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TERRA UAC Water and gas lines are 2 to 3 feet below ground surface and have little or no gradient other than topographical variations. Electrical, cable and phone services are all overhead. i As stated above the depth of groundwater has fluctuated approximately 12 feet during the 10 years the site has been under investigation. The shallowest recorded depth to groundwater, on site, was measured in well MW-1 in May 1996 at a depth of 10.05 feet below ground surface. Available data indicates that the deepest subsurface utility adjacent to the site is the 27 inch diameter storm drain, approximately 8.02 feet below ground surface at the intersection of 4 Country Club Boulevard and Franklin Avenue (Figure 3). Recorded invert elevations indicate storm drain flow to-wards Smith Canal, to the south. Based on the relati�,e depth of groundwater and storm drain, Terra Vac concludes that the existing subsurface utilities do not act either as boundaries that influence groundwater flow or preferential pathways for free phase transport away from the site. Plume Attenuation by Bio-Uptake An evaluation of data collected during the fourth quarter 1996 groundwater monitoring event indicates that biodegradation of the benzene plume is occurring in several areas of the site. The ' following summarizes the results of the evaluations of each of the indicators of bio-activity in groundwater beneath the study area. DO levels recorded in groundwater from wells MW-10 through MW-12 indicates background levels from 1.54 to 3.35 mg/l . These levels are above the minimum level of DO to support bio- activity (1 mg/1). Chart 1 illustrates the relationship of DO levels to detectable concentrations of benzene in groundwater samples from the fourth quarter 1996 sampling event. In general, wells reported as containing detectable levels of benzene show lower DO levels than wells reported as non detect (ND) for benzene. This inverse relationship is consistent with what would be observed if bio- activity were occurring (Table 2). In groundwater with depressed DO levels, anaerobic conditions drive bio-activity. As oxygen is removed from groundwater, oxidation is replaced by chemical reduction. Chart 2 illustrates the relationship of DO levels to ORP. The relationship of DO to ORP predicts that an inverse relationship between ORP and benzene would exist. Chart 3 illustrates this relationship. Again, as with the DO/benzene relationship, the { general trend is consistent with aerobic and anaerobic groundwater conditions. With the depletion of oxygen, nitrates become the preferential terminal acceptor. Nitrate concentrations change inversely to the level of detectable benzene in groundwater (Table 2). Chart 4 illustrates the relationship of benzene to nitrate available for bio-activity. Once again the observed trend is consistent with what would be observed if biodegradation were occurring. F , Project 30-0212 April 17, 1997 5 i