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CLEAWRT-ER ' G R O U P • Environmental Servre[B The extent of the residual dissolved-phase plume is substantially delineated by the existing ' monitoring well network, and the plume does not extend significantly off-site, although it is migrating in an off-site direction (Figure 6) Groundwater sample analytical results of this ' sampling event are summarized on Table 3 ' These groundwater sampling results suggest that additional plume delineation is not required at this time r 3.0 SUMMARY OF HYDROGEOLOGY AND CONTAMINATION 3.1 Hydrogeology ' The site is underlain predominantly by sediments characterized by silts, clays and silt, clay and sand mixtures and discontinuous sand lenses (Figures 4A & 4B) Based on field observations lie and literature values, the average hydraulic conductivity of subsurface sediments is likely on the order of 10-3 cm/sec (Table 4 5, Fetter, C W "Applied Hydrogeology", 1988) Groundwater is present in unconfined conditions beneath the site Depth to water ranges from approximately 8 to 12 feet bgs annually The water table is historically at its highest elevation in ' the late spring Groundwater flow direction is typically toward the southeast at a gradient of approximately 0 0025 ft/ft ' 3.2 Contaminants of Concern ' Cumulative soil and groundwater sample analytical data indicate that gasoline hydrocarbons (1 e TPHg and BTEX) are the contaminants of concern beneath the site The gasoline additive MTBE has not been detected in soil samples collected to date MTBE has only been detected ' once at trace concentrations in water samples collected throughout the course of the investigation Considering that it has not been detected with confirmation at or above laboratory ' method detection concentrations (EPA Method 8020 and confirmation by EPA Method 8260 have been used) MTBE is not a contaminant of concern at this site 1 1 ' ZB 178C CAP 6 November 20,2002