Thermal Stratification and Its Influence on Water Quality of Plain Reservoir in Subtropical Region

Abstract

Thermal stratification is an important factor causing deterioration of water quality in source water reservoirs. To explore the occurrence of thermal stratification and its impact on water quality of plain reservoirs in subtropical region, water temperature and main water quality parameters of Reservoir A, B, and C in City S (South China) were continuously monitored from March to July 2021. The results showed that thermal stratification occurred in all the three reservoirs with different stability. Thermal stratification of Reservoir A and C was relatively weak due to shallow water depth (<10 m) and the influence of water diversion, whereas thermal stratification was more stable in Reservoir B with a water depth of approximate 12 m. Thermal stratification hindered transfer of dissolved oxygen (DO) from surface water to the lower water layer, and oxygen consumption by water and sediment in plain reservoirs in subtropical region was relatively high due to high temperature and microbial activity. Therefore, DO concentration dropped rapidly with water depth, leading to anaerobic layers with heights of 2-4 m at the bottom of the three reservoirs. As a result, release of nitrogen, phosphorus, iron and manganese from the sediments was significant, especially in Reservoir B. In early July, the concentrations of total nitrogen, ammonium, total phosphorus, iron and manganese in the bottom layer have all substantially exceeded the standard limits for drinking water source. In general, water quality deterioration caused by thermal stratification of plain reservoirs in subtropical region cannot be ignored, and measures should be taken to combat the hypoxic environment and pollutants release at the bottom layer.