For years now, invasive water hyacinth plants have clogged up the North West’s Hartbeespoort Dam, which lies downstream from Pretoria and Johannesburg. In new research, hydrogeologists from the University of Pretoria (UP) have used the internal workings of the plants themselves to reiterate that the infestation is fueled by below-par sewerage works and inadequate sanitation facilities for informal settlements upstream.  

The study, by Ryno Germishuys, a MSc student in Hydrogeology at UP, and his supervisor, Dr Roger Diamond of the UP Department of Geology, was published in the South African Journal of Science. It supports previous findings that the poor water quality of the Hartbeespoort Reservoir is mainly caused by effluent from sewage works, mainly those servicing Johannesburg.

According to Dr Diamond, water hyacinth (Eichhornia crassipes) is “the number one water weed in the world.” The plant aggressively infests water bodies and negatively impacts water quality.

“Water quality in the Hartbeespoort Reservoir has been a problem for many decades,” adds Germishuys. “It causes excessive growth of algae and water hyacinth, both of which further degrade water quality, impact indigenous plants and fish, and can be a health hazards to recreational users such as fishermen and skiers.”

Other researchers have shown that despite ongoing efforts to curb the inflow of contaminated water rich in phosphorus and nitrates from industrial sources and mines, the dam’s water-quality problems, including algal blooms, are still dire. Many water quality parameters exceed irrigation guidelines.

Germishuys collected plant and water samples from the dam itself, and from the inflowing Crocodile River and nearby boreholes. The dam’s surface water was found to contain high volumes of faecal bacteria such as Escherichia coli (E.coli) that are typically found in mammal and bird waste and untreated sewage. When people drink such contaminated water, they can experience severe diarrhoea.

Germishuys also conducted a nitrogen isotope-related analysis of the plant material. His results indicate that human faeces and manure, rather than industrial or agricultural causes, are currently the major sources of large volumes of growth-stimulating nitrogen still flowing into the dam. This is worrying because the build-up of nitrogen and other nutrients in water causes eutrophication, which depletes oxygen levels and impacts lifeforms such as fish and plants.

According to Dr Diamond, Germishuys’ study is the first to use nitrogen isotopes in plants to trace the source of water pollution in South Africa. “Water hyacinth as problem plants have been studied widely across the world. Ryno’s study is possibly the first to use the plants themselves to pinpoint what is causing their excessive growth.”

At one stage hyacinths covered almost 80% of the surface of the Hartbeespoort Dam. The floating mats of plants stop sunlight from penetrating into the water around the shoreline and destroy existing water plants growing there. It has a negative impact on indigenous fish species and causes plant debris to build up on the dam floor. Over the years, efforts to physically remove the plants or use chemicals proved futile.

Germishuys says the recent release of weevils by the Centre for Biological Control (CBC) at Rhodes University has reduced the area covered by water hyacinth to below 5%, some of the lowest levels in a long time. “Results seem very promising, but continuous work and control is needed to keep the hyacinth in check and to prevent them from spreading rampantly again.”

Dr Diamond believes such efforts are like “putting a plaster on a wound”, as they do not address the root of the water-quality problem: sanitation upstream. He says the dam water’s high nitrate levels must be fixed before any real headway can be made in successfully controlling the water hyacinth.

“The only way to really control Hartbeespoort’s water hyacinth problem is to maintain, upgrade, and expand municipal sewage treatment works, and to supply better sanitation facilities to informal settlements in the Crocodile River.” He notes that the sewage issue is more than just a “toilet water” problem, as soapy water that was used for baths, showers, and washing also contain high volumes of nitrates. “Water users can help too, by using less water or recycling it. Often our existing sewerage works simply cannot handle the large volumes of water coming in to be cleaned.”

Click on the next page to learn more about water hyacinths, the Hartebeespoort Reservoir and isotopes. 

Quick facts on water hyacinths, Hartebeespoort Reservoir, and isotopes

More about water hyacinths (Eichhornia crassipes):

• This floating freshwater plant originally came from South America. It is an aggressive invader in warm regions.  
• Since being introduced to South Africa about 100 years ago as an ornamental garden plant, it has become a well-established weed in many waterways.
• Water hyacinth prefers warm, nutrient-rich water. The higher the concentration of nitrogen the water contains, the more vigorously the plants will grow.

More about the Hartbeespoort Reservoir:

Situated in South Africa’s North West province, about 30km west of Pretoria, it is mainly fed by the Crocodile River (including the Jukskei and Hennops Rivers) and Magalies River.

More about isotopes:

Researchers use different types of isotopes (such as nitrogen or carbon isotopes) as a way of fingerprinting, identifying, or tracing the natural processes and origins of plants, animals, water, and soil. Isotopes contain nanoscale tell-tale bits of details. These can shed light on the origin, growth and even age of, for instance, plants and animals. In the case of animals, one is able to trace what the dominant type of food is that they have eaten. Nitrogen (N) isotopes trapped inside aquatic organisms offer a way of differentiating between sources of dissolved nitrate species in water.