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Former Discovery Chevrolet June 5, 1998 Tracy,California Corrective Action Plan 4.21 Initial Horizontal and Vertical Extent of Hydrocarbon-Impacted Groundwater The initial horizontal and vertical extent of hydrocarbon-impacted groundwater was previously discussed in the Problem Assessment Report (Horizon, March 20, 1998). In September 1994, the depth to groundwater was initially measured between approximately 10 to 11 feet bsg, and the groundwater gradient was initially calculated to be approximately 0.003 toward the north-northwest. Groundwater monitoring has been performed quarterly at the site by GeoStrategies since March 1995. Based on the groundwater data collected at the site between 1994 and 1998, it appears that the groundwater impacted by gasoline hydrocarbons was primarily in the area of the former USTs pit, and the groundwater gradient has remained fairly constant toward the north-northwest. 4.2.2 Current Horizontal and Vertical Extent of H drocarbon-Impacted Groundwater The current vertical and horizontal extents of hydrocarbon-impacted groundwater was previously discussed in the Problem Assessment section of the PAR. Analytical results for the groundwater samples collected on February 19, 1998 from the 10 Geoprobe®borings and the December 18, 1997 quarterly groundwater sampling of monitoring wells MW-1 through MW-5 were used to construct a TPHg Groundwater Isoconcentration Map and a Benzene Groundwater Isoconcentration Map included in the PAR. The isoconcentration maps indicated that the lateral extent of hydrocarbon impact to the shallow groundwater beneath the site had been adequately defined. Analytical tests of the groundwater for MTBE and other oxygenate compounds indicated MTBE concentrations ranging from 0.53 ppb to 6.4 ppb, well below the interim action level of 35 ppb (Department of Health Services, November 10, 1994, Summary of California Drinking Water Standards), and below the proposed action level of 14 ppb. The latest quarterly groundwater samples were collected on March 6, 1998 (GeoStrategies, April 1998). During the March 6, 1998 sampling, benzene was reported in groundwater monitoring wells MW-1, MW-2, and MW-4 at concentrations of 270 ppb, 1,000 ppb, and csr 0.8 ppb, respectively, and below the laboratory detection limit of 0.50 ppb in groundwater 3001-21.cap 8 HORIZON ENVIRONMENTAL INC.