Mercury dynamics in sulfide-rich sediments: Geochemical influence on contaminant mobilization within the Penobscot River estuary, Maine, USA

Abstract

Research concerning the fate and biogeochemical cycling of mercury (Hg) within coastal ecosystems has suggested that microbially mediated diagenetic processes control Hg mobilization and that ligands with strong affinity for Hg, such as dissolved inorganic sulfide (S(-II)) and dissolved organic matter (DOM), control Hg partitioning between the dissolved and particulate phases. We have studied total Hg cycling in the sediments of the Penobscot River estuary using a combination of equilibrium porewater samplers and kinetic modeling. The Penobscot estuary has been subject to Hg contamination from multiple industries including a recently closed chlor-alkali production facility. The Hg concentration within the estuary surface sediments ranges from 1.25 to 27.5 nmol Hg g⁻¹ sediment and displays an association with sediment organic matter and a concentration maximum within 3 cm of the sediment–water interface (SWI). Porewater profiles for the Penobscot estuary are divisible into three kinetically discrete intervals with respect to Hg dynamics. Beginning at depth in the sediment and moving upward toward the SWI we have defined: (1) a zone of net Hg solubilization at depth, with a zero-order net Hg production rate $(R_{\text{net}}^{{\text{Hg}}_{\text{T}}})=3.7''–''5.2\times {10}^{-20}{\text{mol cm}}^{-3}{\text{s}}^{-1}$, (2) a zone of net Hg consumption within the zone dominated by FeS_(s) precipitation with $R_{\text{net}}^{{\text{Hg}}_{\text{T}}}=-0.75\mathrm{to}-1.4\times {10}^{-20}{\text{mol cm}}^{-3}{\text{s}}^{-1}$, and (3) a zone of net diffusive transfer within the vicinity of the SWI. Zone 1 is characterized by dissolved S(-II) concentrations ranging from 400 to 500 μM. Equilibrium modeling in this zone suggests that inorganic S(-II) plays the dominant role in both mobilization of sediment-bound Hg and complexation of dissolved Hg. In zone 2, FeS_(s) precipitation occurs concomitant with Hg consumption. Net transfer within zone 3 is consistent with the potential for ligand-mediated Hg efflux across the SWI. S(-II)-mediated Hg mobilization at depth in Penobscot estuary sediments suggests a broadening of the depth interval over which biogeochemical Hg cycling must be examined. Our results also show that, while estuary sediments act as a net sink for particulate Hg inputs, they may also function for a considerable time interval as a source of dissolved Hg.

Introduction

Whereas the dominant global transport mechanism for mercury (Hg) is through atmospheric dispersion (Mason et al., 1994), other mechanisms increase in relative importance within coastal zones. Through factors including the erosion of floodplain soils and the traditional waterfront siting of industrial facilities, rivers serve as major transport conduits for both particulate matter and a range of particle-reactive contaminants. The potential delivery of river-borne contaminants to the coastal ocean is further mediated via the chemical, physical and biological interactions that occur within estuaries (Coquery et al., 1997). Studies of Hg transport across the estuaries of large, industrial rivers have demonstrated that while fluvial Hg transport may be significant (exceeding 3 nmol g⁻¹ suspended sediment), Hg is effectively trapped and recycled within the estuarine zone (Guentzel et al., 1996, Stordal et al., 1996, Turner et al., 2001, Laurier et al., 2003).

In stressing the storage potential of estuaries, it is important to consider whether these environments ultimately serve as long-term sinks or sources of Hg contamination. Specifically, it is important to assess the extent to which estuarine storage may affect the speciation and biological availability of the introduced Hg pool. The relationship between storage and speciation is crucial as studies of Hg transfer within food webs suggest that Hg is biologically available, and that methylmercury (MeHg), the toxic organic species of Hg, biologically magnifies (Mason et al., 1996, Pickhardt et al., 2002). Moreover, as inorganic Hg (Hg_i) is the geochemical precursor to MeHg, Hg speciation may strongly influence both the rate and extent to which methylation occurs (Gilmour et al., 1998).

In coastal marine sediments it has been proposed that solid-phase organic matter controls the partitioning of Hg between sediment and aqueous phases (Hammerschmidt et al., 2004, Hammerschmidt and Fitzgerald, 2006). This control is defined by correlations between (1) the distribution coefficient for Hg_i versus total sediment organic matter (K_DHgi versus LOI; Hammerschmidt and Fitzgerald, 2006) and (2) the distribution coefficient for Hg_i versus the distribution coefficient for organic matter (K_DHgi versus K_DOM; Hammerschmidt et al., 2004). Following this model, the partitioning of Hg between sediment and aqueous phases must result from the partitioning of Hg-complexing organic ligands.

Other research has documented an apparent sulfide-mediated control on porewater total Hg (Hg_T). Benoit et al. (1998) observed that along an upper to lower estuary gradient, shallow (<4 cm) porewater Hg_T increased concomitantly with increasing porewater S(-II). They concluded that inorganic ligands such as S(-II) influence the porewater Hg concentration by increasing porewater Hg_T and decreasing porewater MeHg. As conditions within estuaries and salt marshes are conducive to the production of S(-II) and DOM, it is important to assess the extent to which depth-dependent variations in controlling ligands may influence the porewater concentration and potential bioavailability of Hg_T. Such work is crucial as estuaries and salt marshes may prove adept at facilitating geochemical transformations (i.e., precipitation, soluble complexation, and microbial methylation) that affect long-term Hg_T storage.

This paper examines the speciation and mobilization potential of Hg_T within sediments of the Penobscot River estuary in Maine, USA (Fig. 1). The Penobscot River drains a watershed of approximately 19,350 km² and represents the second largest river system in New England. The lower Penobscot River is defined by a long narrow estuary (mean width <0.75 km), with measurable tidal influence extending 35 km upriver to the city of Bangor. Annual river discharge varies seasonally between ∼100 m³  s⁻¹ in the summer to ∼1000 m³ s⁻¹ in the spring and may reach 2500 m³ s⁻¹ during exceptional spring freshets. As well as upriver papermill activity, several potential point sources of Hg pollution exist within the estuary, including an operating waste incinerator and a recently (2000) closed chlor-alkali production facility. Sediment Hg concentration upstream of the limit of tidal influence ranges between 0.25 and 0.50 nmol Hg g⁻¹ dry wt. sediment (defined throughout the paper as g⁻¹) (Smith, 1998), comparable with the freshwater reaches of other large New England rivers (Morgan, 1998, Livingston, 2000). As this sediment Hg concentration appears consistently across a range of New England rivers, it may represent the background Hg concentration resulting from riverine industrial discharge and regional atmospheric deposition. Surface sediment Hg concentrations in the Penobscot estuary generally range between 1.25 and 27.5 nmol Hg  g⁻¹ with an extreme hot-spot (2300 nmol Hg  g⁻¹) within the chlor-alkali plant discharge zone (Morgan, 1998). As sediment Hg concentration exceeds, in places, 3.5 nmol Hg g⁻¹ (i.e., the NOAA-defined median effect burden), the Penobscot estuary is a site of potential biological concern.