Altitudinal and temporal variation of surface water quality: An assessment in Badulu Oya Catchment, Sri Lanka

Rivers are one of the main surface water resources representing a geographical unit that fulfills wide array of economic and ecological values. Quality of river water is vital for health of river ecosystem and maintain its functions. Climate and altitude are 2 of the main natural factors that influence on quality of the river waters. The study assessed altitudinal and temporal variation of surface water quality in the Badulu Oya catchment. The main river of the catchment was categorized into 3 segments based on altitudinal gradient. Ten physicochemical parameters of stream water were monitored at 14 sampling locations along these segments every other month for 18-month period following standard analysis methods. Results revealed that the observed water quality parameters are significantly varying (<0.05) both seasonally and altitudinally. Results of General Linear Model (GLM) revealed that the electrical conductivity (EC), total dissolved solids (TDS), turbidity, sulphate, nitrate, phosphate and biochemical oxygen demand (BOD) significantly vary (<0.05) seasonally.  Further, temperature, dissolved oxygen (DO), sulphate, turbidity, TDS and EC were significantly different (<0.05) between the three river segments. During dry season highest average BOD, pH and phosphate were recorded in the midstream segment indicating possible high urban waste discharges. In the upstream segment, seasonal cultivations and excessive agrochemical usage in sloping lands appear to cause recorded highest EC, TDS, turbidity, and sulphate levels of upstream water. Except for a few sampling locations where BOD and turbidity were higher, all other monitored water quality parameters fall within the guideline ranges of ambient water quality.