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09 September 1996 AGE-NC Project No 95-0173 ' Page 7 of 8 ' S.O. CONCLUSIONS AND RECOMMENDATIONS 5 1 HYDROCARBON-IMPACTED SOIL. The Iaboratory data and field data collected for therelimin site assessment indicates that elevated P �3' concentrations of petroleum hydrocarbons exist beneath the site The data indicates that the soil from ' 15 to at least 60 feet bsg at the northern area as derived from analytical results of B-2 was the most severely impacted It is likely that B-2 was drilled close to the source of the hydrocarbon release The impacted interval in the remaining soil borings to the west, east and south of the former tank area is ' relatively limited between 20/25 and 50 feet in addition to containing lower concentrations ' Based on the existing subsurface conditions, it appears that further assessment of impacted soil is warranted north of the site We recommend to address off-site areas by using the direct push technology of a Geoprobe 5 2 HYDROCARBON-D"ACTED GROUND WATER As anticipated, the down-gradient wells from the former UST area, MW-3 and MW-4, indicated the highest concentrations of dissolved hydrocarbons in ground water The presence of hydrocarbons in MW-1, albeit at a significantly lower concentration, may be attributed to either another hydrocarbon plume further up-gradient or the up-gradient nugration of hydrocarbons by diffusion from the former tank area Migration of hydrocarbons in all directions, even in the up-gradient direction, is a common phenomenon at sites with low to moderate hydraulic gradient The non-detection of MTBE in the ground water samples suggests that the leak may have occurred prior to the 1980's Based on the petroleum hydrocarbon concentrations detected in ground water samples, further assessment is needed to delineate the vertical and horizontal extent of impacted ground water Further ground water assessment is recommend to be performed in two phases The phased approach appears to be more cost-effective by minimizing the number of costly ground water monitoring wells The first phase would include installation of—ater sam.—ling paints uS.ng a Geoprobe, upon `vtil'ucl'i ground water monitoring wells are installed at strategically placed locations as the second phase The ground water sampling points are proposed in the Little Caesars parking lot located north of the former UST area, and in the city right-of-way located on Pacific Avenue and Castle Street Field and laboratory data derived from this sampling methodology will provide a more efficient and cost effective approach for designing the number and locations of off-site ground water monitoring well(s)