Estimating the influence of increased nitrate loads from wastewater on phosphorus mobility in river sediments

Research on the sediment-surface water transition zone in three study site regions, different in hydrological conditions, was conducted to estimate to which extent nitrate in surface water can contribute to riverbed sediment oxidation and thus prevent release of sediment phosphorus to surface water. Consequently, the Dutch Flow Model (DUFLOW) based water quality model "Spreewald" and results from the emission model "Modelling Nutrient Emissions in River Systems" (MONERIS) were used to estimate to which extent wastewater treatment plants (WWTPs) could contribute to the NO 3 - concentration in surface waters if they were operated without denitrification. It is demonstrated that an effective phosphorus retention in sediments by means of surface water NO 3 - is possible only under specific conditions, i.e. (i) a sufficient supply of surface water to the sediment by advective fluxes, (ii) a sufficient amount of sediment iron to provide phosphorus binding sites in the case of sediment oxidation, (iii) a redox system not leading to a rapid NO 3 - and FeOOH depletion and to phosphorus release from organic pools by high microbial activities. Model results show that in-stream denitrification processes counteract a significant increase of NO 3 - surface water concentrations from WWTPs operated without denitrification during summer, when eutrophication risk through phosphorus is highest. The increase of NO 3 - surface water concentrations in winter due to decreased denitrification in the surface water is of minor relevance for phosphorus release from sediments.