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12220 Loma Vista Dr. Wm. J. Hunter & Associates1 Registered Geologists _ t Sacramento, CA 95825 ,r Petroleum &Mineral Appraisers i. ';} j (916) 972-7941 FAX (9161972-1683 August 17, 1998 Jeffrey Wong, EHS, Trainee San Joaquin County PHS Environmental Health Division LOP/Site Mitigation Unit W P.O. Box 388 Stockton, CA 95201-0388 RE: Second quarter 1998 groundwater monitoring report for Valley Wholesale Drug Co., 14011 W. Fremont St., Stockton - Site Code 1782 We are enclosing a report covering the results of the eighth sampling event at the above site. It includes the field logs, laboratory results, and maps showing the hydraulic gradient & lines of equal concentration of benzene, ethylbenzene, toluene, xylene, and TPHg based on the latest data. The field work was performed on July 17, 1998 by Del-Tech Geo - technical as directed by our office. PLATE I shows the hydraulic gradient based on the latest work. It continues to be to the north/northwest at about .01'/ft. The water table was measured at an average depth of 7.29' msl datum, a drop of.90' since the April sampling. The groundwater surface continues to be below the tops of the screens in the 3 monitoring wells -they are at 5'bgl. The results of the analyses of water samples from the three monitoring wells since start of the project are summarized on TABLES I& U. Levels of BTEX measured in MW-1 decreased significantly since April. Benzene dropped from 3970 ug/L to 462 ug/L; ethylbenzene from 1630 to 301 ug/L; toluene from 1630 to 99 ug/L; and xylene from 3550 to 550 ug/L. TPH as gasoline increased slightly from 2.72 mg/L to 6.03 ug/L. MW-2 has not measured any analytes since December, 1995. Levels of BTEX& TPHg in MW-3 have also decreased since the last sampling. Benzene went from 1.0 ug/L to non-detect; ethylbenzene dropped from 106 to a0 ug/L;toluene from 4.0 to 0.4; xylene from 36 to 5.0 ug/L; and TPH as gasoline from 1.53 to .341 mg/L. The drop in the water table combined with the large reduction in levels of contaminates suggests that natural attenuation may be taking place. The upcoming placement of three soil/water probes should provide further information. As approved by the County EHD, we have discontinued analyzing for oxygenates. PLATES U through VI are maps showing the approximate lines of equal concentrations of benzene, ethylbenzene, toluene, xylene, and TPH as gasoline.based upon this latest ampling event. They reveal that the limits of groundwater contamination have been well defined to the east and south, but have not been located to the north and west. The hydraulic gradient (refer to PLATE I,) has been quite stable to the north/northwest, but the plume appears to 1