Abstract
Multivariate statistical analysis was used to investigate changes in water chemistry at 5 river sites in the Vaal Dam catchment, draining the Highveld grasslands. These grasslands receive more than 8 kg sulphur (S) ha-1·year-1 and 6 kg nitrogen (N) ha-1·year-1 via atmospheric deposition. It was hypothesised that between 1991 and 2008 concentrations of dissolved mineral salts, sulphate, nitrate and ammonium would increase as a result of the S and N deposition received. Significant spatial differences were found, by analysis of covariance, between sites within the catchment. Canonical correspondence analysis (CCA) showed that the environmental variables used in the analysis, discharge and month of sampling, explained a small proportion of the total variance in the data set – less than 10% at each site. However, the total data set variance, explained by the 4 hypothetical axes generated by the CCA was >93% for all 5 sites. Sulphate, nitrate-plus-nitrite, ammonium and phosphate concentrations increased at 1 site each, between 1991 and 2008. Over the same time frame, acid-neutralising capacity was decreased significantly at 1 of the 5 river sites. The concentrations of the ions analysed, with rare exception, were within the limits set by the national drinking water guidelines, between 1991 and 2008. Nitrogen and sulphur concentrations at the five selected river sites within the Vaal Dam catchment did not show a statistically significant increase between 1995 and 2008.
Highlights
The Vaal Dam and Vaal Barrage supply Gauteng, and beyond, with water for domestic and industrial use
This study quantified the changes of water quality variables at 5 sites in the Vaal Dam catchment between 1991 (when Fey and Guy (1993) sampled) and 2008, using multivariate statistics to assess the change in water chemistry under elevated S and N deposition rates
The spatial differences between the sites, highlighted by the output from the analysis of covariance (ANCOVA) analysis, confirm the variability between rivers within the catchment (Day et al, 1998), and, together with the correspondence analysis (CCA), suggest that additional environmental variables, including land use, point-source pollution and diffuse inputs from agricultural sources, are stronger influences on the quality of water in the Vaal Dam catchment when compared with S and N deposition
Summary
The Vaal Dam and Vaal Barrage supply Gauteng, and beyond, with water for domestic and industrial use. This study quantified the changes of water quality variables at 5 sites in the Vaal Dam catchment between 1991 (when Fey and Guy (1993) sampled) and 2008, using multivariate statistics to assess the change in water chemistry under elevated S and N deposition rates.
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