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Technical Summary & Report of 4TH Qtr Monitoring Apnl 2, 1992 Diamond Walnut Growers, 1050 South Diamond Street, Stockton, CA W0564 i According to the San Joaquin County Flood and Water Conservation District data, groundwater beneath the site is estimated to be at 70 feet below grade. This is consistent with measurements from monitoring wells on site. ' 3.0 SUMMARY OF PREVIOUS WORK Several phases of subsurface soil and groundwater investigation have been conducted at the subject property since December 1988 This work was performed by Weiss Associates (WA) ' of Emeryville, California. ' The three groundwater monitoring wells were constructed to monitor the upper-most water- bearing zone, which was encountered at a depth of approximately 71 feet below grade in each ' boring The wells were screened at a minimum of 56 feet and at a maximum of 89 feet below grade. On January 31, 1991, after appropriately developing each well, WA personnel collected groundwater samples from each well. Samples collected from monitoring wells MW-1 and MW- 2 (adjacent to Vaults 1 and 6, respectively) were analyzed by GC/FID for ethylene and ' propylene glycol The water sample collected from MW-3, adjacent to Vault 8, was analyzed for total soluble chromium, sodium and calcium (EPA method 6010), hexavalent chromium ' (EPA method 7196) and chlorides (EPA method 325 2) All constituents detected were below the California MCL or below the EPA secondary standard (aesthetic); with the exception of total ' (.10 mg/f) and hexavalent ( 11 mg/f) chrome in well MW-3. These results are reported on Table 1. In order to assess the validity of the MW-3 results for chrome species, a verification round of ' sampling was performed in August of 1991 by WA. Water samples were collected from monitoring well MW-3, and City wells WP-1 and WP-2. Each sample was analyzed for total and hexavalent chromium by Clayton Environmental Consultants. The results, which are reported on Table 1, indicate both City wells are below method detection limits for total (.05 mg/P) and hexavalent ( 01 mg/P) chrome However, total and hexavalent chrome levels of .07 R011792 SAY 3 1