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'r Del Monte Foods 1� One Market ( 1114111 MII Ri P.O. Box 193575 !� . San Francisco, CA 94119-3575 Telephone:(415)247-3004 Steven P. Ronzone Director/Real Estate & Facilities (415) 247-3520 (Direct) (415) 247-3540 (Facsimile) January 20, 2000 Ronald Rowe San Joaquin County Public Health Services LOP/Site Mitigation Unit Environmental Health Division 304 E. Weber Ave., Third Floor Stockton, CA 95202 Re: November 1999 Groundwater Monitoring Results Disco Property, Stockton, California Dear Mr, Rowe: Enclosed please find the November 1999 groundwater monitoring report for the Disco site. The groundwater sample results and the groundwater flow direction continue to be consistent with previous sampling events. Oxygenates were not detected during this event, therefore oxygenates have not been detected in any well at the site in the past five, or more, consecutive sampling events. Although not requested, the laboratory analyzed and reported 1,2-Dibromoethane, also known as ethylene dibromide (EDB), and 1,2-Dichloroethane (1,2-DCA) during this sampling event. These compounds are frequently reported with the oxygenates listed above when analyzed by EPA Method 8260. These compounds are known as lead scavengers and were a common gasoline additive. EDB was detected in NP-1- 90', NP-1-80', NP-1-70', SVE-North, and SVE-South. EDB was not detected in MW- DIS-1 or MW-DIS-2. 1,2-DCA was detected in MW-DIS-1 at a concentration of 150 jig/L, and was not detected in other wells. The Disco site has accumulated a large data set of groundwater sample results since monitoring began in 1991. Monitoring wells containing petroleum hydrocarbons have either stable or reducing concentrations. Monitoring wells outside of the dissolved hydrocarbon plume have been consistently non-detect. We presented Groundwater Monitoring Frequency Decision Tree (Figure 4 in this report) in the groundwater monitoring reports for February (submitted August 9, 1999) and July 1999 report (submitted October 1, 1999), to determine the frequency of sampling for monitoring wells at this site. As defined on the decision tree, the frequency of analysis was re-evaluated at the end of 1999. During 1999, all wells within the groundwater plume were sampled on a quarterly basis. Given the demonstrated stability of the groundwater flow direction and the consistent analytical sample results of existing monitoring wells, we recommend that wells MW-DIS-1, MW-DIS-2, NP-1-70, -80, -90, SVE-South, and SVE-North be sampled semi-annually. Semi-annual sampling will provide sufficient information l for assessing the extent and concentration trends of the plume. Semi-annual groundwater sampling events will be scheduled in the spring (April) and fail L1Re1San Jose#331Phs Rowe Grndwatering Results,Doc