Objective  The aim of this study was to understand the characteristics and release factors of manganese pollution in soil and sediment of a reservoir in Hunan Province, analyze the causes of this event, and evaluate the level and potential ecological risk of manganese pollution in soil and sediment.

  Method  The manganese content in soil and sediment of the study area was determined by small scale experiment, field monitoring, historical data investigation and field investigation, and the relationship between the release of manganese in soil and sediment and its influencing factors was analyzed, and the desorption kinetics was explored. The seasonal variation law of reservoir water quality was investigated, the variation law of meteorological data and the surrounding environment were analyzed, and the causes of the winter manganese anomaly event in a reservoir in Hunan Province were comprehensively analyzed. Soil accumulation index and potential ecological risk coefficient were used to evaluate the manganese pollution level and potential ecological risk in soil or sediment in the study area.

  Result  The contents of manganese in soils or sediments in the study area all exceeded their respective background values. The release of manganese from soil or sediment is closely related to the changes of pH, dissolved oxygen, water temperature, electrical conductivity, disturbance and other physical and chemical properties. The desorption process of manganese from soil or sediment in the study area was in accordance with the quasi-first-order kinetic model except for few experimental group. The desorption time, space environment, manganese content and morphology had different effects on the desorption process. The equilibrium amount of Mn desorption is between 0.026 mg g⁻¹ to 0.195 mg g⁻¹ and higher than their static condition under the condition of oscillation at each sample. The degree of soil or sediment pollution in the study area ranged from pollution-free to mildly polluted, and the potential ecological risks were all at low risk.

  Conclusion  The pollution of endogenous manganese in the study area is serious. The dramatic changes of physical and chemical properties of the reservoir and the construction disturbance during the occurrence of abnormal events promote the release of endogenous manganese. In addition, the limitation of historical monitoring data, the seasonal variation of reservoir capacity and the disappearance of thermocline also lead to the abnormal variation of manganese concentration monitoring values. The result of risk assessment shows that the risk of manganese pollution in soil or sediment in the study area is generally controllable.