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' HURRAY & ASSLSMATES � ENVIRONMENTAL SERVICES �r James M. Langston April 7, 1992 Page 4 Due to the highly porous and permeable nature of the soil (Figure 3), it appears that the gasoline seeped downward directly towards the water table with very little lateral movement. Evidence of this is found in the low PID readings associated with SB-4 and the lack of contamination present in SB-7- Information from nearby monitoring wells suggest that the direction of groundwater flow is primarily to the north with a very flat gradient (low flow velocity). A northward migration of groundwater at the site is supported by the presence of contamination in the 17-foot soil sample collected from SB-2, located immediately north of the Langston's store and approximately 50 feet north of the center of the former tank. Boring SB-3, however drilled an additional 30 feet north of SB-2, and thus 80 feet "down-gradient" of the failed tank registered no evidence of contamination, suggesting that the leading edge of the contaminant plume in somewhere between SB-2 and SB-3 (Figure 4). In addition, looking at the PID readings obtained from soil samples collected from the 17-foot depth in borings SB-5, SB-1 and SB-2 it is apparent that the level of contamination is diminishing in the northward direction. This northward decrease in PID values supports the concept of a low gradient, resulting in the very slow but persistent migration of the contaminant groundwater plume to the north. Conclusions and Recommendations Approximately 1,200 gallons of leaded gasoline was lost from a 4,000 underground fuel tank in March, 1991. Much of this gasoline has seeped through the near surface vadose zone soil to reach the shallow water table at a depth of 17.5 feet below the ground surface. The contaminant plume has developed in the shallow groundwater beneath the site and is moving at a relatively slow velocity to the north. Based on this investigation it is believed that the contamination has not migrated off-site, however, the presence of high concentration of gasoline remaining in the soil column beneath the former tank will continue to contribute contamination to the water table resulting in further northward migration of the contaminant plume. For these reasons Murray & Associates recommends the following remedial actions be initiated: o Installation of three to four groundwater monitoring wells to monitor the movement of the contaminant plume, and for the collection of water samples for chemical analysis. o Excavation and on-site treatment of the contaminated to remove the source �w.- of groundwater contamination.