Self-purification in a lotic ecosystem: a model of dissolved organic carbon and benthic microorganisms dynamics

Abstract

Several studies have been performed since 1982 on a small organically polluted stream, the Albenche river (Savoie, France). In this work, a mathematical model was developed to simulate self-purification mechanisms that occur in the ecosystem. The global model was based on dissolved and particulate organic carbon transfers. The physical part of the model included a hydrodynamical component derived from Saint-Venant's equations, coupled to a transport model based on a convection-dispersion equation under non-uniform, unsteady flow conditions. As the main detritical processes that occur in small streams take place in the sediment, the model was applied to simulate the dynamics of both dissolved organic matter and benthic microorganisms. The simulation of the biodegradation mechanisms was based on the similarity between detritic processes in streams and biological activities of microbial biomass in waste water treatment plants. Monte Carlo simulations have been performed to take into account uncertainties and variabilities of the Albenche system. The results emphasized the role of transport mechanisms (convection and longitudinal dispersion) and the role of benthic biofilm which accounts for 35–40% of the organic load removal.