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LIIJ City of Stockton July I8, 1997 A68107.OIB LPage 9 6.0 EVAL&UA D-Q—N 6.1 SO Characte�M Diesel range constituents (TPH-D) were detected in soil samples collected at soil probe locations SP-1 and SP-2. The greatest concentration of TPH- D (4,100 mg/kg) was detected in the soil sample collected at 13 feet BSG from soil probe location SP-1. TPH-D was detected in the soil samples collected at 16 feet BSG in soil probe location SP-1 and at 7 feet in soil probe location SP-2 at a concentration of 11 mg/kg. Based on the assumption the UST excavation was backfilled with clean fill, the soil above 12 feet BSG is presumed to be free of TPH-D. The analytical results from the soil sample collected at 16 feet in SP-1 (11 mg/kg) suggest that TPH-D concentrations attenuate to levels below the detection limits (less than 10 mg/kg) a very short vertical distance below 16 feet BSG. Therefore the vertical extent of TPH-D migration appears to extend between approximately 12 and 17 feet BSG. The analytical results from the soil sample collected at 7 feet in SP-2 (11 mg/kg) suggest that TPH-D concentrations attenuate to levels below the detection limits (less than 10 mg/kg) a very short lateral distance beyond SP-2. Based on the assumption the TPH-D has migrated in a uniform manner around the former UST, the lateral extent of TPH-D migration appears to extend laterally approximately three feet beyond the limits of the former UST excavation. Therefore, the lateral extent of TPH-D migration extends over an area approximately 13 feet long and 10 feet wide or 130 square feet. Using the vertical and lateral extent estimation, there appears to be approximately 650 i PP y cubic � feet of soil contaminated with TPH-D beneath the former UST location. Using an average of 110 pounds per square foot, this volume of soil is estimated to weigh approximately 36 ! tons. Conservatively estimating that the average TPH-D concentration is 4,100 mg/kg(0.4% by weight) and the diesel fuel weight to volume ratio of 6.9 pounds per gallon, the estimated volume of TPH-D remaining in the soil beneath the former UST location is calculated to be approximately 40 gallons. i } 6.2 Droundwat r haracterization: The analytical results of the groundwater sample L collected from the soil probe SP-1 suggest that groundwater has been impacted by toluene, xylenes, and TPH-D. Benzene, and ethylbenzene were not detected in concentrations above Lthe detection limits in the groundwater sample analyses. TPH-D at a concentration of 72 micrograms per liter (,ug/L) was detected in the groundwater sample collected from soil probe location SP-1. 6.3 n ' vi n 1r Groundwater was encountered at approximately 40 feet BSG beneath this site. Analytical results from the groundwater sample collected detected concentrations of TPH-D at 72 micrograms per liter(ug/L). The U. S. EPA's taste and odor threshold for diesel fuel in drinking water is 100 pg/L (California Regional Water i� Quality Control Board - Central Valley Region, 1995). L