Laserfiche WebLink
� S"A <br /> • December 16, 1992 Working To Restore Nature <br /> Unocal Service Station No 6981, Stockton, California <br /> SUMMARY AND RECOMMENDATIONS <br /> The following summarizes the results of investigation <br /> o The site is underlain predominantly by silt Lenses of poorly graded sand and <br /> silty sand are present and are probably responsible for the majority of the <br /> water production <br /> o During the tank removal in 1988, approximately 1,600 cubic yards of soil were <br /> excavated from the tank and line areas, aerated onsite and disposed of at a <br /> Class III landfill <br /> o During subsequent investigations, nineteen borings have been drilled on or <br /> offsite Thirteen of these borings are monitoring wells (five of which are <br /> currently dry), four vapor extraction wells, one pump-test well, and one <br /> backfilled boring <br /> o In February 1990, depth to water at the site was approximately 50 feet below <br /> lie ground surface In September 1992, depth to water was approximately 56 feet <br /> below ground surface Direction of ground-water flow at the site has <br /> consistently been to the northeast The gradient has varied from 0 004 in <br /> August 1992 to 0 010 in March 1992 Pump test data indicate that the aquifer <br /> beneath the site is heterogeneous, anisotropic, and unconfined with a <br /> transmissivity of 0 4 to 0 8 ft2/min and a specific yield of 5 to 10 percent <br /> o Residual gasoline hydrocarbons are present in the soil beneath the tanks and <br /> the dispenser islands Soil containing TPHg is delineated laterally and a small <br /> volume may be present offsite, east of the dispenser islands Concentrations <br /> r beneath the tank pit ranged from 1,900 ppm TPHg and 84 ppm benzene to <br /> below the laboratory's limits of detection Analytical results from soil beneath <br /> the dispensers ranged from 5,300 ppm TPHg and 17 ppm benzene to below <br /> the laboratory's limits of detection <br /> o Free hydrocarbons have not been observed on ground water The lateral <br /> r <br /> extent of dissolved hvdrocarbons in ground water has been defined Ground <br /> water containing dissolved hydrocarbons appears to be present offsite Initial <br /> levels of 13,000 ppb TPHg and 1,400 ppb benzene were reported in ground- <br /> water in March 1992 Concentrations reported in the same well have <br /> decreased to 240 and 35 ppb for TPHg and benzene respectively in the <br /> 6 October 1992 sampling This was associated with a ground-water level <br /> 38032-14 <br /> 17 <br />