Non-conservative mixing behaviors of mercury in subterranean estuary: Coupling effect of hydrological and biogeochemical processes and implications for rapidly changing world

Coastal ecosystem is an important zone of mercury (Hg) storage and hotspot of neurotoxic methylmercury (MMHg) production and bioaccumulation. The releases of Hg from coastal aquifer or subterranean estuary (STE) via submarine groundwater discharge (SGD) to coastal waters provide an important source of Hg from land to seas. However, the transport and biogeochemical transformation of Hg in STEs are less quantified. In this study, we documented total Hg (THg) and MMHg dynamics in two distinct STEs (a sandflat at an open coast versus a mudflat at a bay) during two different seasons (December versus August) in the temperate coast of northern China, and further applied the salinity-based conservative mixing model (CMM) to quantify the coupling effect of hydrological and biogeochemical processes on STE Hg cycle. Our field data presented large variations of THg and MMHg concentrations and%MMHg/THg of groundwater and sediment in both STEs over time and space. The CMM results clearly displayed substantial divergences of dissolved THg and MMHg from salinity in groundwater between sites and seasons, and the concentration and percent deviations in the Hg-rich mudflat were significantly higher than those in the Hg-poor sandflat. Our findings indicate the non-conservative mixing behaviors of Hg along the groundwater flow paths of both STEs, and the Hg-rich intertidal zone could be hotspot for the production and source of dissolved THg and MMHg to coastal waters via SGD. Our results provide field evidence to highlight that the hydrological shifts and biogeochemical processes collectively drive complex transport and biogeochemical transformation of Hg in STEs. The non-conservative mixing behaviors of Hg in STEs also highlight that, for more accurately calculating SGD-derived Hg fluxes to coastal seas, we need to carefully select the groundwater zonation of STE to better represent the output endmember. Our findings also address that human activities and climate change will profoundly alter the Hg biogeochemical cycle and toxicology in global coastal aquifers.