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�"�` ' 31 October 1996 - AGE-NC Project No. 95-0190 Page 6 of 7 _ In AGE's opinion, the-available data does not demonstrate;a significant threat to ground water resulting from the hydrocarbon release from the former.two 1,000-gallon USTs. 5.0. CONCLUSIONS AND RECOMMENDATIONS 5.1. CONCLUSIONS Based upon results of the investigation, AGE has the following conclusions: L "Laboratory analysis of soil samples collected during the installation of four monitoring wells and one soil boring, have indicted that hydrocarbon-impacted soil, the result of a release associated with the removed USTs, is limited to the vadose soil zone in the general area of the UST excavation: • Analytical results from ground water samples collected from MW-1 through MW-4 over a three-year time span indicate that dissolved petroleum hydrocarbons related to the two former 1,000-gallon USTs have remained undetectable or have generally declined to lower concentrations. • TPH-g concentrations in the more contaminated well, MW-2, located somewhat down - gradient from the UST excavation, have decreased to concentrations of 150 ppb. Benzene and ethylbenzene have remained undetected in MW-2 `while the trace quantities of toluene and xylenes hover near or below the method detection limit. • Matrix interference from the HVOC compound 1,2- dichloropropane in samples collected from monitoring well MW-5, may have occurred during laboratory analysis, yielding false - TPH-g concentrations from that well. • Based on the ground water flow direction, AGE's understanding.of past site usage;and the 'possibility that the reported TPH-g in MW-5 may be a matrix effect, AGE believes that the petroleum hydrocarbons reported for monitoring well MW_-5 are not related to the former two 1,000-gallon USTs. • TPH-g and BTE&X in MW-1,MW-3 and MW-4 were"at less than detectable concentrations at the time of the 1995 and 1996 monitoring events. • Based on the low concentrations of TPH-g and the low toL undetectable concentrations of BTE&X in the ground water at the site, the remaining impacted ground water does not appear to pose a significant threat to ground water. • Soil stockpiled on-site has been analyzed for TPH-g and BTE&X compounds. Residual ` hydrocarbons were not present at detectable concentrations in the stockpiled soil sample.