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6-CYA bw A S S O C I A T E S I N C L bioremediation occurrence at the site. In addition, grab groundwater samples were collected from soil borings during this time period and were also analyzed for specific geochemical parameters. Based upon geotechnical and geochemical data collected from groundwater and/or soil underlying the project site, this study concluded intrinsic bioremediation may be taking place in the contaminant plume beneath the site. Biodegradation appears to be continuing in an aerobic and anaerobic environment. Chemical data suggests the contaminant plume is stable since it appears that petroleum hydrocarbon concentrations are not significantly increasing in the wells. A more detailed discussion of the natural attenuation at this site is contained in Smith Technology #"' Corporation's Additional Soil and Groundwater Report, Fayette Manufacturing Corporation, 7675 West Eleventh Street, Tracy, California, dated January 31, 1997. Geochemical trends from the results are summarized in Table 5. On March 9, 1999 ATC supervised the advancement of three additional soil borings (MW-4, MW-5, and MW-6) to evaluate the extent of impacted soil and groundwater northwest of the former USTs. These soil borings were completed as groundwater monitoring wells to depths of approximately 26 feet below ground surface (bgs). Wells MW-4, MW-5, and MW-6 were placed in locations where elevated concentrations of petroleum hydrocarbons were detected during the y subsurface investigation performed in September 1996. Of the soil and water samples collected from these wells/borings, only low to nondetectable concentrations of total petroleum hydrocarbons as gasoline (TPHg), toluene, ethylbenzene, and xylenes were detected from water samples obtained from wells MW-4 and MW-5. The very low to nondetectable concentrations of petroleum hydrocarbons currently detected at these locations appear to confirm that intrinsic ,.., bioremediation is reducing the petroleum hydrocarbon plume beneath the site. This is especially evident in the vicinity of well MW-4 where free product was detected in September 1996. This work is summarized in ATC's Summary Report of Additional Subsurface Investigation dated a. May 12, 1999. Since the installation of the groundwater monitoring wells in July 1995, sixteen quarterly groundwater monitoring events have been performed at the site. The groundwater gradient has been predominantly north-northwest during these monitoring events. Groundwater elevation data is summarized in Table 4 and copies of the gradient maps are included in Appendix C. L 2.0 SUBSURFACE CONDITIONS ` 2.1 Regional and Site GeoloQy Tracy, California, is located in San Joaquin County and lies on the west flank of the large northwest trending asymmetrical structural trough called the Central Valley, adjacent to the Sierra-Nevada geomorphic provinces. The Central Valley contains a thick sequence of sediment that in places reaches a depth of approximately 10 miles bgs. These sediments range in geologic age from Jurassic (205 million years before present) to Recent (present time) and include both marine and continental deposits. V w:V5122\rep \Request for NFAdoc 3 V