The Role Wetlands Play in Water Reclamation

Michelle Potgieter
Department of Hydrology, University of Zululand, P. Bag X1001, KwaDlangezwa, 3880

World Wetland day is being celebrated between 28th January and 2 February 2002 this year. Wetlands reclaim water, and provide a natural, free service in water purification, combating water-borne disease. World Wetland day focuses our attention on the conservation of these natural purification plants. Earlier articles provide insight into wetlands and cholera and in this article, a deeper look into how wetlands function and reclaim water is provided. For a list of events marking this day, visit the Events Listing.

All aquatic ecosystems have in common the fact that the indigenous species of animals, plants and microorganisms are generally inescapably immersed in a water medium throughout their lives. Since wetland ecosystems involve complex interactions of biotic and abiotic factors it is often difficult to understand the response of a system to pollution or a specific chemical.

Natural wetlands and constructed wetlands consisting of reeds and other emergent hydrophytes, have been used for point source pollution treatment with various degrees of success. Functioning of wetlands in the landscape is strongly influenced by local and regional-scale environmental factors related to climate, geomorphology and source of water. In addition, human alteration of wetlands and the surrounding landscape can have considerable influence on wetland functioning.

Wetlands and Pollution Wetlands are recognised as areas where water is the dominant factor determining development of soils and associated biological communities and where, at least periodically, the water table is at or near the surface (Smith, 1989; Jenssen, Mæhlum & Krogstad, 1993), and the land is covered by shallow water. Pollution is the introduction of any substance into the environment that has or results in direct harmful effects to humanity or the environment, or that it makes the environment less fit for its intended use.

Photo taken at Hlobane Mine (1998), Kwa-Zulu Natal, indicating the effects of acid drainage in a stream. (Potgieter, Van Rensburg 2001)

A major problem contributing to contamination of water supplies is leaking sewer lines, land application of sludge, partially treated waste water, acidic runoff from dumps and the establishment of informal/squatter settlements near the water areas (with insufficient sanitation infra-structure). A common effect of water toxicity is fish mortality, immobility of invertebrates and loss of equilibrium, and algae blooms (Rand & Petrocelli, 1985).

Processes Occurring in the Water

Water flows through wetlands via two mechanisms (see image below). They include surface flow (SF) and subsurface flow (SSF). A wetland can incorporate one or both mechanisms. A surface flow system consists of a cell or cells with wastewater routed at shallow depths over a substrate supporting emergent vegetation. The shallow depth, low flow velocity, and the plant stems and litter in surface flow and subsurface flow systems control the flow of the water in the image below.

(A) free water surface flow and (B) subsurface horizontal flow. (Adopted from Brix & Schierup, 1989).

When water comes in contact with mineral carbonates [especially calcite (CaCO₃) and dolomite (Ca, MgCO₃) in the watershed soil, bedrock or aquatic sediment, a large quantity of bicarbonate alkalinity is generated, and the acid-neutralizing capacity of the water is large. Alkalinity is produced by the dissolution of these minerals: CaCO₃ + H₂O + CO₂ " Ca²⁺ + 2HCO₃^- This process could have the capacity to effectively neutralize the acid inputs of acid runoff water. This in situ production of alkalinity is also enhanced by the biological reduction of sulphate and nitrate, and the exchange of H⁺ for Ca²⁺ in sediment (Schindler, Turner, Stainton & Linsey, 1986).

The Importance of Wetlands

Wetlands are valuable because they greatly influence the flow and quality of water (see diagram below). They improve water quality by intercepting surface runoff and removing or retaining inorganic nutrients, processing organic wastes, and reducing suspended sediments.

The removal processes involved in wetlands could be summarized as follows:

The mechanisms involved in removing pollutants within the wetland. (Potgieter et al 2001)

Another outstanding feature of wetlands and the aquatic inhabiting organisms are the capacity of living cells to take up substances from the environment and use them for the synthesis of their own cellular components or as an energy source. Thus reducing elements/nutrients in their immediate surroundings to a certain degree. The mechanism of nutrient removal in reed bed systems involves physical, chemical and biological processes occurring in the soil-water matrix and in the plant rhizosphere (Gersberg, Lyon, Brenner, & Elkins, 1989). The degradation of wastewater constituents that takes place in an artificial reed bed system is the result of microbial activity in the soil layer under the influence of the reed roots or rhizomes. The reeds provide oxygen to the rhizosphere through their leaves, branches and stem, thus creating an area around the roots in which aerobic bacteria are able amongst others, to oxidise ammonia to nitrate (Markantonatos, Bacalis, Lazarus & Angelis, 1996). Bioremediation is based on natural biological and physical principles, which have been responsible for the deposition of ore bodies. In these geological historic processes, biologically active microbes/microorganisms have fostered the accumulation and deposition of minerals and metals by removing them from their dissolved states in water and fixing them in the underlying soils (in a insoluble state), or by incorporating them into cell structures. Thus bioremediation could be utilised to effectively correct the existing imbalances and eliminate the need for chemical treatment (Davidson, 1993).

Nutrient removal and storage capacity in wetlands is controlled by the interaction of a number of physical, chemical and biological processes in the soil and biota. The net result of these processes determines the potential of a wetland to serve as a filter or sink for nutrients.

In addition to improving water quality through filtering, some wetlands maintain stream flow during dry periods; others replenish groundwater. Because of their low topographic position relative to uplands (e.g. flood plains), wetlands store and slowly release surface water, rain, groundwater, and flood waters. Wetland vegetation impedes the movement of flood waters and distributes them more slowly over floodplains. In addition to this wetlands also provides wildlife a valuable aquatic habitat. Wetlands are a key to providing potable water in the long term. They have been called the "kidneys of the planet" because of the natural filtration processes that occur as water passes through. It has been calculated that one hectare of tidal wetland can do the job of R13.5 million worth of state-of-the-art wastewater treatment (Lum, 1998).

Wetlands are now recognised globally as a cornerstone and focal point of economic development in both the developed and developing worlds. Fish production is leading the way as an income earner but it is closely followed by ecotourism. Wetlands such as Kakadu National Park in Australia, the Okavango in Botswana and Lake St Lucia in South Africa are visited by hundreds of thousands of tourists each year and through this the local economies benefit. In many countries the harvesting of reed beds from wetland areas for producing paper and basketry is a vital part of local economic growth.

Conclusion

Wetland's microbes, plants, and wildlife are part of global cycles for water and nitrogen. Wetlands constitute an interesting alternative for the upgrading of wastewater with minimal maintenance. Wetlands are among the most biologically productive natural ecosystems in the world. They are vital to the survival of various animals and plants. Wetlands often function like natural tubs or sponges, storing water (flood water etc.) and slowly releasing it, thus reducing water's erosive potential. Wetlands help improve water quality by intercepting surface water runoff and removing or retaining nutrients, processing organic wastes, and reducing sediments before it reaches open water.

More Information

References:

BRIX, H. & SCHIERUP, H.-H. 1989. The use of aquatic macrophytes in water pollution control. Ambio 18(2): 100-107.

DAVIDSON, J. 1993. Successful acid mine drainage and heavy metal site bioremediation. (In Moshiri, G. A., ed. Constructed wetlands for water quality improvement, London: Lewis Publishers, 632p.)

GERSBERG, R. M., LYON, S. R., BRENNER, R. & ELKINS, B. V. 1989. Integrated wastewater treatment using artificial wetlands: a gravel marsh case study. (In Hammer, D. H., ed. Constructed wetlands for waste water treatment: municipal, industrial and agricultural. Michigan: Lewis Publishers, 831p.)

JENSSEN, P. D., MÆHLUM, T. & KROGSTAD, T. 1993. Potential use of constructed wetlands for wastewater treatment in northern environments. Water Science and Technology Vol. 28 No. 10, pp. 149-157.

LUM, K. 1998. The key role of wetlands in addressing the global water crisis. International Conference, Water and Sustainable Development, Paris - 19-20-21 March 1998 [Available on the net: http://www.ramsar.org/about_global_water_crisis.htm]

MARKANTOMATOS, P. G., BACALIS, N. Ch., LAZARUS, G. & ANGELIDIS, M. O. 1996. Nutrient removal using reed bed systems in Greece. Journal of Environmental Science and Health A31 (6) pp. 1423-1434.

POTGIETER, M & VAN RENSBURG, L 2001. An ecological study into the possible reclamation effect of reedbeds in a coalmine wetland system in Kwa-Zulu Natal. Unpubilshed MSc, University of Potchefstroom.

RAND, G. M. & PETROCELLI, S. R. 1985. Introduction: in fundamentals of aquatic toxicology: methods and applications, (In Rand, G. M. & Petrochelli, S. R., eds. Johannesburg: McGraw-Hill International Book Company, 673p.)

SCHINDLER, D. W., TURNER, M. A., STAINTON, M. P. & LINSEY, G. A. 1986. Natural sources of acid neutralization capacity in low alkalinity lakes of the Precambrian Shield. Science 232: 843-847.

SMITH, A. J. 1989. Wastewater: a perspective. (In Hammer, D. H., ed. Constructed wetlands for waste water treatment: municipal, industrial and agricultural. Michigan: Lewis Publishers, 831p.)

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