Heavy metals in substrates of a freshwater wetland at Kelaniya

Abstract

Many natural wetlands function as successful remediation systems for the environmental impact dueto industrial effluents and domestic effluents from urban and semi-urban areas. However, this protectivefunction of natural wetlands often comes at the cost of substantial environmental degradation. Hence,wetland sediment analysis is essential in the assessment of the impact of industria! effluent dischargeand in the determination processes of remediation of metals. The important fraction played by thesediments of wetlands is the regulation of nutrients (N, P, K) and other trace elements essential forplants as most bio-geo-chemical processes occur in the substrate of wetland to release nutrients andheavy metals.

Iriyawetiya wetland at Kelaniya is closer to the Kiribathgoda Township and it receives many effluentsfrom urban runoff and industry. The urban runoff into wetland may include pollutants such as nutrients,toxic substances, poly-aromatic hydrocarbons, heavy metals, oils, pathogens and particles. Apreliminarystudy showed that this wastewater carries appreciable amounts of toxic metals, especially heavymetals and their concentrations vary from place to place in the wetland. To determine the heavymetal concentrations in sediments of Iriyawetiya wetland, samples were collected from the inlets,outlets and the middle part of the wetland using a HDPE grab and metal concentrations in aciddigestedsamples were measured separ~tely using an atomic absorption spectrophotometer.

Among the tested heavy metals AI, Fe, Cr, Zn, Cd, Mn and Ni (except Cu, Pb, Sn) were found inhigher amounts in sediments at the inlet 0 I area than the outlet area. AI, Cu, Sn, Cr, Zn, Cd, Pb, Mnand Ni in sediment of inlet 2 area were recorded comparatively lower values than the outlet area dueto lesser loading of effluents in that area. The middle area of the wetland recorded higher amounts ofall measured ten heavy metals than inlet 02. AI, Fe, Cu, Sn, Mn gave the highest values in the middleopen areas of the wetland. This shows that pollutants in urban waste, when dispersed in to the aquaticenvironment, they accumulate primarily in water and sediments which accelerates the contaminationof other water sources including groundwater and drinking water. Further it provides the mechanismsneeded to be used for remediation methods of waste water using natural wetlands.