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Building capacity to monitor for pesticide pollution in Western Cape rivers
Author: Wiida Fourie-Basson, Faculty of Science (media)
Published: 05/12/2022

Geoenvironmental scientists at Stellenbosch University (SU) are working with the Swiss Federal Institute of Aquatic Research (Eawag) in Switzerland to develop in-house capacity to monitor for aquatic pesticide pollution.

Dr. Reynold Chow, a hydrogeologist in the Department of Earth Sciences at SU, says South Africa is the leading pesticide user in Sub-Saharan Africa, but due to the cost and expertise associated with this kind of monitoring, there is limited data from most developing countries on pesticide pollution.

To address this gap, Eawag has donated a set of chemical standards to the Mass Spectrometry Laboratory in SU's Central Analytical Facilities (CAF). These chemical standards are very expensive, but necessary to develop the chemical analytical methods to detect and quantify these compounds in environmental samples.

Dr. Chow says we should be as concerned about pesticide pollution as the residue on exports: “While there are strong regulations regarding the levels of pesticide residue on exports, there are no regulations in South Africa for restricting levels of pesticide pollution in the environment, particularly in water. This is a problem for the environment, as well as rural communities in South Africa that rely on groundwater for drinking water.

“In other words, we have to live with the consequences of environmental pesticide pollution in our local ecosystems, while countries that import South African produce do not. This means they are literally externalizing the environmental impacts of pesticide pollution to South Africans," he warns.

In a recent study, Dr. Chow worked with scientists from Eawag, the Swiss Tropical and Public Health Institute, the University of Basel and the Centre for Environmental and Occupational Health Research at the University of Cape Town to monitor pesticide pollution of waters in three agriculturally intensive river catchments in the Western Cape: Grabouw, Hex River Valley, and Piketberg. These areas are well-known for its wheat, deciduous fruit, wine, and citrus industries.

Over a sampling period of more than a year and testing for 101 pesticides, they found that surface waters across all three catchments contained at least three pesticides, while the majority (83%) of the samples contained five or more pesticides. These analyses were done at Eawag.

Two of the three catchments, Hex River Valley and Piketberg, raised concern because of high concentrations of certain pesticides over longer time periods. Analysis of samples from the Hex River Valley showed the persistence of terbuthylazine and imidacloprid for 22 weeks, while terbuthylazine and metsulfuron-methyl persisted for up to four weeks in the Piketberg catchment.

Terbuthylazine is a herbicide for the control of the emergence of a broad spectrum of annual weeds in apples, vines and citrus. Metsulfuron-methyl is a herbicide which kills broadleaf weeds and some annual grasses. Imidacloprid is an insecticide that mimics nicotine, which is naturally found in many plants and which is toxic to insects. Since 2013 it has been banned as a pesticide of concern in the European Union, as it has been found in honey worldwide and as an insecticide may be responsible for declining honey-bee populations.

Dr. Chow says current thinking is that the pesticides found in the rivers are unrelated to agricultural spraying (in the case of terbuthylazine and imidacloprid): “There are either other non-agricultural sources, such as forestry or urban areas, that we are unaware of, or these pesticides are leaching into the groundwater where they are gradually transported to rivers all year round."

To better understand the potential environmental and human health risks, they propose continuous and consistent monitoring programs for pesticide pollution in agriculturally intensive catchments, coupled with a recording system of pesticide use by farmers. “Only then will we be able to develop context-specific risk thresholds and sustainable agricultural practices that will benefit both the environment and our economy," he concludes.

The study, titled “Seasonal drivers and risks of aquatic pesticide pollution in drought and post-drought conditions in three Mediterranean watersheds", was published in the journal Science of the Total Environment recently. It is open-access and available online at:

​On the photo above: MSc-student Emma Davies is currently completing an internship at Eawag where she is receiving training in the methodology of pesticide analysis on an Agilent 6495C triple quadrupole Liquid Chromatography/Mass Spectrometry (LC/MS) system. Here she is with her co-supervisor Dr Christian Stamm. Image provided