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r - t t I r 30 November 2005 , AGE-NC=Project No 00-0769 Page 7 of 8 I r I ' feet in the water table wells to 15 55 feet below MSL in wells 200 feet bsg, representing a vertical gradient below the site • The HU-1 potentiometric surface slopes uniformly to the east, ground water is inferred to - ' flow along this plane in an easterly direction at a hydraulic gradient between 0 002 ft./ft and 0 003 ft/ft Ground water in HU-2 generally flows to the west, south, and east, away from a „ north-south trending ridge in the potentiometric surface Hydraulic gradients in HU-1 range from 0 002 to 0 003 ft/ft Ground water in HU-3 flows generally towards the southwest and southeast at a gradient of approximately 0 009 ft/ft Ground water in HU-4 generally flows to the northwest and northeast, away from a northwest-plunging ridge in the potentiometric ' surface, at gradients between 0 008 ft/ft and 0 009 ft/ft • _ The most highly impacted ground water appears' to be located near the former USTs ' Laterally, analytes of concern were,detected at or above laboratory reporting limits in all wells except multi-level wells MW-8 and MW-9, vertically,hydrocarbon-impacted ground water was detected in each of the screened intervals of multi-level wells MW-4 through ' MW-7 except interval MW-6A (20'-40') • The lateral and vertical extents of petroleum hydrocarbon-impacted ground water have not • been defined The vertical extent of the fuel additive 1,2-DCA is not defined, however, the lateral extent of 1,2-DCA appears to be fairly well-defined except to the southeast ' BTEX compounds and 1,2-DCA are present in on-site monitoring wells at concentrations that exceed the DHS's Maximum Contaminant Levels (MCLs) for these contaminants in drinking water E 5.0. RECOMMENDATIONS Based on this and previous site investigations, AGE recommends I I Continued quarterly monitoring of the well network The third quarterly monitoring event took place on 08 ands09 September 2005 _ Conduct additional contaminant assessment east of the current monitoring well network, utilizing grab ground water sampling for accurate placement of future monitoring wells • In the context of the exceptionally high concentrations of dissolved hydrocarbons with a very y i stable average concentration,that may represent hydrocarbon concentrations above dissolved solubility levels for the typical compounds, the secondary source of hydrocarbons (soil and , a v shallow ground water)should immediately be treated on-site to prevent the continual vertical and lateral migration of dissolved hydrocarbons off the site towards wells MW-6 and MW-7 Advanced GeoEnvironmental,Inc r r I I L