EUTROPHICATION OF WATER RESOURCES - CAUSES AND IMPACTS

Eutrophication is a global water quality problem which has its roots in all water use sectors, and impacts on all water users. It is defined as the enrichment of water resources with plant nutrients, principally nitrogen and phosphorous, and results in the excessive growth of algae and aquatic macrophytes (plants) in surface water bodies. The Hartbeespoort Dam in South Africa is a well known example of a eutrophic water body and has been the subject of numerous cleanup efforts over the years.

A eutrophic river In the Crocodile West Water Management Area,
South Africa. Note the excessive vegetation on the river banks.

The natural environment has an inherent capacity to assimilate nutrients, but the eutrophication problem occurs where these nutrients are discharged into the environment at levels that are in excess of this capacity. Nutrients enter water resources both by natural means and through the action of man (the latter is termed “cultural eutrophication”). In the agricultural sector, eutrophication occurs as a result of the excessive leaching of fertiliser and animal feed components into receiving waters, as well as from runoff containing animal wastes. The domestic sector contributes nutrients via sewage treatment plants (dysfunctional ones are obviously an even bigger challenge), a lack of adequate sanitation infrastructure,  and chemicals such as household detergents. Industry’s contribution occurs in the form of untreated or partially treated organic effluents from industries such as food manufacturing facilities and other agro-processors. Nitrogen oxide emissions from the combustion of fossil fuels also contribute to the eutrophication problem due to aerial deposition.

Anyone who has experience with eutrophic water bodies will testify to the fact that they can be unsightly and have an unpleasant odour, reducing their recreational value. The water typically appears dark green in colour and there is generally excessive vegetation on the water surface. The water can also foam a lot, and in severe cases become quite viscous. Eutrophication does however have far more sinister consequences than these aesthetic ones.

Eutrophication impacts on the environment as well as on the users of water. Oxygen uptake by algae during respiration, but more particularly from increased oxygen demand when algae die, deprive aquatic species of oxygen, leading to their decline or in some cases elimination from the ecosystem. For industries and municipal treatment plants, algae can block pipes, clog filters and interfere with any operations requiring water free of particulate matter. I once was engaged in a water conservation project on an industrial site in which cooling tower blow-down was driven not by salt concentrations but by the amount of algae which was in the cooling water circuit, a consequence of nutrients in the incoming raw water. Local treatments applied by users of eutrophic water can also lead to the further pollution of receiving waters with hazardous materials such as algacides and biocides, exacerbating the risks to downstream water users. For industrial users and municipalities, the costs of treating water containing large amounts of algae are generally higher than in other instances, not least because of the additional filtration processes needed and the costs of sludge disposal. Aquatic macrophytes also deplete water resources through transpiration. Perhaps most troubling are the human (and animal) health issues associated with eutrophication. When water rich in organic materials is chlorinated, there is the potential for the formation of toxic and carcinogenic trihalomethanes, which will ultimately end up in the purified drinking water produced from such sources. Cyanobacterial blooms also produce genotoxic and potentially carcinogenic substances.

How then is this serious problem of eutrophication to be addressed? The issue clearly involves multiple stakeholders, encompassing water users in all sectors (as individuals and in organised formations), local government, national government, the scientific community, neighbouring countries which share water resources (together with their internal stakeholders) and of course the wider global community, given the widespread nature of this problem. Source-directed measures, for example the use of a phosphorous standard for municipal wastewater treatment plants, are well intended, but must be strictly enforced if they are to add value. Clean-up projects for affected water resources also cannot do any harm, but are an end-of-pipe approach. Like most sustainability challenges, enforcement without a change in mindset among those responsible for the problem will never achieve lasting results. More communication about this issue needs to take place so that polluters are more aware of the consequences of their actions. In addition, the issue needs to be integrated into water management strategies and policies, a matter which requires a systems view of the eutrophication challenge. One of the challenges we face in South Africa is that in arid parts of the country, at times wastewater discharge volumes can actually exceed natural streamflow volumes, leaving little room for dilution and assimilation of excess nutrients. This situation arises due to the many inter-basin transfers that take place across the country. The Waste Discharge Charge System, which the Department of Water Affairs is now in the process of implementing, is a positive step which will deal with many forms of point and diffuse source pollution, eutrophication included. As always however, the true test will lie in implementation.

REFERENCES:

1.   Walmsley, RD (2000). A Review and Discussion Document Perspectives on Eutrophication of Surface Waters: Policy/Research Needs in South Africa. Report No. KV129/00. Water Research Commission.