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Laboratory analysis of soil samples may be necessary to establish foc, which is useful for modeling sorption Hydraulic conductivity may be obtained as an estimate from the literature based on soil type (for homogeneous lithologies), or by performing an aquifer test(slug or pump) For plumes under steady-state conditions, contaminant transport models such as the Domenico Transport Equation can be modified to include the processes of dispersion and sorption to predict contaminant concentrations at a given distance from the source (Connor et al, 1995) In addition, biological and chemical degradation may be collectively modeled by a first-order decay function requiring assignment of a literature-based decay half-life value (in days)for each contaminant Conservative decay half-life default values from Connor et al (1995) are provided in Table 1 Alternatively, for most realistic results, biodegradation may be modeled based on actual concentrations of electron acceptors,by determining the biodegradation capacity(BC)for each electron acceptor and contaminant concentration(Connor et al, 1995) The biodegradation capacity is a measure of the actual potential of an electron acceptor(n)to remove contaLmmant mass The BCS, is calculated for each contaminant and electron acceptor by dividing the concentration of the acceptor in the groundwater by its utilization factor(UFS,) The UF„can be easily derived from the stoichiometric equation for the particular degradation reaction and represents the ratio of mass of electron acceptor utilized to the mass of hydrocarbon degraded(Wiedemeier, 1995) Values of UF„for benzene for each pathway are presented in Table 2 The sum of the BC„ values obtained for the principal electron acceptors is the total biodegradation capacity of the groundwater(BC) (Connor et al, 1995) This datum is necessary in contaminant fate and transport models to realistically evaluate the potential for plume attenuation resulting from intrinsic biodegradation MTBE is almost completely recalcitrant to biological degradation and does not sorb onto the soil Due to these properties, MTBE concentrations generally numic non-attenuated plume transport Therefore,MTBE may be used as a conservative tracer or"internal standard"for modeling plume transport with no attenuation Confirming Natural Attenuation To best confirm natural attenuation in anticipation of site closure, the assessment and monitoring activities should confirm the following plume characteristics 1) Fieldscale contaminant mass has been reduced (based on historical groundwater analyses) Figure 1 illustrates a generally accepted methodology for calculating residual dissolved contaminant mass 2) Microbial activity is occunng in the plume(based on microbial counts) 3) The less recalcitrant compounds are reduced in concentration and extent relative to the more recalcitrant compounds The approximate order of increasing recalcitrance for BTEX aromatics is toluene, o-xylene, m- and p-xylene, benzene, ethylbenzene That is, toluene concentrations should be most attenuated, ethylbenzene least attenuated 3) Electron acceptors such as DO, nitrate and sulfate are depleted within the plume 4) Metabolic end-products such as carbon dioxide, hydrogen sulfide and methane have accumulated within the plume relative to outside of the plume References Arulanantham, R and Salhotra, A, 1996 Risk and Decision Making at Petroleum Contaminated Sites University of California Extension,Programs in Environmental Management Barker, G et al, 1995 "Assessment of Natural Hydrocarbon Bioremediation at Two Gas Condensate Production Sites" In R E Hinchee, J T Wilson, and D C Downey(Eds),Intrinsic Bioremediation, pp 181-188 Battelle Press, Columbus,OH Cookson, J T, Jr, 1995 Bioremediation Engineering Design and Application (Ed Nalven, G) McGraw-Hill, Inc, New York,NY CLEARWATER GROUP(NATURAL ATTENUATION) 6 revised October 3,2002