Temporal and spatial variation in water quality in the Three Gorges Reservoir from 1998 to 2018.

The Three Gorges Reservoir (TGR) underwent staged impoundment of water from 135 m to 175 m between 2003 and 2010. Periodic water impoundment was divided into drainage (March to early June), low water level (June to August), impoundment (September to October), and high water level (November to February) period. However, the impact of the Three Gorges Dam (TGD) and staged impoundment on water quality, especially in the long term, remains unclear. Herein, hydrological, pollution load, nutrient, and biochemical indices were determined for the TGR during 1998-2018. The Canadian Council of Ministers of the Environment Water Quality Index, a K-means clustering algorithm, and the Mann-Kendall (MK) test were applied to this data to explore the spatial and temporal distribution of water quality. The results show that water quality was good overall, but it before the full impoundment stage (2010) was worse than after that. The low water level period had the worst water quality among the four periods, and spatially, midstream was worst. Among water quality indices, the median total nitrogen (TN) and total phosphorus (TP) were in the range of 1.505-2.303 and 0.071-0.176 mg/L, respectively, and these were the key pollution indices. In addition, due to differences in hydrological and hydrodynamic conditions, and the regional distribution of pollution sources, water quality in the TGR displayed temporal and spatial heterogeneity. TN, TP, potassium permanganate index (COD), five-day biochemical oxygen demand (BOD) and Escherichia coli (E. coli) were maximal during the low water level period, and TN, TP and E. coli were highest in midstream. MK test results revealed that nutrients pollution became worse midstream, and a gradual increase in TP caused severe algal blooms downstream. Therefore, nutritional water treatment and non-point source pollution control should be the focus of future work.