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f �4VJ� 4 A S S O C I A T E S I N C contained calcium chloride and sodium dichromate chill water solution Vaults 7 and 8 had 3,000 and 5,000-gallon capacities, respectively, and were located against the north wall of the processing ' building (Figure 2) All of the vaults were taken out of service and cleaned in June 1990 (Weiss Associates, April 1991) ' Several phases of subsurface soil and groundwater investigation have been conducted at the subject property since December 1988 Thus 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 nummum of 56 feet and at a maximum of 89 feet below ground surface (bgs) 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 maximum contaminant level (MCL) or below the EPA secondary standard (aesthetic), with the exception of total(0 10 milligrams per liter(mg/1))and hexavalent(0 11 mg/1) chrome in well MW-3 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 chronuum by Clayton Environmental Consultants The results indicated both City wells were below method detection limits for total (0 05 mg/1) and hexavalent (0 01 mg/l) chrome However, total and hexavalent chrome levels of 0 07 mg/l were recorded for the MW-3 sample - In an effort to determine the significance of the recorded concentration, a quarterly ' monitonng/samplmg program was implemented to include all three shallow on-site monitoring wells in addition to the four adjacent City wells ' Historical monitoring data are included in Appendix A 2.0 METHODS AND PROCEDURES To evaluate the extent of impacted soil and groundwater at the site, five shallow soul borings were advanced and completed as monitoring wells and two deep soul borings were advanced in ' order to collect groundwater samples from the deeper water-bearing zone All field activities were performed in accordance with ATC's Workplan for Additional Well Installation at Diamond Walnut Growers, 1050 South Diamond Street, Stockton, California, dated May 2002 and subsequent addendum dated August 2002 wIwordpro1625831reportslWell_install_rpt doc 2