A two-year study of the quality of irrigation water from Loskop Dam and its effect on crop production in the Groblersdal area, funded by the Loskop Irrigation Board (LIB), has shown very low levels of E. coli and no detectable levels of other disease-causing bacteria.

However, warns CSIR limnologist Dr Paul Oberholster, Loskop Dam’s water quality is deteriorating fast, with permanent blooms of cyanobacteria visible on the surface water throughout the year since 2008. The dam provides water to the Loskop Irrigation Board situated downstream – one of the largest irrigation schemes in South Africa.

The main crops produced in the irrigation area and analysed by the research team include fresh produce, maize, citrus, grapes and wheat.

According to a research report from the CSIR and the University of Stellenbosch, to date none of these nuisance and potentially toxic cyanobacteria had been detected in the irrigation canal water or on irrigated crops.

A more immediate problem and cause for concern is the proliferation of nuisance filamentous algae with strings of up to 15 metres long, caused by the high levels of nutrient enrichment detected in the canals.

According to Oberholster the filamentous macroalgae decrease the carrying capacity of the canals.

“Detached algae continuously drift down the canals, clogging the control gates and crop sprayers causing economical losses. The irrigation board then has to bear the costs of removing these nuisance algae,”he said.

Diek Engelbrecht, chair of LIB, said while they are relieved about the results from the study in the short term, they are still concerned as the dam’s water quality problems and the causes thereof have not been resolved.

Loskop Dam is fed by the heavily contaminated Olifants River. Over the past 15 years, the dam has had a history of isolated incidents of fish mortality. These incidents have escalated in recent years and are believed to be linked to crocodile mortalities and a decline in the crocodile population.

Since the dam’s construction in 1938, it has played an important role in supplying irrigation water to the agriculture sector. Currently the dam supports an export market to the European Union (EU) valued at R1 billion a year. The Loskop Irrigation Board – one of several downstream of the dam – provides irrigation water to 16 000 hectares of agricultural land by means of irrigation channels with a total length of more than 550 kilometres.