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I - - ' Page 2 • Ms Margaret Lagono ' Quarterly Groundwater Monitoring Report - September 1996 (Third Quarter) October 28, 1996 ' 6,000-gallon gasoline tanks were located near the southwest corner of the main building A 20,000-gallon diesel-fuel tank, was located beneath the parking lot on the east side of the facility (see Figure 2) 1 The gasoline tanks near the southwest building corner were removed in April 1987 The westernmost gasoline tank failed a tank integrity test in December 1986 The easternmost gasoline tank had previously been abandoned in place, prior to 1986, by removing the fuel 1 and filling the tank with sand The two diesel-fuel USTs were removed in September 1989 1 Approximately 450 cubic yards of soil was excavated during removal of the two USTs in 1987, of wluch approximately 120 cubic yards were impacted by gasoline The soil was disposed of onsite with guidance from the San Joaquin Air Pollution Control District 1 In July and September 1987, Environmental Science and Engineering, Inc installed three groundwater monitoring wells (MW-1 through MW-3) In January 1988, Anderson Consulting Group (ACG) installed wells MW-4 and MW-5 ACG installed wells MW-6 1 and MW-7 in February 1990, and MW-8 in September 1990 In March 1994, Philip (formerly Burlington Environmental) installed wells MW-9 through MW-13 in five boreholes that were drilled at the facility Soil samples were collected from the boreholes l i for chemical analysis Soil sample hydrocarbon concentrations were all below laboratory method detection limits with the exception of the 60-foot depth sample from borehole MW-13, which contained 10 micrograms per kilogram(gg/kg) of benzene This sample was collected at a depth just below the groundwater table; and was probably contaminated 1 by hydrocarbon-impacted groundwater at that location The EHD-SMU letter to Earthgrains dated July 16, 1996 1) asked for clarification regarding previous groundwater sampling of Monitoring Wells MW-5, MW-7, and MW- 8, 2) indicated that Philips conclusions of decreasing concentrations were difficult to make based on the historical data, and 3) indicated that the options of"active source 1 remediation" or"long term groundwater monitoring" (pending changes in California's remediation guidelines) needed to be evaluated 1) Monitoring Wells N4W-5, MW-7, and MW-8 were not sampled during the first two ' quarterly events of 1996, but have been sampled during this third quarterly event, and will be sampled during future quarterly events as applicable For Monitoring Wells MW-5 and MW-7, sampling will depend on the availability of a sufficient source of groundwater 1 Regarding Monitoring Well MW-8, the data reported in the first quarter 1996 report was in error, and was removed from the corresponding table(s) in the second quarter report 2) Histonc groundwater elevations and contaminant concentration tables have been 1 modified to include a chart illustrating the historic data Philip used to draw the conclusion of decreasing contaminant levels 1 i 1 1