2003-03-13_REVISION - M1992069 - Laserfiche WebLink

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Ms. Kate Pickford/DMG - F Stree[ Soil Contamination/Closure Request Project Number 1001018 Pl1R/~Ci~N March 11, 2003 Page 2 Risk-based modeling using BP RISC was performed to estimate the required benzene- toluene-ethylbenzene-xylenes (BTEX) and total petroleum hydrocarbons (TPH) "source" cleanup concentrations in soil in the spill area to achieve a carcinogenic risk level of 1x10-6 and hazard index of 1 in groundwater at the downgradient property boundary. The model output, including the model inputs, is attached to this letter. Inputs to the model included a saturated soil source with a length of 32 meters, a width of 39 meters and a thickness of 3 meters. It is our opinion that these are conservative inputs for the volume of the soil source since soil abatement activities were previously performed at the site and the amount of residual contamination which remains at the site appears to be smaller than the model input. The distance to the downgradient property boundary was input as 152 meters. As seen from the model output, the estimated soil ``source" concentrations for benzene, toluene and ethylbenzene are approximately 120; 576,000 and 452,000 µg/Kg, respectively, to achieve the target risk levels at the downgradient property boundary. For TPH and xylenes, the BP RISC model output indicated "(a) Risk goal not achieved at residual concentrations." This means that even at levels above saturation (the point at which free product forms), the model did not predict concentrations high enough at the downgradient property boundary to meet the target risk (hazard index). In other words, the model predicts that the source does not pose a significant risk even with free product present. The analytical results for soil samples collected from the site during this project are summarized in Table 1 which is attached to this report while the soil sample locations are shown on Figure 3. Samples which were collected from areas with apparent residual contamination consisted of the West Wall North End. Bottom #1 and South Wall Middle. As seen from Table 1, benzene was not observed above the laboratory detection limit of 2 micrograms per kilogram (µg/Kg) in the West Wall North End, Bottom #1 and South Wall Middle soil samples; toluene concentrations ranged from not observed the laboratory detection limit of 2µg/Kg to 2,422 µg/Kg in the West Wall North End, Bottom #1 and South Wall Middle soil samples; and ethylbenzene concentrations ranged from 606 to 7,250 µg/Kg in the West Wall North End, Bottom #1 and South Wall Middle soil samples. The benzene, toluene and ethylbenzene concentrations observed in soil samples collected from areas with residual contamination (West Wall North End, Bottom #1 and South Wall Middle) were well below the estimated "source" concentrations. Also, free-phase product was not observed in the six (6) wells located m the spill area during the five (5) groundwater monitoring events. Historical groundwater elevation data is summarized in Table 2 which is attached to this report.