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groundwater for sulfate reduction is determined as follows EACs = 0 21 (SB-SM) Where EACs = Expressed Assimilative Capacity, sulfate reduction 0 21 = mg/L BTEX degraded per mg/L sulfate consumed ratio SR = background sulfate concentration (mg/L) ' SM = sulfate concentration in plume (mg/L.) A reduction in sulfate and BTEX conc,entrdtions within an existing BTEX plume is a strong ' indication that indigenous anaerobic microbes are established and actively biodegrading petroleum hydrocarbons via sulfate reduction ' Groundwater was also tested for aerobic and anaerobic biodegradation indicators (dissolved oxygen, nitrate as nitrogen, Ferrous iron, and sulfates) to identify the occurrence ofintrinsic biodegradation at the site No detectable concentrations of dissolved BTEX were identified in ' monitoring wells MW1, MW3, MW4, MWS, MW6, MW7, MW9, MWl 1, and MW13 and biodegradation indicator results did not support intrinsic, biodegradation ' Groundwater sample results collected in January 1999 identified BTEX in monitoring wells MW2, MW8, MW10, and MW12 exclusively at concentrations of 154 pg/L, 1332 µg/L, 42 ug/L, and 270 ug/L, respectively The relatively low dissolved oxygen concentrations in the shallow groundwater, ranging from 0 0 mg/L to 2 2 mg/L, support that aerobic biodegradation is not a significant terminal electron acceptor process at the site Of the above wells, only momtonng wells MW2 and MW12 identified sufficient dissolved oxygen concentrations to sustain aerobic biodegradation Fecal coloform was identified from groundwater testing performed in 1991 which may have influenced the levels of dissolved oxygen Furthermore, aerobic biodegradation is favorable at a redox greater than 500 mV yet the redox potential at the site ranged from 140 mV to -114mV which favors anaerobic biodegradation Groundwater sample results collected from MW2, MW8, MW10, and MW12 identified dissolved BTEX with corresponding decreased nitrate, elevated ferrous iron, and/or decreased sulfate levels relative to those wells which had no dissolved BTEX Average site concentrations of Ibiodegradation indicators were as follows 1 2 mg/L, as nitrates, 0 8 mg/L as ferrous iron, and 57 2 mg/L as sulfates Groundwater from monitoring well MW2 contained 154 ug,IL as BTEX 0 5 mg/L as nitrates, and 1 36 mg/L as ferrous iron Nitrate levels are decreased and ferrous iron levels are elevated compared to the site wide average Groundwater testing from MW8 identified decreased nitrate levels (0 mg/L), elevated ferrous iron levels (3 19 mg/L), decreased sulfate levels (53 mg/L), and BTEX (1332 ug/L) Monitoring well MW10 which contained 42 ug/L as BTEX identified elevated ferrous iron concentrations (I l mg,/L) and decreased sulfate concentrations (48 mg/L) compared to the site avec age Groundwater testing from MW12 identified decreased nitrate levels (0 3 m« ), elevated ferrous iron levels (1 15 mg/L), decreased sulfate levels (18 mg/L), and BTEX(270 ua) which supports the occuirence of anaerobic 18