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1 ' AR CAD1S GERAGHTY&MILLER 6.3 LATERAL EXTENT OF SITE PETROLEUM HYDROCARBON IMPACTS ' 6.3.1 Soil Impacts ' Previous investigations conducted at the Site indicate that hydrocarbon concentrations in the soil appear to be limited to the area around the location of the tanks that were excavated from the southeast corner of the site in 1967 No offsite soil impacts have been observed or documented in the shallow soil Field work reported herein yielded data consistent with the finding of the previous consultants Additionally, this report finds that the depth of maximum of TPHg and BTEX impact is at 60 feet bgs Consistent with the previous investigation findings, the TPHg concentration at the 60-65 foot bgs level declines sharply from a high of ' 420 mg/kg at NP-1 to 1 5 mg/kg at NP-2 Soil sample data from NP-1 and NP-2 indicate that lateral impacts tosoils depths beneath_the_Site_are_hmited and have been adequately_ characterised ' 6.3.2 Shallow Groundwater Impacts Previous investigations have provided sufficient data to allow identification of the lateral impacts of petroleum hydrocarbons to shallow groundwater in the vicinity of the Site ' The Site_momtoring_well array_prov_ides data which allows interpretation of the impacts to_ 2 shallow groundwater above the MCLs ' The February 5, 1999 quarterly monitoring data in shallow groundwater indicate that the downgradient TPHg impacts to groundwater (define by the 10 ug/L contour) extend ' approximately 200 ft (Figure 6-1) Figure 6-1 illustrates a log-linear interpolation and extrapolation of the TPHg plume and represents a conservative estimate of the lateral extent of ' the plume The southerly extent of the plume interpretation as depicted in Figure 6-1 is likely more extensive than the real situation However, the large distance between data points in the southerly direction, and the application of a log-linear interpretation, result in the depicted ' 35