Water quality indices in the upper Vaal river catchment

Abstract

The main objective of this research was to assess the change in the quality of surface water and groundwater in the Upper Vaal River Catchment (UVRC), both spatially and temporally. The UVRC is one of South Africa's most reliable freshwater sources; however, it is stressed by multiple pervasive land use activities. Therefore, water quality variability should be understood in relation to land use types in the catchment. This study aims to determine the change in water quality with time and space and how land uses have also contributed to the deterioration or improvement in water quality. Surface water and groundwater historical data were acquired from the Department of Water and Sanitation. In addition, the land use data were obtained from the Department of Forestry, Fisheries and Environment. Daily water quality data were averaged into monthly data to calculate the weighted arithmetic Water Quality Index (WQI) for the Vaal River, its main tributaries and aquifers in the UVRC. At the same time, the land use data were used to calculate the percentages of land use changes with the years in the catchment. The WQI was calculated for the Vaal, Klip, Mooi, Suikerbosrant, Sterkfontein, Wilge, Rietspruit, Blesbokspruit, Leeuspruit, Wonderfonteinspruit, Waterval and Tweefontein rivers. The WQI for all the rivers showed a relative seasonal control of water quality, with the wet months having lower WQI than the dry months. This was attributed to the dilution by rainwater. The water quality for all the rivers also deteriorated over time; for example, from 2000 to 2004, rivers mainly had ‘poor’ (WQI = 51– 75) to ‘very poor’ (WQI = 76 – 100) water quality. On the contrary, the water quality for the same rivers is mainly classified as ‘very poor’ to ‘unfit for consumption’ (WQI > 100) from 2012 to 2016. The generated pollution flowed downstream along the Vaal River, resulting in higher WQI values downstream. Vaal F is the most downstream point and was classified under ‘unfit for consumption’ almost all year round. There is a massive difference between surface water and groundwater quality in the UVRC. There was seasonal variation in surface water quality, as most dry months and years were characterised by higher WQI. In contrast, the groundwater WQI did not show seasonal variations. The Vaal F, Wilge, Suikerbosrant, Waterval, Mooi and Wonderfonteinspruit boreholes (named after rivers closest to them) showed an average of ‘excellent’ (WQI = 0 – 25) to ‘good’ (WQI = 26 – 50) water quality conditions. However, Borehole Waterval (BH ii Waterval) had a deterioration of water quality as it started with ‘good’ WQI in 2000 and then ranged between ‘poor’ to ‘very poor’ from 2002 until 2016. The land uses in the UVRC were dominated by vegetation cover in 2000, while agriculture dominated the catchment land uses in 2016. The drastic changes in land use between 2000 and 2016 coincided with an increase in the WQI of rivers during the same period. Therefore, it was concluded that the land use types significantly influence the WQI of the UVRC surface water sources