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,i j II. INDICATOR PARAMETERS Dissolved Oxygen (O=or DO) rDissolved oxygen is the most thermodynamically favored electron acceptor used in the biodegradation of petroleum hydrocarbons. Dissolved oxygen concentrations can be used to ' estimate the mass of contaminant that can be biodegraded by aerobic processes. During aerobic biodegradation, dissolved oxygen levels are reduced as aerobic respiration occurs. If sufficient dissolved oxygen (> I to 2 mg/L) is present in groundwater, aerobic biodegradation compounds such as benzene, toluene, ethylbenzene, and leases can degrade x3' (BTFrX) (McAllister and Chiang 1994). It is difficult to reliably measure dissolved oxygen concentrations less than about 0.5 mg/L. Because most well purging techniques can allow aeration of collected groundwater unportant to minimize potential aeration by taking the precautions described in the Seecction,hplesit is Sampling and Analysis Under saturated conditions, dissolved oxygen can occur at concentrations as high as 8 to 13 mg/L. ' Oxidation-Reduction Potential (ORP or Eh) The oxidation-reduction(redox)potential of groundwater is a measure of electron activity and is an indicator of the relative tendency of a solute species to accept (gain) or transfer (lose) electrons '• Oxidation is defined as the loss of an electron(s) from a species, while reduction is the gain of an electron(s) from a species. Redox reactions in groundwater are usually biologically mediated and therefore the redox potential depends upon and influences rates of biodegradation. the redox potential of Knowledge of po groundwater is also important because some biological processes only operate within a prescribed range of redox conditions The redox potential of groundwater can be used as an indicator of certain geochemical processes such as sulfate reduction. The redox potential of groundwater generally ranges from -400 millivolts (mV) to +800 mV. Under oxidizing conditions the redox potential of groundwater is positive; reducing conditions are characterized by negative readings. Reducing conditions may be measured where anaerobic biodegradation (lacking oxygen) is occurring. The redoxtential of Po groundwater inside the contaminant plume should be somewhat less than that measured outside the plume. To map the redox potential of groundwater it is unportant to have at least one measurement from a well upgradient of the plume ' Because most well purging techniques can allow aeration of collected groundwater can affect redox potential measurements), it is important to minimize potential aeration by following the steps outlined in the Sampling and Analysis section pH PH is defined as the negative logarithm of the i hydrogen solution is acidic H on activity and describes whether a PH of groundwater has�an effect�onther basic ��7) Domenico and Schwartz 1990). The and activity of microbialpopulations in groundwater. Microbes capable of degrading petroleum hydrocarbon compounds generally prefer 5