Effect of high temperature on water quality of drinking water source reservoirs in large cities in lower reaches of Yangtze River: A case study of Nanjing

Abstract

[Objective] Under the background of global warming, the frequent occurrence of high-temperature events has posed a serious threat to the water quality safety of urban drinking water sources. The objective is to clarify the mechanisms by which high-temperature events affect the water quality of urban drinking water source reservoirs, and to explore the effects of reservoir storage capacity and scheduling strategies on water quality changes during high-temperature periods. [Methods] Taking 16 drinking water source reservoirs in Nanjing, a major city in the lower reaches of Yangtze River, as the research objects, the monthly long-term data from 2000 to 2023, on 24 physicochemical water quality indicators and meteorological data were used. Spearman correlation analysis was conducted to reveal the long-term associations between key water quality indicators and temperatures. The response mechanisms and spatial variation patterns of reservoir water quality were analyzed in combination with the extreme high-temperature event in 2022. [Results] The result showed that only 8 indicators—water temperature(T), pH, permanganate index(COD_Mn), dissolved oxygen(DO), fecal coliform(FC), total phosphorus(TP), total nitrogen(TN), and ammonia nitrogen(NH₃-N)—were significantly correlated with temperature in certain reservoirs(p<0.01). The typical high-temperature events in August 2022 led to an average increase of 6.5% in T and a decrease of 5.9% in DO across the reservoirs, while TP and TN increased by 18.8% and 15.6%, respectively. Spatial variation analysis revealed that reservoirs in northern Nanjing, characterized by smaller average storage capacity and greater water level decline, had significantly higher increases in TN and TP(21.8% and 15.5%) compared to those in the southern region. In contrast, reservoirs in southern Nanjing with larger average storage capacities showed more pronounced changes in DO, pH, COD_Mn, FC, and NH₃-N. [Conclusion] High temperatures exacerbate water quality deterioration by altering thermal stratification structures and biogeochemical processes in water bodies. Reservoir storage capacity and water scheduling strategies significantly regulate water quality response, with small-capacity and rapid water-level-declining reservoirs being more vulnerable to high-temperature stress. It is recommended that a classification-based management model should be established based on scheduling functions and response characteristics, and differentiated regulation strategies should be developed to enhance the climate resilience of water supply systems in large cities.

Keywords

high temperature / drinking water source / reservoir / water quality / lower reaches of the Yangtze River / climate resicience / reservior capacity regulation