A two-dimensional, multispecies reactive solute transport model with sequential aerobic and anaerobic degradation processes was developed and tested. The model was used to study the field-scale solute transport and degradation processes at the Bemidji, Minnesota, crude oil spill site. The simulations included the biodegradation of volatile and nonvolatile fractions of dissolved organic carbon by aerobic processes, manganese and iron reduction, and methanogenesis. Model parameter estimates were constrained by published Monod kinetic parameters, theoretical yield estimates, and field biomass measurements. Despite the considerable uncertainty in the model parameter estimates, results of simulations reproduced the general features of the observed groundwater plume and the measured bacterial concentrations. In the simulation, 46% of the total dissolved organic carbon (TDOC) introduced into the aquifer was degraded. Aerobic degradation accounted for 40% of the TDOC degraded. Anaerobic processes accounted for the remaining 60% of degradation of TDOC: 5% by Mn reduction, 19% by Fe reduction, and 36% by methanogenesis. Thus anaerobic processes account for more than half of the removal of DOC at this site. Introduction Experience obtained from remediation efforts at contaminated groundwater sites has demonstrated the limitations of pump-and-treat technology, especially at sites contaminated with nonaqueous phase liquids (NAPLs). Increasing effort isbeing devoted to the development and testing of alternative technologies [MacdonaM and Kavanaugh, 1994]. A promising alternative to traditional pump-and-treat methods is intrinsic bioremediation, a method that relies on the naturally occurring biodegradation processes at a site [Lee et al., 1988; Madsen, 1991; Bouwer and Zehnder, 1993; Salanitro, 1993]. Both aerobic and anaerobic biodegradation processes can be effective at removing hydrocarbons from the environment [Grbi&Gali•, 1991; Barker et al., 1987; Wilson et al., 1987; Chiang et al., 1989; Cozzarelli et al., 1990; Wilson et al., 1990; Acton and Barker, 1992; Lyngkilde and Christensen, 1992b; Thieftin et al., 1993; Baedecker et al., 1993; Eganhouse et al., 1993]. Long-term, detailed monitoring of the groundwater plume caused by a crude oil spill near Bemidji, Minnesota, has documented the importance of both aerobic and anaerobic biodegradation at this site [Baedecker et al., 1993]. The availability of electron acceptors determines the sequence of biodegradation processes. Based on the thermodynamics of reactions and redox potential, the theoretical sequence is aerobic degradation, followed by denitrification, manganese and iron reduction, sulfate reduction, and then methanogenesis. This sequence may cause zonation of a contaminant plume with different biodegradation processes dominating in each redox zone [Baedecker and Back, 1979; Chapelle This paper is not subject to U.S. copyright. Published in 1995 by the American Geophysical Union. Paper number 95WR02567. and Lovley, 1992; Lyngkilde and Christensen, 1992a; Vroblesky a...

> Abstract We conducted a plume-scale study of the microbial ecology in the anaerobic portion of an aquifer contaminated by crude-oil compounds. The data provide insight into the patterns of ecological succession, microbial nutrient demands, and the relative importance of free-living versus attached microbial populations. The most probable number (MPN) method was used to characterize the spatial distribution of six physiologic types: aerobes, denitrifiers, iron-reducers, heterotrophic fermenters, sulfate-reducers, and methanogens. Both free-living and attached numbers were determined over a broad cross-section of the aquifer extending horizontally from the source of the plume at a nonaqueous oil body to 66 m downgradient, and vertically from above the water table to the base of the plume below the water table. Point samples from widely spaced locations were combined with three closely spaced vertical profiles to create a map of physiologic zones for a cross-section of the plume. Although some estimates suggest that less than 1% of the subsurface microbial population can be grown in laboratory cultures, the MPN results presented here provide a comprehensive qualitative picture of the microbial ecology at the plume scale. Areas in the plume that are evolving from iron-reducing to methanogenic conditions are clearly delineated and generally occupy 25-50% of the plume thickness. Lower microbial numbers below the water table compared to the unsaturated zone suggest that nutrient limitations may be important in limiting growth in the saturated zone. Finally, the data indicate that an average of 15% of the total population is suspended.http://link.springer-ny.com/link/service/journals/00248/bibs/37n4p263.html

Under some conditions, a first‐order kinetic model is a poor representation of biodegradation in contaminated aquifers. Although it is well known that the assumption of first‐order kinetics is valid only when substrate concentration, S, is much less than the half‐saturation constant, K s, this assumption is often made without verification of this condition. We present a formal error analysis showing that the relative error in the first‐order approximation is S/Ks and in the zero‐order approximation the error is Ks/S. We then examine the problems that arise when the first‐order approximation is used outside the range for which it is valid. A series of numerical simulations comparing results of first‐ and zero‐order rate approximations to Monod kinetics for a real data set illustrates that if concentrations observed in the field are higher than Ks, it may be better to model degradation using a zero‐order rate expression. Compared with Monod kinetics, extrapolation of a first‐order rate to lower concentrations under‐predicts the biotransformation potential, while extrapolation to higher concentrations may grossly over‐predict the transformation rate. A summary of solubilities and Monod parameters for aerobic benzene, toluene, and xylene (BTX) degradation shows that the a priori assumption of first‐order degradation kinetics at sites contaminated with these compounds is not valid. In particular, out of six published values of Ks for toluene, only one is greater than 2 mg/L, indicating that when toluene is present in concentrations greater than about a part per million, the assumption of first‐order kinetics may be invalid. Finally, we apply an existing analytical solution for steady‐state one‐dimensional advective transport with Monod degradation kinetics to a field data set.

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Wastes from a wood preserving plant in Pensacola, Florida have contaminated the near‐surface sand‐and‐gravel aquifer with creosote‐derived compounds and pentachlorophenol. Contamination resulted from the discharge of plant waste waters to and subsequent seepage from unlined surface impoundments that were in direct hydraulic contact with the ground water. Two distinct phases resulted when the creosote and water mixed: a denser than water hydrocarbon phase that moved vertically downward, and an organic‐rich aqueous phase that moved laterally with the ground‐water flow. The aqueous phase is enriched in organic acids, phenolic compounds, single‐ and double‐ring nitrogen, sulfur, and oxygen containing compounds, and single‐ and double‐ring aromatic hydrocarbons. The ground water is devoid of dissolved O2, is 60–70% saturated with CH4 and contains H2S. Field analyses document a greater decrease in concentration of organic fatty acids, benzoic acid, phenol, 2‐, 3‐, 4‐methylphenol, quinoline, isoquinoline, l(2H)‐quinolinone, and 2(lH)‐isoquinolinone during downgradient movement in the aquifer than could be explained by dilution and/or dispersion. Laboratory microcosm studies have shown that within the study region, this effect can be attributed to rnicrobial degradation to CH4 and CO2. A small but active methanogenic population was found on sediment materials taken from highly contaminated parts of the aquifer.