Mercury Speciation in Saltmarsh Porewater, Groundwater, and Surface Water: Cape Cod, MA

Abstract:

We are assessing variability in mercury species in porewater, groundwater, and surface water at Sage Lot saltmarsh along the south shore of Cape Cod, MA. This relatively undisturbed coastal marsh is part of the Waquoit Bay National Estuarine Reserve and is surrounded by undeveloped wooded uplands. The marsh substrate is comprised of an organic-rich peat layer that reaches a maximum thickness of ~2 m at the water’s edge and overlays the regional sand groundwater aquifer. We collected porewater depth profiles within the peat layer along an approximately 120 m transect perpendicular to the shoreline (see figure). Total mercury (HgT) concentrations in porewater ranged from 2 to 6 pM in filtered (< 0.2 µm) water and from 10 to 20 pM in unfiltered water. Mercury concentrations in groundwater collected from a well installed about 20 m upgradient from the marsh were an order of magnitude higher (~ 130 pM and 170 pM in filtered and unfiltered water, respectively). Although ~70% of this mercury occurred in the presumably mobile (< 0.2 µm) fraction, the elevated groundwater concentrations did not appear to enhance HgT within the peat layer. Groundwater in sand immediately below the sand-peat interface, however, typically had HgT concentrations similar to those observed in peat, suggesting porewater infiltration to the underlying aquifer. We also collected time series surface water samples over tidal flooding and draining cycles. Filtered and unfiltered surface water HgT were similar to concentrations observed in peat porewater and did not change in response to the tidal cycle. Preliminary data suggests methylmercury (MeHg) in filtered porewater was about 1 pM (i.e., ~20% MeHg), indicating enhanced mercury methylation within the anoxic peat layer. If the similarity between peat porewater and marsh surface water HgT concentrations is a result of hydraulic exchange between the porewater and offshore water, the saltmarsh may be an important source of bioavailable mercury to the local ecosystem.