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15 June 2009 AGE-NC Project No.05-1306 r6w Page 11 of 33 4.4. MASS OF DISSOLVED PETROLEUM HYDROCARBONS REMAINING The dissolved mass was estimated utilizing the March 2009 analytical ground water data, and the distribution of impacted ground water(Cross Section Figures 9 and 10,and TPH-d Map Figure 11). The vertical distribution of impacted ground water is modeled to extend 45 feet under the former UST area (MW-7). The lateral distribution of impacted ground water is modeled to extend approximately 20 feet towards the northeast and southwest, and approximately 50 feet towards the southeast, surrounding the former UST. The current dissolved mass of TPH-d is estimated to be 0.873 pounds. The mass and volume calculations are presented in Appendix G. Based on maximum dissolved petroleum hydrocarbon concentrations reported in ground water samples, AGE estimates approximately 41.89 pounds of TPH-d were initially present in ground water on the site, which is a volume of approximately 5.74 gallons (Appendix G). 5.0. CORRECTIVE ACTION 6W In the H&H-prepared Corrective Action Plan Addendum (CAP-A), dated 16 March 2005 and submitted to the EHD,three soil remedial alternatives were proposed: • Excavation to 14 to 17 feet into ground water table and laterally to monitoring well MW-4, removing approximately 850 cubic yards of impacted soil and 506 cubic yards of overburden; this alternative will require shoring and de-watering. • Excavation to 5 feet below ground water table and laterally to MW-3, removing approximately 627 cubic yards of impacted soil and 672 cubic yards of overburden; this ,r alternative will not require shoring, but may require de-watering. • Excavation to 14 to 17 feet below ground water table and laterally to MW-3, removing approximately 961 cubic yards of impacted soil and 672 cubic yards of overburden; this `~ alternative will require shoring and de-watering. H&H recommended alternative 2 as the most cost-effective solution because shoring would not have been necessary, the hydrocarbon mass removed was comparable to alternatives 1 and 3, and the removal efficiency of alternative 2 would address the free product and petroleum material at the soiI/water boundary. However,in the subsequent 10 May 2005 EHD directive letter,the EHD,with concurrence of the CVRWQCB, rejected alternative 2 in favor of alternative 1; the concentrations of petroleum hydrocarbons as diesel (TPH-d) remained high, as much as 14,000 mg/kg, at a depth ` 20 feet bgs. This soil remediation work plan was based on EHD guidance and recommendations. w rr Advanced GeoEnvironmental,Inc.