Influence of hydrological regime on spatiotemporal distribution of boron in sediments in the Three Gorges Reservoir, China

Hydropower dam construction can alter the continuity of the river flow, thereby influencing geochemical cycle of seldom monitored trace elements due to the change of hydrological regime by reservoir regulation. Here, we investigated the response of hydrological regime on migration of boron (B) in the sediments of Three Gorges Reservoir (TGR) using multiple geochemical approaches. Although B concentrations in TGR sediments increased slightly during five consecutive water seasons, the potential ecological risk exhibited relatively low. The spatial distribution patterns of B with high heterogeneity in TGR sediments were influenced by periodic water-level fluctuation. This high spatial heterogeneity significantly correlated with the distribution of fine-grain sediments, which was attributed to the change of hydrological regime (including the decrease of water flow velocity, sediment accumulation, and extended residence time of suspended fine-grain sediments). In order to remove the sediment size effect, the geochemical normalization approach was applied, and revealed that the slope of normalization curve remarkably decreased from 0.9402 to 0.3682. This indicated that a small amount of B was still accumulated in the mid- and downstream of TGR after removing the hydrological regime effect. Further source identification using geochemical baseline model and principal component analysis, exposed that this slight accumulation of B in sediments originated from the anthropogenic input (8.45%). Our findings provide a geochemical insight on the environmental impacts of reservoir operation on toxic element cycling, with potential implications for ecological assessment of hydropower worldwide.