SITE INFORMATION AND CORRESPONDENCE

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PUBLIC EtALTH SERVIES SAN JOAQUIN COUNTY ENVIRONMENTAL HEALTH DIVISION Karen Furst, M.D., M.P.H., Health Officer 304 East Weber Avenue, Third Floor • Stockton, CA 95202 209/468-3420 DONNA SANTINI GARDNER 1206 STRUCK ORANGE CA 92667 RE: Donna Gardner Property 26056/26080 Thornton Road Thornton CA 95686 SEP 2 2 1997 Site Code: 1864 San Joaquin County Public Health Services, Environmental-Health Division (PHS/EHD) has reviewed the "Problem Assessment Report - Interim Remedial Plan" dated September 4, 1997 which was prepared by Smith Environmental Technologies. PHS/EHD has the following comments for your consideration and response. The interim remedial plan (Plan) included the well construction information for the Thornton Municipal Supply Well #2 (Supply Well) which is located 85 feet north of monitoring well, MW4. The Supply Well is cased to a depth of 245 feet below ground surface (bgs) and perforated at three intervals beginning at 135 feet bgs. A sanitary seal of the 30 inch conductor casing extends to 70 feet bgs. A field inspection in 1982 by the California Department of Health Services indicated that the static groundwater level was 34.9 feet bgs and that when the pump was in operation that the depth to water dropped to 77.4 feet bgs. The Supply Well currently serves 264 connections in the Thornton area. Following the 1995 excavation attempt which removed 1,400 cubic yards of contaminated soil, an investigation was conducted to determine the extent of the remaining soil contamination. Soil contamination is evidenced between 11 and 30 feet bgs and is estimated to extend to within 60 feet of the Supply Well. During the excavation attempt 10,000 gallons of contaminated groundwater were removed and remediated. Grab groundwater samples were collected during the subsequent investigation beyond the existing monitored area. The groundwater plume is estimated to extent to within 30 feet of the Supply Well. Please submit a corrected Table 1 which reflects the actual results as reported by the laboratory for the soil samples during the November 1996 investigation and an amended Table 2 which summarizes the grab groundwater analytical results obtained. The Plan further evaluated remedial alternatives because contaminants remaining in the vadose and saturated zones appear to threaten the mass reduction of dissolved petroleum hydrocarbons in the vicinity of MW4 and because the close proximity of the dissolved petroleum hydrocarbon plume provides the reasonable potential to impact the Supply Well's water quality, thereby potentially affecting the health of the residents served by this well. Natural attenuation was considered initially, but was not recommended due to the extent of remaining soil contamination and the proximity of the groundwater plume to the Supply Well. Additionally, it must be noted that in 1985 the tanks, which were subsequently removed in 1989, were considered abandoned. The site was last an active station sometime prior to 1985. Further excavation was not recommended due to the presence of multiple utility lines within the area of the remaining contamination. The Plan recommended that a pilot scale test of soil vapor extraction and air sparge be implemented to determine the effectiveness of these remedial options. The Plan proposed to install three vapor extraction wells, one air sparge well, and a sentinel monitoring 1 A Division of San Joaquin County Health Care Services