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' each contarnmant Conservative decay half-life default values from Connor et al (1995) are provided on Table 1
<br /> Alternatively, for most realistic results, biodegradation may be modeled based on actual concentrations of electron
<br />' acceptors, by determining the biodegradation capacity (BC) for each electron acceptor and contaminant
<br /> concentration(Connor et al, 1995)
<br /> The biodegradation capacity is a measure of the actual potential of an electron acceptor(n) to remove contaminant
<br />' mass The BCn is calculated for each contaminant and electron acceptor by dividing the concentration of the
<br /> acceptor in the groundwater by its utilization factor(UFn) The UFn can be easily derived from the stoichiometric
<br /> equation for the particular degradation reaction and represents the ratio of mass of electron acceptor utilized to the
<br />' mass of hydrocarbon degraded (Wiedemeter, 1995) Values of UFn for benzene for each pathway are presented in
<br /> Table 2 The sum of the BCn values obtained for the principal electron acceptors is the total biodegradation capacity
<br /> of the groundwater(BCt) (Connor et al, 1995) This datum is necessary in contaminant fate and transport models to
<br />' realistically evaluate the potential for plume attenuation resulting from intrinsic biodegradation
<br /> MTBE is almost completely recalcitrant to biological degradation and does not sorb onto the soil Due to these
<br /> properties, MTBE concentrations generally minuc non-attenuated plume transport Therefore, MTBE may be used
<br />' as a conservative tracer or"internal standard" for modeling plume transport with no attenuation
<br /> Confirming Natural Attenuation
<br /> To best confirm natural attenuation in anticipation of site closure, the assessment and monitoring activities should
<br />' confirm the following plume characteristics
<br /> 1) Fieldscale contaminant mass has been reduced (based on historical groundwater analyses) Figure 1
<br />' illustrates a generally accepted methodology for calculating residual dissolved contaminant mass
<br /> 2) Microbial activity is occurmg in the plume(based on microbial counts)
<br /> 3) The less recalcitrant compounds are reduced in concentration and extent relative to the more recalcitrant
<br /> compounds The approximate order of increasing recalcitrance for BTEX aromatics is toluene, o-xylene,
<br /> in- and p-xylene, benzene, ethylbenzene That is, toluene concentrations should be most attenuated,
<br /> ethylbenzene least attenuated
<br />' 3) Electron acceptors such as DO,nitrate and sulfate are depleted within the plume
<br />' 4) Metabolic end-products such as carbon dioxide, hydrogen sulfide and methane have accumulated within the
<br /> plume relative to outside of the plume
<br /> References
<br /> Arulanantham, R and Salhotra, A, 1996 Risk and Decision Making at Petroleum Contaminated Sites University
<br /> of California Extension,Programs in Environmental Management
<br />' Barker, G et al, 1995 "Assessment of Natural Hydrocarbon Bioremediation at Two Gas Condensate Production
<br /> Sites " In R E Huichee, J T Wilson, and D C Downey (Eds), Intrinsic Bioremediation, pp 181-188 Battelle
<br /> Press, Columbus, OH
<br />' Cookson, J T, Jr, 1995 Bioremediation Engineering Design and Application (Ed Nalven, G) McGraw - Hill,
<br /> Inc,New York,NY
<br /> Connor, J,Nevin, P J, Fisher, R T,Bowers, R L, and Newell, C J, 1995 Tier 2 Guidance Manual for Risk-Based
<br />' Corrective Action,Appendix A Groundwater Services, Inc,Houston,TX
<br /> Hem, J, 1985 Study and Interpretation of the Chemical Characteristics of Natural Water US Geological Survey
<br /> tWater-Supply Paper 2254 US Government Printing Office
<br /> McAllister, PM and Chiang, C Y, 1994 "A Practical Approach to Evaluating Natural Attenuation of
<br /> Contaminants in Ground Water" In Ground Water Monitoring and Remediation, Spring 1994
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<br />' CLEARWATER GROUP(NATURAL ATTENUATION) 6 revised October 3,2002
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