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. 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. Fourteen physico-chemical parameters, i.e., pH, DO, BOD5, COD, N-NH4+, N-NO3−, N-NO2−, N-total, P-total, SO42−, Cl−, Fe-total, Zn2+ 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. Indices results showed that the water analysed was ranked into different water quality classes, although the same dataset was used. These differences were due to the contribution of each parameter taken into account in the calculation formula. Thus, the WQI scores were mostly influenced by those parameters whose maximum allowable concentration was low (e.g., heavy metals, N-NO2−), while the WPI and CCME-WQI scores were influenced by those parameters which exceeded the maximum allowable concentration (BOD5, DO, COD, N-NO3−, N-NO2−). Based on the WQI results, the water was ranked into quality classes II and III. WPI and CCME-WQI assessed water only in quality class II, with one exception in the case of CCME-WQI when water was ranked into quality class III. The temporal assessment identified the seasons in which the water quality was lower, namely summer and autumn. The variation of the indices values between the sampling stations demonstrates the existence of pollution sources in the study area. Moreover, the indices results illustrated the contribution of the main tributaries (Rivers Siret and Prut) to the Danube River water quality. The appropriate applicability of the three indices was also discussed in this study.

中文翻译：

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用于评估多瑙河下游水质的一系列理化质量指标的比较方法

水质指数是用于评估水质的合适工具，因为它们能够将大量水质指标简化为一个定义水质等级的值。在本研究中，水质指数（WQI）、水污染指数（WPI）和加拿大环境部长理事会水质指数（CCME-WQI）被用于评估水质的季节和空间变化。罗马尼亚下游多瑙河部门。十四种物理化学参数，即 pH、DO、BOD5、COD、N-NH4+、N-NO3-、N-NO2-、N-total、P-total、SO42-、Cl-、Fe-total、Zn2+ 和在 2018 年秋季和 2019 年夏季之间的四个季节中，沿多瑙河航道（距离约 120 公里）监测 Cr-total，以计算上述三个指数。指数结果表明，尽管使用了相同的数据集，但分析的水被分为不同的水质等级。这些差异是由于计算公式中考虑的每个参数的贡献。因此，WQI 得分主要受那些最大允许浓度较低的参数（如重金属、N-NO2−）影响，而 WPI 和 CCME-WQI 得分受那些超过最大允许浓度（BOD5 、DO、COD、N-NO3-、N-NO2-)。根据 WQI 结果，水质等级为 II 级和 III 级。WPI 和 CCME-WQI 仅对水质等级为 II 的水进行评估，但 CCME-WQI 的情况除外，当时水被列为质量等级 III。时间评估确定了水质较低的季节，即夏季和秋季。各采样站间指标值的变化表明研究区存在污染源。此外，指数结果说明了主要支流（Siret 河和Prut 河）对多瑙河水质的贡献。本研究还讨论了这三个指标的适当适用性。