Declines in Riverine Mercury Export from China Driven by Water Pollution Controls

Although recent pollution controls in China have reduced anthropogenic mercury (Hg) release into air and water, their effects on riverine Hg export remain unknown. We measure Hg concentrations and isotopic compositions in 12 major Chinese rivers during 2021-2022 to constrain riverine Hg sources and evaluate trends and drivers of national riverine Hg export. We determine a two- to twentyfold decrease in THg levels in 2021-2022 (mean: 6.69 ± 4.74 ng L^-1) compared with previous observations since 2008. The median δ²⁰²Hg, Δ¹⁹⁹Hg, and Δ²⁰⁰Hg values of riverine THg are -1.22‰, 0.01‰, and 0.01‰, respectively. Hg isotope mixing model results reveal that soil erosion, wastewater discharge, and rainfall input contribute ∼51%, ∼33%, and ∼16%, respectively, to riverine Hg. The estimated riverine THg export in 2021-2022 was 45.1 ± 12.1 Mg yr^-1, ∼46% lower than the 2016 estimate. We combine source identification and atmospheric Hg observations to demonstrate that water pollution controls contribute ∼95% to recent riverine THg export declines. Using new riverine Hg isotope data, we estimate that riverine export contributes ∼67% and ∼45% to estuarine and continental shelf sediment Hg, respectively, in East Asia. These findings highlight the co-benefits of water pollution controls in reducing riverine Hg pollution, potentially mitigating Hg risks in coastal ecosystems.