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PUBLIC HLALTH SERVICES a��N P SAN JOAQUIN COUNTY ENVIRONMENTAL HEALTH DIVISION Karen Furst, M.D., M.P.H., Health Officer - - �- 304 East Weber Avenue, Third Floor • Stockton, CA 952021, 4.�•�o` a� • r 2091468-3420 THOMAS J NOKES SEP 0 2 1997 1650 PARK ST ALAMEDA CA 94501 RE: DISCOVERY CHEVROLET SITE CODE: 2595 1615 WEST 11TH STREET TRACY CA 95376 San Joaquin County Public Health Services, Environmental Health Division (PHSIEHD) has recently reviewed the June 6, 1997"Report of Additional Subsurface Investigation" prepared by Horizon Environmental and the July 7, 1997"Second Quarter 1997 Status Report" prepared by GeoStrategies Inc. for the above referenced site. The June report summarizes the results of a workplan implemented on April 3, 1997, to comply with the recommendations in the PHSIEHD correspondence of September 9, 1996. In this letter PHSIEHD requested a complete soil and groundwater assessment of contamination at the site. The required data was to include the lateral and vertical extent of soil contamination in the former common tank location, the downgradient extent of the groundwater plume, an evaluation of natural attenuation and a well survey. Hand auger borings were used to define the extent of soil contamination around the former tank pit. The boring logs from the hand augered borings were not included in the report. The hand augered borings were unable to identify the lateral or vertical extent of soil contamination to the west. The lateral extent of soil contamination east of thebrmer tank pit appears to have been identified. None of the hand augered samples were able to identify the vertical extent of the contamination. During the September 10, 1994, installation of monitoring well 1 soil samples were taken from 6.5', 10.5', 15' and 20' below ground surface(bgs). Only the 10.5' sample was submitted for analysis. The Photo Ionization Detector(PID) reading from this sample was 1149 ppm. The analytical results of this sample confirmed the presence of gasoline.contamination. PID readings from the 15' and the 20' sample were 356 ppm and 449 ppm respectively. MW-1 was installed near the dispenser lines. The vertical extent of contamination in this area has not been defined. r -- The September 10, 1994, boring log for MW-2 recorded the PID readings as 1281 ppm at 10.5', 11.5 ppm at 15', 1.3 ppm at 17.5' and 2.4 ppm at 20'. Although only the 10'sample was analyzed, based on the PID readings, gross contamination does not appear to extend beyond 15 feet on the north side of the former tank excavation. Laboratory analyses of samples obtained from the hand augered borings combined with the monitoring well data implicate the f r dis eraser and line as a source of contamination. Obtaining soil data from beneath the former dispenser and lines is necessary to determine the size of the soil plume. The size of the soil plume is necessary to determine the probability of groundwater impact and to determine the feasibility of various cleanup options. It may be necessary to obtain samples from soil deeper than static water to define the vertical extent of soil contamination in this area. Since gradient is north west additional monitoring wells may be necessary north west of the building to monitor the groundwater plume that would have resulted f A Division of San Joaquin County Health Care Services