 and 8, 2001, investigation at each facility. 2.1 Regional Geology and Hydrogeology The Lathrop and French Camp facilities are located about 6 miles south of Stockton, California, within the Central Valley geomorphic province. The Central Valley geomorphic province is divided into the Sacramento Valley and the San Joaquin Valley. The Central Valley is a large, northwestward- trending, asymmetric structural trough that has been filled with several miles thick of sediment (USGS 1986). The United States Geological Survey (USGS) describes the boundaries of the P:\wp\lobs\20587\LP—FCGW.doc—DCN 102300 June 2001