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' each contaminant Conservative decay half-life default values from Connor et al (1995) are provided in 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 (LTF„) The UF,, 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 (Wiedemeier, 1995) Values of UF,,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 (BCI) (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 mimic 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 /> Confirnung 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 /> 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, 1995 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 Hmchee, 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 U S 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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