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TERRA VAC l DO levels decrease rapidly with depth below the water table surface. As a result most aerobic biodegradation takes place in the unsaturated zone, capillary fringe and within a few inches of Qthe water table surface. Studies have shown a statistical correlation between the level of DO and the benzene concentration at the sampling ,point (Me Allister, P.M., Chiang, C.Y., Groundwater monitoring and Remediation, 1994, A Practical Approach to Evaluating Natural Attenuation of Contaminants in Groundwater, p 170). Groundwater from wells containing benzene would have lower DO levels than groundwater from wells that were reported as not containing detectable + levels of benzene. Aerobic biodegradation consurnes oxygen, resulting in lower DO levels where there is a hydrocarbon nutrient source. Oxygen Reduction Potential Oxygen Reduction Potential (ORP) is the measure of the relative tendency of a solution to accept Yg Y P or donate electrons. This is useful in determining the stability of minerals in contact with groundwater and is a direct result of DO levels in groundwater, the higher the DO level the greater the ORP (Fetter, C.W., 1990,Applied Hydrogeology, p 341). Positive values indicate that I the solution is in a state of oxidation. Negative values indicate the solution is in a state of reduction (Fetter, p 342). The resulting chemical conversion can affect the solubility of inorganic f chemicals making them more available as nutrients for bio-uptake (Peavy, H.S., etal, 1985, Water Resources and Environmental Engineering, p 74). For example, ferric iron (Few) reduces to ferrous iron (Fe++) as DO becomes limiting and pH moves from neutral to basic (Fetter, p 343, Figure 9.2). ORP data reported for the fourth quarter monitoring event indicates positive ORP values in wells MW-10 through MW-12. Negative ORP values are reported for the remaining wells (Table 1). pH The measure of hydrogen ions for chemical conversion, pH, is an important indicator of the potential for bio-activity. As stated above, pH can affect the availability of inorganic nutrients for bio-uptake. Microorganism growth is typically within a pH range of 6.5 to 8.5 (Baker, K.H., Herson, D.S., 1994, Bioremediation, p 181). Groundwater monitoring data show pH to range from 7.2 to 8.6. A summary of pH data is presented in Table 1. Temperature a As with pH, microorganisms have a tolerance to extremes in temperature (Baker, p 217). Temperatures in the range of 20 to 40 °C promote microbial incubation, and therefore biodegradation of hydrocarbons (Lyman, p 298). However, biodegradation of hydrocarbons is maximized at 27 °C (Baker, p 217). Temperature of the groundwater in the study area ranges from 18 °C to 22 °C. Temperature data are summarized on Table 1. F Project 30-0212 April i 7, 1997 Q3