Nutrient Dynamics at the Sediment-Water Interface: Influence of Wastewater Effluents

Abstract

Uptake and regeneration fluxes and concentrations of nutrients, i.e., nitrate (NO3−), ammonium (NH4+), phosphate (PO43−) and dissolved organic carbon (DOC), were evaluated upstream and downstream of a wastewater treatment plant (WWTP) in the River Wandle, UK, from July to October 2019. Using chamber techniques, water-specific nutrient concentrations were measured at two exposures (3 and 10 min) to calculate fluxes. The WWTP effluent contributed to elevated concentrations and modified flux rates, resulting in significant differences at the study sites. Compared with summer, the concentrations of NO3− and DOC increased while NH4+ and PO43− decreased in autumn. Nutrient fluxes varied both temporally and spatially in uptake (i.e., storage in sediments) or regeneration (i.e., release into river water). Under the actions of physical and biological processes, the fluxes of NO3− and NH4+ showed opposite flux directions. Dissolved oxygen (DO) and bioabsorption mainly affected PO43− and DOC fluxes, respectively. Specifically, across all sites, NO3− was −0.01 to +0.02 mg/(m2 s), NH4+ was −29 to +2 μg/(m2 s), PO43− was −2.0 to +0.5 μg/(m2 s), and DOC was −0.01 to +0.05 mg/(m2 s). Further, we did find that these variations were related to nutrient concentrations in the overlying water. Our results provide further evidence to show that reductions in river nutrients are paramount for improving river ecological conditions. Additionally, we suggest that more research is needed to evaluate chamber-based experimental approaches to make them more comparable to in-situ flux methods.Highlights• Sewage effluent resulted in elevated nutrient concentrations and modified fluxes.• Flux was affected by initial nutrient concentrations, DO and microbial activity.• Inexpensive approaches to study nutrient dynamics are needed for river restoration.