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1 September 30, 1999 474911 Harding Lawson Associates Margaret Lagono, Supervising REHS ' San Joaquin County Public Health Services Environmental Health Division Page 5 ' Remaining Contaminant Mass Calculations ' The TPHd mass vathm the remaining impacted soils near the courtyard tanks and sump is conservatively estimated at approximately 215 kilograms, or approximately 68 gallons of diesel At the former 210,000 gallon UST location, the TPHd mass within the two remaining areas of impacted soil are conservatively estimated to ' total approximately 31 kilograms, or 10 gallons of diesel The sections below present the calculations and the basis for the assumptions used in calculating the remaining contaminant mass Courtyard Tanks and Sump. This section estimates the remaining contaminant mass in proximity to the courtyard tanks and sump Contaminant mass calculations were performed with conservative assumptions based on field observations and analytical data collected during HLA's 1998 field investigations, and on previous work performed at the site As indicated in HLA's 1998 summary report (HLA 1998a), and as shown on Plate 3,the lateral extent of impacted soil in the vicinity of the courtyard tanks and sump is approximately 630 square feet As shown on Plate 4 and 5, the maximum vertical extent of impacted soil is approximately 18 ' feet Using these assumptions, the volume of soil within the soil plume is approximately 420 cubic yards Within the soil plume,the concrete-filled USTs and sump occupy approximately 105 cubic yards The remaining volume of impacted soil within the plume is approximately 315 cubic yards Assuming 3,000 pounds per cubic yard(silty clay/clayey silt soil), this is approximately 945,000 pounds or 430,000 kilograms Assuming an average TPHd concentration of 500 mg/kg in the impacted soil, this yields a TPHd contaminant mass of approximately 215 kilograms, or approximately 68 gallons of diesel (assumes specific gravity of 0 84) ' Former 210,000-Gallon UST This section estimates the remaining contaminant mass in proximity to the former 210,000-gallon UST location Two areas of impacted soil are shown on Plate 6, on the northeast and northwest sides of the former UST location On the northeast, the lateral extent is approximately 70 square feet ' (Plate 6)and the vertical extent is constrained by sample T-23 to a depth of less than 11 5 bgs The bottom of the excavation was approximately 8 feet bgs, therefore the vertical thickness of impacted soil is 3 5 feet Using these assumptions,this is an approximate volume of 9 cubic yards Assuming 3,000 pounds per cubic yard for the silty clay/clayey silt soil,this is approximately 27,000 pounds or 12,275 kilograms Assuming an average TPHd concentration (sample T-22 12 mg/kg) of 12 mg/kg in the impacted soil, this yields an approximate mass of 0 15 kilograms, or approximately 0 05 gallons of diesel ' On the northwest,the lateral extent is approximately 350 square feet (Plate 6) The soil samples shown in Plate 6 are those remaining after the second and final phase of overexcavation The vertical extent of impacted soil ' on the northwest can be estimated from concentration gradients defined by soil samples collected at different depths within the impacted soil on the northwest The sample depths and analytical results (HLA, 1998b)on samples T-9 (TPHd concentration of 8,100 mg/kg at 8 5 feet bgs) and T-18 nearby (TPHd concentration of 270 mg/kg at 9 5 feet bgs) indicate a concentration gradient of approximately 7,830 mg/kg/foot Similarly,the results on samples T-3 (TPHd of 1,800 at 8 0 feet bgs) and nearby T-8 (TPHd of<1 0 mg/kg at 10 0 feet bgs), located near the impacted area indicate a concentration gradient of approximately 900 mg/kg/foot These data ' indicate, the concentration gradients in the northwest impacted area range from 7,830 to 900 mg/kg/foot The sample collected at the greatest depth in the northwest impacted area is T-20 with a TPHd concentration of 280 mg/kg collected from a depth of 11 0 feet Using a conservative approach, impacted soils in the northwest are ' assumed to extend to a depth of 15 feet bgs The excavation bottom was approximately 8 0 feet bgs, this gives 1 �