Abstract
Abstract Anoxia is a common phenomenon at the bottom of large reservoirs during thermal stratification. In an anoxic environment, an increasing amount of reducing substances and nutrients are released and settle at the hypolimnion of the reservoir, leading to water quality deterioration and eutrophication. This work presents a case study on Daheiting Reservoir, a part of the Water Diversion Project from the Luanhe River to Tianjin city. With the monitored data of the water temperature and dissolved oxygen content in the reservoir, and based on the mechanism of redox reactions, the water quality response to the hypolimnetic anoxia in Daheiting Reservoir was systematically analyzed. It was found that the release of total phosphorus from the sediments in Daheiting Reservoir was a joint effect of the biological and chemical processes, and the redox reaction in the anoxic zone boosted release of phosphorus. Anoxia in the reservoir caused the ammonia nitrogen released from sediments in the reservoir to accumulate at the hypolimnion, which increased the concentration of ammonia nitrogen in the water. Anoxia in the reservoir led to an increase in the concentration of iron and manganese, which accounts for the major driving factor of release of iron and manganese.
Highlights
It is common to observe an anoxic zone at the hypolimnion of deep-water lakes or reservoirs during thermal stratification in summers (Zhang et al 2015; Du et al 2019)
Elci (2008) made a case study on the Tahtali Reservoir in Turkey, and found that the thermal stratification in the reservoir led to a sharp drop of dissolved oxygen (DO) at the hypolimnion and reduced the water quality there
Xu et al (1999) studied the vertical distribution pattern of iron and manganese in Aha Reservoir in Guiyang, Guizhou province, China with seasonal anoxia, and revealed the layered distribution of water temperatures along the vertical direction; they found that high concentrations of iron and manganese occurred at the metalimnion and hypolimnion of the reservoir during thermal stratification, and thermal stratification was closely related to hypolimnetic anoxia, decline of pH scale, and secondary pollution induced by release of iron and manganese from sediments
Summary
It is common to observe an anoxic zone at the hypolimnion of deep-water lakes or reservoirs during thermal stratification in summers (Zhang et al 2015; Du et al 2019). Xu et al (1999) studied the vertical distribution pattern of iron and manganese in Aha Reservoir in Guiyang, Guizhou province, China with seasonal anoxia, and revealed the layered distribution of water temperatures along the vertical direction; they found that high concentrations of iron and manganese occurred at the metalimnion and hypolimnion of the reservoir during thermal stratification, and thermal stratification was closely related to hypolimnetic anoxia, decline of pH scale, and secondary pollution induced by release of iron and manganese from sediments. Hu et al (2019) explored the correlation between the DO in the water body and its physical influencing factors These aforementioned works have revealed that anoxia during thermal stratification in reservoirs would lead to substantial release of pollutants from sediments and worsen the water quality. This research will be able to more accurately and intuitively explain the effects of hypoxia on water quality and provide solutions to the problem of worsening water quality in reservoirs during thermal stratification
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