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r k ----- Attenuated Contaminant R=3, a=0 0't day� 2004 feet downgradient of the source, it would not be Conservative 'tracer- R=1, 7►=0 impacted by the attenuated plume However,the plume lOD of the conservative compound would reach the receptor within five years 75 To select the most appropriate remedial response � + and to focus enhanced remediation efforts on the highest s°n 50 priority sites, it is important to understand natural o - attenuation and be able to determine the extent to which r 25 it is occurring at individual sites If natural processes m m Yr 10 Y16 are likely to prevent migration of significant levels of m 0 contaminants to sensitive receptors, then a monitoring c only plan may be sufficient for a site However,if natural -25 attenuation will not prevent significant impacts on 50 human health or the environment, then an enhanced e m vx = 1 fYlday soil and/or ground water remediation system should be N 75 implemented Even when enhanced remediation is C required, the extent of natural attenuation should still T be considered in selecting the most appropriate reme- -140000 0 i00p 2000 3000 40Q0 50QC1 diation technology For example enhanced remediation x, Longitudinal Distance from Source (ft) i or containment of the high concentration source area combined with natural attenuation of the downgradient Figure 1.contaminant plan son of a cons y be an appropriate and cost-effective attenuated at cative rate a traceri percent/day a soluble plume maretardation tactor of 3 0 The results are calculated from an i alternative Thus the first objective of this work is to analytical solution to the three-dimensional transport equation evaluate the different biological physical and chemical with first-order decay of the contannnant. phenomena which can result in natural attenuation Biological Processes Based on an understanding of natural attenuation mech- anisms and contaminant fate and transport in ground At many sites where natural attenuation has been iodegradation of BTEX indigenous, water the second objective is to derive indicators and documented, b methods to determine when natural attenuation is subsurface microbes appears to be the primary mecha- occurring and to what extent The third objective is to nism Wilson (1993) has summarized two patterns of present sevzral site-SPCC'f'c examples where natural natural bioattenuation for aromatic hydrocarbons aero- attenuation has been documented to illustrate apphca- bic and anaerobic Mikesell et a! (1993) have proposed _f tion of the natural attenuation indicators and methods biodegradation under hypoxic conditions(low dissolved of demonstration The overall objective 's to develop a oxygen but not strict/} anaerobic) as a third possibility i ' practical approach for evaluating natural attenuation that Aerobic Biodegradation is batted on knowledge of natural attenuation mechanisms if readily obtainable field data and field tested indicators if sufficient dissolved oxygen (� 1 to 2 mg/L} is I tion primary focus of this paper present in of natural attenuaground water, aerobic biodegradation can The prim ` i5 on the aromatic do hydrocarbons ben/enC toluene ethyl- degrade the BTEX components according to the stoi- ben/Lne and o m and p-xylene(BTEX) which arc fre- chiometr'c ratio of dissolved oxygen to BTEX at higher quently of regulatory importance at sites where reltases rates than normally achieved anaerobically In this pro- of petroleum hydrocarbons have occurred However cess indigenous microbes utilize dissolved oxvgen - many of the analyses described here could be applied to (bD O )as an substrate electron ctron acceptor torom degrade O data are ocar- 1- other attenuated compounds as well _ I able an inverse correlation(Chiang et al.19$9)between D O and aromatic hydrocarbon concentrations has been observed suggesting that aerobic biodegradation Natural Attenuation MechanlSMS is a significant mechanism for natural attenuation of Y Possible natural attenuation mechanisms include BTEX Aerobic biodegradation has been well _ biological processes such as aerobic or anaerobic bio- mented and demonstrated in laboratory studies annddt aat degradation physical phenomena such as dispersion field sites Available data from several sites suggest that volatilization and sorption and chemical reactions such natural attenuation due to aerobic biodegradation as hvdrolysis and dehydrohalogenation Brief descrip- occurs with attenuation rate constants of 0 3 to 13 per- tions of each of these mechanisms and the potential of cent per day for BTEX compounds when modeled as each for natural attenuation of BTEX are given in a first order process (Chiang et al 1989, Kemblowski Table i and the following paragraphs The intent of the et al 1987, Salanitro 1992) _ following summaries is not to provide thorough reviews Anaerobic Biodegradation of each mechanism but rather to identify the processes When D O is depleted in an aquifer,anaerobic con- which have the potential for natural attenuation and prevail for biodegradation to occur, an alterna- discuss the key aspects in relation to natural attenuation ditions p g