Environ. Sci. Technol. 2009, 43, 415–422
Relative Importance of Atmospheric and Riverine Mercury Sources to the Northern Gulf of Mexico GLENN E. RICE,* DAVID B. SENN, AND JAMES P. SHINE Harvard School of Public Health
Received March 7, 2008. Revised manuscript received July 21, 2008. Accepted October 20, 2008.
A box model was developed to quantify the major sources and dominant fates of inorganic mercury (Hg) in the Mississippi River-influenced area of the northern Gulf of Mexico (nGOM). Riverine (75%) and direct atmospheric deposition (25%) deliver 9.7 t Hg y-1 to this productive fishery; most (80%) accumulates in bottom sediments where it can be methylated and enter foodwebs. Although riverine inputs dominate atmospheric deposition, 75% of the riverine sediment-associated Hg accumulates in only ∼8% of the study area. Atmospheric deposition can explain most of the Hg accumulating in sediments of the remaining area. Considering the differences in temporal responsiveness of riverine (centuries) and atmospheric (years) Hg inputs to anthropogenic emissions changes, the spatial limits of the riverine Hg source and the potential dominance of atmospheric deposition over large areas could have implications for the timing of benefits from policies reducing anthropogenic Hg emissions.
Introduction Globally, marine fish consumption is the dominant human exposure pathway for methylmercury (MeHg) (1, 2), a fetal neurotoxicant (3). While MeHg typically comprises only a small fraction of the total mercury (Hg) in marine systems (4-7), foodwebs can biomagnify MeHg concentrations in top predator fish to levels 1 000 000 times higher than those in the water column (4). Hg dynamics in coastal and continental shelf ecosystems deserve special attention because these areas receive inputs from both atmospheric deposition (HgATM) and terrestrial (HgTER) sources, and they account for a disproportionate fraction of marine productivity (8, 9). In addition, MeHg that enters coastal foodwebs has the potential to “bioadvect” to open ocean and migratory species via overlapping foodwebs (4, 10). Thus characterizing the sources and fate of Hg in coastal and continental shelf systems is an essential component of accurately evaluating the benefits of reducing anthropogenic Hg emissions (11). Quantifying the relative magnitudes of HgATM and HgTER input rates to coastal ecosystems is important because their response times following changes in atmospheric Hg emissions can differ by orders of magnitude. HgATM deposition rates can respond rapidly to changes in emissions because reactive gaseous Hg (RGM) and elemental Hg (Hg0) have atmospheric half-lives of less than a month and less than a year, respectively (12). Hg deposited in the terrestrial ecosystem is subject to watershed processes involving runoff, * Corresponding author phone: (513) 569-7813; fax: (513) 4872539; e-mail:
[email protected]. 10.1021/es800682b CCC: $40.75
Published on Web 12/09/2008
2009 American Chemical Society
erosion, and riverine transport prior to entering coastal regions. Due to the particle-reactivity of Hg, these processes will respond over much longer time-scales, and could extend through multiple centuries depending on watershed characteristics (13-15). Most of the MeHg in coastal and continental shelf systems is produced in situ (4, 6, 16). Numerous studies point to Hg methylation in coastal sediments as a major source of MeHg (6, 7, 16, 17), although Hg methylation in water columns might contribute (18, 19). Therefore, the fate of inorganic Hg inputs, primarily whether most of the Hg remains in the system (i.e., the sediments) or is advected offshore or evaded, needs to be determined. Once produced, MeHg biomagnifies in benthic, epibenthic, and pelagic foodwebs (6, 20) leading to human exposures and, potentially, adverse health effects. Using a mass balance box model approach, this paper examines the relative importance of Hg sources and sinks in the northern Gulf of Mexico (nGOM) near the Mississippi (MR) and Atchafalaya River (AR) outlets, a productive fishery (21-24). Data from several biogeochemical studies provided model inputs. The model was used to explore the following questions: What is the dominant fate of Hg entering the nGOMsaccumulation in sediments, evasion, or advection to the larger GOM? What are the contributions of directly deposited HgATM and HgTER delivered by the rivers to the nGOM and how do they vary spatially? Finally, considering regulatory efforts to reduce atmospheric Hg emissions and response times associated with Hg inputs from direct atmospheric deposition and the rivers, what policy implications arise from this analysis of nGOM?
Methods Setting and Approach. The Mississippi and Atchafalaya River System (MR/AR) drains 40% of the conterminous United States (24). About 500 km upstream of the MR delta, approximately one-quarter (15-29%) of the MR flow is diverted to form the AR (25). Both the AR and the MR flow seaward entering the nGOM along the Louisiana coast (Figure 1). While the main outlet of the MR discharges into deep outer continental shelf waters near the shelf break, the AR discharges into a shallow bay adjacent to the broad shallow inner continental shelf. The river plumes generally advect westward along the coast (26) and their discharges of fresh water and suspended sediments (SS) dominate the coastal margin. The nGOM water column was used as the control volume of the box model. The boundaries of the control volume were the sediment:water interface extending to the coast and river outlets; the air:water interface; and the water column boundary between the study area and the larger Gulf of Mexico, defined as the vertical projection of the 2500 m depth contour. Figure 2 presents key components of the box model. Suspended sediment associated Hg (HgSS) was the dominant form of Hg (>90%) entering the system from the rivers; therefore, a well-constrained sediment mass balance was a crucial component of the Hg mass balance, and also is presented in detail. SS Inputs and Outputs to the nGOM Water Column. Sediment loads carried by the MR and AR have been regularly monitored since the mid-20th century (27, 28). These data point to seasonal and interannual variability in sediment loads and highlight a long-term decline (∼50%) beginning in 1953 that is attributed to dam construction on tributaries of the MR (27). We used SS load estimates between the years 1980-1992 to calculate average SS loads for the MR and AR (Supporting Information Section S1), because this time period VOL. 43, NO. 2, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Northern Gulf of Mexico Study Area. The study area is divided into six zones that are analogous to those in Gordon and Goni (33): Eastern Atchafalaya Bay comprises Zone 1; the inner shelf (10 to 200 to
