Hydrological Controls on Methylmercury Distribution and Flux in a Tidal Marsh

Abstract

The San Francisco Estuary, California, contains mercury (Hg) contamination originating from historical regional gold and Hg mining operations. We measured hydrological and geochemical variables in a tidal marsh of the Palo Alto Baylands Nature Preserve to determine the sources, location, and magnitude of hydrological fluxes of methylmercury (MeHg), a bioavailable Hg species of ecological and health concern. Based on measured concentrations and detailed finite-element simulation of coupled surface water and saturated-unsaturated groundwater flow, we found pore water MeHg was concentrated in unsaturated pockets that persisted over tidal cycles. These pockets, occurring over 16% of the marsh plain area, corresponded to the marsh root zone. Groundwater discharge (e.g., exfiltration) to the tidal channel represented a significant source of MeHg during low tide. We found that nonchannelized flow accounted for up to 20% of the MeHg flux to the estuary. The estimated net flux of filter-passing (0.45 μm) MeHg toward estuary was 10 ± 5 ng m(-2) day(-1) during a single 12-h tidal cycle, suggesting an annual MeHg load of 1.17 ± 0.58 kg when the estimated flux was applied to present tidal marshes and planned marsh restorations throughout the San Francisco Estuary.

Effects of unstable flow on solute transport in the marsh soil and exchange with coastal water

“…While a number of studies have been carried out to examine solute transport in creek water [Haines, 1979;Jordan et al, 1983;Childers et al, 2000], fewer investigations on the subsurface transport process have been conducted. Recent field investigations, laboratory experiments, and numerical modeling have revealed the complexity of pore water flow in the marsh soil under the influences of tidal fluctuations [Wilson and Gardner, 2006;Xia et al, 2010;Xin et al, 2010;Wilson and Morris, 2012;Zhang et al, 2014], evapotranspiration [Ursino et al, 2004;Marani et al, 2006], soil property variations [Gardner, 2007;Xin et al, 2009;Wilson and Morris, 2012;Xin et al, 2012], inland fresh groundwater input [Nuttle and Harvey, 1995;Gardner et al, 2002], topography [Moffett et al, 2010;Xin et al, 2011;Moffett et al, 2012;Xin et al, 2012], and salinity differences between surface water and pore water [Shen et al, 2015]. In particular, many studies focussed on tide-induced pore water circulation near the creek, as a strong mechanism for solute transport through the marsh soil and exchange with creek water [Wilson and Gardner, 2006;Xin et al, 2009].…”

Effects of salinity variations on pore water flow in salt marshes

“…These ecological functions of marsh systems are affected by various physical and biogeochemical processes, among which pore water flow and associated solute transport play an important role in determining the soil condition and salinity distribution for marsh plant growth, and the material exchange between the marsh sediment and seawater [Gardner, 2007;Marani et al, 2006;Ursino et al, 2004;Wilson and Gardner, 2006;Wilson et al, 2014;Xin et al, 2013b;Zhang et al, 2014].…”

Relating salt marsh pore water geochemistry patterns to vegetation zones and hydrologic influences

“…In the study by Zhang et al . at this field site [ Zhang et al ., ], the methylmercury concentration was also found to be more elevated in persistently unsaturated areas of the marsh compared to areas that remained saturated. This was likely due to a different mechanism, however, whereby labile organic root exudates promote Hg methylation via changes to the local rhizospheric microbial metabolism [ Windham‐Myers et al ., ], which process was perhaps more active in the persistently unsaturated areas than in the saturated areas of the marsh.…”

“…Such variations might take the form of horizontal marsh pore water differentiation between vegetation zones or vertical geochemical differentiation within the root zone. The likelihood for such geochemical differentiation to exist is strongly suggested by empirical data and modeled values of the ecohydrological zonation of salt marshes [ Moffett et al ., ; Cao et al ., ; Xin et al ., ; Wilson et al ., ] and of the role of both rooting depth and sediment hydraulic properties on the concentration of aqueous constituents such as methylmercury [ Zhang et al ., ]. These studies also suggest that such geochemical differentiation, if present, may correspond to a combination of the spatial patterns of vegetation zonation and of hydrologic influences [ Moffett et al ., ; Wilson et al ., ].…”