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' 9110 Thornton Road, Stockton, California April 27, 1994 The Southland Corporation Page 7 from monitoring wells MW-9, MW-11, MW-14 and domestic wells 9172 Thornton Avenue and 9156 Connie Avenue Groundwater sample collected from MW-1 contained dissolved BTEX and TPH-G ' concentrations of 1, 1, 0 7, 4 and 34 Ng/L, respectively The laboratory report and the chain-of- custody manifest for the groundwater sampling on January 24, 1994 Is presented in Attachment 5 Analytical results of groundwater samples collected from domestic wells on January 24, 1994 are summarized in Table 8 ' Results of the groundwater sampling conducted by the CRWQCB-CVR are presented In Attachment 6 iRATIONALE FOR SITE CLOSURE ' The scope of work for site cleanup confirmation was described in Groundwater Technology's Site Closure Work Plan, dated March 1991, in the Proposal to Implementation Site Cleanup ' Confirmation, dated July 7, 1992, and in an amendment letter dated July 31, 1992 This scope of work included drilling nine soil borings, collecting soil samples, submitting the soil samples for laboratory analyses The results of these activities were presented in Groundwater Technology's Closure Report, dated May 21, 1993 The Closure Report also presented historical groundwater analytical data and confirmation soil boring results (fable 9) Based on the soil and groundwater data presented, a request for site closure was made ' Results of site remediation activities both soil and groundwater, have documented the decrease of petroleum hydrocarbons in the subsurface conditions beneath the site The presence of separate-phase hydrocarbons in soil or groundwater is no longer detected The detection of low levels of BTEX and TPH-G concentrations in groundwater samples has been reported at random intervals since 1990 Results of sampling indicated levels of hydrocarbon constituents have been at or below California action levels for drinking water since 1989 California action levels for hydrocarbon constituents in drinking water are 1 part per billion (ppb) benzene, 100 ppb toluene, 680 ppb ethylbenzene, and 1,750 ppb xylenes Water Quality Clean-Up Levels have not been established for the site The quality of the ' groundwater at the site ("background") is unknown If "background" conditions are considered to be at nondetectable levels, typically, hydrocarbon concentrations at the low levels currently reported will require years to asymptotically reach nondetectable levels To operate the sod vapor extraction system or groundwater treatment system for the estimated time to decrease the hydrocarbon levels to below detection limits is not an economical option The State Water Resources Control BoardSWRCB ( ) Leaking Underground Fuel Tank (LUFT) field manual provides a calculation table to estimate the concentrations of TPH and BTEX that can be left GROUNDWATER � { TECHNOLOGY