Constraining release of pollutants from anoxic bottom sediment via water-lifting aeration in a source water reservoir, East China

Abstract

Seasonal hypoxia in water bodies can increase levels of reduced chemical species in the hypolimnion as they are released from anoxic bottom sediments. Water-lifting aeration (WLA) is a technology known to solve this problem by elevating near-sediment dissolved oxygen (DO) and increasing mixing in the water column in a canyon reservoir with deep water (>20 m). The influence of WLA in reservoirs with large surface area and small water depth has not been adequately evaluated. For this purpose, the efficiency of an innovative WLA system was reviewed in Zhoucun Reservoir (ZCR), a mildly eutrophic reservoir with a mean depth of 15 m. Three years of field experiment was performed in ZCR. Water and sediment samples were collected before and after the operation of the WLA system to explore the causes for the deterioration of water quality in ZCR, and the efficiency of the innovative WLA system installed in 2015. With the use of a microporous aerator and a kinetic energy dissipator, the new-style WLA system is different from WLA studied before. As a result of thermal stratification and the long duration of anaerobic environment in the hypolimnion, the sediment oxygen consumption rate in ZCR was calculated to be 16.14 mg m−2 h−1, which was much higher than other reservoirs, and the main pollutants in ZCR were ammonia, phosphorus, manganese and sulphide. After the operation of the new WLA system, the anaerobic environment in the bottom water was completely reversed and the DO concentration in the hypolimnion water has been maintained at more than 6 mg L−1. The release of pollutants from sediments, such as ammonia, phosphorus, manganese and sulphide, was inhibited significantly. Ammonia, phosphorus, manganese and sulphide in water decreased by 89%, 84%, 97% and 87%, respectively. The results of this study demonstrate that employing WLA systems to oxygenate and mix stagnant water bodies is a viable and potentially favourable management strategy for reservoirs with large surface area and relatively small water depth.