Temporal and spatial variation and risk assessment of soil heavy metal concentrations for water-level-fluctuating zones of the Three Gorges Reservoir

Abstract

This study aimed to reveal the temporal and spatial variation of soil heavy metal concentrations in the Three Gorges Reservoir area (TGR) water-level-fluctuating zone (WLFZ) and evaluated its pollution status and potential ecological risks and provide scientific basis for ecological risk prevention and ecological restoration of the TGR. This study was based on long-term monitoring of soil heavy metals (Cu, Pb, Cd, and Cr) before water level fluctuation (2008) and after 1 (2009), 4 (2012), or 7 (2015) cycles of water level fluctuation at the altitude of 155–172 m in the Wushan (WS) and Zigui (ZG) sections of the TGR, and pollution status and potential ecological risks of each heavy metal element were evaluated by index of geoaccumulation and potential ecological risk index. The Cd concentration increased with the increase in the number of reservoir water level fluctuations, whereas the concentrations of Cu, Cr, and Pb varied with the monitoring site. The Cd showed clear horizontal transfer characteristics. Moreover, with the increase of the frequency of water level fluctuations the Cd concentration at ZG (near the dam) were higher than those at WS (away from the dam). After 7 cycles of water level fluctuation, the concentrations of most soil heavy metal were not obvious differences between soil layers (except Pb). Before and after the reservoir water level fluctuation, Cd contamination level changed from pollution-free to strong or extremely polluted, Cu contamination level changed from pollution-free to moderately polluted, and Cr and Pb were pollution-free. Before the fluctuation of the reservoir water level, the potential ecological risk of Cd in the WS reached a classification of strongly polluted, whereas pollution at ZG was considered to be low level. However, after 4 cycles of water level fluctuation the Cd pollution level increased to a very high level, whereas Cu, Cr, and Pb remained consistently low. There is an obvious temporal and spatial variation of heavy metal concentrations for WLFZ of TGR. Cd concentration increased with the increase in the number of reservoir water level fluctuations. Heavy metal concentrations changed from WS > ZG to WS < ZG after the impact of water level fluctuations. After 7 cycles of water level fluctuation, the distribution of heavy metals in different soil layers tends to be uniform (except Pb). Cd pollution is more serious, and there is a strong potential ecological risk.