Impacts of irrigation return flows on the quality of the receiving waters: A case of sugarcane irrigated fields at the Royal Swaziland Sugar Corporation (RSSC) in the Mbuluzi River Basin (Swaziland)

Abstract

In Swaziland sugarcane is grown on more land than any other crop. It is irrigated and on average it consumes about 10,000 m 3/ha. Nevertheless, limited studies have been conducted on the water quality status of return flows from the sugarcane fields and the resultant impact on the Mbuluzi River. The Mbuluzi River is of particular interest because the River has two main functions; first is a source of drinking and irrigation water to both Swaziland and Mozambique, and secondly as a sink that receives contaminated flows from irrigated fields. Samples were collected each month between the period March 2003 and March 2005 from drainage streams that also receive water returning from irrigated fields at the Royal Swaziland Sugar Corporation (RSSC). The samples were analysed to determine the extent of pollution resulting from irrigation return flows. The RSSC is comprised of two main estates known as Mhlume and Simunye which are referred in the study as Groups 1 and 2, respectively. Group 1 comprised of three sampling sites (sites 1–3), while Group 2 comprised of seven sampling sites (sites 4–10). Ten parameters namely TDS, SS, Na, Ca, Mg, COD, K, EC, SAR, and pH were measured. The results from both groups indicated elevated TDS, Na and Mg levels in the drainage streams when compared to the control sites upstream of the river. Results from Group 1 samples showed average TDS concentrations of 300 mg/l (standard deviation 169), average Na concentrations of 60 mg/l (standard deviation 25.1), average Mg concentrations of 48 mg/l (standard deviation 16.7), COD concentrations of 60 mg/l (standard deviation 77.8). Results from the Group 2 sites showed average TDS concentration of 700 mg/l (standard deviation 274), average Na concentration of 140 mg/l (standard deviation 69.2), average Mg concentration of 53 mg/l (standard deviation 31.7) and COD concentration of 27.2 mg/l (standard deviation 11.6) The results are indicative of the fact that TDS, Na and Mg concentrations exceeded the Swaziland guidelines for drinking water and the South African standard for irrigation water for infield drainage streams. The summer and spring seasons recorded significantly higher TDS, Na and Mg concentrations in both groups. The field drainage streams were salt affected and may deem water unpalatable for domestic and irrigation use downstream in the near future.