A spatial analysis of eutrophication in dam reservoir water on the Molopo River at Mafikeng, South Africa

Abstract

In water-scarce environments, surface water bodies serve both as water sources and waste disposal channels. The Molopo River that drains through Mafikeng, South Africa, has been dammed for agricultural and municipal water supply, resulting in four reservoirs near Mafikeng. It receives municipal waste water discharges from point pollution source sewage processing plants. In this study the spatial variation in the manifestation of eutrophication in the reservoir water was examined. Surface water samples were collected from the four reservoirs in July (dry season), and analysed for nutrient (NO₃⁻, PO₄³⁻) concentrations. The dry season low river flow period was perceived as the period when the eutrophication problem is more acute. Near infrared (NIR) reflectance from the reservoirs, due to algae and macrophytes like the water lettuce (Pistia stratiotes), was used as manifestation of eutrophication on satellite imagery. The presence of these plant NIR reflectors in the water bodies was assessed using near-concurrent multispectral SPOT 5 images. The image digital number (DN) values were converted to reflectance values. Geostatistical analysis indicated autocorrelation of algal abundance as indicated by the near infrared (NIR, 0.78–0.89 μm) reflectance. Ordinary kriging interpolation indicated change in algal abundance away from the point nutrient sources. Reservoirs with direct effluent point sources had higher nutrient concentrations and NIR reflectance than those without direct point sources. The results indicate the reservoirs to range between eutrophic and hypertrophic, and show the suitability of SPOT 5 imagery for use in the assessment of eutrophication in support of environmental quality assessments for inland water bodies.