Article pubs.acs.org/est
Hydrological Controls on Methylmercury Distribution and Flux in a Tidal Marsh Hua Zhang,† Kevan B. Moffett,‡ Lisamarie Windham-Myers,§ and Steven M. Gorelick*,† †
Environmental Earth System Science, Stanford University, Stanford, California 94305-2115, United States Geological Sciences, University of Texas at Austin, Austin, Texas 78712, United States § United States Geological Survey, Menlo Park, California 94025, United States ‡
S Supporting Information *
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 finiteelement simulation of coupled surface water and saturatedunsaturated 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.
■
INTRODUCTION Mercury is a global pollutant that affects human and ecosystem health.1,2 Among its various chemical species, methylmercury (MeHg) is of most concern because it can be readily biomagnified in food webs.3,4 It is recognized that coastal environments significantly contribute to the net production of MeHg and global mercury cycling.5,6 Elevated environmental mercury exposure directly places coastal wildlife at risk and threatens human populations that rely on seafood as an important protein source.1,7 Potential sources of MeHg in coastal environments include atmospheric deposition and stomatal uptake,8−10 riverine loads,7 influx from ocean currents,11 production in estuarine sediments,12 and loads from coastal wetlands5 and groundwater.13 Although a generalized understanding of mercury cycling has been achieved, many essential aspects of the fate and transport of MeHg in coastal environments remain incompletely understood, among them the role of tidal marshes in coastal food web MeHg exposure.5,14,15 Tidal marshes possess many environmental characteristics that promote microbial methylation at the field scale,14,16 suggesting that tidal marshes are potential “hot spots” for MeHg production. However, it is less clear whether tidal marshes should be generally considered as sinks or sources of MeHg at the regional scale, as the exchange of MeHg between © 2014 American Chemical Society
tidal marshes and estuaries is highly variable across different regions and times.5,16−18 Comprehensive knowledge has developed over decades for the biological and geochemical factors that affect the production and transport of MeHg in tidal marshes.14,16,18−21 In contrast, few studies have examined the role of hydrological processes in controlling MeHg fluxes within or from tidal marshes. These hydrological controls include (i) exchange between marsh surface water and groundwater, (ii) interaction between marsh vegetation and soil water, (iii) water exchange between the marsh and estuary, and (iv) water exchange between the marsh and atmosphere. Such processes may not only directly contribute to the transport and accumulation of MeHg across a tidal marsh, but may also have indirect effects on MeHg flux through biogeochemical controls on methylation dynamics.17,22,23 For example, frequent inundation in saline environments can help maintain highly reduced sulfidic conditions that may limit mercury solubility and availability for methylation;24 in comparison, on the banks of tidal creeks where surface sediment may be exposed for significant periods, net Received: Revised: Accepted: Published: 6795
February 14, 2014 May 13, 2014 May 14, 2014 May 14, 2014 dx.doi.org/10.1021/es500781g | Environ. Sci. Technol. 2014, 48, 6795−6804
Environmental Science & Technology
Article
Figure 1. San Francisco Estuary tidal marsh study area and field sampling locations. Red site outline indicates the extent of the coupled surface watergroundwater numerical model domain. Bordered by an intertidal creek and levees, the studied marsh footprint (inside red box) has a total area of 0.022 km2 and an average elevation of 1.02 ± 0.52 m above mean sea level. Five tidal loggers and three piezometers were deployed at eight locations across the marsh to record the responses to tidal forcing of hydraulic head, temperature, and salinity, for use in understanding ambient marsh dynamics and parametrizing the hydrological model. Surface water was sampled hourly at five locations of tidal channels. Pore water was sampled in January at nine locations at depths of 8, 20, 32, and 44 cm and in September at eight locations at the depth of 26 cm.
■
demethylation or adsorption may become dominant.25 Therefore, it is important to consider the effects of hydrological controls on MeHg distribution and fluxes in tidal marshes. Current methods used to estimate marsh-scale MeHg budgets fall short in ways that more careful consideration of hydrological dynamics can help remedy. In several studies,26−29 tidal channel MeHg fluxes are derived as the product of water flux and MeHg concentrations and then converted to marshwide fluxes by dividing by the contributing area, which is either assumed to be the total marsh area28 or estimated as a function of the height and volume of flooding water.26 However, even with marsh-scale auxiliary information such as measured precipitation, evapotranspiration and freshwater input,29 or soil porosity and moisture content,26 the estimated water fluxes do not balance over a tidal cycle. This suggests a need to account for (i) flow bypassing main channels via small channels or the marsh surface and (ii) hydrological variability caused by the heterogeneous hydraulic and vegetation conditions in the tidal marsh. A few studies were undertaken to generate better physically based characterization of tidal marsh hydrology for MeHg flux analysis. For example, Pato et al.30 applied a 2-D vertically integrated hydrodynamic model over an entire marsh, and Black et al.31 measured radium isotopes to estimate submarine groundwater discharge. However, none of these prior studies has been able to address the combined effect of spatially variable plant−water interactions, 3-D variably saturated groundwater hydrology, and tidal flooding on MeHg distribution and flux in tidal marshes. To analyze the control of MeHg mobility by water exchanges among the marsh subsurface, surface, vegetation, atmosphere, and estuary, this study presents field measurements, geochemical analyses, and 3-D surface water-groundwater numerical modeling results for a tidal saltmarsh in the San Francisco Estuary over two tidal cycles.
MATERIALS AND METHODS Study Site. The San Francisco Estuary, California has a legacy of Hg contamination from historic gold mining in the Sierra Nevada and Hg mining in the Coast Range Mountains. Large amounts of inorganic Hg have been transported to the estuary where it resides in the sediments and is converted to MeHg through microbial processes.20,21 The estuary is spatially heterogeneous in terms of physical, chemical, and biological properties. In particular, unlike its northern counterpart, south San Francisco Bay receives less than 5% of the estuary’s total freshwater inflow, which is insufficient to develop gravitational circulation driven by salinity differences.24 Consequently, the southern estuary resembles a tidally oscillating lagoon, leading to long residence times for particle-associated contaminants such as Hg.24,32 The estuary is perhaps the most thoroughly studied estuary in the world with regard to Hg contamination.22,33 The study site is located in the Palo Alto Baylands Nature Preserve (37°27′54”N, 122°6′58”W) in the southern estuary (Figure 1). Based on data provided by the estuary’s Regional Monitoring Program for Water Quality (2005−2012) for this area,36 the ambient MeHg concentration is 0.65 ± 0.12 ng/g (mean ± SD, across all marsh locations) in nearby estuarine sediment and 0.05 ± 0.03 ng/L in estuarine water. This tidal marsh has been extensively studied for intertidal ecohydrological processes in recent years.34,35,37,38 Tides are mixed semidiurnal with a mean tide range of 1.94 m. Low tides retreat beyond the marsh, emptying the tidal channels. The higher of two daily spring tides generally floods the marsh, whereas neap tides may not exceed bank-full channel capacity. There is no fresh groundwater or surface water discharge into this marsh. Near-surface sediments (0−30 cm) are texturally clay, with 61.8% clay, 35.5% silt, and 2.6% sand, on average, and overlap the uppermost of a series of alluvial aquifers interbedded with marine clay.34,35 The marsh plain and tidal channels are 6796
dx.doi.org/10.1021/es500781g | Environ. Sci. Technol. 2014, 48, 6795−6804
Environmental Science & Technology
Article
Figure 2. MeHg concentrations in the tidal marsh: (a) comparison of MeHg concentrations in surface water and pore water; and (b) variation of pore water MeHg concentrations with depth. In box plots (left), the median is represented by the middle line of each box, hinges represent the 0.25 and 0.75 quartiles, and whiskers represent the minimum and maximum values. In line plots (right), the pocket means a persistent unsaturated zone in the shallow root zone sediments, discussed in the subsection Effects of Groundwater-Vegetation Interactions on MeHg in Pore Water. All of the results shown are based on samples from January 2011.
extensively vegetated, with Salicornia pacif ica, Spartina foliosa, and Distichlis spicata as dominant species.35 Field Measurement, Sample Collection, and Laboratory Analysis. Figure 1 shows the locations of field measurements from January 26 and September 26, 2011. Tidal loggers and piezometers were deployed across the marsh. Surface water was sampled in tidal channels. During high tide, whole-water samples were collected using combusted 1 L amber bottles submerged 10−15 cm below the water surface. During low tide, samples were collected from seepage flow running in the channel as a small trickle; details are provided in the Supporting Information (SI). Pore water was sampled under tension using field conditioned stainless-steel sippers and polyethylene syringes. Each surface water and pore water sample was immediately analyzed for pH, redox potential, alkalinity, and electrical conductivity in the field using portable meters. All samples were syringe-filtered immediately after collection and preserved in the field with 0.2% H2SO4 (v/v) and stored in a refrigerated cooler. MeHg and THg analyses were run within 6 months of collection. MeHg and THg contamination were assessed with intermittent field blanks (n = 6) over the 12 h sampling period. Field blanks were below detection for MeHg (DDL = 0.004 ng/L) and THg (DDL = 0.02 ng/L). Aqueous samples were analyzed for dissolved (
