Abstract
Little is understood on the interaction between irrigated agriculture and surface water quality in South African catchments. A case study was conducted on the Middle Olifants Catchment, which contains the second largest irrigation scheme in South Africa. Dams, rivers, irrigation canals, and drainage canals were sampled between the Loskop and Flag Boshielo Dams. Results were compared to historical water quality monitoring data from the Department of Water and Sanitation (DWS). While DWS data indicate that phosphate-phosphorus (PO4-P) does not pose a eutrophication risk, our monitored data were above the eutrophication threshold for the majority of the sampling period. In general, phosphorus (P) pollution is a bigger issue than nitrogen (N), and concentrations of these nutrients tend to be higher during the summer rainfall months, potentially indicating a link to agriculture and fertilization events. We estimated that waste water treatment works (WWTW), which are currently systematically failing in South Africa, have the potential to pollute as much P as irrigated agriculture. Electrical conductivity levels increased downstream, moving from the acceptable towards the tolerable category, while the sodium adsorption ratio (SAR) presents a moderate risk of infiltrability problems. The pH values were generally in the ideal range. This study has highlighted existing and looming water quality issues for irrigation and the environment in the Middle Olifants. Similar scoping studies are recommended for other intensively-irrigated catchments in the region to identify issues and allow timely intervention.
Highlights
South Africa is located in a predominantly semi-arid part of the world with an average rainfall of approximately 450 mm yr−1, well below the world average of 860 mm yr−1
Eutrophication is the process of nutrient enrichment and the associated excessive algal growth in water, and is recognized as a major threat to water security in South Africa [7,8] and globally [9,10,11,12] in areas with intensive agriculture
The Middle Olifants covers an area of 22,550 km2 and receives an average of 500 mm of rainfall yr−1 [28]
Summary
South Africa is located in a predominantly semi-arid part of the world with an average rainfall of approximately 450 mm yr−1, well below the world average of 860 mm yr−1. Eutrophication is the process of nutrient enrichment and the associated excessive algal growth in water, and is recognized as a major threat to water security in South Africa [7,8] and globally [9,10,11,12] in areas with intensive agriculture. One of the major drivers of eutrophication is phosphorus (P) originating from both point and nonpoint sources [10,13] as it is most often the limiting nutrient in freshwater systems [14]. Addressing this critical issue requires urgent and sustained management action based on sound information. A study by Matthews and Bernard [15] used satellite remote sensing to assess the trophic status of 50 of South Africa s largest water bodies, and results showed that the majority are impacted by eutrophication, with 62% even being hypertrophic and all of these having cyanobacterial blooms
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