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i IV• DATA EVALUATION This section describes some techniques to evaluate the analytical data generated by the groundwater sampling protocol. Demonstration of contaminant losses is the first lane of evidence described by the National Research Council (NRC 1993). Plots of contaminant ' concentration versus time and/or distance are useful in this type of evaluation (Buscheck et al 1993). Data evaluation should also include a comparison of contaminant concentrations and biological indicator data It may also include plotting certain constituents to evaluate spatial trends across a dissolved plume. As discussed in Section II, during biodegradation certain electron acceptors (DO, nitrate, and ' sulfate) are consumed (reduced). If a particular electron acceptor is reduced as part of microbially-catalyzed biodegradation, one would expect to observe an inverse correlation between the contaminant and the electron acceptor (declining concentrations of the electron acceptor with increasing contaminant concentrations) Ferrous iron (Fey) is the product of ferric iron (Fe3) reduction. If ferric iron is reduced, ferrous iron will increase, particularly in the middle of a contaminant plume. ' This evaluation is intended to be qualitative The objective is to demonstrate strate multiple lanes of evidence for the occurrence of intrinsic bioremediation. DO concentrations varying by >2 ppm '. across a contaminant plume suggest the potential for aerobic biodegradation. Concentrations of alternative electron acceptors varying by more than several ppm may suggest anaerobic processes are also contributing to the configuration of the contaminant plume. There are several graphical techniques whack can assist an the evaluation of intrinsic bioremediation data. The first technique requires a minimum of three monitoring wells oriented along the groundwater flow direction. By plotting the dissolved contaminant concentration (preferably for an individual compound) on one y-axis and one of the indicator parameters on the ' other y-axis, for each of the monitoring wells, potential spatial trends across the dissolved plume will become apparent. The dimension on the x-axis is distance. This technique is illustrated with data obtained from the Fairfax, VA terminal. Figure 3 is a site map highlighting six monitoring wells which are oriented along the groundwater flow direction. Contaminant concentrations suggest a source in the vicinity of MW-15, which is designated as the origin on the following figures. Figure 4 is a plot of benzene and dissolved oxygen concentrations versus ' distance, illustrating the inverse correlation suggestive of aerobic biodegradation. Figure 5 is a plot of benzene and ferrous iron for the same wells. The increased concentrations of ferrous iron in the middle of the plume suggest anaerobic biodegradation may be contributing to the 1 configuration of the contaminant plume. 1 i 12 I