SITE INFORMATION AND CORRESPONDENCE_FILE 1

Home Browse Search

io" Iwo 4 . DISCUSSION No detectable hydrocarbons were found in any soil samples above the ground water from new or old boreholes (Table 2 ) . The only ` contaminated soil sample was collected in the borehole MW8 below ground-water level at a depth of 28 . 5 feet, at a concentration of 3, 200 mg/kg of TPH. Hydrocarbon contamination below the ground-water level at depths from 25 to over 30 feet in the upgradient well MW8 suggests that major fuel losses may have occurred at a time when the ground- water level was 10 to 15 feet lower than at present, at a loca- tion upgradient of the former Exxon site. Lost fuel may have infiltrated to the previous water table and then moved laterally following regional water flow. Rising ground water then trapped the fuel in the soil below the water table. Records of the San Joaquin County Flood Control and Water Conservation District ( FCWCD ) show that in the Stockton area during the period from 1977 to 1982 the water table was 15 to 20 feet lower than at present . A considerable rise ( around 10 feet ) in the water table occurred between fall 1982 and spring 1983 ( FCWCD 1985 ) ( Figure 4 ) . r On 11 April 1988, 0 . 6 feet of free product was detected in well MW6, for the first time in 10 months. Its occurrence might be related to product moving upward with ground water from a deep zone of contamination followed by a fall in ground-water level ( 2 feet in two months ) . In this situation product may be seeping into the well from a recently desaturated zone. The supposition that the source of free product in the well is the former tank field at the Exxon site is less probable, because neither free product nor hydrocarbons were detected in soil sam- ples in the excavation ( see Table 2 ) or in the monitoring wells closer to the tank field. r e20/73942 8