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1 4.0 SOURCE OF HYDROCARBON PRESENCE The Phase I groundwater gradient interpretation (based on monitoring wells MW-1, MW-2 and MW-3) suggested that the groundwater beneath the site flowed toward the northeast, that the southwesternmost well, MW-1, was a "clean" upgradient well and that the former tanks on the corner of Washington and Sutter Streets were the source of the hydrocarbon impact to groundwater in wells MW-2 and MW-3 However, instead of confirming this interpretation, the Phase II groundwater investigation has revealed complexity in the groundwater flow direction tand indicates that hydrocarbons are present in nearly all of the new wells One of the most significant findings from Phase II is the presence of 8 /cg/L of benzene and 520 ' µglL of TPH-G in well MW-4 This well is located in what was thought to be the upgradient location, based on the Phase I groundwater gradient interpretation and as such, the presence of hydrocarbons in MW-4 would have suggested an upgradient, off-site hydrocarbon source But ' based on the Phase II gradient data which indicates that this well is in a side gradient location, the significance of hydrocarbons in this well is less certain ' The center of hydrocarbon impact appears to be in the vicinity of well MW-7, based on 190 µg/L of benzene and 1700 µg/L of TPH-G in this well As with MW-4, the Phase I gradient ' interpretation of groundwater flow from southwest to northeast would have supported the contention that MW-7 was a downgradient well and hence the hydrocarbon source would have been located between the upgradient well (MW-4) and the downgradient well (MW-7), the location of the former tank site But as with MW-4, the Phase 11 interpretation of groundwater flow direction suggests that MW-7 is not directly downgradient of the tank site which leaves the question of the hydrocarbon source unresolved ' The non-planer nature of the groundwater gradient beneath the site and the unexpected hydrocarbon presence in MW-4 casts doubt on the conclusion that the former tank site is the sole ' source of the hydrocarbon impact and raises the question of a possible unknown source or sources in the site vicinity Until a more consistent groundwater flow direction is determined for the site, the source or sources of the hydrocarbon impact will remain in question ' 5.0 CONCLUSIONS ' Although the Phase II groundwater investigation was designed to confirm the hypothesis that the former tank site was the source of the hydrocarbons in the groundwater, results of this study cast doubt on this conclusion The Phase II groundwater investigation has revealed the presence of ' gasoline-related hydrocarbon compounds in groundwater beneath the site in four of the five new monitoring wells These results are in addition to the previous evidence of these compounds in ' two of the three Phase I wells, two of which are now dry and one of which is no longer in existence This study has also revealed that the groundwater flow direction and gradient beneath the site is irregular, possibly due to the influence of tidal effects, which makes the interpretation of the hydrocarbon source indeterminant ' MSISIc WP 21 Wnm