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1 <br />' 28 September 2000 <br /> • AGE-NC Project No 95-0128 <br />' Page 4 of 5 <br />' 4.0. SUMMARY AND CONCLUSIONS <br /> A summary of findings for the 18 August 2000 event indicated the following points <br /> • Monitoring wells on-site have been impacted by detectable concentrations of various BTEX <br /> compounds and petroleum hydrocarbons The wells on-site were also impacted with <br />' moderate levels of MTBE <br /> • Four of the five off-site monitoring wells yielded samples with the highest concentrations of <br />' MTBE detected during the sampling event <br /> • The deeper-screened monitoring wells yield samples with lower concentrations of MTBE as <br /> compared to the shallow-screened wells <br />' a The data suggests that vertically downward migration of petroleum hydrocarbons is minor <br /> yet persistent and also indicates that deeper hydrocarbon-impacted soil horizons are not <br />' significantly impacting the ground water with petroleum hydrocarbons This also suggests <br /> that the more significantly petroleum hydrocarbon-impacted ground water occurs in the <br /> upper twenty feet of the ground water column <br /> • MTBE detections in the former UST/dispenser area and in the down-gradient area suggesting <br /> preferential horizontal migration of MTBE compared to vertical migration <br /> • The vertical extent of TPH-impacted ground water is close to being delineated, however, the <br /> horizontal extent of MTBE-impacted ground water is not, especially toward the north The <br /> current ground water monitoring well system appears to be situated near the MTBE-impacted <br /> Iground water plume margins on the west and south However, the northern edge of the <br /> network may likely be within the core of the dissolved MTBE, requiring additional well or <br /> soil borings to delineate the northern and eastern edges of the dissolved MTBE Figure 5 <br /> depicts the known extent of dissolved MTBE exceeding 100 gg/L <br /> 5.0. RECOMMENDATIONS <br /> Based on the results of this and previous investigations, AGE recommends <br /> • Continued quarterly monitoring of the ground water monitoring network, <br /> IImmediate initiation of ground water extraction from MW5 to prevent further off-site <br /> migration of MTBE-impacted ground water, <br /> • <br /> Advanced GeoEnvironmental Inc <br /> I <br />