John F. DeWild

Litterfall mercury dry deposition in the eastern USA

Mercury (Hg) in autumn litterfall from predominately deciduous forests was measured in 3 years of samples from 23 Mercury Deposition Network sites in 15 states across the eastern USA. Annual litterfall Hg dry deposition was significantly higher (median 12.3 micrograms per square meter (μg/m(2)), range 3.5-23.4 μg/m(2)) than annual Hg wet deposition (median 9.6 μg/m(2), range 4.4-19.7 μg/m(2)). The mean ratio of dry to wet Hg deposition was 1.3-1. The sum of dry and wet Hg deposition averaged 21 μg/m(2) per year and 55% was litterfall dry deposition. Methylmercury was a median 0.8% of Hg in litterfall and ranged from 0.6 to 1.5%. Annual litterfall Hg and wet Hg deposition rates differed significantly and were weakly correlated. Litterfall Hg dry deposition differed among forest-cover types. This study demonstrated how annual litterfall Hg dry deposition rates approximate the lower bound of annual Hg dry fluxes.

Mercury export from the Yukon River Basin and potential response to a changing climate

We measured mercury (Hg) concentrations and calculated export and yield from the Yukon River Basin (YRB) to quantify Hg flux from a large, permafrost-dominated, high-latitude watershed. Exports of Hg averaged 4400 kg Hg yr(-1). The average annual yield for the YRB during the study period was 5.17 μg m(-2) yr(-1), which is 3-32 times more than Hg yields reported for 8 other major northern hemisphere river basins. The vast majority (90%) of Hg export is associated with particulates. Half of the annual export of Hg occurred during the spring with about 80% of 34 samples exceeding the U.S. EPA Hg standard for adverse chronic effects to biota. Dissolved and particulate organic carbon exports explained 81% and 50%, respectively, of the variance in Hg exports, and both were significantly (p < 0.001) correlated with water discharge. Recent measurements indicate that permafrost contains a substantial reservoir of Hg. Consequently, climate warming will likely accelerate the mobilization of Hg from thawing permafrost increasing the export of organic carbon associated Hg and thus potentially exacerbating the production of bioavailable methylmercury from permafrost-dominated northern river basins.

Mercury Loading and Methylmercury Production and Cycling in High-Altitude Lakes from the Western United States

Studies worldwide have shown that mercury (Hg) is a ubiquitouscontaminant, reaching even the most remote environments such ashigh-altitude lakes via atmospheric pathways. However, very fewstudies have been conducted to assess Hg contamination levels ofthese systems. We sampled 90 mid-latitude, high-altitude lakes from seven national parks in the western United States during afour-week period in September 1999. In addition to the synoptic survey, routine monitoring and experimental studies were conducted at one of the lakes (Mills Lake) to quantify MeHg fluxrates and important process rates such as photo-demethylation. Results show that overall, high-altitude lakes have low total mercury (HgT) and methylmercury (MeHg) levels (1.07 and 0.05 ng L-1, respectively), but a very good correlation of Hg to MeHg (r2= 0.82) suggests inorganic Hg(II) loading is a primary controlling factor of MeHg levels in dilute mountain lakes. Positive correlations were also observed for dissolved organic carbon (DOC) and both Hg and MeHg, although to a much lesser degree. Levels of MeHg were similar among the seven national parks, with the exception of Glacier National Park where lowerconcentrations were observed (0.02 ng L-1), and appear to berelated to naturally elevated pH values there. Measured rates ofMeHg photo-degradation at Mills Lake were quite fast, and thisprocess was of equal importance to sedimentation and stream flowfor removing MeHg. Enhanced rates of photo-demethylation are likely an important reason why high-altitude lakes, with typically high water clarity and sunlight exposure, are low in MeHg.

Methylmercury in flood-control impoundments and natural waters of northwestern Minnesota, 1997-99

We studied methylmercury (MeHg) and totalmercury (HgT) in impounded and natural surface waters innorthwestern Minnesota, in settings ranging from agriculturalto undeveloped. In a recently constructed (1995) permanent-pool impoundment, MeHg levels typically increased from inflowto outflow during 1997; this trend broke down from late 1998 toearly 1999. MeHg levels in the outflow reached seasonal maximain mid-summer (maximum of 1.0 ng L-1 in July 1997) andlate-winter (maximum of 6.6 ng L-1 in February 1999), andare comparable to high levels observed in new hydroelectricreservoirs in Canada. Spring and autumn MeHg levels weretypically about 0.1–0.2 ng L-1. Overall, MeHg levels inboth the inflow (a ditch that drains peatlands) and outflowwere significantly higher than in three nearby referencenatural lakes. Eleven older permanent-pool impoundments andsix natural lakes in northwestern Minnesota were sampled fivetimes. The impoundments typically had higher MeHg levels(0.071–8.36 ng L-1) than natural lakes. Five of six lakesMeHg levels typical of uncontaminated lakes (0.014–1.04 ngL-1) with highest levels in late winter, whereas ahypereutrophic lake had high levels (0.37–3.67 ng L-1)with highest levels in mid-summer. Seven temporary-poolimpoundments were sampled during summer high-flow events. Temporary-pool impoundments that retained water for about 10–15days after innundation yielded pronounced increases in MeHgfrom inflow to outflow, in one case reaching 4.6 ng L-1,which was about 2 ng L-1 greater than the mean inflowconcentration during the runoff event.

Spatial distribution of mercury in southeastern Alaskan streams influenced by glaciers, wetlands, and salmon

Southeastern Alaska is a remote coastal-maritime ecosystem that is experiencing increased deposition of mercury (Hg) as well as rapid glacier loss. Here we present the results of the first reported survey of total and methyl Hg (MeHg) concentrations in regional streams and biota. Overall, streams draining large wetland areas had higher Hg concentrations in water, mayflies, and juvenile salmon than those from glacially-influenced or recently deglaciated watersheds. Filtered MeHg was positively correlated with wetland abundance. Aqueous Hg occurred predominantly in the particulate fraction of glacier streams but in the filtered fraction of wetland-rich streams. Colonization by anadromous salmon in both glacier and wetland-rich streams may be contributing additional marine-derived Hg. The spatial distribution of Hg in the range of streams presented here shows that watersheds are variably, yet fairly predictably, sensitive to atmospheric and marine inputs of Hg.

Permafrost Stores a Globally Significant Amount of Mercury

Changing climate in northern regions is causing permafrost to thaw with major implications for the global mercury (Hg) cycle. We estimated Hg in permafrost regions based on in situ measurements of ...

The Effect of Natural Organic Matter on Mercury Methylation by Desulfobulbus propionicus 1pr3.

Methylation of tracer and ambient mercury (200Hg and 202Hg, respectively) equilibrated with four different natural organic matter (NOM) isolates was investigated in vivo using the Hg-methylating sulfate-reducing bacterium Desulfobulbus propionicus 1pr3. Desulfobulbus cultures grown fermentatively with environmentally representative concentrations of dissolved NOM isolates, Hg[II], and HS− were assayed for absolute methylmercury (MeHg) concentration and conversion of Hg(II) to MeHg relative to total unfiltered Hg(II). Results showed the 200Hg tracer was methylated more efficiently in the presence of hydrophobic NOM isolates than in the presence of transphilic NOM, or in the absence of NOM. Different NOM isolates were associated with variable methylation efficiencies for either the 202Hg tracer or ambient 200Hg. One hydrophobic NOM, F1 HpoA derived from dissolved organic matter from the Florida Everglades, was equilibrated for different times with Hg tracer, which resulted in different methylation rates. A 5 day equilibration with F1 HpoA resulted in more MeHg production than either the 4 h or 30 day equilibration periods, suggesting a time dependence for NOM-enhanced Hg bioavailability for methylation.

Mercury accumulation and the mercury-PCB-sex interaction in summer flounder

Patterns in the relative differences in contaminant concentrations between the sexes of mature fish may reveal important behavioral and physiological differences between the sexes. We determined whole-fish total mercury (Hg) concentrations in 23 female summer flounder (Paralichthys dentatus) and 27 male summer flounder from New Jersey coastal waters. To estimate the change in Hg concentration due to release of eggs at spawning, Hg concentration in the somatic tissue and ovaries of 5 of the 23 female summer flounder were also determined. To ascertain whether most of the Hg in the summer flounder was methylmercury (MeHg), whole-fish MeHg concentrations were determined in all 50 summer flounder. Whole-fish Hg concentrations averaged 113 ng/g for females and 111 ng/g for males. Thus, females were 2% higher in Hg concentration than males, on average, but the difference was not statistically significant. Based on Hg determinations in the somatic tissue and ovaries, we predicted that Hg concentration of females would increase by 3.7%, on average, immediately after spawning due to release of eggs. On average, 92% of the Hg in the summer flounder was MeHg. To determine whether the effect of sex on Hg concentration was significantly different from the effect of sex on polychlorinated biphenyl (PCB) concentration, we paired our Hg determinations with PCB determinations from a previous study, and applied regression analysis. Sex significantly interacted with contaminant type (Hg or PCBs), as males were 43% higher in PCB concentration than females, whereas females were 2% higher in Hg concentration than males. Males eliminating Hg from their bodies at a faster rate than females was a likely explanation for this discrepancy between the two contaminant types. Overall, the Hg and PCB concentrations in the summer flounder were relatively low, and therefore our findings also had implications for continued operation of the summer flounder fishery.

Assessing bias in total mercury results after removing a subsample from the bottle

ABSTRACT U.S. EPA Method 1631 for total mercury (THg) analysis in water recommends that bromine monochloride (BrCl) be added to the original bottle in which the sample was collected, to draw into solution any Hg that may have adsorbed to the bottle walls. The method also allows for the removal of a subsample of water from the sample bottle for methylmercury (MeHg) analysis prior to adding BrCl. We have demonstrated that the removal of a subsample from the sample bottle prior to THg analysis can result in a positive concentration bias. The proposed mechanism for the bias is that ‘excess’ inorganic Hg, derived from the subsample that was removed from the bottle, adsorbs to the bottle walls and is then drawn into solution when BrCl is added. To test for this bias, we conducted an interlaboratory comparison study in which nine laboratories analysed water samples in fluorinated polyethylene (FLPE) bottles for THg after removing a subsample from the sample bottle, and analysed a replicate sample bottle from which no subsample was removed. We received seven complete data sets, or 63 unique sample pairs. The positive concentration bias between the bottles was significant when comparing all samples in aggregate (1.76 ± 0.53 ng/L after subsample removal, 1.57 ± 0.58 ng/L with no subsample removal, P < 0.05), however when comparing each of the three samples individually, the only significant bias was in the saline sample (Site UJ; 1.51 ± 0.31 ng/L after subsample removal, 1.32 ± 0.47 ng/L with no subsample removal, P < 0.05). Based on the findings presented here, we conclude that water chemistry, volume of water poured off, and the sample storage temperature explain some but not all of the observed bias, and we recommend collecting THg and MeHg samples in separate bottles whenever possible.

Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of the Laurentian Great Lakes

Identifying the sources of methylmercury (MeHg) and tracing the transformations of mercury (Hg) in the aquatic food web are important components of effective strategies for managing current and legacy Hg sources. In our previous work, we measured stable isotopes of Hg (δ202Hg, Δ199Hg, and Δ200Hg) in the Laurentian Great Lakes and estimated source contributions of Hg to bottom sediment. Here, we identify isotopically distinct Hg signatures for Great Lakes trout ( Salvelinus namaycush) and walleye ( Sander vitreus), driven by both food-web and water-quality characteristics. Fish contain high values for odd-isotope mass independent fractionation (MIF) with averages ranging from 2.50 (western Lake Erie) to 6.18‰ (Lake Superior) in Δ199Hg. The large range in odd-MIF reflects variability in the depth of the euphotic zone, where Hg is most likely incorporated into the food web. Even-isotope MIF (Δ200Hg), a potential tracer for Hg from precipitation, appears both disconnected from lake sedimentary sources and comparable in fish among the five lakes. We suggest that similar to the open ocean, water-column methylation also occurs in the Great Lakes, possibly transforming recently deposited atmospheric Hg deposition. We conclude that the degree of photochemical processing of Hg is controlled by phytoplankton uptake rather than by dissolved organic carbon quantity among lakes.