Evaluation of Hg methylation in the water-level-fluctuation zone of the Three Gorges Reservoir region by using the MeHg/HgT ratio

In the recent decade, the hydroelectric reservoir is identified as a methylmercury (MeHg) hotspot and gained much attention. The artificial water level management in the Three Gorges Reservoir (TGR) in China formed a water-level-fluctuation zone (WLFZ) undergoing flooding drying rotations annually. However, the mercury (Hg) methylation and major geochemical driving factors at different elevations in the WLFZ remain unclear. Here we use total Hg (Hg-T) normalized MeHg (MeHg/Hg-T ratio) to evaluate Hg methylation degree in a one-year field study at 155, 165 m elevations in the WLFZ and with > 175 m elevation as the reference. Results demonstrate that MeHg/Hg-T ratio at the WLFZ could reach 4.1% in soils, and both 155 and 165 m elevations have a higher Hg methylation degree than the > 175 m elevation. However, the differences in MeHg/Hg-T ratios both in soils and waters between 155 and 165 m elevations are not significant. This indicates the influence of different submerging periods on the MeHg/Hg-T at the WLFZ elevations is not observed. The significant correlation between the MeHg/Hg-T ratio and soil organic carbon (SOC) content implies a MeHg retention in re-exposed soils after flooding. Decoupling of MeHg/Hg-T ratios between submerged soil and overlying water are found at both elevations and therefore make MeHg/Hg-T in waters alone cannot be used to evaluate Hg methylation degree in this study. The calculation of Hg T and MeHg partitioning coefficient (K-d) found an immobilization of MeHg by submerged soils at the WLFZ during the flooding period. Major geochemical factors, determined through principal component analysis (PCA), in affecting Hg methylation are the redox cycling of sulfur and the distribution of organic matters in the WLFZ.