MODELING REDOX DISEQUILIBRIUM AND BIOGEOCHEMICAL BEHAVIOR OF METALS IN LAKE SEDIMENTS

The mobility of metals in riverine, estuarine, and lacustrine sediments is affected by complex coupled biotic and abiotic geochemical processes that reflect redox disequilibrium and cannot be modeled using conventional equilibrium-based models. Competing mechanisms in these environments include the mobilization of sorbed metals by reductive dissolution of Fe(III) (hydr)oxides and the precipitation of metal sulfides upon reaction with biogenic sulfide. In addition, the toxicity of metals in polluted environments can have a significant effect on microbial activity and thus indirectly affect metal biogeochemical behavior. We are investigating these processes with a numerical model and data from Lake Coeur d'Alene from an area heavily impacted by upstream mining activities. Model results indicate that the relative rates of Fe(III) versus sulfate reduction may be an important factor controlling pH and types of Fe(II) minerals precipitation at depth. Upon reductive dissolution of Fe(III) hydroxides, numerical experiments indicate that a delicate balance takes places between FeS and FeCO3 precipitation, which compete for aqueous Fe(II), and the formation of aqueous (bi)sulfide complexes, which compete with the precipitation of FeS and other metal sulfides for biogenic sulfide. The model incorporates a multicomponent biotic reaction network representing multiple terminal-electron accepting processes by a consortium of anaerobic microbial species, via nonlinear kinetics to capture electron-acceptor limitations on degradation rates as well as inhibition by presence of alternative electron-acceptors. In addition, we have constructed and applied "dose-structured" kinetics to quantify the dynamics of microbial populations when their degradation rate is affected by the presence of toxic metals. We use an exposure-time approach to express the limitation and are applying this approach to laboratory data. The metal toxicity imparts a lag in microbial metabolism of the electron donor and a reduction in the specific growth rate as evidenced by the reduced slope of the biomass curve during exponential growth.