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

Introduction

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 et.al, 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).