A re-analysis of nutrient mass balances in the Hartbeespoort Dam

Abstract

The Hartbeespoort Dam, located 40 km west of Tshwane on the Crocodile River, has recently been described as the most eutrophic dam in Africa. The dam is situated in one of the most economically active areas of South Africa and receives a high nutrient input from 11 wastewater treatment plants, leaking sewers and urban and agricultural runoff. The Metsi a Me programme, which ran from 2006 to 2016, aimed to mitigate in-lake nutrient stocks using biomanipulation, including the physical removal of water hyacinth and algae. This study reassesses the debate regarding nutrient stocks and fluxes in the Hartbeespoort Dam. Using Department of Water and Sanitation water quality and flow data, the annual input and output fluxes of total nitrogen (TN) and total phosphorus (TP) to the Hartbeespoort Dam from 2010 to 2018 were calculated, which formed the principal focus of this study. Through literature review and previous studies the relative importance of nutrient removal from biomass harvesting in relation to retained nutrients was assessed. Between 2010 and 2017, the range of P to the Hartbeespoort Dam from rivers was 578 to 610 t/a and the range of N was 4437 to 4687 t/a. On average, the P inflow flux increased by 54.2 to 77.8 t/a and the N inflow flux increased by 368 to 465 t each year, a reversal of a long-term decreasing trend. A generous estimation of the total annual nutrient removal from hyacinth and algae harvesting combined is 1% of the nutrient influx. Much of the nutrient flux to the dam is sedimented. A summary of the flux calculations of this study is shown on the diagram on the next page. Thus, the upgrading of WWTWs in the catchment and the refurbishing of leaking and overflowing sewers is the most appropriate long-term solution to the eutrophication problem at the Hartbesspoort Dam.