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1 September 11, 1997 Harding Lawson Associates 38737 002 ' Ms Diary Meays San Joaquin County Public Health Services Environmental Health Division Page 4 Groundwater samples were collected from all wells with a steam-cleaned or TSP-cleaned and deiomzed ' water triple-rinsed, stainless steel bailer and decanted into laboratory-prepared containers The sample containers were immediately sealed, labeled, and placed in a cooled ice-chest for transport to Chromalab Incorporated in Pleasanton, California All 40-milliliter sample containers were visually inspected to be certain that no bubbles were contained after sealing ' During the sampling event the casing on MW-1 was fractured, arrangements are being made to fix the well In addition based on apparent erroneous elevation data points for well MW-3 and MW-4, and on review of previously generated groundwater contour maps, top of casing elevations will be resurveyed Chenucal Analysis of Groundwater Samples During the third quarter of 1997, one water sample from each well sampled was analyzed for TPHg,by ' EPA Method 8015 modified, and for benzene, toluene, ethylbenzene, and total xylenes (BTEX)by EPA Method 8020 RESULTS ' Groundwater Flow Water,level data collected on August 7, 1997 during the third quarter indicates that the shallow groundwater beneath the site flows to the south-southwest The water levels indicated a gradient of approximately 0 00197 feet per foot Water level data are summarized in Table 2 A groundwater surface map for water levels obtained on August 7, 1997 is presented on Plate 4 Water levels for wells MW-3 and MW-4 were apparently erroneous and were not incorporated in the groundwater contour map ' Groundwater Analyhcal Data Laboratory results are presented in Table 1 and copies of the laboratory analytical results are included in Appendix B TPHg and BTEX were not detected in excess of the reporting limits in wells MW-1, MW-2, MW-3, MW-5, or MW-6 In well MW-4, TPHg was detected at a concentration of 0 054 mg/L,benzene was detected at a concentration of 0 068 micrograms per liter(ugIL), and total xylenes were detected at a ' concentration of 0 5 7 ug/L DISCUSSION ' In general, the analytical results indicate that dissolved petroleum hydrocarbon concentrations in the groundwater have fallen below the lowest previous levels in the period for which groundwater samples have been collected, first quarter 1988 through second quarter 1994 The benzene concentration of 0 68 ug/L is the lowest benzene concentration yet detected and is below the California Primary Maximum ' Contaminant Level of 10 ug/L This sampling event occurred after a period of over three years during which active remedial measures were discontinued This provides an opportunity to assess the efficacy of a potential remedial alternative, ' insitu bioremediation, prior to proposing remedial alternatives for the Site The three year period could be interpreted as a full-scale bench test of passive bioremediation Based on these analytical results, insitu bioremediation appears to be an effective remedial measure at this site Demonstration of the effectiveness of proposed remedial alternatives is one of the RWQCB criteria for categorization as a low risk groundwater site 1 iii