LINKING SUBMARINE GROUNDWATER DISCHARGE TO MERCURY BIOGEOCHEMICAL CYCLING AT THE COASTAL MARGIN

Submarine groundwater discharge (SGD) is a mixture of terrestrial groundwater and recirculated seawater that discharges along the coastal margin. SGD provides nutrients to nearshore marine waters and can be critical to supporting productive coastal ecosystems. However, studies across the globe have documented contamination in coastal groundwater due to anthropogenic development, and along many shorelines SGD introduces excess nutrients and heavy metals to the ocean. This impact is of particular concern when considering mercury (Hg), which can be converted to an organic bioaccumulative form by anaerobic bacteria in low oxygen environments. Because groundwater often contains low dissolved oxygen and high nutrients relative to coastal seawater, redox and nutrient gradients at the groundwater-seawater interface can support anaerobic bacterial respiration. These conditions promote the production of monomethylmercury (MeHg), which is primarily formed by sulfate-reducing bacteria. From previously published literature, MeHg concentrations in coastal groundwater have been documented for 8 sites globally (France, Israel, Korea, USA), with MeHg concentrations in groundwater up to 8-times greater than in overlying seawater. Among these sites, MeHg in SGD ranged from <0.04 to 1.2 pM, with the highest concentrations documented in coastal slough and lagoon environments at sites with no Hg point sources. In contrast, sites where groundwater moves through karst topography, MeHg in SGD was consistently below the detection limit (<0.04 pM). We append these 8 sites with results from Hood Canal in Puget Sound, WA. SGD-derived MeHg at Hood Canal remained low (≤ 0.11 pM) throughout a ~4 m falling tide. We hypothesize that like the karstic system, the rapid flow of groundwater through hydraulically conductive glacial deposits limits anaerobic respiration, thereby limiting the production of MeHg. The Puget Sound study provides an important contribution to our understanding of the interplay between watershed conditions, subsurface geology, and climate with respect to global Hg biogeochemical cycling in nearshore marine environments.