SITE INFORMATION AND CORRESPONDENCE

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Former Town& Country_Chevron -2- _ 1/23/98 F t. III L. 1 Y w i,-. aF µ San Joaquin County' - _: Y. xylene. Therefore, water in the vadose zone is leaching contamination from the soil at 7.5 to 9.0 ft bgs, and spreading the__rota—inationto'16'ft'bgs where-it is reabsorbed onto the soil above detectable levels" The soil data show a definite decreasing trend in the concentration of contaminants from the waste oil tank. Measurable amounts of petroleum contaminants probably occur in this water at depths greater than 16 ft bgs. However, the concentration of contaminants is too low to be absorbed onto the soil in measurable amounts.. Therefore;the concentration in the vadose zone water is probably too low to impact ground water(70 to 100 ft bgs). ' - ., �� .. , t -` .,t,� s . Soil samples were collected from the bottomz of the gasoline tank excavation at 13 - 14 ft bgs, and at 5 ft intervals while advancing the borings. The analytical results of these samples shows that contaminated soil remains from 13 ft bgs to greater than 26 ft bgs. At the bottom of the gasoline tanks, soil samples Y lad an average concentration of 553 mg/kg%TPH=g, 0.42 mg/kg benzene;79$ mg/kg toluene, 2.56 mg/kg ethylbenzene, and 34:80 mg/kg xylene: The samples were nondetect for lead and ethylene 200 mg/kg TPH- , 0.005 - 6.7 mg/kg dibromide. At 20 ft bgs contamination vanes from nondetect to 3, g g g _ _ benzene, nondetect to 32 mg/kg toluene, non detect to 58 mg/kg ethylbenzene, and 0.008=220 mg/kg xylene. The concentration of petroleum contaminants increases with depth within the sandy clay bed. One soil sample, collected from boring B-1 in the clayey sand layer at 29 ft bgs had 2.3 mg/kg TPH-g, 0.49 mg/kg benzene, 0.074 mg/kg toluene, 0.066 mg/kg ethylbenzene, and 0.24 mg/kg xylene. The increase in the concentration of contamination within the sandy clay bed at depth, indicates the original contamination may have extended through this clay bed to an unknown depth within the vadose zone or to the water table. Because petroleum contaminants do not adhere to sand and silt as readily as clay, water percolating through the vadose'zone would leach the petroleum contaminates from the sand and silt beds more readily than from the clay bed. This is'possibly'the reason the soil'sample from the'ciayey sand bed showed a significant decrease in the concentration of petroleum contaminants.- Yet, significant contamination may remain in deeper deposits where petroleum contaminants may have been absorbed onto the surface of the "saprolitic"bed or migrated through fractures in this bed and absorbed onto deeper'clay deposits. Conclusions: The extent of soil contamination beneath the waste oil tank has been determined: Soil contamination was detected in the deepest analyzed samples from borings, B-1 and B-5, and the concentration of contarr i ation increases-witlf-deptl within the sandy clay-bed: Therefore;'the extent-of.. contamination was not:determined beneath the gasoline tanks.' iL k Recommendations: The lateral and vertical extent of col tarrrination needs to Wdetermined beneath the gasoline tanks. Additional soil samples should be collected at 5 ft intervals or at lithologic changes from.. the depth of the last petroleum contaminated sample into and possibly through the "saprolitic".bed to the nearest clay bed. These samples should be analyzed for petroleum contanninants to determine the°4 vertical extent of contamination. if water is not encountered, a computer model (ex, VLEACH or SESOIL) and Waste Extraction Test (WET) or Toxicity Characteristic Leaching Procedure (TCLP)tests analyses are needed to determine the probability this contamination will impact groundwater. if water is encountered; samples should be collected and analyzed far'petroleum contaminants including methyl a tertiary-butyl ether(MTBE) and the other oxygenates using EPA`Method 8260. This additional information- is required to calculate the mass of contamination remaining on-site. ' ' -