Microbial reactions, chemical speciation, and multicomponent diffusion in porewaters of a eutrophic lake

A mathematical model for the study of early diagenesis in lake sediments is presented. The computer program STEADYQL has been extended into a multibox model that allows the calculation of porewater profiles at steady state. The model includes microbial and geochemical reactions, inorganic chemical speciation of the dissolved components, and diffusion of the individual species between the boxes. The application of the approach to a set of porewater data from the eutrophic Lake Sempach (Switzerland) yields the following results. The assumption of solubility equilibrium with respect to calcite produces a pH profile that is in close agreement with the measurements. The model predicts calcite precipitation in the zone of Fe reduction. The contributions of methanogenesis, sulfate reduction, denitrification, and Fe reduction to anaerobic mineralization are calculated as 47, 27, 17, and 9%, respectively. Based on the assumptions of Redfield ratios, the model predicts the profiles of ammonium and phosphate remarkably well. The analysis of the diffusion pathways shows that 35% of the Fe(II) flux across the sediment-water interface occurs in the form of inorganic complexes. However, effects of complex formation on diffusive fluxes are smaller than the experimental error of the measured chemical gradients.