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9110 Thornton Road, Stockton, California September 30, 1993 <br /> The Southland Gorporation Page 2 <br /> Historically, the groundwater has been calculated to flow to the east and southeast. Based on the <br /> groundwater flow direction, wells 9156 and 9172 (IVY) are cross-gradient of the site. The:groundwater <br /> samples collected from these domestic wells have not been above the action levels since July of 1986 <br /> for well 9156 and February of 1988 for well 9172 (IVY). We do not concur with the recommendation to <br /> continue the monthly domestic well sampling of wells 9156 and 9172. <br /> The PHS/EHD requested quarterly sampling of domestic wells 1912,8968, 9036, and 9140. Based on <br /> the data presented below we do not concur with the continued sampling of these domestic wells. <br /> 1912 ■ Well 1912 is located up-gradient of the site and the analytical results Indicate that <br /> hydrocarbon constituents have not been detected since August of 1989. The analytical <br /> results indicate that the action levels have only been exceeded onetime (benzene <br /> detected at 1.1 part per billion (ppb) on March 30, 1987). <br /> 8968 ■ This well is located 400 feet southeast of the site. Based on the historical groundwater <br /> flow direction it is cross-gradient or down-gradient of the site. Hydrocarbon constituents <br /> have not been detected in the groundwater samples since November 1988 and only <br /> once was a constituent (benzene at 1.5 ppb in October of 1987) detected above the <br /> action levels. <br /> 9036 ■ This well is located southeast (down-gradient) of the site. The last reported <br /> concentration above the action levels was a benzene concentration of 62 ppb in <br /> February 1988. <br /> 9140 ■ This well is located down-gradient and cross-gradient of the site and has not had any <br /> dissolved hydrocarbon concentrations reported above the action levels since 1986. <br /> Groundwater Monitoring Well Sampling: <br /> The PHS/EHD letter requests continued sampling of all groundwater monitoring wells. Based on the <br /> historical analytical results (Table 3) concentrations have been below the action levels in all of the <br /> groundwater monitoring wells since 1990 with the exception of monitoring well MW-18. We do not <br /> concur with continued quarterly groundwater monitoring and sampling of all groundwater monitoring <br /> wells. <br /> _ �GROUNDWATER <br /> TECHNOLOGY <br />