Mercury speciation and transport at the land-sea margin

Groundwater discharge and seawater recirculation along the coastal margin, referred to as submarine groundwater discharge (SGD), deliver dissolved chemicals, such as nutrients, to nearshore seawater and are essential to the health of many coastal ecosystems. However, urban and agricultural development can impair coastal aquifers and SGD is now recognized as a potential vector for transporting terrestrial contaminants to the ocean. Recently, SGD has been identified as a source of mercury to coastal habitats. Although data quantifying the SGD-flux of mercury and processes that influence mercury bioavailability at the freshwater-saltwater interface are limited, studies confirm that, on a local scale, groundwater-seawater interaction can be an important component of nearshore mercury biogeochemical cycling and budgets. To construct a more comprehensive view of mercury dynamics in response to tidal perturbations we collected hourly groundwater and seawater samples for approximately half a tidal cycle (i.e., 12-hour period) in a range of environments. At most locations we measured total mercury (HgT) as well as monomethylmercury (MMHg), the more toxic, bioaccumulative species of mercury. In coastal systems, MMHg is predominantly generated by anaerobic bacteria; therefore, the interface where oxygen-depleted, nutrient-rich groundwater mixes with seawater is a likely zone of MMHg production. Our results support this hypothesis -- furthermore, groundwater discharge appears to transport MMHg to nearshore habitats. The bioavailability of mercury in seawater may also be enhanced by Hg(II) partitioning into the dissolved phase when groundwater discharge is greatest, typically at or near low tide; however, the mechanism driving this phenomenon is not clear. Zones of increased MMHg in groundwater depth profiles from a coastal lagoon site correlated with increased temperature and dissolved organic carbon, and decreased dissolved oxygen and nitrate, providing evidence of MMHg production by anaerobic bacteria. MMHg concentrations in coastal lagoons (1 to 5 pM) were elevated relative to coastal groundwater (∼0.5 to 1 pM) and seawater (< 0.2 pM), suggesting coastal lagoons represent an additional source of bioaccumulative mercury at the land-sea margin.