Increasing nutrient influx trends and remediation options at Hartbeespoort Dam, South Africa: a mass-balance approach

The Hartbeespoort Dam, located 40 km west of Tshwane on the Crocodile River, is an extremely eutrophic water body. Situated in one most economically active areas South Africa, it receives a high nutrient input from wastewater treatment works (WWTW), leaking sewers, as well urban and agricultural runoff. Metsi Me programme, which ran 2006 to 2016, aimed mitigate in-lake stocks using biomanipulation, including physical removal Eichhornia crassipes (water hyacinth) Microcystis aeruginosa (blue-green algae). Using Department Water Sanitation quality flow data, annual influxes outfluxes total nitrogen (TN) phosphorus (TP) Dam were calculated. Through literature review comparison with previous studies, relative importance biomass harvesting relation retained nutrients was assessed. average influx rivers during hydrological years 2010/11 2016/17 582 t∙a−1 TP 4 687 TN, trends for both TN being significantly positive over this period. increased by 77.8 every year 456 t∙a−1, reversing long-term negative trend. Average dam retention + (calculated difference between river inputs outputs, i.e., sedimentation, denitrification losses) 358 t P 2 195 N. A best estimation hyacinth algal 2.1 11.5 N, 3.9 respectively. An estimated 341 674–1 288 N sedimented. Denitrification losses are poorly quantified but possibly comparable sedimentation. River 28.4 110 smaller rates than influxes, suggesting increasing per annum. Upgrading WWTWs catchment refurbishing overflowing sewers appropriate solution.