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,r 12 March 1999 <br /> AGE-NC Project No. 95-0128 <br /> Page 4of5 <br /> 4.0. SUMMARY AND CONCLUSIONS <br /> Based on results of this investigation AGE concludes: <br /> .� All properly screened wells on site yield ground water samples that have been impacted by <br /> detectable concentrations of petroleum hydrocarbons. <br /> • The three monitoring wells with deeper-screened intervals yield samples with minimal or <br /> undetectable concentrations of petroleum hydrocarbons, demonstrating that vertically <br /> downward migration of petroleum hydrocarbons is insignificant, and also indicating that <br /> deeper hydrocarbon-impacted soil horizons are not significantly impacting the ground water <br /> with petroleum hydrocarbons. This also suggests that the more significantly impacted ground <br /> water is confined to the upper twenty feet of the ground water column. <br /> The oxygenating fuel additive MTBE was detected in ground water samples collected from <br /> all the wells with the exception of MW 1. The lowest concentrations of MTBE (7.5 ,ug/l and <br /> 17,ug/1) were detected in the deeper-screened MW2 and MW4, respectively. Compared to <br /> the 97 Aegll to 950 /..eg/l MTBE detected in the properly screened wells in the core area and <br /> in the down-gradient area, the deeper concentrations are suggestive of minor vertical <br /> migration of MTBE. <br /> • The MTBE concentration in the up-gradient well, MW7, was 29 4gll, slightly greater than <br /> the concentrations detected in samples from the deeper-screened wells. <br /> a.. <br /> • The order-of-magnitude decline of dissolved hydrocarbon concentrations in samples from <br /> MWb and VW1, coupled with the order-of-magnitude increase in samples from MW4 and <br /> MW5 compared to the June 1998 monitoring event are consistent with a shift of ground <br /> water flow direction from northerly in June to easterly-northeasterly in December 1998. <br /> 5.0. RECOMMENDATIONS <br /> &. <br /> Based on the results of this investigation, AGE recommends that quarterly monitoring of the ground <br /> water monitoring network on the site continue. AGE has been informed during conversations with <br /> +- PHS-EHD personnel that a letter directing additional assessment for MTBE will be issued in the near <br /> ` future.AGE will make additional recommendations once the directive letter has been reviewed based <br /> on site conditions and requirements directed by the FHS-EHD.AGE believes that the site is currently <br /> sufficiently assessed such that preparations for on-site remediation of soil and ground water can <br /> proceed. <br /> Advanced GeoEnvironmental Inc. <br /> it <br />