Heavy metals in the water-level-fluctuation zone soil of the three Gorges Reservoir, China: Remobilization and catchment-wide transportation

Abstract

• Selective HMs remobilization was observed in WLFZ soils. • Catchment-wide flood promotes the transport of HMs from the WLFZ soil into the TGR. • Erosion, runoff, and leaching released 36.61 Gg of HMs in soils in 2020. • Runoff-induced labile metals accounted for approximately 10% of fluxes in the TGR. • WLFZ soils act as a potential source of HMs in the TGR. Water-level-fluctuation zone (WLFZ) is a sensitive zone influencing the transformation between sink to source for heavy metals (HMs), which suffers from cyclic inundation and exposure during different hydrological regimes. To better understand the geochemical cycling of HMs in the WLFZ, we performed temporal analysis and transport quantification of HMs in the world’s largest freshwater WLFZ: Three Gorges Reservoir (TGR), China. From 2002 to 2020, HM contents in WLFZ soils fluctuated and gradually stabilized. In 2020, selective remobilization of HMs was observed, which were mainly influenced by four environmental factors: Fe and Mn oxides, soil pH, dissolved organic carbon, and soil mobility-resupply ability. Multiple models were developed to quantify the HM fluxes in 2019 and 2020 induced by soil erosion, runoff, and leaching. In 2020, HMs liberated in the TGR region catchment increased from 13.35 (2019) to 36.61 gigagram (Gg), including 6.90 Gg of non-residual fraction and 1.34 Gg of labile metals. Otherwise, metal fluxes of WLFZ soils into the TGR increased 1.96 times, which were attributed to integrated factors, including the increase of runoff, soil erosion, sediment load, and remobilization of HMs resulting from the catchment-wide flood in 2020. In particular, the runoff-induced labile metals only accounted for ∼ 10% of HM fluxes entering the TGR. We inferred that catchment-wide flood was an important control on HM transport in terrestrial and aquatic systems of the TGR region catchment. This study makes a novel contribution to the understanding of the transformation between sink to source of the WLFZ, and for targeted management of its ecological and environmental impact.