Determination of Retention Efficiency of Kimondi Wetland in North Nandi District in Kenya

Determination of Retention Efficiency of Kimondi Wetland in North Nandi District in Kenya

Shadrack. M. Mule (University of Eastern Africa, Kenya) and Charles. M. Nguta (Egerton Univerity, Kenya)
DOI: 10.4018/978-1-61520-907-1.ch021
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Wetland buffers may play an important role in the retention of nitrogen (N) and phosphorus (P) which are released in large quantities from agricultural, municipal and industrial sources with run-off from agricultural lands being a common source of such nutrients to wetland ecosystems. Wetlands receiving crop field drainage are shown to lower nitrogen and phosphorus in water of such ecosystems. The main objective of the study was to determine the retention efficiency of Kimondi wetland in terms of nitrogen and phosphorus. Results of the study show that it the wetland has mean retention efficiency of 90% and 95% for nitrogen during rainy and dry seasons respectively and mean retention efficiency of phosphorus of 80% and 93% during rainy and dry seasons respectively an indication that the wetland has high retention efficiency and its buffering ability has not been exceeded in both seasons.
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In recent years, wetlands have been applied to address water quality degradation associated with municipal and industrial wastes, and agricultural run-off, Nutrient run-off from agricultural fields in the form of nitrate-nitrogen and phosphate The enrichment of surface waters with pollutants from municipal, industrial, and agricultural sources have produced concern on the quality of water in water bodies. This has resulted in quite a number of studies to ascertain the level of pollution in such bodies (Goody D.C, 2002; Griffioen, 2006). Agricultural run-off has been cited as the major point source of nutrients into water bodies due to the ecological impacts they have on both freshwater and estuarine systems. A number of studies have compiled evidence linking excessive nutrient loading, particularly nitrates and phosphates from anthropogenic sources to eutrophication of lakes, rivers dams and wetlands (Cooper., et al. 1987; Heathwaite., et al, 2006). As pollution point sources have become easier to identify and control, attention is now being focused towards the contribution of such nutrients from agricultural catchments to water quality and the use of wetland ecosystems to improve water quality and retain pollutants. These vegetated riparian zones appear to act as a natural buffer for nitrogen, phosphorus and suspended sediment, thus controlling nutrient movement from the drainage area into the lake (Kadlec, 2003; Lucassen., et al, 2006).

Research has suggested that wetland areas can play a critical role in reducing water pollution, especially by intercepting surface runoff, subsurface flow, and certain ground-water flows. Their role in water quality improvement includes processing, removing, transforming, and storing such pollutants as sediment, nitrogen, phosphorus, and certain heavy metals (Cooper., et al. 1987; Marschner, 2003). Nutrients, mainly nitrogen and phosphorous from agricultural sources, human wastes and industrial discharges, may be retained by wetland ecosystems by accumulating in the wetland sub-soil, where they are transformed by chemical and biological processes or they can be taken up by wetland vegetation which can then be harvested and effectively removed from the system The efficacy of wetlands in removing pollutants from the upslope surface and groundwater is highly dependent upon hydrology For effective removal of pollutants, flow must occur as sheet flow rather than highly focused flows, and advance at a slow velocity and shallow enough depth to allow interaction with the rooting zone of wetland vegetation (Chescheir., et al, 1991). High flows especially during rainy season may even lead to net nutrient release through the flushing of nutrient rich soil water, desorption processes and sediment erosion (Marschner, 2003). Longer residence times experienced during the dry season, however, promote removal processes, particularly denitrification, plant uptake and sedimentation, effectively increasing nutrient retention (Howard-Williams, 1985; Healy & Cawley, 2002; Haycock and Pinay, 1993).

Key Terms in this Chapter

Nutrient: A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism’s metabolism which must be taken in from its environment.

Nitrate: A nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms (NO3-). In organic chemistry the esters of nitric acid and various alcohols are called nitrates.

Wetland: A wetland is an area of land whose soil is saturated with moisture either permanently or seasonally. Such areas may also be covered partially or completely by shallow pools of water. Wetlands include swamps, marshes, and bogs, among others. The water found in wetlands can be saltwater, freshwater, or brackish.

Concentration: In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent.

Retention: The capacity to hold or retain a pollutant.

Efficiency: The state or quality of being efficient; competency in performance.

Phosphorus: A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid.

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