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Many hydrocarbons and related organic contaminants are rapidly degradable in the presence of oxygen. Unfortunately, exchange of oxygen with subsurface contaminant plumes is often slow. In this paper, equations are developed for simulating the simultaneous growth, decay, and transport of microorganisms, as well as the transport and removal of hydrocarbon and oxygen. These equations are solved by conventional numerical techniques to study the impact of microbial kinetics, horizontal mixing, adsorption, and vertical exchange with the unsaturated zone on biodegradation. In the region near the hydrocarbon source, any available oxygen will be rapidly consumed. In the body of the hydrocarbon plume, oxygen transport will be rate limiting and the consumption of oxygen and hydrocarbon can be approximated as an instantaneous reaction. The major sources of oxygen to the plume appear to be transverse mixing, advective fluxes when adsorptioh is significant and vertical exchange with the unsturated zone. In a companion paper (R. C. Borden et al., this issue), hydrocarbon transport is simulated at a hazardous waste site where oxygen-limited biodegradation is known to occur. INTRODUCTION Current research at Rice University through the NationalCenter for Ground Water Research and the R. S. Kerr Laboratories of the U.S. Environmental Protection Agency has been aimed at developing mathematical models for simulating the degradation processes that occur in shallow aquifers and defining the natural assimilative capacity of those aquifers. Modeling work to date has focused on simulating the degradation of petroleum hydrocarbons. In many aquifers, oxygen, which is the primary electron acceptor in hydrocarbon degradation, is absent or present in low concentrations. Contact between oxygen and the hydrocarbon will then be the rate limiting step in biodegradation.The United Creosoting Company, Inc. (UCC) site in Conroe, Texas, has been studied to gain an understanding of the major processes affecting the transport and biodegradation of dissolved hydrocarbons and to aid in the development and testing of physically and microbially realistic simulation models. The site is well-characterized hydrologically and microbiologically. Bedient et al. [1984] described the site geology, hydrology, and the major contaminants present. Lee et al. [1984] and Wilson et al. [1985] have characterized the subsurface microbiology using aseptic sampling techniques and provided estimates of the rates of microbial degradation. Recent field work has suggested that lack of the required electron acceptor, oxygen, is limiting the microbial degradation of aromatic hydrocarbons present in the shallow aquifer. Inorganic nutrients such as nitrogen, phosphorus, sulfur, and iron are all present in significant concentrations in the plume and have been shown to be nonlimiting. The organic hydrocarbons are also present at substantial concentrations I-Bedient et al., 1984; Lee and Ward, 1984].In this paper, after a brief review of current microbiology as it relates to the subsurf...

Abstract. Three-dimensional field monitoring of a gasoline plume showed rapid decay of toluene and ethylbenzene during downgradient transport with slower decay of xylenes, benzene, and MTBE under mixed aerobic-denitrifying conditions. Decay was most rapid near the source but slower farther downgradient. Effective first-order decay coefficients

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A detailed field investigation has been completed at a gasoline‐contaminated aquifer near Rocky Point, NC, to examine possible indicators of intrinsic bioremediation and identify factors that may significantly influence the rate and extent of bioremediation. The dissolved plume of benzene, toluene, ethylbenzene, and xylene (BTEX) in ground water is naturally degrading. Toluene and o‐xylene are most rapidly degraded followed by m‐, p‐xylene, and benzene. Ethylbenzene appears to degrade very slowly under anaerobic conditions present in the center of the plume. The rate and extent of biodegradation appears to be strongly influenced by the type and quantity of electron acceptors present in the aquifer. At the upgradient edge of the plume, nitrate, ferric iron, and oxygen are used as terminal electron acceptors during hydrocarbon biodegradation. The equivalent of 40 to 50 mg/I of hydrocarbon is degraded based on the increase in dissolved CO2 relative to background ground water. Immediately downgradient of the source area, sulfate and iron are the dominant electron acceptors. Toluene and o‐xylene are rapidly removed in this region. Once the available oxygen, nitrate, and sulfate are consumed, biodegradation is limited and appears to be controlled by mixing and aerobic biodegradation at the plume fringes.