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each contaminant Conservative decay half-life default values from Connor et at (1995) are provided in Table I .
<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 at, 1995)
<br /> The biodegradation capacity is a measure of the actual potential of an electron acceptor (n) to remove contaminant
<br /> mass The BC,, is calculated for each contaminant and electron acceptor by dividing the concentration of the
<br /> acceptor in the groundwater by its utilization factor (UF,,) 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 BC„values obtained for the principal electron acceptors is the total biodegradation capacity
<br /> of the groundwater(BC,) (Connor et at, 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 /> Confirmeng Natural Attenuation
<br /> To best confirm natural attenuation in anticipation of site closure, the assessment and monitoring activities should
<br /> confinrit the following plume characteristics
<br /> I) 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 occuring 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 /> m- 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 Hinchee, J T Wilson, and D C Downey (Eds), Intrinsic Bioremediation, pp 181-188 Battelle
<br /> Press, Columbus, OR
<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 /> Hen-4J, 1985 Study and Interpretation of the Chemical Characteristics of Natural Water US Geological Survey
<br /> Water- Supply Paper 2254 U S Government Printing Office -
<br /> McAllister, P M 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
<br /> CLEARWATER GROUP(NATURAL ATTENUATION) 6 revised October 3,2002
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