SITE INFORMATION AND CORRESPONDENCE

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CURRENT AND HISTORICAL DEPTH TO WATER The current and historical depth to water at the site both have considerable impact on the extent of soil and groundwater contamination and the efforts to investigate and remediate it. Groundwater elevations were considerably lower during the operation of the UST at the site than the present, creating the potential for deeper zones of impacted soil. The current high groundwater levels, however, prevent the effective use of vapor extraction to remove this secondary source of contamination. WEGE used historical groundwater elevations recorded at selected wells in the site vicinity by the California DWR, the San Joaquin County Flood Control District, and California Water Services to determine the historical groundwater elevation minimum at the site for the years 1965 -1999. A review of this data shows the interval September 15 through October 15, 1977 to coincide with the lowest groundwater elevations for the majority of the wells. The elevations at each well (in feet above mean sea level) for the period September 15 through October 15 1977 were plotted on a base map provided by the San Joaquin County Flood Control District and used to establish five foot groundwater elevation contour intervals (See Figure 5). Figure 5 shows the site to be located at the -35 foot groundwater elevation contour interval, indicating that the top of groundwater reached a maximum depth (minimum elevation) of 40 feet below ground surface (bgs). WEGE's investigation of March 26, 1998 included the collection of hydropunch samples to 45 feet bgs. TPH-G, BTEX, and MTBE groundwater contamination, but no soil contamination was observed in these lower samples. Future monitor wells at the site should include provisions to sample groundwater to a maximum depth of 40 feet bgs. Effective use of vapor extraction requires that the groundwater levels remain below the zone of contaminated soil. The top of contaminated soil at the site begins somewhere between 10 and 15 feet bgs. Table 1 shows the measured depth (from top of well casing) to the top of groundwater at monitor well MW1 from April 1993 to the most recent quarterly measurement on March 3, 1999. The groundwater elevation shows a steady increase over the six-year period, reaching a current level of 8.24 feet below the top of casing (approximately 8.5 feet bgs). The last time the top of groundwater was deeper than 15 feet bgs was in December of 1977. The long-term trend towards an increase in groundwater elevation in the site vicinity makes it unlikely that groundwater will recede to levels that would provide efficient use of vapor extraction as a remedial alternative anytime in the near future. 2