The USGS is sponsoring the workshop in recognition that public concern for fish and wildlife and human health from mercury toxicity has increased substantially over the past 5 to 10 years. These concerns are manifested primarily from the issuance of fish consumption advisories in the majority of U.S. states, Canada, and several European countries due to high levels of mercury in game fish. Consumption advisories have been issued for systems as small as individual seepage lakes in Wisconsin (where a neighboring lake does not exhibit elevated levels in fish), to sub-regional areas as large as the Florida Everglades where a complete ban on game fish consumption is in effect. Although the precise causes for this contamination are poorly understood, it appears to result from both source and ecosystem-specific factors.
Until recently, attempts to unravel this environmental contamination problem have been frustrated by both sampling and analytical barriers. For most aquatic ecosystems, atmospheric deposition is the primary source of mercury (although there are numerous instances of geologic and anthropogenic point-source contamination cases) and the resulting aqueous concentrations of total mercury are generally less than 10 nanograms/liters. The challenge to scientists is to explain the series of processes that lead to toxic or near-toxic levels of mercury in organisms near the top of the food chain (the bioaccumulation process), when aqueous concentrations and source-delivery rates are so low. To adequately understand this phenomenon an interdisciplinary approach is requisite. Due to recent great strides in sampling and analytical techniques, scientists can now routinely collect representative air, water, tissue, and sediment samples, and perform mercury species-specific analysis. The resultant data have provided new insights into the transport, cycling, and fate of mercury in systems as small as individual seepage lakes, to as large as Lake Michigan and oceanic-scale studies. In addition, new techniques that employ isotopic tracers have provided new insights about the specific processes at the root of this contamination problem: mercury methylation and demethylation.
Prior concern about mercury in aquatic ecosystems has stemmed primarily from the potential adverse health effects of consuming fish contaminated with methylmercury, a neurotoxin. Nearly all (95-99%) of the mercury present in fish is methylmercury, which is obtained mostly from the diet. Dietary methylmercury is efficiently assimilated across the fish gut, binds to red blood cells, and is rapidly transported via the circulatory system to all internal organs, readily crossing internal cellular membranes. Most of the methylmercury in fish eventually accumulates in skeletal muscle, where it is bound to sulphydryl groups in protein. Methylmercury in fish is neither readily eliminated, bound to metallothioneins, nor demethylated, and storage in muscle may serve as the primary protective mechanism against methylmercury. Methylmercury biomagnifies in aquatic food chains, and the structure of aquatic food webs can greatly influence mercury concentrations in fish. Furthermore, the fraction of total mercury existing as methylmercury typically increases progressing up aquatic food chains from primary producers to fish. The potential consequences of methylmercury contamination of aquatic food webs were first recognized in the 1950s and 1960s in Minamata and Niigata, Japan, where human consumers of contaminated fish were severely poisoned. This awareness prompted widespread reductions in direct releases of mercury into surface waters, and mercury levels in fish inhabiting affected waters typically declined in the years after decreases of point-source inputs. More recently, high concentrations of mercury have been found in fish from low-alkalinity lakes, newly flooded reservoirs, and wetland ecosystems. Applications of recently developed methods for contamination-free sampling and analysis of air, rain, and water have shown that seemingly small amounts or inputs of mercury can cause significant contamination of fish in these ecosystems. In fact, many fresh waters with fish-consumption advisories can be characterized as lightly contaminated ecosystems in which inorganic Hg(II) is readily converted to methylmercury. Moreover, some human activities, such as the construction of new reservoirs, increase mercury levels in fish by creating environmental conditions that enhance the microbial methylation of existing inorganic Hg(II). Conversely, mercury levels in fish are generally low in fresh waters having low mercury-methylation rates, even in some ecosystems with large inventories of inorganic mercury. Fishery management and health agencies in many states have responded to the current fish-mercury problem by surveying and monitoring mercury levels in key sport fishes and by providing advice concerning the consumption of fish.
Recent evidence suggests that current consumption advisories for methyl mercury (MeHg) in fish are adequate to protect human health. Resource managers in both government and industry now need to understand the hazard posed to the most "at-risk population" that being fish-eating wildlife. Top predator piscivorous wildlife such as common loons, mergansers, osprey, eagles, kingfishers, mink, otter, and wading birds may be at greatest risk to the toxic effects of MeHg in aquatic systems. MeHg readily bioaccumulates and can cause neurotoxicity at low concentrations in vertebrates. Piscivorous mammals, birds, and reptiles consistently have the greatest tissue Hg concentrations in aquatic systems. Several ongoing field research projects are investigating the impact of elevated MeHg exposure on wildlife health and reproductive fitness on inland lakes of Wisconsin, Minnesota, Ontario, the maritime region of New Brunswick and the northeastern U.S., hydroelectric impoundments in northern Ontario and Quebec, and the Everglades ecosystem in Florida. In Wisconsin, common loons may be at greatest risk because they are known to be sensitive to the toxic effects of MeHg and frequently nest on acidic lakes where fish MeHg concentrations are greatest. An epidemiological approach is being used to test whether the reduced common loon breeding success documented on acidic lakes in Wisconsin is related to elevated loon Hg exposure also documented on acidic lakes. Since 1992, WDNR has captured >320 common loon adults and chicks using nightlighting techniques; blood and feathers are collected and analyzed for Hg content and each individual is marked with 4 colored leg bands to facilitate reidentification in future years. Annual return rates and reproductive performance are currently being documented for each adult loon and related to levels of Hg exposure. Preliminary results from 1992-95 field seasons will be presented. A preliminary evaluation of Wisconsin bald eagle Hg exposure and its relationship to productivity will also be presented.
Atmospheric deposition of Hg to natural waters has been identified as an important research area under Section 112 of the 1990 Clean Air Act Amendments. The recent findings that many pollutants, including Hg, are at levels of concern in the waters, and that they reach these bodies via atmospheric deposition has complicated the policy makers attempts to find effective control strategies for these pollutants. Identifying the sources of Hg to the atmosphere and quantifying their emissions by chemical and physical form emitted has only recently been attempted. Identifying the sources of Hg to an aquatic ecosystem and determining the relative importance of the atmospheric emissions is a difficult task. In the case of Hg in the Everglades this problem is even more complicated as the loading of Hg to the aquatic system likely contains a fraction which is locally derived from agricultural activities, a fraction which is transported short distances through the air, and the potential for a fraction which was emitted into the atmosphere at long-distances from South Florida. Development of effective control strategies for improving water quality and lessening ecosystem Hg burdens can not take place until we understand the relative importance of each of the potential contributors. While quantification of the Hg emissions by chemical and physical form is critical to our ability to understand and model the deposition of Hg to natural waters, the final solution to the problem depends just as much on understanding the mesoscale atmospheric transport, chemistry, and deposition processes prevalent in the airshed being studied.
The National Atmospheric Deposition Program/Mercury Deposition Network (NADP/MDN) began formal operation in the Spring of 1995 as a transition network. It is the largest network of its type in the country. The network now consists of 14 sites, and anticipates having 20-25 sites by the end of 1996. The objective of the NADP/MDN is to establish a national monitoring network to better understand the spatial and temporal deposition of mercury in precipitation. The NADP/MDN is a subnetwork within the NADP. The NADP/MDN data will be of the same high quality as is typical of the NADP, and will make use of the data quality criteria and sample validation procedures already in place for NADP. Quality assurance and quality control protocols are standardized for the field, laboratory, and data management operations of the NADP/MDN. Results of the transition network have been published (Vermette et al., 1995). Total mercury concentrations measured by network sites from December 1995 through the end of May 1996 range from about 1.0 ng/L to over 50 ng/L. Laboratory data and QA/QC results will be presented for discussion.
The past year (1995-1996) has seen a slowing in the number of new developments in low level mercury speciation--an indication that the current technology is maturing. Several significant developments are discussed, however: Two methods for the determination of gaseous Hg(II) in air at ambient levels have been fully developed, and are currently being intercompared. These are the mist chamber scrubber and the ion exchange membrane techniques. Although these methods differ significantly in principle, integration time, and specific limitations, both have been showing similar results, indicating atmospheric Hg(II) levels of approximately 0.01 to 0.1 ng/m3. Another recent finding of importance is that the distillation procedure commonly used to extract MMHg from waters and sediments can create a positive MMHg artifact through the action of natural organic matter with inorganic Hg. The risk of error is greatest in samples with a very low natural MMHg/Total Hg fraction (< 0.1%>, such as Hg(II) contaminated waters and sediments. Another significant development is the availability of a growing choice in very sensitive automated mercury analyzers (both AFS and AAS based), which have estimated detection limits as low as 1-2 ng/L Hg. Although insufficient for low-level research, these instruments are proving popular for monitoring purposes, where the required MDL is typically 12 ng/L Hg. Finally, it is noted that with the partial support of the Florida DEP, a twice-yearly ambient level inter-laboratory intercomparison program for Hg and MMHg in water is now available through FGS.
Since the late 1960s it has been suspected that anoxic aquatic sediments represented the sites where inorganic forms of mercury underwent methylation by resident bacteria, but it was not clear at the time which group of microbes were responsible. Because methanogenic bacteria are important members of anaerobic food webs and that they contain abundant B-12 coenzymes, it was reasoned that methylcobalamin-mediated reactions in methanogens were responsible for the biomethylation of Hg (II) in nature. Indeed, because this reaction was demonstrated in vitro with cell extracts the concept that methanogens were important mercury methylators in the environment was the dominant paradigm in the literature for over a decade. However, mercury methylation could not be found in whole cells (in vivo) of methanogens or in assays of with strongly methanogenic systems, such as sewage sludges. Subsequent investigations with marine sediments revealed a capacity for biogenesis of methylmercury (MeHg) and that activity was abolished by molybdate, an inhibitor of sulfate-reducing bacteria (SRBs). It has now been well-established that certain types of SRBs form MeHg via cobalamin-linked reactions, and that this occurs in both freshwater and marine sediments. In situ measurements of sediment Hg (II) methylation have been refined by employing high-purity radioassays with 203Hg (II), thereby allowing for methylation activity to be assessed at near-ambient levels of Hg (II). In contrast to the methylation reaction, much more is known about the mechanisms for bacterial demethylation of MeHg. MeHg represents a potent microbial toxin which must be neutralized in order for bacteria to grow. In some bacteria the presence of MeHg activates the mer operon which is composed, in part, of mer A and mer B genes. The mer B gene encodes for the formation of an organomercurial lyase enzyme which cleaves MeHg into methane and Hg (II). The mer A component codes of the formation of mercuric reductase which by volatilization removes Hg by reducing Hg (II) to Hg (0). Although the mer operon is contained on a plasmid and can be inserted into various bacteria, cells which have mer B do not necessarily have mer A. Studies with freshwater systems have detected demethylation activity as well as the presence of mer operon genes in the resident bacterial populations. However, it is not clear if mer operon-mediated reactions are the dominant mechanism of MeHg demethylation in anoxic sediments. Assessments of both methylation and demethylation capacity of such systems is necessary in order to determine whether a net methylation (or demethylation) occurs. The employment of high purity Me203Hg will facilitate such determinations.
A second mechanism for microbial demethylation was recently discovered whereby MeHg mimics a 1-carbon organic substrate, such as methanol. This process is termed oxidative demethylation, and was discovered in sediment incubations with 14C-MeHg. Instead of 14C-methane, 14C-carbon dioxide was found to be the major product, especially in systems which harbored active populations of sulfate reducers. However, 14C-methane was also an important endproduct (in addition to 14C-carbon dioxide) in sediments which had high methanogenic activity. The addition of molybdate to inhibit sulfate-reducers or of bromoethanesulfonic acid to inhibit methanogens shifted the ratio of CO2/CH4 formed from MeHg. These results indicated that both methanogens and sulfate-reducers are involved in oxidative demethylation in nature. Highest potential activity was found to occur in systems having extensive mercury-contamination from mine tailing point sources. This was shown to occur in the Carson River, Nevada. The oxidative demethylation can also be detected in Everglades sediments, but at much slower rates. Future challenges for this research will be to demonstrate that: 1) oxidative demethylation occurs at near-ambient MeHg levels; 2) the product is Hg (II) or Hg (0); and 3) high rates of oxidative demethylation can be achieved in novel bacterial isolates which will clarify the underlying mechanism(s).
Sediment cores can provide a detailed record of mercury fluxes to a waterbody over time. Stratigraphic studies of Hg in sediment cores are useful for the reconstruction of: (1) historical time trends of atmospheric Hg deposition, (2) trends in the geographic pattern of atmospheric Hg deposition, and (3) the history of point-source discharges of Hg to lake and river systems.
A recent publication argues that sediments do not provide an accurate historical record of atmospheric pollution due to a number of geological phenomena, including relatively high background Hg concentrations from local bedrock and soils, and diagenesis that promotes the movement of Hg to the top of the sediment column. These criticisms reinforce the need for clear explanations of the utility and limitations of stratigraphic data and appropriate protocols for data collection and interpretation. Such information will soon be available in the proceedings of a EPRI-sponsored workshop on the subject ("Protocol for Estimating Historic Atmospheric Mercury Deposition"), which the following partially summarizes.In most geographic regions, background Hg inputs from local bedrock and soil are small relative to atmospheric Hg inputs. However, where background Hg is high or atmospheric Hg inputs are relatively low, some data analysis procedures, such as the use of enrichment factor ratios, are inappropriate.
Vertical Hg movement due to diagenesis is only a issue in sediments with low organic carbon content, and therefore may be a concern in the Great Lakes and some marine systems. Experiments with high-organic sediments from smaller lakes show no evidence of diagenetic movement. The results of stratigraphic Hg analyses may be numerically represented either in units of concentration (micrograms per g dry sediment) or as rates of accumulation (micrograms per square meter-year). As long as sediment accumulation rates have not changed appreciably over time, both representations show the same relative changes over time. On the other hand, if inputs of organic or inorganic matter change over time, only Hg accumulation rates yield an accurate portrayal of Hg loading to the waterbody. The accuracy of data interpretation in non-constant systems is therefore highly dependent on the accuracy of the sediment dating.
There are several ways that the history of atmospheric deposition rates can be described from sediment accumulation rates. The ratio of Hg accumulation in a sediment core between modern and reference (typically preindustrial) conditions provides a quantitative measure of the magnitude of change over time. This ratio is broadly comparable among sites and geographic regions, because most local processes, such as sediment focusing, cancel out in the calculation of the ratio. Ratios can be misleading if Hg inputs from local bedrock and soils are high.
The absolute atmospheric Hg deposition rate can be calculated if lake-wide Hg loading rates are available from a cluster of lakes that possess a range of catchment-to-lake ratios. Lake-wide loading can be estimated from either (1) multiple cores within one basin or (2) Hg accumulation rates from a single core that is corrected by the core s lead-210 flux relative to the regional mean.
Three of the essential aspects of assessing health risk to human and non-human populations from emissions of mercury are: 1) quantifying the magnitude of the emissions relative to natural sources, 2) estimating how far different forms of emitted mercury are transported, and 3) defining the form of the relationship -- whether linear or non-linear -- between inputs to aquatic systems and the production of methylmercury. Ongoing research sponsored by EPRI, DOE, EPA, USGS, and others is aimed at reducing the significant uncertainties in these areas.This paper focuses on a modeling analysis aimed at further quantifying natural Hg emissions, which is essential for assessing the benefit derived from reducing anthropogenic emissions.
Accurate anthropogenic emissions data are just now becoming available in the US for the trace element mercury (Hg). These emissions estimates can be used to constrain risk assessments, to bound emissions for countries that lack data, and to further constrain global Hg budgets. However, these emissions estimates are incomplete because significant uncertainty about the magnitude of natural sources remains. Moreover, it has become clear that mankind's use of Hg has varied geographically and with time. Because of re-emissions of Hg from terrestrial and marine environments to the atmosphere, past activities continue to affect current atmospheric Hg concentrations.
The Global Mercury Cycling Model has been used to simulate historical rates of Hg deposition. The model predicts that peaks associated with gold mining during the late 1800's should be a significant fraction of modern deposition rates in sedimentary records analyzed with adequate temporal resolution. There is some evidence that this is the case, implying that anthropogenic emissions then and now are significantly greater than natural emissions. More detailed analyses of sediment cores and modeling of this peak are needed to further quantify the background rate of natural Hg emissions.
As recently as eight years ago, scientists were making novel contributions by using "clean" sampling methods and sensitive analytical techniques for establishing true concentrations of mercury (Hg) in freshwater aquatic ecosystems without contamination artifacts. As clean techniques have become standard practice, aquatic-Hg investigators have focused efforts on important biogeochemical processes and cycling pathways of Hg. Of particular interest has been an examination of external mono-methylmercury (MeHg) sources to aquatic ecosystems such as precipitation, runoff, and groundwater, as well as internal processes that result in the production mmHg in the water column and at the sediment/water interface. The emphasis placed on this area of research is due to two factors: 1) the observation that virtually all Hg in fish tissue is mmHg, and 2) and the development of highly sensitive techniques for measuring mmHg concentrations and Hg-methylation rates which permit process-level research at sub ng/L mmHg levels.
Recent studies on lakes have shown that system-scale parameters such as lake size and depth, littoral-zone area, epilimnetic temperature, and differences in methylation rates determined on incubated cores all point to the sediment/water interface in littoral zones as an important site for mmHg production. This observation corroborates with findings of previous researchers who have demonstrated that sandy littoral-zone sediments are a primary site for sulfate reduction, and that sulfate-reducing bacteria are important Hg methylating agents in aquatic ecosystems.
A detailed examination of the sediment/water interface as a methylmercury generation site and its relation to groundwater flow, is the focus of this research. The challenge in conducting Hg studies at the groundwater/lake interface is two fold: difficulties of collecting contamination free samples at small (cm) intervals in sandy sediments, and quantifying the natural variability in groundwater-flow rates that exist over relatively small areas. The study site for this research is Pallette Lake, a seepage lake in northern Wisconsin. One of the products of this study is a novel method for estimating net Hg-methylation rates in sediments where mmHg profiles and groundwater-flow rates have been measured. The methylation rates estimated from this study are compared to rates estimated using an independent, radio-tracer (203Hg) method.
It is well recognized that the chemical forms of mercury in the water column and sediments are intimately related to its overall effects on living organisms. Interactions of mercury and dissolved organic matter may play an important role in controlling both the availability of mercury for uptake by living organisms, and the types of chemical reactions that can occur with mercury. For instance, the association of mercury with dissolved organic carbon (DOC) has been proposed as a primary mechanism for the transport of mercury in aquatic systems. This association has been proposed on the basis of a strong correlation between dissolved mercury and DOC concentrations in ground, lake and stream waters. Little is known, however, about how mercury interacts with DOC or how strong these interactions are. It does appear, however, that interactions with DOC may be significant in controlling the transport and bioavailability of mercury. The important questions to be addressed are: 1) By what mechanisms and how strongly does mercury interact with DOC, and 2) How do these interactions control the effects that mercury has on living organisms. The goal of our current research is to provide information about the interactions of mercury and dissolved organic matter that will better define this important, albeit, poorly understood process. Ultimately, this research will lead to a more complete model of mercury behavior in the environment. Our research will focus on the effect of DOC on the transport and reactivity of mercury through a combined field and laboratory study. The underlying hypothesis of this research is that the chemistry and structural characteristics of organic matter in the Everglades have a strong influence on the processes that control mercury cycling in the environment.
Sediment cores were collected from lakes in the Devils Lake Basin in North Dakota to determine if mercury (Hg) accumulation chronologies from sediment-core data are good indicators of variations in Hg accumulation rates in saline lakes. Sediment cores from Creel Bay and Main Bay, Devils Lake were selected for detailed analysis and interpretation. The maximum Hg concentration in the Creel Bay core was 0.15 micrograms per gram at 8 to 9 centimeters. The maximum Hg concentration in the Main Bay core was 0.07 micrograms per gram at 5 to 7 centimeters. The general decreases in Hg concentrations with depth are attributed to historic variations in atmospheric Hg deposition rate. Hg stratigraphies combined with 210Pb and 137Cs dating analyses yield Hg chronologies that indicate a general increase in Hg accumulation rates in Devils Lake since the middle of the 19th century.Mean modern Hg accumulation rates in Creel Bay were 4.9 nanograms per square centimeter per year, and rates in Main Bay were 1.8 nanograms per square centimeter per year.
Mean preindustrial Hg accumulation rates in Creel Bay were 1.2 nanograms per square centimeter per year, and rates in Main Bay were 1.6 nanograms per square centimeter per year. Relatively low Hg concentrations in recent sediments in the Devils Lake Basin, along with similarities in Hg accumulation rates between lakes in the Devils Lake Basin and other lakes in the northern interior of North America, indicate that local sources of Hg are not important sources of Hg.
Results of the study indicate that accurate Hg chronologies are discernible in sediment cores collected from saline lakes. However, spatial and temporal variations in lake level and water chemistry common to saline lakes make interpretation of radioisotopic and geochemical chronologies difficult. Hg geochemistry in Devils Lake, and presumably in other saline lakes, is dynamic. The results of this study indicate that the absolute amount of sediment transported to Devils Lake, along with the associated Hg and total organic carbon, and the distribution of sedimentation patterns in Devils Lake may be affected by changing lake levels.
Factors associated with high concentrations of mercury in fish have been established in previous investigations: low pH, soft water, high color content, high chloride. Although atmospheric deposition of mercury has increased during industrial times, it has been difficult to establish that increased mercury deposition has lead to increased mercury in fish. Several hardwater sites contaminated by point sources have been investigated to determine whether increased mercury exposure results in increased fish contamination. Here we report on a mercury point-source contaminated softwater system in a region prone to high concentrations of mercury in fish.
The Sudbury River, a system of free-flowing river reaches, impoundments, and riparian wetlands in eastern Massachusetts, was contaminated over a 60-year period with mercury originating from the Nyanza Superfund Site in the town of Ashland. We used measurements of total mercury (SHg) and monomethylmercury (MeHg) concentrations and streamflow data collected for 18 months at several locations to determine how mercury transport is affected by contamination from the Superfund site. The mean daily load of SHg increased seven-fold as the river flowed past the Superfund site, indicating that the site currently is a source of SHg to the system. The mean daily SHg load at the first sampling location downstream from the Superfund site was 4.5 times greater than a background load estimated by multiplying streamflow at that location by the mean SHg concentration at an upstream reference location. About 1.5 g SHg per day entered the first reservoir in the systemæan amount more than 100 times greater than the estimate for wet deposition on the reservoir surface. About 27% of the mean daily SHg load was removed by sedimentation in the reservoir. However, the load remaining at a sampling location 14 km farther downstream was 48% higher than the estimated load due to atmospheric deposition. Farther downstream in the wetland reach, mean daily SHg loads again increased, indicating the possible presence of a second mercury point source
Transport of MeHg did not increase as the river flowed past the Superfund Site, but did increase significantly in wetland-associated reaches. Although sediments in the reservoir immediately downstream from the Superfund Site were severely contaminated with both SHg and MeHg, net production of MeHg, estimated at 3.7 mg m-2 y-1, was not much greater than that reported for water bodies receiving only atmospheric contamination.
In the late 1980's it was revealed that much of the remaining 900 million acres of the Florida Everglades contained game fish (Bass) that exceeded the State of Florida's "do not consume" advisory limit of 1.5 ppm mercury. Mercury levels observed in game fish from this region are some of the highest ever observed for a large area not directly affected by a point source. In response to this, several State and Federal agencies joined together to initiate several mercury studies in this region to provide a better understanding of the causes of this problem. This study is part of a large, multi-agency effort to understand the nature of mercury cycling in the water, sediments and biota of the Florida Everglades. This project has a strong interdisciplinary component. Currently there are several sub-projects that focus on wide ranging topics, including: 1) process-level studies of mercury species and phase transformations on spatial and temporal scales, 2) mercury methylation and demethylation studies, 3) DOC-mercury interaction studies, 4) sulfur cycling studies, 5) mercury accumulation and diagenetic processes in peat, 5) biological uptake of mercury and lower food chain transfer pathways of mercury, and 6) groundwater/surface water exchange studies. While each of these projects has several specific objectives, the overall project objective is to provide resource managers responsible for making restoration decisions, with scientific information on the hydrologic, biologic, and geochemical processes controlling mercury cycling.
Data from periphyton samples collected in the south Florida ecosystem in 1995 indicate that: (1) The percent nitrogen in periphyton is directly related to the concentrations of total mercury and methyl mercury found in periphytic material. (2) There is a difference between laboratory experiments, where the removal of methyl mercury from solution is related to the organic carbon content of the plant material, and natural concentrations of methyl mercury, which are related to the percent nitrogen in the periphytic material. (3) There is a gradient from larger to smaller concentrations of total mercury and methyl mercury in periphytic material that generally follows a north to south direction in the ecosystem. (4) The methyl mercury gradients for locations with a Water Conservation Area or within the Big Cypress National Preserve are influenced by water column pH with larger concentrations of methyl mercury in periphyton occurring when pH values are approximately neutral.
High concentrations of mercury have been found in fish and fish-eating birds in areas of Maine that are remote from industrial and population centers. Bald eagle nestlings from inland waters contain feather mercury concentrations up to 33 mg/g, the highest reported for the species. Fish mercury concentrations commonly exceed 1 mg/g (wet weight) and range up to 4 mg/g; more than half the samples in a recent survey of fish from 125 randomly-selected lakes exceeded 0.5 mg/g. Analysis of the distribution of lakes in Maine containing fish with high levels of mercury indicates that geological deposits or industrial discharges are not likely sources of this mercury contamination. Deposition from the atmosphere has been implicated as a major source of mercury to lakes. The mercury content of rain and snow in Maine ranges 5-15 ng/L and wet deposition is about 8-10 mg/m2/yr, which is similar to that in other regions where fish are contaminated with mercury. Evidence from lake sediment and ombrotrophic peat bog cores suggests that pre-industrial deposition of mercury to these areas was substantially lower (by a factor of three or more) than present deposition. A survey of mercury content in snowpack and Sphagnum from southern New England to the northern Canadian Maritimes indicates that mercury deposition is highest in south-western locations, which are closest to major population and industrial centers. Present research is focusing on determination of chemical speciation and concentration of inorganic mercury and organo-mercury complexes in water and sediment, and in prey organisms eaten by larger fish, in lakes lacking known local mercury inputs, with the goal of elucidating the source and route of exposure of large fish to mercury.
Mercury bioaccumulation was examined in muscle tissue of walleye (Stizostedion vitreum) from Lake Roosevelt, a large (80,000 acres) reservoir on the upper Columbia River. This system has historically received metal contaminated wastes from mining and associated processing activities. Walleye were collected from three reaches to determine if the concentrations of mercury were at levels that pose a threat to human health and to determine if a spatial difference in mercury bioaccumulation could be predicted by spatial patterns of mercury in bed sediment. We collected 272 walleye and separated them into four size classes (25.4-33 cm, >33-40.6 cm, >40.6-48.3 cm, and >48.3-55.9 cm). Individual samples within each size class consisted of a composite of eight fillets. Overall, concentrations of mercury in muscle tissue ranged from 0.11 to 0.44 mg/kg wet weight (n=34) and were positively correlated with age, weight, and length of walleye. Concentrations of mercury in walleye varied spatially within the reservoir, with the highest concentrations occurring in fish from the lower and middle reaches. Mercury in surficial sediments ranged from <0.05 to 2.70 mg/kg, and generally decreased from the upper to lower reaches. Mercury bioaccumulation could not be predicted by spatial differences in sediment concentrations of total mercury. We suggest that observed spatial patterns of mercury bioaccumulation may be related in part to preferential utilization of specific regions for spawning and foraging, and that these areas likely differ in mercury bioavailability.
Mercury and other toxic metals are introduced into the Sulphur Creek drainage by inactive mercury-gold mines and numerous hot springs which vent directly into the creek water. The hot spring waters have high salinity and are actively depositing mercury, gold, and native sulfur. Ranges in Hg content are: 0.7 to 61 ppb in hot spring waters; 0.2 to 1.1 ppb in Sulphur Creek waters; 22 to 220 ppm in hot spring sediments; and 0.95 to 150 ppm in creek sediments. Mercury is transported in Sulphur Creek as fine grained particles of cinnabar, in suspension, and in solution. The suspension consists of amorphous particles comprised of variable concentrations of iron-sodium-aluminum-silica-chlorine-sulfur, and have a typical grain size of 0.1 to 0.2 microns with some anhedral grains up to several microns in width. Cinnabar and barite grains up to 0.2 microns in length are inter grown with the amorphous particles. Mercury concentration in creek water and sediments decrease rapidly downstream from hot spring and mine areas indicating that mercury is not effectively transported during low stream flow.
Concentrations of Hg exceeded the Maximum Contaminant Level for drinking water of 2 ug/L in over 10 percent of the more than 2,000 (mostly domestic) wells completed in the unconfined Kirkwood-Cohansey aquifer system sampled by various state and county agencies; 31 distinct areas in seven counties were identified where elevated dissolved Hg was detected. Background levels of dissolved Hg in the aquifer system have been determined to be about 10 ng/L. Former land use at the at the majority of the sites was agricultural with residential development in the last 20 to 30 years. Mercurial pesticides and fungicides used in agricultural areas may be an important source of the dissolved Hg, although numerous other point and non-point sources of Hg may also be important at various sites. An ultra-clean sampling technique for collection of ground-water samples for analysis of trace metals and mercury was designed. No blank sample contained detectable mercury. Hg was detected in 4 of 25 public supply wells completed in the aquifer that were sampled using the ultra-clean sampling technique, but none of the samples had an Hg concentration in excess of 2 ug/L. Data from domestic wells and from 2 borehole sites where samples were collected in 10-foot increments below the water table indicate that the highest concentrations of Hg were found at depths between 75 and 110 feet below land surface. The apparent age of water from these depths determined by the tritium/helium-3 dating technique was about 25 to 35 years. This distribution indicates that mercury, possibly introduced at the land surface several decades ago, was mobilized by as yet unidentified chemical mechanisms.
Elevated concentrations of mercury in the Carson River Basin, Nevada, were first reported by A.S. Van Denburgh of the U.S. Geological Survey in 1973.His study showed high concentrations of mercury in samples of whole-water and bottom sediment downstream from pre-1900 ore milling sites. Some of these mercury-rich samples exceeded 200-times background concentrations. Subsequent investigations have further documented the extent and severity of the human-caused contamination problem. Prior to construction of Lahontan Dam in 1915, as part of the DOI Newlands Irrigation Project, much of the mercury stored in mill tailings was flushed downstream to the Carson Desert wetlands by episodic floods. After 1915, Lahontan Reservoir provided an effective trap for fluvial sediment and associated mercury. The vast network of canals, laterals, and drains of the near-century-old Newlands Project downstream from Lahontan Dam has effected the wide distribution of mercury-contaminated fluvial sediment throughout the Carson Desert area. As a consequence of biomagnification of mercury up the food chain, public-health warnings were issued for the consumption of fish from Lahontan Reservoir and of shovler ducks from Carson Lake. Maximum recorded mercury concentrations in sampled media in the Basin are: soils, 180 ug/g (d.w.); whole water, 61 ug/L; bottom sediment, 156 ug/g (d.w.); and aquatic biota, 48 ug/g (d.w. crayfish).
We are evaluating environmental hazards at inactive mercury mines in southwestern Alaska. Mercury concentrations were measured in vegetation, soil, and stream-water samples collected from sites near the Cinnabar Creek and Red Devil mines, as well as from regional background sites. Mercury concentrations in all samples collected near the mines are elevated over those in background samples. Vegetation samples collected near the surface workings at the mines contain as much as 970 ppb Hg, whereas background vegetation samples contain up to 190 ppb Hg. Soil samples collected from the mines contain as much as 1,500 ppm Hg, but background soil samples contain only up to 1.2 ppm Hg. Concentrations of the methylmercury are low in vegetation (less than or equal to 37 ppb) and soil samples (less than or equal to 132 ppb) from the mine sites, however the ratio of methylmercury to total Hg is higher in samples collected from background areas than in samples from the mine sites. Stream-water samples collected downstream from the mines contain a maximum of 0.28 ppb Hg and pH values are near neutral, ranging from 6.4 to 7.6. All stream-water mercury concentrations are well below the 2.0 ppb Hg drinking water standard recommended by the State of Alaska.
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