Unveiling Hidden Mercury and Methylmercury Sources: The Role of Submarine Groundwater Discharge in Coastal Lagoons

Mercury loads from Submarine Groundwater Discharge (SGD) may represent an overlooked source of methylmercury (MeHg) to the ocean, affecting human and ecosystem health. The SGD process involves the flow of fresh, saline, or mixed groundwater from coastal aquifers into the oceans classified in different components. Existing studies rarely report the fluxes supplied by the different SGD components; therefore, the relevance of SGD as a source of mercury remains unclear. We aimed to quantify SGD-driven mercury/methylmercury fluxes to the coast, focusing on the largest coastal lagoon in the western Mediterranean. We measured total dissolved mercury and MeHg in surficial and porewaters during the summer and autumn. Porewaters were enriched in total dissolved Hg and MeHg compared to the lagoon. Lagoon shore waters and porewaters with a high concentration of labile dissolved organic matter were prone to MeHg formation and thus had higher MeHg concentrations. The mercury input through SGD to the Mar Menor lagoon (4300 mmol year^-1) was similar to atmospheric deposition and 1 order of magnitude greater than the stream input. Among different components of the SGD, long-scale lagoon water recirculation dominated. These findings have substantial implications for the regional Hg budget and raise awareness of the importance of considering the different SGD components for an accurate estimation of SGD-based Hg input to the coastal ocean.