| Summary: | <p>Malwathu Oya is a perennial river in the Dry Zone of Sri Lanka. Water of the nearby paddy lands is diverted to the river from different locations during cultivation seasons. These anthropogenic activities cause stresses on Malwathu Oya by impairing the quality of its water. The main objective of this study was to assess the seasonal fluctuation of nitrate, phosphate, chloride, dissolved oxygen (DO) and turbidity of river water in different locations of Malwathu Oya. The study was carried out in the upper part of Malwathu Oya from Ritigala to Nachchaduwa in the Anuradhapura District. Data were collected continuously for twelve months from March 2012 to February 2013. Factorial analysis was carried out to study the interactions among different factors (study site location, climatic season, channel geomorphic unit and river segment). The mean nitrate, phosphate, chloride and DO concentrations ranged from 1.05 to 5.28, 0.004 to 0.043, 2.63 to 8.72, 3.11 to 8.50 mg/L, respectively, and the mean turbidity level ranged from 81.75 to 256.10 NTU. The nitrate, phosphate and chloride concentrations were significantly higher (p<0.05) in all the segments and channel units during the first inter-monsoon season (March - April) in the Paddy growing area. The highest mean DO and turbidity values were observed in the northeast monsoon season. Significant difference was not observed (p>0.05) in DO and turbidity in paddy and non-paddy areas. The results concluded that the water quality parameters except turbidity in the investigated area of Malwathu Oya were within the threshold limits for dinking purposes.</p><p> </p><p>Tropical Agricultural Research Vol. 25 (2): 158 – 170 (2014)</p>
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