Dissolved Total Mercury and Methylmercury Fluxes to the Coastal Ocean via Non-Hydrothermal Submarine Groundwater Discharge

Abstract

Coastal ecosystems are hotspots for biogeochemical cycling of mercury (Hg) and human exposure to neurotoxic methylmercury (MMHg). However, coastal aquifer Hg biogeochemistry remains understudied at a global scale, and the contribution of submarine groundwater discharge (SGD) to global coastal Hg budgets has long been overlooked. In this study, based on a compilation of 44 new observations in the coastline of China and 20 existing observations from around the world, we investigate the distribution and transformation of Hg within global coastal aquifers and estimate non-hydrothermal SGD-derived dissolved total Hg (DTHg) and MMHg (DMMHg) flux to the global coastal ocean. We find that almost all coastal aquifers are identified as non-conservative for both DTHg and DMMHg, and the productive behavior is the most prevalent. We estimate that non-hydrothermal SGD-derived DTHg (Monte Carlo mean = 901.8; median = 339.0; interquartile range = 46.6–1005.3 Mg yr⁻¹) rivals riverine discharge, and is ∼3 times and 14 times greater than atmospheric deposition and porewater diffusion from coastal sediments to the global coastal ocean. Notably, non-hydrothermal SGD is identified as the known largest external source of DMMHg (mean = 77.3; median = 43.4; interquartile range = 14.4–97.7 Mg yr⁻¹) to the global coastal ocean. Hydrothermal SGD systems with elevated Hg concentrations constitute important sources of Hg to specific coastal environments, but their contribution to the global coastal Hg budgets represents a critical knowledge gap. We highlight that SGD-derived Hg discharge exerts broader and a more direct toxicological pressure on global coastal ecosystems under global change.