[Pollution Sources and the Stratification Effects on Water Quality in Lijiahe Reservoir]

An in-depth understanding of the source of pollutants and the evolution of water quality is a prerequisite for water pollution control in source water reservoirs. Lijiahe Reservoir is an important water source for Xi'an. To strengthen the protection of Lijiahe Reservoir, water quality monitoring was carried out monthly from December 2016 to December 2017. The results showed that the main pollutant source was upstream water, and its percentage contribution to the bulk pollutants was 99.52% for permanganate index, 99.41% for TN, and 99.23% for TP. The main source of pollution in the upstream Dongcaiyu was the upper reach with Gepai Primary School and the turf service area. The main source of pollution in Xicaiyu was the upper reach with a chicken farm and fishery. In summer and autumn, Lijiahe Reservoir exhibited thermal stratification. During the stable stratification period, the maximum concentrations of TN, TP, permanganate index, TOC, Fe, and Mn in the water body were as high as 3.32, 0.177, 5.21, 3.01, 0.21, 0.235 mg·L^-1, respectively, under the dual effects of exogenous pollution and bottom anaerobic release. The maximum concentrations of TN, TP, Fe, and Mn exceeded the Class Ⅲ water limit of Surface Water Environmental Quality Standards (GB 3838-2002). The higher nutrient levels promoted large-scale reproduction of algae. The density of algae in the upper water reached 2.18×10⁸ cells·L^-1, and the dominant algae species were Microcystis aeruginosa and Aphanizomenon flosaquae, which pose a significant threat to water quality. Therefore, the primary goal for water pollution control in Lijiahe Reservoir is to take effective measures to control algae blooms, and at the same time, reduce the input of upstream pollutants.