Assessment of the extent of the impact of agricultural activities on different water uses in the Mooi River

Abstract

The quality of water in the Mooi River is deteriorating due to anthropogenic activities practiced in the catchment area. The most practiced activities in the catchment include amongst others mining which is dominating the upper catchment (Wonderfontein Spruit), urban and industrial activities, agricultural activities such as crop and cattle farming and chicken breeding, as well as abattoirs. The Mooi River is used for domestic water supply, crop irrigation, livestock watering and recreational use. Anthropogenic activities are having a serious impact on the quality of water and makes it less fit for the intended water uses. Several studies were conducted in the catchment to identify and quantify pollution emanating from mining activities and urban pollution but a little has been done to determine the extent of agricultural pollution in the area. The main aim of this research is the assessment of the extent of the impact of agricultural activities on different water uses in the Mooi River. The objectives of the study were achieved by sourcing data for two time periods (2014 – 2015) and (2018 – 2019) from the Department of Water and Sanitation’s water quality management system, the data was therefore plotted on line graphs to determine the trends and changes that occurred between the study period. The data was analysed and compared with water quality guidelines for domestic, agricultural and aquatic ecosystem use. The data was also compared with the resource quality objectives for the Mooi River catchment. The degree of change in the water quality and the relationship between the agricultural related measured parameters were determined using analysis of variance (ANOVA) and principal component analysis biplots (PCA) methods. With the aid of the land-use map seven water quality monitoring points located within the Mooi River and its tributaries were selected from the existing DWS monitoring programme and used for the present study. Analysis of the results demonstrated that the upper parts of the Mooi River just below the Klerkskraal Dam is still complying with the water quality guidelines, however, trend analysis demonstrated that the lower catchment is mostly affected by agricultural activities, this is based on the increasing trends in phosphate (PO43-), magnesium (Mg2+), chloride (Cl-), potassium (K+) and nitrate-nitrite (NO3-NO2). The other sites along the Mooi River demonstrated an increase in Mg2+ and Cl-, electrical conductivity (EC), PO43- and sulphates (SO42-). An increase in Cl-, Mg2+ and SO42- were also noted for the Mooi River tributaries (Loop Spruit and Wasgoed Spruit). For the sites located on the Mooi River after confluence with the Wonderfontein Spruit trend analysis have also shown some increase in EC and SO42-, which is an indication of the acid mine water release from the west basin mines. Based on this study the agricultural water use has an impact in the water quality of the Mooi River, however, it is recommended that further studies be conducted to determine the impacts of pesticides and metals, as well as other parameters such as sodium, calcium and pathogens, which did not form part of this study.