This study aimed to demonstrate efficiency of documented index method “universal water quality index-UWQI” to evaluate surface water quality and investigate seasonal and temporal changes, in the case of Gediz River Basin Turkey. UWQI expressed results relative to levels according to criteria specified in European legislation (75-440 EEC). The method produced a unitless number ranging from 1 to 100 and a higher number was indicator of better water quality. Water quality is classified into five classes and index scores between 95-100 represent excellent and lower than 24 represent poor quality. In the study, dissolved oxygen-DO, pH, mercury-Hg, cadmium-Cd, total phosphorus-TP, biochemical oxygen demand- BOD and nitrate nitrogen-NO3-N have been chosen as index determinants. Samples analyzed for these variables were collected from five stations on monthly basis along two years. Based on UWQI classification scheme, water quality at sampling stations had scores below 40 and assigned to “marginal” which is between fair and poor quality class. On the other hand sub-indices of water quality determinants showed seasonal differences for some parameters. Cd concentrations were higher in “high flow” and lower values were observed in “low flow” periods. This was explained by negative impact of urban runoff on water quality. On the other hand DO concentrations were higher in “high flow” period. Under “low flow” conditions water quality at upstream stations (where the industrial density is low) was comparably better than downstream part. The study showed that index approach can be efficient tool to: a) evaluate water quality, b) investigate spatial and seasonal variations and finally, c) extract required information from complex data sets that is understandable by non-technical people.
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