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j6m Mr. Bob Fratto July 27. 1993 Page 6 hydrocarbon constituents analyzed for. Groundwater analytical results are summarized in Table 3. DISCUSSION AND RECOMMENDATIONS A total of five soil borings were drilled and sampled for soil and/or groundwater around the open excavation to define the lateral and vertical extent of hydrocarbons in soil from the 'r former underground storage tank. Four of the borings were drilled into groundwater, temporary wells were installed. and water Ievel measurements and groundwater samples were collected. LM k One soil sample, TW I-15-15.5, contained trace levels of benzene. TW 1 is located on the north side of the excavation between the existing tank farm and the excavation. Soil has been visibly affected by hydrocarbons to approximately 43 feet in the vicinity of boring TW4, which was originated west of the excavation and extended to approximately 81 feet below ground surface beneath the center of the excavation. Some surficial soil (upper 2 feet) in TW2 was documented as being hydrocarbon-affected. i-� Groundwater collected from TW4 did contained low levels of gasoline constituents. Hydrocarbons were not detected in the other groundwater samples analyzed. It is estimated that groundwater flows to the southwest under a very low hydraulic gradient. It is possible groundwater has been affected by releases at this site. Even though the oil stored within the former UST was viscous and relatively immobile and insoluble, free oil occurring as droplets and within fractures in the sediment was documented to approximately 33 vertical feet in TW4. Visible signs of soil discoloration extended to approximately 44 y, vertical feet in TW4. Due to the occurrence of fractured silt and clay and root traces, which may supply vertical migration pathways, hydrocarbon migration to groundwater cannot be completely dismissed. It is possible, but unlikely, migration occurred to and affected ,.. groundwater beneath the site. Hydrocarbon concentrations detected in groundwater from TW4 may be from an up-gradient off-site source. i- Based on the above discussion, further excavation of visibly hydrocarbon-affected soil will be necessary to remove the primary source area. At least three permanent groundwater monitoring wells should be installed to better quantify groundwater flow direction, hydraulic gradient, and to monitor groundwater quality. An upgradient well would be helpful in defining a possible off-site hydrocarbon source. 4 d/ L.. ` Brown and Caldwell &M Consultants 7603\CORRES PA7603-01.L-t