Modeling reactive transport of organic compounds in groundwater using a partial redox disequilibrium approach

Abstract

The chemical transformation of organic contaminants in natural groundwater systems is clearly dependent upon local geochemistry which determines the thermodynamically favorable degradation reactions and the nature of local microbial populations. Conversely, groundwater geochemistry may be impacted significantly in terms of pH and redox couple speciation by the chemical transformation of sufficient quantities of organic compounds. Therefore an understanding of the coupling between degradation reactions, local geochemistry, and chemical transport is essential in predicting the chemical evolution of contaminated aquifers. Equilibrium‐based reactive chemical transport models are usually not utilized for problems involving the transport of degradable organic compounds due to slow reaction kinetics and the persistence of intermediate degradation products. In this study we propose a reactive geochemical transport model which considers these types of degradation reactions. An expert system approach is used to postulate a set of sequential, first‐order degradation reactions for the organic compounds based upon thermodynamic considerations and user‐defined rules. Redox disequilibrium provides the driving force for the abiotic or microbially mediated transformation of the organic compounds as well as the associated response of groundwater geochemistry. Coupling between local inorganic geochemistry and reacting organic compounds is achieved by assuring conservation of operational valence and mass balance. The composite geochemical model is in turn coupled with an integral finite difference transport algorithm using a two‐step sequential solution approach. The transport equation is solved separately for each inorganic aqueous species, complex, and dissolved organic species, allowing a high degree of flexibility in problem definition. We apply the model to an illustrative example problem concerning the introduction of aromatic hydrocarbons and chlorinated ethenes into an initially oxidizing aquifer. Modeling results agree well qualitatively with field and laboratory observations reported in the literature in terms of degradation patterns and the effects on groundwater geochemistry.

Kinetic control of oxidation state at thermodynamically buffered potentials in subsurface waters

“…Recognition that these general kinds of reactions can take place has led to modeling efforts based on local partial equilibrium (LPE) in which species subsets, often highconcentration solutes, are modeled as in equilibrium with each other (e.g., Morel and Hering, 1993;Lichtner, 1996). However, success of LPE models has been limited because understanding of specific interactions among couples remains elusive (e.g., McNab and Narasimhan, 1994).…”

Calibration of subsurface batch and reactive-transport models involving complex biogeochemical processes

“…Common transformation pathways for nitrate and sulfate include degradation via microbially mediated redox processes that utilize an organic carbon substrate [47,54,63,72]. In subsurface environments, numerous processes influence microbial degradation by controlling substrate or electron-acceptor concentrations [4,31,51,79,97].…”

Trace Metal Bioremediation: Assessment of Model Components from Laboratory and Field Studies to Identify Critical Variables

“…The kinetics of such microbial-mediated redox reactions has been expressed using various formulations that usually are a function of the concentrations of the electron donors and/or acceptors. Monod-type formulations have been widely used to describe the consumption/production rates of electron donors and acceptors in microbiological reactions [Widdowson et al ., 1988;Semprini and McCarty , 1992;McNab and Narasimhan , 1994;Salvage and Yeh , 1998;Smith and Jaffé , 1998]. Due to substantial amounts of external carbon sources supplied into the contaminated aquifer during biostimulation, and because high level of nitrate are common at many sites contaminated with uranium, the rate of biodegradation of the dissolved organic substrate can be formulated using a dual-Monod kinetics as: …”

“…Different chemical species have usually been simulated as either single or multiple limiting components [Borden and Bedient, 1986;Molz et al, 1986;Widdowson et al, 1988]. In recent years, models have been developed that included biologically mediated redox dynamics in terms of the sequential utilization of different electron acceptors during the degradation of an organic substrate [Rabouille and Gaillard, 1991;Sweerts et al, 1991;Matsunaga et al, 1993;McNab and Narasimhan, 1994;Van Cappellen and Wang, 1995;Dhakar and Burdige, 1996;Park and Jaffé, 1996]. Some of these models have been extended to include abiotic redox reactions and geochemical processes such as speciation and precipitation/dissolution [Wang and Van Cappellen, 1996;Hunter et al, 1998;Smith and Jaffé, 1998].…”