Hydropower reservoirs enhanced the accumulation of heavy metals towards surface sediments and aggravated ecological risks in Jiulong River Basin, China

Abstract

In complex watersheds with multiple types of waterbodies, the effect of river damming on the distribution pattern of heavy metals in sediments and their potential ecological risks are inconsistent, leading to an inability to accurately manage heavy metal pollution. The aim of this study was to reveal the driving mechanisms of hydropower reservoirs on the distribution of heavy metals in sediments and their ecological risks. Characterization of the spatial distribution patterns of Cd, Cr, Cu, Ni, Pb, and Zn and evaluation of their ecological risks in surface sediments were conducted to explore the role of dam construction and operation in modulating heavy metal pollution in the Jiulong River Basin (JRB) using sequentially extracted European Community Bureau of Reference (BCR) extraction scheme, geo-accumulation index (Igeo), and potential ecological risk (RI). The results showed that hotspot patchiness pattern of sediment heavy metal contents highly matched the reservoir distribution but not for the non-dammed south river of the JRB. Ecological risk assessment demonstrated the RI of sediment heavy metals in the reservoir was markedly higher than those in the mainstream and its tributary (p < 0.05), and the potential ecological risks were dominated by Cd, followed by Pb in the sediments of three waterbodies (reservoir, mainstream, tributary). Moreover, BCR heavy metal fractionation showed bioavailability of heavy metals in Xipi Reservoir sediments was much greater than that in downstream reservoir sediments. The reason for this phenomenon was mainly attributed to the elongation of hydraulic residence time (HRT) in this reservoir, which promoted suspended solid sorting by grain size and consequent deposition, facilitating adsorption of finer sediments, with more bioavailable heavy metals falling onto the upper sediment layer in the reservoir. In brief, dam construction and operation reinforced the accumulation of heavy metals and aggravated potential ecological risks in reservoir surface sediments. These findings deepen current understanding of the driving mechanism of reservoirs for sediment heavy metal pollution and confer the probability to precisely control sediment heavy metal pollution for management practices in the multitype waterbody catchment.