Water quality evaluation, pollution sources apportionment, and environment management strategies in plain reservoirs: A case study of Tianhe Lake, China

Tianhe Lake serves as the sole emergency reserve drinking water source for Bengbu City’s 1.16 million residents. Recently, the lake’s water has shown significant signs of eutrophication and contamination. In order to understand the change rules and driving factors behind of water quality degradation, this study conducted water quality monitoring and evaluation of the lake. The study results revealed that the water quality index (WQI) of Tianhe Lake ranged from 60.560 to 81.724, indicating overall water quality fluctuating between “good” and “moderate”. The average trophic level index (TLI) of lake water was 63.23, characterized by moderate eutrophication. Influenced by climatic factors and anthropogenic activities, water quality exhibited significant seasonal and spatial distribution characteristics. Principal component analysis/factor analysis (PCA/FA) results indicated that the key water quality indicators included Chlorophyll-a (Chla), five-day biochemical oxygen demand (BOD5), ammonium nitrogen (NH3-N), total nitrogen (TN), and total phosphorus (TP). The minimal water quality index (WQImin) models developed using these five indicators and relative weights exhibited higher accuracy in water quality assessment, with the coefficient of determination (R2) and Root Mean Square Error (RMSE) values of 0.886 and 2.586, respectively. Pollution source apportionment analysis suggested that agricultural runoff and domestic sewage discharge were the predominant contributors to pollutant loads. Endogenous release of nitrogen (N) and phosphorus (P) from lake bottom sediments was identified as a secondary source. This study can enhance public awareness regarding safe water supply from small-scale reservoirs and provide new references and benefits for the environment management of plain reservoirs.