Abstract: Clarifying the distribution of heavy metals in water and sediment across different seasons and evaluating their pollution are helpful to better understand the spatiotemporal dynamics of heavy metal pollution and its environmental effects in lakes and reservoirs. We analyzed the vertical distribution and the pollution evaluation of eight heavy metals in stratified water and sediments of different seasons in Aha Reservoir. The concentrations of eight heavy metals in the stratified water were in an order as Ni>As>Zn>Cu>Cr>Co>Pb>Cd. The increases in the concentrations of heavy metals in the hypoxic bottom water in summer and autumn were related to the release of heavy metals caused by the reduction of iron and manganese in the surface sediments. In the lake water mixing period of winter and spring, there was limited vertical change of heavy metals in the stratified water. The concentrations of eight heavy metals in the sediments were following an order of Zn>Ni>Cr>Co>Cu>As>Pb>Cd. The seasonal variation of the peak concentrations of heavy metals in the active sediment layer (above 12 cm) was related to the combined effects of the redox status of the sedimentwater interface and the corresponding seasonal variation of ironmanganese reduction and sulfate dissimilatory reduction. The concentration, pollution degree, and potential ecological risk of heavy metals (mainly Co, Ni, As and Cd) in the active layer of sediments in the Aha Reservoir were higher in summer and autumn than that in winter and spring. The influence of pore water diffusion into overlying water was very limited. The concentrations of heavy metals in the water in all the four seasons of Aha Reservoir did not exceed the standard limit specified in the “Surface Water Environmental Quality Standard” (GB 3838-2002) Class I standard and “Drinking Water Sanitary Standard” (GB 5749-2006), indicating that the water of Aha Reservoir had no heavy metals pollution.

Key words: stratified water, sediment, heavy metal, distribution characteristics, pollution assessment, Aha Reservoir.