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petroleum hydrocarbon constituents have been detected above method detection limits at any time during the groundwater sampling program. The historical analytical results of groundwater samples collected from well MW 1 are summarized in Table 2. 4.0 EXISTING CONDITIONS m 4.1 Soil Soils beneath the site are fine grained, consisting of clays and silts with some silty sand. Residual soil contamination remains beneath the facility building and sidewalk. Analytical data indicates residual impacted soils occupy a lateral semi-circular configuration with an estimated maximum radius of approximately 12 to 14 feet from the northern extent of the former excavation. Sample F (SP-NW- 5.5'-W), which contained significant levels of petroleum hydrocarbons, was collected prior to removal of the product vent lines. The lines were removed during the excavation and an additional two feet from the north wall was excavated, effectively removing a significant portion of contaminated soil in the vicinity of Sample F. No soil contamination was apparent below the layer of fat clay (approximately 10 feet below grade) based upon field observations, PID readings, and analytical results. The zone of vertical impaction appears to exist primarily between depths of approximately three to eight feet below grade. The calculated mass and volumes of petroleum hydrocarbon constituents are summarized below (RESNA, June 1994). .v. _ Total Mass of TPHg beneath facility=241 lbs Total Mass of TPHd beneath facility= 195 lbs Total Mass of benzene beneath facility=0.14 lbs _ Total Mass of toluene beneath facility=8.5 lbs Total Mass of ethylbenzene beneath facility= 8.7 lbs ' Total Mass of xylenes beneath facility=30 lbs This residual soil contamination is estimated to occupy an area of approximately 300 to 350 square feet between the depths of three and eight feet below grade. The volume of contaminated soil is estimated at a maximum of approximately 56 to 65 cubic yards, containing an estimated 241 pounds (36 gallons) of gasoline range hydrocarbons and 195 pounds (28 gallons) of diesel range hydrocarbons. These conservative totals were estimated as "worst case scenario" and are likely higher than actual conditions due to an overlap of gasoline and diesel hydrocarbons reported by the laboratory (diesel values consisted of "non-diesel mix + discrete peaks"). In addition, the estimates assume a uniformly impacted area between three(the depth of the reported leak) and eight feet in depth (the depth of PID — readings of zero). The actual affected area(and thus, the mass of contaminants) could be significantly less than our estimates. The affected area is covered with concrete. Rain water cannot leach into the soil of this area. The presence of fat clay from approximately 7 to 10 feet below grade further reduces the potential for vertical migration of residual contaminants. .J w:\94368\repoM\=oi1.doc 4 _ SMTH r