Article pubs.acs.org/est
A Fluvial Mercury Budget for Lake Ontario Joseph S. Denkenberger,* Charles T. Driscoll, and Edward Mason† Department of Civil and Environmental Engineering, 151 Link Hall, Syracuse University, Syracuse, New York 13244, United States
Brian Branfireun Department of Biology and Centre for Environment and Sustainability, Room 2064, Biology and Geological Sciences Building, University of Western Ontario, London, ON N6A 5B7, Canada
Ashley Warnock Department of Biology and Centre for Environment and Sustainability, Room 5 Biotron, University of Western Ontario, London, ON N6A 5B7, Canada S Supporting Information *
ABSTRACT: Watershed mercury (Hg) flux was calculated for ten inflowing rivers and the outlet for Lake Ontario using empirical measurements from two independent field-sampling programs. Total Hg (THg) flux for nine study watersheds that directly drain into the lake ranged from 0.2 kg/yr to 13 kg/yr, with the dominant fluvial THg load from the Niagara River at 154 kg/yr. THg loss at the outlet (St. Lawrence River) was 68 kg/yr and has declined approximately 40% over the past decade. Fluvial Hg inputs largely (62%) occur in the dissolved fraction and are similar to estimates of atmospheric Hg inputs. Fluvial mass balances suggest strong in-lake retention of particulate Hg inputs (99%), compared to dissolved total Hg (45%) and methyl Hg (22%) fractions. Wetland land cover is a good predictor of methyl Hg yield for Lake Ontario watersheds. Sediment deposition studies, coupled atmospheric and fluvial Hg fluxes, and a comparison of this work with previous measurements indicate that Lake Ontario is a net sink of Hg inputs and not at steady state likely because of recent decreases in point source inputs and atmospheric Hg deposition.
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INTRODUCTION Emissions of mercury (Hg) occur globally in the form of primary anthropogenic emissions from waste incineration, industrial processes, mining, and the burning of fossil fuels, from natural primary emission sources, and from the reemission of previously deposited Hg.1,2 The increased global Hg pool that results from current and legacy anthropogenic emissions is chiefly responsible for elevated Hg levels in aquatic ecosystems.3,4 Methyl Hg (MeHg) is of particular interest, as it is the form which most readily bioaccumulates and contributes to exposure of biota.2 In wetlands, Hg methylation is supported via anaerobic pathways.5 As a result, higher ratios of MeHg to total Hg have been observed in water draining watersheds with substantial wetland coverage. Fish consumption advisories due to Hg occur throughout the United States (US) and Canada.6,7 These advisories emphasize the need for quantitative knowledge regarding Hg fate and transport in rivers and lakes. The concentration, speciation, and fluxes of Hg in tributaries are commonly governed by watershed processes.8,9 Hurley and others10 note that in watersheds characterized by urban, agriculture, or forest lands, the oxidized form of Hg (i.e., Hg[II]) is the dominant fraction and is most commonly associated with particulate or dissolved organic matter. As a result of the particulate association, high discharge events that © 2014 American Chemical Society
mobilize particulate matter serve as an important transport mechanism for Hg. Resuspension of sediment-bound Hg can also represent an internal source of Hg to lake waters.11 The Great Lakes region is an important economic and cultural resource.12 Elevated levels of emissions and deposition of Hg occur in the region and worldwide.13−18 There is a large body of work on the impacts of Hg on some of the Great Lakes (e.g., refs 10, 19−22). However, very few Hg studies have focused on Lake Ontario. Lai et al.23 investigated atmospheric exchange of Hg to and from Lake Ontario but did not consider the fluvial component of Hg transport. Riverine inflows may represent a significant portion of the Hg loads to nearshore zones and also may contain elevated bioavailable MeHg as a result of watershed processes. Ethier et al.24 developed a model of transport and transformations of Hg in Lake Ontario and found that the concentration of Hg in inflow water has one of the largest impacts on the simulated Hg mass balance. Despite this importance, fluvial inputs and export of Hg from the lake have not been empirically characterized; sediment Hg data are Received: Revised: Accepted: Published: 6107
January 9, 2014 April 29, 2014 May 1, 2014 May 1, 2014 dx.doi.org/10.1021/es500128f | Environ. Sci. Technol. 2014, 48, 6107−6114
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separation, and detection by CVAFS.31−33 Particulate Hg (THgP) concentrations, flux, and yield were calculated as the difference between whole- and filtered-sample results; consequently, some reported THgP concentrations are below the THg detection limit (0.20 ng/L). Ancillary water quality measures were analyzed using Standard Methods:34 for DOC, the persulfate−ultraviolet oxidation method (5310C) and for TSS, method 2540D. All analytical observations are provided in Tables S2−S12, Supporting Information. For calculation of means, medians, standard deviations, and mass balances, nondetect samples were assigned a value of onehalf the detection limit of 0.20 ng/L for THg and 0.02 ng/L for MeHg. This approach was used to fill data gaps for less than 9% of all Hg analyses. The mass transport of Hg species was estimated by coupling fluvial concentration measurements with discharge data available from US Geological Survey (USGS) and Environment Canada stations. Calculations were performed using FLUX32 software, version 3.03.35,36 FLUX32 utilizes daily discharge rates (where available) in conjunction with analyte concentrations to estimate material fluxes. Multiple calculation methods are provided in the software package that may be chosen from to best suit the flow and concentration characteristics of each tributary. Continuous flow data are available for most of the tributaries in this study, with the exceptions of the St. Lawrence River (weekly data only) and the Trent River. Daily discharge for the Trent River was composited from a gauged subwatershed and discharge data provided by Ontario Power Generation. Estimates of fluxes for Hg species in this study generally had only small relative variations among most calculation methods (disregarding Twenty Mile Creek, which has a small watershed and flashy flow characteristics; Table S1, Supporting Information). Flux estimates for Twenty Mile Creek varied considerably (i.e., over 100%) due to differences in how each calculation method handles the concentration−flow relationship. For each study tributary, the calculation method producing the lowest coefficient of variation (as calculated by FLUX32) was reported. It is appropriate to select different calculation methods for different rivers, as concentration−flow relationships vary because of watershed characteristics. Although quantification error exists in all the FLUX32 methods, the load values provide, at a minimum, best estimates of THg loads. Retention coefficients (R) for Lake Ontario were estimated for all analytes using the following equation:
also lacking. A recent review of the literature found record of only three dated sediment cores from Lake Ontario that include Hg analysis.25 Hg trends observed in these cores indicate that while Hg deposition has decreased in recent years, it remains elevated in Lake Ontario relative to values expected in lakes without point sources. French et al.26 suggested that while slight declines are occurring in fish tissue Hg concentrations in Lake Ontario, overall levels remain elevated, lending support for the development of a more robust empirical budget for Hg. The objective of this study is to improve the understanding of Hg dynamics for Lake Ontario by quantifying fluvial loading of Hg, previously identified as a significant, but unquantified, component of the Hg mass balance. These critical data are then integrated with previous observations to develop a comprehensive Hg budget for the lake.
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EXPERIMENTAL SECTION Lake Ontario is the fourth largest of the Great Lakes with respect to volume (1640 km3) and the fifth largest with respect to surface area (18 960 km2). The direct watershed area of the lake is 64 030 km2, for a watershed area to lake surface area ratio of 3.4 (the highest of the Great Lakes). The mean hydraulic residence time is estimated at 6 years, which is the second shortest of all the Great Lakes.27 Water samples were collected twice per month from ten major rivers discharging into Lake Ontario and from the outlet at the St. Lawrence River. Rivers sampled included the Niagara, Twenty Mile Creek, Credit, Humber, Ganaraska, and Trent in Canada and the St. Lawrence, Black, Salmon, Oswego, and Genesee in the US (Figure S1, Supporting Information). The volume discharged to Lake Ontario by the tributaries studied represents approximately 90% of the flow at the St. Lawrence River, with the Niagara River contributing approximately 91% of the measured tributary inflow during 2009. Samples were collected over a broad range of recorded flows, with no emphasis on any specific flow regime (Figure S2, Supporting Information). Mean discharge during the sampling period was similar to the period of record for the US tributaries, while the Canadian tributaries averaged approximately 30% higher flows during the study period than the long-term mean (Table S1, Supporting Information). The Canadian tributaries were sampled for Hg analysis from June 2008 to December 2009, whereas the US tributaries were sampled from June 2009 to May 2010. Discharge for all tributaries was generally higher in 2008 than either 2009 or 2010, supporting the higher relative flows recorded in the Canadian rivers over the study period. Sample collection was performed using trace metal clean protocols.28 Either in the field or upon arrival at the laboratory, 250−500 mL water sample aliquots were filtered through precleaned 0.45 μm filters. Whole and filtered water samples were then preserved using 4 mL/L 11.6 M hydrochloric acid. Sample analysis included ancillary chemistry [e.g., total suspended solids (TSS), dissolved organic carbon (DOC)]. All analyses of US river samples were conducted at Syracuse University; analyses of Canadian tributary samples were conducted at the University of Toronto. Both laboratories used methods for Hg analysis that were consistent with EPA Methods 1630 for THg and 1631 for MeHg. Total Hg (THg) analysis was conducted on whole and filtered water samples using cold vapor atomic fluorescence spectrometry (CVAFS29,30). MeHg determination was performed on whole and filtered water samples via aqueous phase ethylation with sodium tetraethylborate [NaB(C2H5)4], purge and trap
R = (flux in − flux out)/(flux in)
(eq 1)
Note that R values were calculated on a solely riverine (Rriverine) basis, including dissolved (Rdissolved), particulate (Rparticulate), and MeHg fractions (Rmethyl), on an atmospheric basis (Ratmos), and on an atmospheric plus riverine (Rtotal) basis. Analysis of watershed land cover characteristics was performed with a geographic information system (GIS) approach and is summarized elsewhere and in Table S13, Supporting Information. 37 Statistical analysis was performed using SigmaPlot 11.0,38 and alpha was set to 0.05 for significance in all statistical tests.
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RESULTS AND DISCUSSION Total Mercury Flux. The yearly THg flux estimates were highly variable among the study rivers (Figure 1; Table 1). While some of this variation can be attributed to differences in Hg concentration, the major driver of Hg loading rates to the lake is river discharge. Although the Niagara River has the 6108
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Table 2. Summary of Flux Estimates and Retention Coefficients for Lake Ontario basis riverinea
atmospheric
total a
flux in (kg/yr)
flux out (kg/yr)
122 74 22 197 233
67 1 17 68 0
0.45 0.99 0.22 0.65 1.00
300 533 730
410 410 478
−0.37 0.23 0.35
THgD THgP MeHg THg HgP + Hg(II)b Hg0 c THg THg
retention coefficient (R)
This study. bCMAQ.60 cLOADS.23
Dissolved Mercury Flux. Estimates of THgD flux are similar to THg flux estimates because the total annual THgD load is also dominated by the Niagara River (105 kg/yr). As observed with THg estimates, the next highest contributor is the Black River (5.5 kg/yr) and the lowest contributor is the Ganaraska River (0.1 kg/yr). THgD export via the St. Lawrence River (67 kg/yr) constitutes over 95% of total fluvial Hg loss from Lake Ontario. Unlike most of Lake Ontario’s tributaries, the Hg in the St. Lawrence River consists almost entirely (>90%) of the dissolved fraction. The riverine mass balance of THgD for the lake suggests fluvial retention of approximately 56 kg/yr (Rdissolved = 0.45; Table 2). Particulate Mercury Flux. THgP loads to the lake are also dominated by the Niagara River (50 kg/yr), despite the Niagara having the lowest mean THgP concentration of all sampled tributaries (0.3 ng/L; Table S14, Supporting Information). Unlike THg and THgD fluxes to the lake, where the contribution of the Niagara River was approximately 15- to 20-fold higher than the next largest tributary source, THgP flux from the Niagara is not as dominant relative to fluvial sources from watersheds adjacent to the lake. The next highest source of THgP to Lake Ontario is the Genesee River at 10 kg/yr, followed by the Black River (7.5 kg/yr) and the Oswego River (3.9 kg/yr). Interestingly, net THgP loads to the lake exceed net
Figure 1. Summary of fluvial Hg flux into and out of Lake Ontario for the study period.
lowest average THg concentration of the inflowing tributaries (0.8 ng/L; Table S14, Supporting Information), its THg flux (154 kg/yr) dominates the fluvial THg load to Lake Ontario, constituting nearly 80% of the annual total. The next closest fluvial source of THg to the lake is the Black River (13 kg/yr). The smallest contributor is the Ganaraska River, which only supplied THg at a rate of 0.2 kg/yr during the study period. Based on these estimates, the total fluvial THg load was approximately 197 kg/yr to Lake Ontario. The St. Lawrence River is the sole fluvial export for Lake Ontario (assuming that groundwater seepage constitutes a negligible mass flux of Hg). THg export from the lake is estimated to be 68 kg/yr. The mean THg concentration for the St. Lawrence River (0.3 ng/L; Table S14, Supporting Information) is lower than any of the inflowing rivers, including the Niagara River. Similar to fluvial THg loads, the magnitude of THg export via the St. Lawrence River is driven by water flux. Summed together, the riverine THg flux estimates indicated net fluvial retention of approximately 129 kg/yr THg (Rriverine = 0.65; Table 2).
Table 1. Summary of Hg Flux and Yields for Rivers of Lake Ontario flux and yield THg
watershed Niagara Oswego Trent Genesee Black Credit Humber Salmon Twenty Mile Ganaraska St. Lawrence
THgPc
THgD
MeHg
watershed area (km2)
kg/yr
g/km2yr
CVa
SDb
kg/yr
g/km2yr
CV
SD
kg/yr
g/km2yr
kg/yr
g/ km2yr
683 000 13 045 12 560 6550 5015 950 910 713 295
154 8.0 6.5 12 13 0.54 0.62 1.2 0.70
0.23 0.61 0.52 1.8 2.6 0.57 0.69 1.6 2.4
0.1 0.2 0.1 0.2 0.2 0.1 0.1 0.1 0.2
15.52 1.55 0.86 2.51 2.19 0.04 0.06 0.07 0.08
105 4.1 4.1 2.0 5.5 0.27 0.26 0.86 0.31
0.15 0.31 0.33 0.31 1.1 0.28 0.29 1.2 1.0
0.1 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1
14.35 0.71 0.63 0.25 0.28 0.03 0.03 0.08 0.04
49 3.9 2.4 10 7.5 0.27 0.36 0.34 0.39
0.072 0.30 0.19 1.5 1.5 0.29 0.40 0.40 1.4
20 0.6 0.6 0.1 0.3 0.04 0.02 0.06 0.01
− 0.05 0.04 0.02 0.05 0.04 0.02 0.08 0.03
0.5 0.48 0.44 0.32 0.24 0.43 0.23 0.24 0.21
9.57 0.35 0.24 0.04 0.06 0.02 0.004 0.01 0.002
275 765 990
0.24 68
0.87 0.089
0.4 0.1
0.10 8.12
0.33 0.087
0.3 0.08
0.02 5.59
0.15 1.0
0.54 0.0012
0.01 17
0.03 −
0.16 0.27
0.001 4.47
0.090 67
CV
SD
a
CV = coefficient of variation. bSD = standard deviation. cTHgP is the calculated difference between THg and THgD. As a result, measures of variation are not reported for THgP. 6109
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watershed yields of MeHg is consistent with others in the literature.44,48−51 The positive link between MeHg and forest cover is not as common in the literature, though some studies suggest that a MeHg pulse occurs from litterfall inputs in forested catchments,52,53 and another study observed higher dissolved MeHg in more forested watersheds.54 In our study watersheds, DOC was not related to land use. However, a positive relationship existed between forest and wetland cover (r2 = 0.44). The correlation between MeHg and forest cover may therefore be coincident with the link observed between forest and wetland cover. The inverse relationship between agricultural land use and MeHg yield seen in this study is not frequently noted in the literature, though Tsui and Finlay54 describe lower dissolved MeHg concentrations in streams draining more agricultural watersheds and suggest a link to lower DOC content and quality relative to forested watersheds. In our study, there was a significant negative relationship between percent wetland coverage and percent agricultural coverage among the Lake Ontario watersheds (r2 = 0.50), as well as a significant negative relationship between percent forest coverage and percent agricultural coverage (r2 = 0.94). The relationship observed between MeHg yield and agricultural land use may therefore be coincident with land use dynamics: study watersheds with higher agricultural land cover had less wetland and forest coverage. Driscoll et al.55 describe a dampening of MeHg production resulting from nutrient loading to coastal ecosystems, where subsequent increases in biomass decrease MeHg trophic transfer by growth biodilution. In addition, they noted that increases in carbon deposition from higher rates of primary production can result in reductions in methylation rates. A similar process may also be occurring in study watersheds with higher relative agricultural land cover. Wetlands are typically important sources of natural organic matter to connected surface waters.56,57 A positive relationship was evident between watershed yield of THgD and percent wetland coverage in our study. Strong associations between THgD and DOC are regularly reported in the literature.58,59 There was also a weak, though significant, positive relationship between DOC and THgD for the entire data set (r2 = 0.27). However, there were no significant relationships between either percent wetland or wetland area and measures of DOC in the study watersheds. We speculate that this lack of a pattern is a result of the relatively small (i.e., ≤6%) amount of wetland coverage for the study watersheds, and that there is likely an additional small amount of wetland coverage that escapes the resolution (i.e., 250 m) of our land cover data or low-lying terrain that escapes wetland classification yet still affects streamwater chemistry and that of adjacent water bodies.49 Atmospheric Mercury Fluxes to Lake Ontario. Unfortunately, detailed measurements of atmospheric exchange of Hg are not available for Lake Ontario for the study period. However, to help put our fluvial measurements into perspective, we summarize reports in the literature on these Hg fluxes. The Community Multiscale and Air Quality (CMAQ60) model has been used to develop estimates of particulate and ionic atmospheric Hg deposition on a 12 km grid. To estimate deposition for Lake Ontario based on the CMAQ output, a GIS approach was used to analyze depositional rates for each of the 12 km squares over the lake based on the Hg emission inventory for 2005. These rates ranged from a low of 7.0 μg/m2-yr Hg (near the centralnorthern shore and the outlet) to a high of 19.4 μg/m2-yr Hg
THgD loads. Gross THgD loads to the lake are approximately 60% higher than gross THgP loads. However, over 95% of the fluvial THgP load is retained in the lake (i.e., 75 kg/yr; Rparticulate = 0.99; Table 2), further emphasizing the role of Lake Ontario as a depositional basin for Hg. Note that while this calculation depicts net THgP transport, some THgP production occurs within the lake. Methyl Mercury Flux. As it represents only a small fraction of THg, MeHg fluxes for the study period are substantially smaller than all other forms of Hg (THg minus MeHg). The MeHg flux from the Niagara River (20 kg/yr) is far greater than any of the other tributaries. The next closest MeHg input is for the Oswego River at 0.6 kg/yr; over 1 order of magnitude less than the Niagara’s contribution. MeHg fluxes from the remaining US study rivers are all higher than that of the Canadian tributaries except for the Trent River. At 0.6 kg/yr, the Trent River contributes approximately the same amount of MeHg as the Oswego River. The observed spatial distribution of MeHg fluxes reflects variation in river discharge. The smallest US tributary (Salmon River) contributed approximately twice the volume of water to Lake Ontario than the second largest Canadian study tributary (Table S1, Supporting Information). All told, the gross fluvial MeHg load to the lake is estimated at 22 kg/yr. An estimate of export via the St. Lawrence River is 17 kg/yr, for a net fluvial retention of 5.0 kg/ yr MeHg (Rmethyl = 0.22; Table 2). As with other Hg forms, while Lake Ontario is a net sink for MeHg inputs, likely some production of MeHg occurs within the water column and surface sediments of the lake, and photodecomposition of MeHg occurs in the water column.39,40 Methyl Hg was largely associated with the aqueous phase. Due to analytical variability and typically very low MeHg concentrations, many filtered samples exhibited MeHg concentrations greater than MeHg concentrations in the corresponding unfiltered samples. As a result, the calculated mean %MeHg in the dissolved phase (i.e., the percent of unfiltered MeHg that is filterable) was virtually 100% in every tributary of Lake Ontario. Watershed Mercury Yield and Land Cover. Annual THg yields from watersheds adjacent to Lake Ontario ranged from a low of 0.5 g/km2-yr (Trent River watershed) to a high of 2.6 g/ km2-yr (Black River watershed), THgD yields ranged from 0.3 g/km2-yr (all watersheds but Black, Salmon, and Twenty Mile) to 1.2 g/km2-yr (Salmon watershed), THgP yields ranged from 0.2 g/km2-yr (Trent watershed) to 1.5 g/km2-yr (Black and Genesee watersheds), and MeHg yields ranged from 0.02 g/ km2-yr (Humber and Genesee watersheds) to 0.08 g/km2-yr (Salmon watershed) (Table 1). Only MeHg yield estimates were normally distributed; all others were positively skewed. The range of THg and MeHg yields observed for our study watersheds are within the range noted by other researchers for the catchments of Lake Superior41 and Lake Champlain,42 for several watersheds with varying land use in Wisconsin,43,44 for several watersheds in Minnesota,45,46 and for watersheds draining into the Chesapeake Bay.47 Watershed yields of THg, THgD, THgP, and MeHg were regressed against percent land cover (forest, agriculture, wetland, urban, open water) for each of the watersheds. Three of the four significant relationships observed suggest land cover controls on MeHg yield. Of these three, two were positive (forest, r2 = 0.39; wetland, r2 = 0.73), and one was negative (agriculture, r2 = 0.43). The observed relationship between increases in wetland coverage and increases in 6110
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contributing to sedimentation is estimated at 38 ng/g. This value is well within the range of PE observed for our riverine data set (median = 4.9 ng/g; mean = 470 ng/g). For many contaminants, the Niagara River is the main source to Lake Ontario.63,64 In contrast, atmospheric exchange appears to be an important input and loss of Hg to and from Lake Ontario. The data collected in this study represent the most comprehensive data set for Hg in Lake Ontario tributaries, and despite uncertainties and year-to-year variation, atmospheric and fluvial fluxes of Hg to and from the lake are similar in magnitude. Lake Ontario is clearly a net sink for Hg inputs (Rtotal = 0.35; Table 2). The Hg fluxes from tributaries are largely driven by water discharge. Therefore, significant variation in inflows from tributary watersheds as a result of wet or dry years will result in widely varying Hg loads. In addition, atmospheric emissions of Hg for the Lake Ontario source area and atmospheric Hg deposition have been decreasing since the mid-1980s.25 These changes have driven decreases in atmospheric Hg loading but also have likely contributed to decreases in riverine inputs. The marked decreases in sediment Hg deposition for Lake Ontario and the other lower Great Lakes are also suggestive of decreases in point Hg inputs. Quémerais et al.66 calculated fluvial Hg losses for Lake Ontario of 112 kg/yr for 1995 to 1996. In contrast, we estimate a fluvial outflow of THg of 68 kg/yr from Lake Ontario for 2008−2010, approximately a decade later. This marked decrease in THg outflow shows the response of Lake Ontario to recent decreases in Hg inputs and may reflect the effects of decreases in net atmospheric emissions associated with control programs in North America6 and decreases in atmospheric concentrations of elemental Hg.67 On the basis of our analyses, Lake Ontario is clearly not at steady state with respect to Hg inputs. Areal estimates of atmospheric THg deposition (21.0 g/km2-yr) to the Great Lakes Basin18 far exceed areal estimates of fluvial THg export (0.089 g/km2-yr), underscoring the nature of the basin as a sink for Hg inputs. The areal riverine THg flux from the Great Lakes upstream of Lake Ontario (0.23 g/km2-yr) is over twice the estimate for the entire Great Lakes Basin at the St. Lawrence River (0.089 g/km2-yr). In Lake Ontario, processes such as sediment deposition and evasion serve to diminish THg export from the Great Lakes Basin. With anticipated future decreases in Hg emissions from the Mercury and Air Toxics Standards and the United Nations Environment Programme (UNEP) Minamata Hg treaty, we hypothesize that both atmospheric and riverine Hg inputs will decrease, but atmospheric inputs will decrease at a more rapid rate than fluvial inputs. Decreases in fluvial inputs will lag behind decreases in atmospheric inputs because of legacy Hg in contributing watersheds. The long hydraulic residence time of the lake (i.e., 6−8 years) and the dynamic physical nature and potential bioavailability of Hg provides for complex cycling patterns that challenge a simplistic input−output view of Lake Ontario.24,62 The data presented here provide robust Hg input estimates for further characterization of the lake’s response to decreases in, and the shifting nature of, Hg inputs. For future studies it will be important to evaluate variation between wet and dry years. Further, a spatially robust sediment sampling program involving dated sediment cores would be helpful to improve the current understanding of Hg sedimentation and resuspension in the lake.
(in the vicinity of the western end of the lake). The resulting mean of all squares covering the lake (12.3 μg/m2-yr Hg) was applied for budget estimates. Based on a total open water area of 18 960 km2 for Lake Ontario, this analysis suggests a particulate and ionic Hg input of approximately 233 kg/yr to the lake via wet and dry deposition. Gaseous elemental Hg (Hg0) exchange was estimated based on a field study performed on Lake Ontario between April 2002 and July 2003. Gross deposition and emission of Hg0 were estimated at 300 and −410 kg/yr, respectively.23 Combining these estimates with the CMAQ values for particulate and ionic deposition results in a net atmospheric THg flux of 123 kg/yr to the lake (Ratmos = 0.23; Table 2), which is similar in magnitude to the fluvial THg flux from the Niagara River (i.e., 154 kg/yr). Sedimentation of Mercury in Lake Ontario. There are few empirical data for lake-wide THg sedimentation rates. Data from a single dated sediment core collected in the western basin of Lake Ontario in 2008 suggest a recent THg net sedimentation rate of 191.8 μg/m2-yr.25 This value results in a THg flux to the sediments on the order of 4000 kg/yr when applied across the entire lake bottom (assuming a lake bottom area of approximately 20 000 km2). This estimate doubles to approximately 8000 kg/yr when using sedimentation rates based on cores collected in 1981.13 Reductions in anthropogenic Hg emissions over the past two decades25 support reductions in THg sedimentation. However, in light of our net estimates of atmospheric plus fluvial THg flux to the lake (total = 252 kg/yr), these values greatly overestimate total net Hg deposition to sediments. Sedimentation across the lake bottom is not homogeneous, making a basin-wide estimate of net THg sedimentation based on sediment cores a challenge. Thomas et al.61 indicate that sedimentation is limited to the three major sub-basins of the lake (i.e., Niagara, Mississauga, Rochester); this has been more recently expanded to include the Kingston sub-basin.62 Oliver et al.63 suggest that little net sediment accumulation occurs outside these sub-basins. Estimates of the combined area of these four sub-basins range from 8660 km2 to 11 390 km 2 . 62,64 Application of the more recent net sedimentation estimate from Drevnick et al.25 to this range of depositional area results in a net THg sedimentation flux of approximately 1660 to 2180 kg/yr. Though this range is approximately half the value of that calculated from application of Hg fluxes from individual cores to the entire lake bottom, it is still high relative to our estimates of fluvial and atmospheric THg loads to the lake. None of the above studies address the effects of sediment resuspension, though it can be assumed that sedimentation rates developed from sediment cores represent net sedimentation. In a recent modeling effort, gross THg sedimentation was estimated at approximately 3530 kg/yr.24 Ethier et al.24 also estimated resuspension of sediment THg for areas of the lake bottom likely to be affected by atmospheric conditions and water currents (i.e., the nearshore zone) and found the total flux of Hg into the water column from sediments in the nearshore zone to be approximately 3370 kg/yr. The balance between these values suggests an approximate net THg sedimentation flux of 160 kg/yr to lake sediments, a value that is on the same order of magnitude as our net sum estimate of atmospheric and fluvial THg flux to the lake (i.e., 252 kg/yr). Further, total solids deposition to Lake Ontario sediments have been estimated at approximately 4.26 × 109 kg/yr.65 Assuming all Hg sedimentation results from THgP, particle enrichment (i.e., PE; the concentration of mercury on particles) for solids 6111
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S Supporting Information *
A map of the study system (Figure S1), flow duration curves (Figure S2), and tables summarizing river discharge characteristics (Table S1), all analytical data (Tables S2−S12), watershed land cover (Table S13), and summary statistics for Hg species (Table S14). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*Phone: 315-443-4121; fax: 315-443-1243; e-mail: jsdenken@ syr.edu. Present Address †
ARCADIS U.S., Inc., 6723 Towpath Rd., Syracuse, NY 13214.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the U.S. Environmental Protection Agency, the Ontario Ministry of the Environment, and the Syracuse Center of Excellence through the CARTI program. Partial funding for this work was provided by the Wen-Hsiung and Kuan-Ming Li Graduate Fellowship. We appreciate the help of Mario Montesdeoca at Syracuse University and Tracie Greenberg, Tom Ulanowski, Britney Meyers, and Geoff Stupple at the University of Toronto. Ram Yerubandi at Environment Canada and Tony Zhang at the University of Toronto aided in development of discharge estimates for the Trent River. Mark Gravelding and Alain Hebert, of ARCADIS U.S., Inc., provided additional support for manuscript development.
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