Migration characteristics of arsenic in sediments under the influence of cascade reservoirs in Lancang River basin

•The concentration of labile-As in XW Reservoir was larger than that in NZD Reservoir.•As had a potential tendency to be released from surface sediments to overlying water.•The development of cascade reservoirs had a certain influence on the bioavailability of As in sediments.•Highlighted the role of reservoir interception and regional sediment. Cascade hydropower development in the Lancang River (LCR) Basin has a complex and far-reaching impact on river ecosystem, which has always been the focus of international attention. In this study, diffusive gradients in thin films (DGT) technique was used to study the reactivation of arsenic (As) in the sediments of Xiaowan (XW) and Nuozhadu (NZD) cascade dams in LCR. Pearson correlation analysis and Redundancy Analysis (RDA) were used to evaluate the bioavailability and spatial distribution characteristics of As. The results showed that the vertical variation of As in DGT at different sampling points presented different activation trends. The content of DGT-labile As (CDGT-As) in NZD Reservoir (36.21 ng/ml) was less than that in XW Reservoir (47.26 ng/ml). However, the difference of total content of As between the two reservoirs was relatively small. The results showed that the development of cascade reservoirs had no significant impact on total As, but had a great impact on DGT-labile As. The diffusion fluxes of the two reservoirs indicated that most of the sediments in the reservoir area were the source of As. Similar distribution characteristics and correlation analysis indicated that the migration mechanism of As was related to Iron (Fe) and manganese (Mn). In addition, on the basis of the results, the sources of As in sediments had certain spatial differences: cascade dam projects had significant accumulation and interception effects on As in the sediments, and at the same time strengthened the role of regional sediment, and this role reduced the risk of As release in downstream NZD Reservoir.