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T In anaerobic environments microorganisms may use other compounds such as nitrate, ferric iron, manganese, sulfate, and sulfide as electron acceptors. Thus, an increase in ferrous iron, carbon dioxide, dissolved manganese (Mn II), and perhaps sulfide and a corresponding decrease in nitrate and/or sulfate within a hydrocarbon plume may indicate biodegradation is taking place. Additional indicators of anaerobic biodegradation include total alkalinity, redox potential (Eh) (Buscheck and O'Reilly, 1995), and methane (Borden and others, 1995). The total alkalinity of a groundwater system is indicative of the water's capacity to neutralize acid. Alkalinity results from the dissolution of rock (particularly carbonate rocks), the transfer of carbon dioxide from the atmosphere, and the respiration of microorganisms. Therefore, an increase in alkalinity within a hydrocarbon plume is potentially an indicator of bioremediation occurring (Buscheck and O'Reilly, 1995). The redox potential of groundwater generally ranges from -400 millivolts (mV) to 800 mV. Under oxidizing conditions the redox potential of groundwater is positive while reducing conditions are negative. The redox potential inside a hydrocarbon plume should be less than that measured outside the plume (Buscheck and O'Reilly, 1995), and generally negative. Methane levels generally increase within the plume as a byproduct of the breakdown of petroleum hydrocarbons under anaerobic conditions (Borden and others, 1995). Indicators of potential intrinsic biodegradation occurring across a dissolved contaminant plume can be summarized by the following trends: A relative decrease in: A relative increase in: =r dissolved oxygen ferrous iron oxidation-reduction potential alkalinity nitrate carbon dioxide sulfate GROUNDWATER SAMPLING AND GRADIENT EVALUATION Smith Technology personnel performed the latest quarterly groundwater monitoring of monitoring wells on March 19, 1997. Field work consisted of measuring depth-to-water levels in all wells and collecting groundwater samples from all wells and submitting groundwater samples for laboratory analysis. Water level measurements were collected from groundwater monitoring wells MWA, MW-2, and MW-3. The depth to water ranged from 7.11 to 8.59 feet below top of well casing elevation. Groundwater elevations have decreased an average of 0.29 feet since December 16, 1996. The water level data were used to develop the groundwater elevation contour map (Figure 2). Assuming that horizontal isotropic conditions prevail, groundwater in the uppermost aquifer beneath the site flowed in a northwest direction. The average hydraulic gradient on March 19, 1997 was calculated to be 0.003 ft/ft or about 16 feet/mile. The data indicates a groundwater gradient that is consistent with the gradient last quarter. A summary of groundwater monitoring data is presented in Table 1. Groundwater elevations are referenced to feet above mean sea level. w:1951221Repor1s4QMR1996.&c 3 • SMTH