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09.August 2000 ; � AGE-NC Project No: 95-0106 ; Page 9 of 10 • Assuming'a plume of impacted soil 85 feetlong and 75 feet'wide, slightly off-center from the former UST excavation, and 8 feet thick(from the bottom of the former UST to the floor sample collected from the excavation), with approximately 61.93 cubic yards of impacted . soil removed through:excavation;approximately 1,422 cubic yards of impacted soil was left in place on the subject site. Assuming further that the plume of impacted soil was uniformly impacted by TPH-g at an average concentration equal to the average TPH-g concentrations detected in.soil samples collected within the plume subsequent to excavation, the mass of TPH-g in the impacted soil would be approximately 4,150 kilograms;,the mass of benzene . in the soil would be approximately 44 kilograms. AGE believes that the actual size of the. impacted.soil plume Js smaller, that the thickness of the plume decreases radially away from ` the former UST pit and that,the hydrocarbon concentrations are currently less than the values utilized for•this•calculation; the resulting masses are probably too high..Assumptions and . calculations of,the hydrocarbon masses-are in Appendix C. 6.0. RECOMMENDATIONS AGE conservatively calculates that a plume of impacted soil 85 feet long, 75 feet wide, 8 feet thick and evenly impacted by an average concentration of 2,383.mg/kg TPH-g and 25.4 mg/kg benzene would contain 4,150 kilograms TPH-g and 44.2'kilograms benzene. The hydrocarbons are primarily ..adsorbed to fine-grained soil that should retard migration of the hydrocarbons. Theplume of impacted soil is probably smaller, thinning toward' the edges, and potentially has had .some hydrocarbon mass reduction through natural attenuation processes. AGE also interprets the results of this and'previous investigations of the site as'to indicate that TPH- g and:benzene have impacted ground water in an approximate area possibly as large as 180:feet by . 110 feet in the soil interval between, 10 feet and 25 feet bsg on and near,the subject property. Although' not fully,delineated, the plume appears to have-limited mobility as evidenced by the relative lack of detectable concentrations of petroleum hydrocarbons'primarily in samples from. , MW-2,`MW-3, MW-5, MW-6and MW-7,monitoring wells that surround the probable source area, . the former UST'pit: A plume thus defined, evenly-impacted by 2 kilograms of gasoline and 0.27 kilograms of benzene, would yield the average concentration§,detected over the last 5 ground I water,monitoring events. The actual mass is.probably less in the area delineated-as-the plume on Figure.5. _ Although it is possible that the plume of impacted groundwater extends'northwest of MW-4, the above calculations of hydrocarbon mass shows that the additional mass is not likely to be significant. AGE recommends that the PHS-EHD consider closure for this site.If the PHS-EHD concurs with this recommendation, AGE'will prepare a'work plan to abandon the monitoring wells; if the I PHS-EHD does not concur,.AGE will immediately schedule.a ground water monitoring event. ' Advanced GeoEnvironmental,Inc.