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' K 4.0 Discussion and Estimate of Residual Contaminant TPHD contaminants were observed in well MW-3. SJC EHD has requested that Wright provide an estimate to quantity the residual dissolved contaminant for this sample event. Monitoring well MW-3 is located adjacent to the former underground tanks pit. TPHD has historically been observed in MW-3; other contaminants including BTEX and MTBE and fuel oxygenates have not been observed(see Table 3). Wright has made the following assumptions regarding the estimate of residual dissolved contaminants in the former tank pit area. The average of all TPHD detections is used since TPHD has been observed at various concentrations over time. Contaminants have not been observed in distant wells so an estimate of that aquifer area near MW-3 was made for a potential groundwater volume. This estimate uses the saturated thickness of the aquifer uses the length of the MW-3 slotted interval (15 feet) in an area (30 feet by 45 feet of the former tank pits) of 20,250 cubic feet and assumed effective porosity of 25% containing contaminated water(5,062 cubic feet)times 7.48 gallons per square foot,or 20,250 fe x 0.25=5,062 fe. 5,062 fe x 7.48 gallons/fe about 37,900 gallons groundwater containing residual contaminants near MW-3. The average of TPHD detections from September 2001 to December 2004 at MW-3 of 3,206 micrograms per liter (ug/1) divided by 8 detections) is 400 ug/l. The TPHG calculation used the only detection of 58 ug/1. It should be noted that changing these assumptions would yield different estimates. However in our opinion these assumptions appear reasonable given source removal and area of the former tank pits, historically observed concentrations of TPHD and lack of other dissolved contaminants. The residual contaminant estimate uses a simple calculation that calculates gallons of TPHD that may remain. Please note the contaminant volumes are rough estimates only of the interpreted contaminant quantity remaining. The quantity of estimated groundwater TPHD contaminant in micrograms per liter potentially present in the groundwater converted to gallons used the following equation: 4.0x10'ug/1(average TPHD concentration)x 3.79x105 gallons x 0.264 liter/gallon 0.004 gallons TPHD residual estimated One detection of TPHG occurred in 2001 and no other detections have occurred since. However,if the same equation is used with the same assumed volumes then, 5.8x19'ug/1 (TPHG concentration)x 3.79x105 gallons x 0.264 liter/gallon 0.00058 gallons TPHG residual estimated 5.0 Conclusions and Recommendations Monitoring well MW-3 located near the former tank pit was sampled during this quarter. TPHD was present at 130 ug/1 in MW-3 and TPHG, BTEX and MTBE and oxygenates were not detected in that well. The laboratory reports that the TPHD in MW-3 does not quantify as a match the laboratory diesel standard. The TPHD values have fluctuated slightly with aquifer recharge and discharge. The TPHD appears degraded and is anticipated to continue to degrade. Page 3 of 6