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These differences were due to the contribution of each parameter taken into account in the calculation formula. Indices results showed that the water analysed was ranked into different water quality classes, although the same dataset was used. Fourteen physico-chemical parameters, i.e., pH, DO, BOD 5, COD, N-NH 4 +, N-NO 3 −, N-NO 2 −, N-total, P-total, SO 4 2−, Cl −, Fe-total, Zn 2+ and Cr-total, were monitored along the Danube course (on a distance of about 120 km), during the four seasons between the autumn of 2018 and the summer of 2019 in order to calculate the three indices mentioned above. In this study, Water Quality Index (WQI), Water Pollution Index (WPI) and Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI) were applied in order to evaluate the seasonal and spatial variation of the water quality in the Romanian Lower Danube sector.
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Water quality indices are suitable tools used for assessing water quality because of their capacity to reduce a large number of water quality indicators into one value which defines the water quality class.
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