B13C-0452
A Paleoenvironmental Record of Atmospheric Mercury Deposition in a Permafrost Core from Northern Alaska
An estimated 13 percent of the earth's land surface is covered by permafrost. Northern regions are experiencing permafrost degradation due to climate change which may have major implications for the cycling of carbon, nutrients, and metals, particularly mercury (Hg) in arctic and subarctic ecosystems. An organic- rich core of perennially frozen soil 50 cm below the soil surface (7 cm in diameter and 72 cm in length) was collected from a forested peatland region of continuous permafrost near Coldfoot, Alaska, in August, 2005 (latitude about 67 degrees). The core was processed using trace metal clean sampling protocols. Forty-five samples cut at 1.6 cm intervals, were sent to the USGS Mercury Lab in Middleton, Wisconsin, for analysis of total Hg using low-level analytical methods. The use of permafrost soil cores for the examination of Hg accumulation histories is not apparent from the open literature. Results from this core reveal a range of Hg concentration from 70 to 250 ng/g. Mercury concentrations are elevated by about 50 percent compared to the mean for the entire core at a depth range of 70 to 95 cm from the surface, suggesting a prolonged event of elevated atmospheric Hg deposition. A Carbon-14 date at 55 cm below surface was 7500 years BP. This preliminary data suggests that: (1) As global warming continues, melting permafrost soils containing a large reservoir with the potential to add to the global Hg cycling pool are a possible future hot spot source of Hg, and: (2) there appear to be pre-industrial periods on the scale of millennia ("hot episodes") when atmospheric deposition of Hg was elevated. Additional radiocarbon dates will be presented along with results from another frozen soil core collected in discontinuous permafrost at a latitude of about 65 degrees.
B13C-0453
Interactions of Stream Flow Dynamics and Mercury Variation During Snowmelt in an Adirondack Catchment
Mercury transport in surface waters is strongly influenced by hydrological controls, especially by the state of saturation in catchment soils and by the hydrologic connectivity of uplands to riparian areas. In mountainous regions of the northeastern US, the period when saturation and connectivity is greatest is typically during spring snowmelt. We studied the transport of mercury during spring snowmelt in the Fishing Brook catchment, a headwater tributary with abundant riparian wetlands in the Hudson River basin of the Adirondack Mountains, New York. This stream has high concentrations of mercury, and values for unfiltered total Hg (UHg) ranged from 1.8 to 3.1 ng/L during the 2008 snowmelt. Dissolved organic carbon (DOC) is believed to be an important geochemical control on the transport of Hg through complexation, and DOC concentrations are strongly related to Hg concentrations in the upper Hudson basin. DOC concentrations were positively related to stream discharge at Fishing Brook during the 2008 snowmelt, and this relation showed counterclockwise hysteresis. Fewer snowmelt samples were analyzed for Hg than for DOC, however, UHg concentrations showed a strong positive nonlinear relation (r2 = 0.82) to discharge, consistent with the DOC - discharge pattern observed, and suggesting a common flushing effect. In contrast, unfiltered methyl Hg (UMeHg) concentrations decreased exponentially with discharge (r2 = 0.76) indicating supply-limited flushing during early spring. Hydrological modeling results using TOPMODEL with snowmelt data from 2007 indicated that UHg concentrations were strongly related to simulated saturated area in the catchment. When this modeled relation was then applied to Hg data from 2008, good agreement between modeled and measured UHg concentrations was obtained. These results indicate that as the water table rises to the surface during snowmelt, DOC and Hg are simultaneously flushed to surface waters, whereas a supply limitation of MeHg results in diminishing concentrations as the snowmelt proceeds. Thus snowmelt can be assumed as a main process to form sufficient supply of inorganic Hg and DOC to the hot spots of methylation.
B13C-0454
Aquatic Ecosystem Exposure Associated with Atmospheric Mercury Deposition: Importance of Watershed and Water Body Hot Spots and Hot Moments
Atmospheric deposition of divalent mercury (Hg(II)) is the often the primary driving force for mercury contamination in fish tissue, resulting in mercury exposure to wildlife and humans. In lake systems associated with small watersheds, direct deposition to the water surface is typically the dominant mercury loading source; however, in lake systems with large watersheds and river systems, these inputs may be relatively small compared to loadings from the watershed via erosion and surface runoff. Within each system, transformation of the deposited mercury into the environmentally relevant form, methylmercury (MeHg), proceeds at different rates largely regulated by physical characteristics such as watershed land use types and water body hydraulic residence times, as water body chemistry, such as pH and trophic status Therefore, to fully represent mercury exposure in aquatic ecosystems, we must couple watershed models with water body models and explore where, why, and when hot spots and hot moments of transformation and transport occur. Here we link the simulated atmospheric mercury deposition results from the Community Multi-Scale Air Quality (CMAQ) model, a spatially distributed grid-based watershed mercury (Hg) model (GBMM), and the Water Quality Analysis Simulation Program (WASP). We use this multi-media modeling framework to simulate mercury species cycling over time for the different river reaches and watersheds within the Cape Fear River Basin, North Carolina. Through these simulations we investigate the importance of specific watershed and surface water system characteristics in simulating MeHg exposure concentrations. Because GBMM is a spatially-distributed model we are able to investigate the importance of such factors (i.e., watershed area, land-use types, and land-use percentages) in transporting and transforming deposited mercury. We present how particular land-use types and land-use change influence total loading and total mercury concentrations, how different hydrological transport and transformation characteristics impact MeHg exposure, and how these relate spatially ("hot spots") and temporally ("hot moments").
B13C-0455
EXAFS of Frozen Elemental Mercury and its Implications for Abandoned Mercury Mine Wastes
Mercury in the environment is a large concern from both ecosystem and human health perspectives. Mine wastes at inoperable mercury mines throughout California can be highly elevated in mercury concentration, with some materials having mercury concentrations above 2% wt. It is of prime importance to know the speciation of Hg within these sediments to gauge potential bioavailability of Hg from these impacted areas of California. In order to assess Hg speciation within impacted sediments, Hg EXAFS studies have previously been conducted at room temperature. However, elemental Hg at room temperature is a liquid, which results in very little structure in the EXAFS region, thus making it difficult to determine relative proportions of elemental Hg in sediments. Hg LIII EXAFS of Hg mine wastes previously analyzed at room temperature were re-analyzed at 77K to determine what proportion of the Hg speciation consists of elemental Hg. By using least squares fitting it is shown that some Hg impacted mine wastes contain up to 25% elemental Hg. The same sediment samples used in the frozen EXAFS studies were also used in measuring the flux of Hg leaving the sediments during both light and dark exposures. Ratios between light and dark exposures for Hg fluxes, when normalized to concentration, showed some samples having nearly 60 times more Hg released when exposed to light versus the dark. The light/dark ratios of the mine sediments did not relate to overall Hg concentrations and could not be explained by Hg speciation determined from room temperature EXAFS. By doing EXAFS at 77K we see that there is a correlation between Hg light:dark fluxes and percentages of elemental Hg, with higher ratios correlating with higher Hgo percentages. Sediment samples with light:dark <3 show no evidence of elemental Hg from EXAFS, while samples that show ratios at nearly 60 contained ~25% elemental Hg. This research illustrates that a significant species of Hg has been neglected from the overall Hg speciation in environmental samples. The correlation between elemental Hg and light:dark fluxes should allow researchers to determine which mine wastes have the highest possibility of gaseous Hg emissions into the surrounding environment. In order to accurately determine speciation of Hg within sediments by EXAFS cryostat temperatures are needed and a rethinking of previous work on Hg speciation in the environment needs to be done.
B13C-0456
Investigation of Automated Sampling Techniques to Measure Total Mercury in Stream- Water During Storm-Events
High-flow events (storms and snowmelt) are a dominant transport mechanism for total mercury (HgT) from the terrestrial to the aqueous environment. High-gradient headwater catchments are a primary source of downstream contamination because they store large pools of Hg in soils and sediments. Consistent, high- frequency event-sampling of headwater streams is rare, however, because of the unpredictability of high flows, remoteness of sites, and the difficulties associated with the ultra-clean sampling procedures. The use of automated sampling techniques with an ISCO® sampler has been demonstrated in several studies for trace metals, but their use for collection of HgT samples has not been systematically evaluated in the literature. Even with clean equipment at deployment, subsequent contamination and loss by evasion are possible considering the bottles, as currently designed, are open to the atmosphere before sampling and until retrieval. Field tests are conducted using an ISCO® sampler retrofitted with pre- cleaned Teflon® sampling lines and glass bottles to determine the relative errors associated with the automated sampling method for a variety of HgT concentrations and preservation techniques. Differences between quality assurance and quality control results for automated and manual sampling are also investigated. Sample containers are filled with known standards of HgT solution and left in the ISCO® containers at the field site and each day (up to 7 days) are capped and returned for analysis. During a storm event, manual samples are taken from the middle of the water column concurrently with the ISCO® at hourly intervals using "clean hands" procedures. Evaluations of results are used to establish quality assurance guidelines for future field campaigns using automated techniques for HgT sampling.
B13C-0457
Fluxes of new and old mercury from mercury amendments to Puerto Rico soil columns
Puerto Rico receives high loading of mercury (Hg) in wet deposition. We hypothesized that high-organic soils with white-rot decomposition would retain more of this mercury than low-organic soils with brown-rot decomposition. In order to test this hypothesis, four paired intact soil cores (15-cm diameter, 20-cm depth) were extracted in acrylic tubes from a hillslope in the low-elevation Tabonuco forest within the Luquillo Experimental Forest in northeastern Puerto Rico. One core of each pair was taken from an organic-rich microsite (above a debris dam) with white rot, and the other was taken from an adjacent bare-soil area (below the debris dam) with brown rot. The soil cores were amended with 384 ng of 202Hg (8 applications of 48 ng, twice per week for 4 weeks, April - May, 2006). Total water added was equivalent to 271 mm rainfall (close to the 4-week average) but Hg loading was about ten times the average loading. Ambient rainfall was excluded. Leachate passed through a funnel to a collection bottle and was composited for each core. Despite rapid throughput of the amendment solution, about 90% of the spike (new) Hg was retained in the soil columns. Our hypothesis was rejected in that there was no difference in retention in the white-rot (89.1 /- 0.8%) and brown-rot (91.5 +/- 2.8%) columns. Moreover, the white-rot soils leached 34% more native (old) Hg than the brown-rot soils. The greater leaching of native Hg is consistent with the higher DOC concentration in the white rot soil leachate (4.1 +/- 1.7 mg/L) compared to the brown-rot soil leachate (1.7 +/- 0.6 mg/L). The amount of native (old) Hg leached from the columns was roughly similar to the amount of spike (new) Hg that passed through the columns. We surmise that the greater DOC mobilized from the white- rot soils complexed and transported greater amounts of Hg from the large pool of native Hg in the soil. The relative amounts of spike Hg throughput and native Hg mobilization are consistent with other recent results from Hg isotope application to an entire hillside.
B13C-0458
The Impact of 100 Years of Wildfires on Mercury Accumulation in Sediment Cores From Two Lakes in Southern California, USA
In southern California, USA, wildfires are a source of mercury (Hg) to local watersheds, most likely due to increased erosion of Hg-laden soils during high-flow sediment transport events. Hg levels were analyzed in dated sediment cores from two southern California lakes, Big Bear Lake and Crystal Lake. Between 1895 and 2006, fires were routinely minimized or suppressed around Big Bear Lake, while fires regularly charred thousands of hectares near Crystal Lake. Crystal Lake sediments were interspersed with distinct sediment layers (e.g., rapid depositional layers), which were characterized by a sudden change in one or more of the following: organic matter, charcoal levels, the shells of oogonia, or percent sand. Although not perfectly correlated, the timing of Hg peaks in Crystal Lake sediments indicated fires significantly increased Hg content (Crystal Lake: 48-370 ng g-1, Big Bear Lake: 13-130 ng g-1). In both lakes, Hg levels were positively correlated with sedimentation rates, which implied sediments were an important source of Hg. Compared to Big Bear Lake sediments, the ratio between Hg flux during erosional and non-erosional periods in Crystal Lake sediments was higher (11 vs. 1.7), reflecting increased substrate susceptibility to erosion following fires, and higher Hg content. The ratio between modern (post 1900s) and pre-industrial (1800s) Hg flux in Crystal Lake was 1.2, which was far lower than values typically reported for lakes (~3.7). This suggests continuous wildfire activity over the last century significantly enhanced Hg accumulation rates at Crystal Lake.
B13C-0459
Mercury in the Air, Water and Biota at the Great Salt Lake (Utah, USA)
The Great Salt Lake, Utah (USA), is the fourth largest terminal lake on Earth and a stop-over location for 35 million birds on the Pacific Flyway. Recently, the Utah Department of Health and Utah Division of Wildlife Resources issued tissue mercury (Hg) consumption advisories for several species of birds that consume the lake's brine shrimp. Sources of Hg to the lake are the watershed and the atmosphere, and we hypothesized that the chemistry of the air above the Great Salt Lake would facilitate atmospheric deposition of Hg to the water. Because little information was available on Hg at the Great Salt Lake, and to begin to test this hypothesis, we measured atmospheric elemental (Hg0) and reactive gaseous mercury (RGM) concentrations as well as Hg concentrations in water and brine shrimp five times over a year. Surrogate surfaces and a dry deposition model were applied to estimate the amount of Hg that could be input to the lake surface. We found that atmospheric Hg0 and RGM concentrations were comparable to global ambient background values and those measured in rural areas (respectively). Both Hg0 and RGM exhibited regular diel variability, and no consistent seasonal periods of depleted or elevated values were observed. Based on these findings, local factors are thought to be important in generating elevated RGM concentrations that could be deposited to the lake. Model estimated deposition velocities for RGM to the lake ranged from 0.9 to 3.0 cm sec-1, with an estimated 19 kg of Hg deposited annually. Total Hg and methyl Hg concentrations in surface waters of the lake were consistent throughout the year (3.8 ± 0.8 ng L- 1 and 0.93 ± 0.59 ng L-1, respectively) and not significantly elevated relative to natural waters; however, the percent methyl Hg to total Hg was high (25 to 50%). Brine shrimp Hg concentrations were 384 ppb and had a statistically significant increase from early summer to fall. Based on modeled dry deposition and estimated wet deposition, the annual input of Hg directly to the lake by these processes is sufficient to account for the Hg in brine shrimp anthropogenically harvested from the lake each year and most likely that consumed by birds.
B13C-0460
Mercury Sources and Species in Waquoit Bay and Other Cape Cod Massachusetts Water Bodies
Previous work in Waquoit Bay, Massachusetts (USA), has found high concentrations of dissolved mercury (Hg) in bay water and implicated groundwater as a potentially important source of this toxic metal to that coastal ecosystem (Bone et al., 2007). On-going research is testing this hypothesis by examining the spatial distribution and temporal variability of Hg within the subterranean estuary, the open water of Waquoit Bay, and in selected water bodies elsewhere on Cape Cod. Survey sampling indicated that Waquoit Bay was unusually elevated in total dissolved Hg concentration when compared to Vineyard Sound, fresh ponds within its watershed and nearby tidal salt ponds. This suggests that groundwater and diffusion from sediments are key sources of Hg to Bay waters. Watershed Hg transfer efficiencies, calculated by comparing rain fluxes measured at the Cape Cod National Seashore Coastal Lab (MA01) with groundwater fluxes at the head of Waquoit Bay, are typical of other temperate systems but lower than those estimated previously for this site. Furthermore, while the total Hg concentrations of local sands and soils from around Cape Cod are often very small and strongly a function of organic carbon (as determined by LOI), the partition coefficients (Kd) were not unusually low. This potentially contradicts our hypothesis that the cause of high Hg concentrations in the Bay are the result of little retention of Hg within the watershed compared to other temperate systems. In addition to total Hg studies, monomethyl Hg (MMHg) fluxes into the Bay with advecting groundwater are being determined. The flux with groundwater will be compared to the flux of mmHg generated in surficial sediments of the central bay, less affected by groundwater, and released via diffusion and/or tidal pumping.
B13C-0461
Measurements of mercury methylation rates and bioavailability in the Allequash Creek Wetland, Northern Wisconsin
Wetlands are known to be hot spots for the production of methylmercury (MeHg) and subsequent export into other aquatic ecosystems. Because MeHg is a bioaccumulative neurotoxin, and because the primary route of human exposure to mercury is through the consumption of contaminated fish, understanding the processes by which MeHg is produced in the aquatic environment is important to the protection of human health. Inorganic Hg(II) is known to be methylated by bacteria in the anoxic zones of wetland sediments, but bioavailability plays a role in this process, as certain chemical complexes of mercury are unavailable to the microbial community. In the Allequash Creek wetland, a strong relationship has been observed between MeHg and Dissolved Organic Carbon (DOC) concentrations, but the observed relationship between MeHg and total Hg is weak. This observation implicates factors other than Hg(II) concentration as drivers of methylation. In this study, depth-resolved estimates of the bioavailability of inorganic Hg(II) were made by measuring the net mercury methylation rate potential in the hyporheic zone of the wetland. Gross mercury methylation was measured in sediment cores amended with stable isotope-enriched Hg(II), by analyzing isotopically-enriched methylmercury produced during an incubation. Demethylation was measured by amending replicate cores with stable isotope-enriched methylmercury and analyzing the amount consumed over the incubation period. Analyses were conducted using an inductively coupled plasma-quadrupole mass spectrometer. A method comparison was made between incubating cores intact, with mercury amendments injected through core tube walls, and incubating sectioned cores, with mercury amendments mixed into homogenized sediments. The value of incubating intact cores is that disturbance to the sediment and the microbial community is minimized, resulting in experimental conditions that more accurately mimic in situ conditions. The value of mixing mercury amendments into homogenized sediments is that this spreads the amended mercury throughout the core section, avoiding injection channel effects. The influence of Hg(II) speciation on methylation is also being examined. Preliminary results show gross mercury methylation occurring at both sites sampled, although net methylation is negative throughout most of the depth profile, suggesting that the chemical conditions controlling the bioavailability of Hg(II) are highly variable. Because this is the first time such measurements have been made in this system, these data provide an important baseline for future studies of mercury bioavailability in northern wetlands.
B13C-0462
Assessing the potential for mercury methylation and mobilization within shallow surface waters during periods of anoxia
Wetlands and stratified conditions within lakes have been demonstrated to enhance Hg species mobilization as well as methylation to the bioaccumulative species, methylmercury. However, few studies have been conducted within small, shallow aquatic systems. Although each system is relatively small in area, the overall number of such systems is large (and increasing) and warrants investigation within overall landscape nutrient cycling. Previous research has documented strong diel stratification cycles in such systems and the frequent development of anoxia within the bottom waters of such polymictic systems compared with larger, dimictic lakes. The increased frequency of anoxic events within shallow systems may exacerbate dissolution of Hg and other trace metals from sediments, potentially enhancing bioavailability. We examined the impact of polymixis and the shorter hydraulic residence time on the bioavailability and the downstream transport of Hg. Filtered and unfiltered lake water samples were collected at 15 and 50 cm above the sediment as well as the surface of the ~1 m deep Mirror Lake retention pond on the University of Connecticut Storrs campus. These samples were analyzed for Hg speciation, metal content and ancillary chemistry. Additional samples were collected from the lake outlet under baseflow and elevated discharge conditions, including the capture of initial mobilization during precipitation events.
B13C-0463
Linking diurnal trends in methylmercury concentration and organic matter photo-reactivity in wetlands of the Yolo Bypass, California
Aqueous concentrations of methylmercury (MeHg) are known to vary temporally and spatially due to multiple concurrent production and loss mechanisms, and due to variations in the hydrologic connectivity between the methylating substrate (most commonly the benthos) and the overlying water compartments. Diurnal trends in MeHg production, bacterial demethylation, photo-demethylation, diffusion and advection transport processes have been identified and investigated; however, the magnitude and relative importance of each process in mediating overlying water MeHg concentrations, is not well known in natural wetland systems. Temporal variations in aqueous MeHg concentrations may impact the biological accumulation of MeHg into the base of the aquatic food chain, and may challenge regulatory efforts designed to mitigate MeHg exports from point and non-point sources. To identify the possible "hot moments" during the diurnal cycle, surface water MeHg concentrations were monitored in two agricultural wetland settings (wild rice and white rice fields) over a 24- hour period within the Yolo Bypass Wildlife Area, California using a combination of in situ optical sensors and traditional surface-water grab samples. In the wild rice field, MeHg concentrations doubled from 1 ng/L to 2 ng/L over the nighttime hours and returned to 1 ng/L during the daylight hours, whereas the white rice field showed no significant variation in MeHg concentration (0.73 +/- 0.08 ng/L) throughout the diurnal cycle. Similar trends were observed when MeHg data was expressed as a percentage of total Hg, with both wetland habitats exhibiting similar levels (20% MeHg) following the nighttime period and the wild rice field declining to 10% in the early evening. Field parameters measured in situ (including: solar radiation, pH, dissolved oxygen, and temperature) exhibited large diurnal trends in both wetlands, whereas optical proxies for dissolved organic matter (DOM) composition mirrored the fluctuations in MeHg concentration and %MeHg, with a strong diurnal trend in wild rice and no apparent trend in white rice. These results suggest a unique link between DOM photo-reactivity and photo-demethylation in these wetlands that challenges traditional monitoring efforts in terms of defining generalized "hot moments" associated with the diurnal cycling of MeHg in shallow agricultural wetland settings.
B13C-0464
Mercury Bioaccumulation Potential from Wastewater Treatment Plants in Receiving Waters
In early 2007, the Water Environment Research Foundation (WERF) mercury bioavailability project was initiated in response to the establishment of mercury Total Maximum Daily Load (TMDL) criteria around the country. While many TMDLs recognize that point sources typically constitute a small fraction of the mercury load to a water body, the question was raised concerning the relative bioavailablity of mercury coming from various sources. For instance, is the mercury discharged from a wastewater treatment plant more or less bioavailable than mercury contributed from other sources? This talk will focus on the results of a study investigating approaches to the estimation of bioavailability and potential bioaccumulation of mercury from wastewater treatment plants and other sources in receiving waters. From the outset, a working definition of bioavailability was developed which included not only methylmercury, the form that readily bioaccumulates in aquatic food chains, but also bioavailable inorganic mercury species that could be converted to methylmercury within a scientifically reasonable time frame. Factors that enhance or mitigate the transformation of inorganic mercury to methylmercury and its subsequent bioaccumulation were identified. Profiles were developed for various sources of mercury in watersheds, including wastewater treatment plants, with regard to methylmercury and inorganic bioavailable mercury, and the key factors that enhance or mitigate mercury bioavailability. Technologies that remove mercury from wastewater were reviewed and evaluated for their effect on bioavailability. A screening procedure was developed for making preliminary estimates of bioavailable mercury concentrations and fluxes in wastewater effluents and in fresh, estuarine and marine receiving waters. The procedure was validated using several diverse river and reservoir data sets. A "Bioavailability Tool" was developed which allows a user to estimate the bioavailability of an effluent and compare it to another, and to mix an effluent in a receiving water to estimate bioavailability in the near- and far-field. As part of this project, a study was undertaken to evaluate methylmercury and reactive mercury in wastewater effluents. Effluent samples from 7 municipal wastewater plants from around the Unites States were collected weekly over a ten week period from late June through August of 2008. These data represent the first comprehensive study of bioavailable mercury in wastewater effluents and have not been published elsewhere. Initial data suggest that bioavailable (methyl plus reactive) mercury is less than 30 percent of total unfiltered mercury. Reactive mercury percentages (relative to dissolved total mercury) are somewhat higher than were initially predicted from theoretical calculations. This presentation will overview the project as a whole with a focus on the bioavailability study of these 7 wastewater plants.