Simulation of reactive processes related to biodegradation in aquifers. 2. Model application to a column study on organic carbon degradation

Abstract The reactive transport model TBC (Transport, Biochemistry, and Chemistry) was applied to a laboratory column study on microbial organic carbon degradation by von Gunten and Zobrist [von Gunten, U., Zobrist, J., 1993. Biogeochemical changes in groundwater-infiltration systems: column studies, Geochim. Cosmochim. Acta, 57 (1993) 3895–3906]. Five microbial groups were considered in the model: facultatively aerobic bacteria or denitrifiers, fermenters, and sulfate-, manganese-, and iron-reducers. By simulating their growth, subsequent consumption of organic carbon and electron acceptors, and production of metabolic products, it was possible to reproduce the observed transient concentration changes of the reactive species. Inhibition and availability of electron acceptors and/or organic carbon governed the transient growth behaviour of the different microbial groups. With the model the much slower growth of sulfate reducers compared to that of facultatively aerobic or denitrifying bacteria could be reproduced. To be able to reproduce the observed spatial sequence of different microbial groups a maximum microbial capacity had to be introduced. It restricts total microbial density in the porous material. Without this restriction the excessive organic substrate supply would have caused the microbial activity to be exclusively located at the immediate vicinity of the column inlet. The distribution of facultatively aerobic or denitrifying organisms and sulfate reducers in the column was determined by the exchange coefficient for the electron acceptor between pore water and biophase. The exchange of sulfate seemed to be much slower than that of O 2 and NO 3 − . The growth of manganese and iron reducers was limited by the availability of the solid electron acceptors Fe(III) and Mn(IV). The testing of two hypotheses concerning the interaction of iron and sulfide in the column suggests that the formation of elemental sulfur during abiotic reductive iron dissolution is an important sink for sulfide. The TBC model has proven to be complex enough to grasp the important reactive processes. The model provides a temporally and spatially resolved quantification of microbial degradation activity. It has shown to be a useful tool in testing alternative hypotheses on processes which could not directly be identified from the observed concentration data.