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Ms. Laurie Cotulla �.✓ July 25, 1989 Page 12 i, benzene are 1.0 ppb and 680 ppb, respectively. The benzene concentration in well ATR-5 is in well ATR-5 is in exceedence of the DHS action level. Figure 3 is a contour map of benzene concentrations in groundwater. Groundwater sampling data from the February 1989 quarterly sampling of wells ATR-1 through ATR-4, as well as the data from wells ATR-5 through ATR-7 in March 1989, were used to generate this contour reap. Groundwater Flow. Upon completion of the three additional wells, water levels were measured in all seven of the wells on the site using an oil/water interface probe to determine the groundwater flow direction and gradient. Figure 4 is a contour map of water levels as determined from water level measurements taken on March 10, 1989. The contour map shows that currentlythe groundwater is flowing east to northeast under a �' g gradient of 0.24 percent, or 12.6 feet per mile. L' Table 4 summarizes the survey data collected from March 1989 (ATR-5 through ATR-7) and April 1987 (ATR-1 through ATR-4). Water levels as measured in March 1989 are also presented. The additional wells were surveyed from the top of casing, as well as from grade. LA The datum used to determine the relative elevations is the top of the casing of well ATR-4. �- CONCLUSIONS �. Based on field observations of groundwater conditions and analytical results of soil and groundwater samples, the following conclusions are presented about the March 1989 investigation: 1. This investigation was limited to the ARCO 2083 property. The lateral extent of hydrocarbons in the soil have not been completely defined with the two on-site investigations. Off-site hydrocarbons contamination in the soil may exist south of the site. L, 2. In the three wells installed in Phase 2, petroleum hydrocarbons in the gasoline range and benzene concentrations that exceeded DHS maximum contaminant levels (MCLS) were identified in well ATR-5, but were not identified in wells ATR-6 or 4 ATR-7. Figure 3 shows that benzene concentrations in groundwater exceeding DHS action levels or MCLs may be found off site to the south and west of the site. 3. Brown and Caldwell recommends that wells ATR-5, ATR-6, and ATR-7 be monitored and sampled for BETX and TPH on a quarterly basis. The monitoring and sampling schedule for the three new wells should coincide with the monitoring t., and sampling schedule for the previously existing wells. LN 16A BROWN AND CALDWELL J� 723 S STREEET SACRAMENTO,CALIFORNIA 95814-7092•(916)444-0123