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The murky waters of the lower Nuwejaars River, Western Cape: A dairy farm case study
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Water pollution is one of the main environmental issues faced across the globe.
The predominant drivers of this pollution are anthropogenic activities such as cultivation, dairy farming and sewage works. The impact of these activities reduces the quality and quantity of our water resources, possibly making them unsafe for use.
My honours research project investigated the influence of a dairy farm on the water quality of the Nuwejaars River. The dairy farm is close to the town of Elim in Cape Agulhas (Western Cape) and is located on the bank of the lower part of the Nuwejaars River.
The river flows into one of South Africa’s largest lakes, Soetendalsvlei, which is home to many diverse ecosystems. Soetendalsvlei then flows into the Heuningnes Estuary, a protected RAMSAR wetland site.
Animal manure and waste products from the dairy farm are stored in a small retention pond on the banks of the river, which overflows into the Nuwejaars when it is full or when it rains. Although these waste products have impacted the downstream visual water quality, it has not been properly measured to date.
Animal manure mainly consists of nutrients rich in nitrogen (N), phosphates (PO4-) and potassium (K). These nutrients are required for plant growth and are naturally found in water. However, their presence in amounts exceeding their natural occurrence may result in eutrophication.
Eutrophication is an overproduction of algal growth in the water that reduces the volume of dissolved oxygen available for aquatic plants and animals. Less oxygen can potentially reduce the diversity of the species, if they are not tolerant to this environment.
Eutrophication is aesthetically unpleasing (see Plate 1) and can pose a health risk to adults and children with weakened immune systems, if consumed. Consequently, the South African National Standard (SANS 241) has limits on the nitrate concentration in drinking water.
Water sampling
During March and July 2018, two of my classmates and I embarked on a sampling trip to the Nuwejaars River in Cape Agulhas. We took basic water quality measurements on site, such as the water temperature, pH, salinity and dissolved oxygen.
These variables were measured to help us understand the general water quality status of the river. In July we also collected some water samples to analyse at the university, measuring nutrients that would affect water quality.
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The stagnant pools of water we monitored in March could have been there for months without any input from another source of water or the dairy, due to extremely low water levels in the system. During the wet season (July in the Western Cape), we would expect higher water levels to dilute the pollutants.
However, due to a drought in the region, this was not the case. The area received below-average rainfall in July, but it was just enough to start washing the pollutants into the river. The measurements taken in July are therefore the most significant for my study, as they include the inputs from the farm.
Plates 2A and 2B below were taken in July at the point where the dairy farm discharges into the river. The colour of the water at that point shows a lot of photosynthetic activity taking place, with algae on the water surface, as seen in Plate 1. The photosynthetic activity was caused by the nutrients from the farm being exposed to the sun in the still water.
In comparison, the water upstream of the farm was less turbid and green. Surprisingly, at the inflow of Soetendalsvlei (about 5 km downstream of the dairy farm), the measurement of dissolved oxygen was higher than that measured higher up the river (10 km above the dairy farm).
Some literature has suggested that rivers have the ability to purify themselves through natural processes, and this can be seen in the Nuwejaars River. The ability of this system to restore itself downstream of the dairy could also be assisted by the natural vegetation belt surrounding the river downstream of the dairy, as the river flows through a protected SANParks area.
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The drought in the Western Cape has had a significant influence on what we observed in the field, and the results we obtained. The most important thing I observed during March was that the dairy farm was not discharging any water (or pollutants) into the river.
In July, after some rains, there were more pools present, some of which were connected. We could see the pollutants from the dairy farm being washed into the river. However, usually the river is flowing at this stage and this would have prevented the accumulation of nutrients just below the dairy, and helped flush the system faster.
Dissolved oxygen was below the recommended concentration for aquatic ecosystems at the dairy farm, but gradually increased further downstream. There was a general improvement in water quality and clarity before the river entered Soetendalsvlei (see Plate 2).
This suggests that the lower Nuwejaars River is purifying the organic pollutants introduced by the dairy farm, and that the protected area is probably playing an important role in facilitating the recovery of the system. Going forward, it is important that the river is monitored regularly so that the medium- to long-term influence of the dairy farm on water quality downstream is better understood across different seasons and water levels.
This will assist in more sustainable and applicable future catchment management practices.
1 Bongiwe Seleka is currently completing her BSc Honours in Environmental and Water Science at the University of the Western Cape. She is supervised by Brigitte Melly at the SAEON Fynbos Node.
