Using the Nemerow's Pollution Index to investigate correlations between land use and water quality across a sample of South African catchments

ABSTRACT

Water Quality Indices (WQIs) enable researchers to combine the measurements of multiple parameters into a single unitless score of overall water quality. The Nemerow's Pollution Index (NPI) is a flexible and intuitive index that uses an ascending scale to indicate overall levels of impairment/contamination. This study demonstrates the utility of the NPI, when used in combination with site-specific water quality guidelines, to investigate correlations between land use and water quality across a sample of 58 sub-catchments located along the western, southern, and south-eastern coast of South Africa. NPI scores for the period 2013–2014 were calculated for each sub-catchment using data from South Africa's National Chemical Monitoring Programme. Land use metrics for urban, agricultural, and natural land cover were estimated from a contemporaneous national land cover map. Correlation analysis revealed statistically significant positive correlations across the sample between pollution levels and proportions of urban and agricultural land (Spearman's ρ = 0.333 and 0.621, respectively), and strong negative correlations between pollution levels and proportions of natural land cover (Spearman's ρ = –0.729). The study concludes that, when used in concert with site-specific water quality standards, the NPI is a useful tool for evaluating overall levels of impairment across a sample of water bodies.