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alkalinity, nitrate, sulfate and ferrous iron by EPA Methods 310 2, 353 2, 375 4, and 200 7, respectively When evaluating intrinsic bioremediation data, the following relationships are indicative of active biodegradation an indirect relationship between hydrocarbon concentrations and DO, ORP, nitrate and sulfate concentrations, and a direct relationship between hydrocarbon concentrations and alkalinity and ferrous iron concentrations These analyses are discussed below Dissolved Oxygen- During aerobic biodegradation, DO levels are reduced as aerobic respiration occurs DO is the most thermodynamically favored electron acceptor used in aerobic biodegradation of petroleum hydrocarbons Active aerobic biodegradation of BTEX compounds requires at least 1 ppm DO in ground water and DO concentrations can be as high as 8 to 13 mg/L in oxygen-saturated ground water that is free of hydrocarbons Observed inverse relationships between DO and hydrocarbon concentrations indicate the occurrence of aerobic degradation, provided that at least 1 to 2 mg/L of DO is present in ground water Oxidation-Reduction Potential. The oxidation-reduction potential (ORP) of ground water is a measure of electron activity and is an indicator of the relative tendency of a solute species to gain or lose electrons The ORP of ground water generally ranges from -400 millivolts (mV) to +800 mV Under oxidizing conditions the ORP of ground water is positive, while under reducing conditions the ORP is usually negative Reducing conditions (negative ORP) suggest that anaerobic biodegradation is occurring Generally, the ORP of ground water inside a hydrocarbon plume should be somewhat less than that measured outside the plume .Alkalinity The total alkalinity of ground water indicates the ground water's ability to neutralize acid High alkalinity (high pH) conditions occur when ground water contains elevated hydroxides, carbonates, and bicarbonates of elements such as calcium, magnesium, sodium, potassium, or ammonia Since these chemical species are created by the respiration of microorganisms, high alkalinity is an indicator of biological activity However, these chemical species may also result from the dissolution of rock (especially carbonate rocks) and the transfer of carbon dioxide from the atmosphere Alkalinity also buffers ground water pH against acid generation by both aerobic and anaerobic biodegradation processes Higher alkalinity in the source area as compared to clean areas suggests that biodegradation is occurring Nitrate After DO has been depleted in the ground water, nitrate may be used as an electron acceptor for anaerobic biodegradation In this denitrification process, nitrate is reduced to nitrite Reduced nitrate concentrations in the source area compared to the clean area suggests that anaerobic biodegradation is occurring 240-0783 27