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Estimate of Residual Mass—DRO and MTBE in Soil Former BP Station No. 11193 3202 West Hammer Lane, Stockton, CA General Assumptions: •T These calculations are based on Site conditions, data, and other information available as of the date-of the .correspondence. ' Closure Solutions has relied on this information as furnished, and is neither responsible for nor has confirmed the accuracy of this information. The accuracy of these calculations is based on the data available and the actual mass maybe over represented or under represented. • The approximate_lateral and vertical extent of impacted soil used to calculate contaminant. volumes was determiried based on information contained in historical site investigation summaries. • To be conservative, a.yertical impact zone from 2 feet bgs (approximate depth of impact from a typical surface release)to 12 feet bgs (depth to groundwater) was used. • No soil analytical table was available. Calculation: Mass in lbs (V) (55 kg/ft3) (C} (2:2 x 10-6 lbs/mg) • l ft3 of soil = 55 kg • 1 Ib =454,000 mg —> 2.2x 10-6 lbs/mg Soil Assumptions DRO (Three discreet areas) • VSi =Volume of impacted soil between 2 and 12 feet bgs is 1,000 feet' (A,, x 11 S�} • AS=Impacted soil area is assumed to be 100 feet2 (10' by 10') • HS—Impacted soil thickness assumed to be 10 feet • Csj =Result of one soil sample from the November 1994 investigation at a concentration of 1 mg/kg of DRO. •. Calculated Residual Mass of DRO =0.12 lbs- • Vsz =Volume of impacted soil between 2.and 12 feet bgs is 1,000 feet' (A 52 x H 52) • AS=Impacted soil area is assumed to'be 100 feet (10' by 10') • H,=Impacted soil thickness assumed to be 10 feet • C52 =Result of one soil sample from the November 1994 investigation at a concentration of 7,600 mg/kg of DRO. • Calculated Residual Mass of DRO =919.6 lbs Attachment A Page 1 of 2 Closure Solutions Inc.