Applied Geochemistry (v.23, #3)
Transport and fate of mercury in the environment by Mae Sexauer Gustin; Allan Kolker; Katarina Gårdfeldt (343-344).
Mercury emission from terrestrial background surfaces in the eastern USA. Part I: Air/surface exchange of mercury within a southeastern deciduous forest (Tennessee) over one year by Todd Kuiken; Hong Zhang; Mae Gustin; Steve Lindberg (345-355).
This study focused on the development of a seasonal data set of the Hg air/surface exchange over soils associated with low Hg containing surfaces in a deciduous forest in the southern USA. Data were collected every month for 11 months in 2004 within Standing Stone State Forest in Tennessee using the dynamic flux chamber method. Mercury air/surface exchange associated with the litter covered forest floor was very low with the annual mean daytime flux being 0.4 ± 0.5 ng m−2 h−1 (n = 301). The daytime Hg air/surface exchange over the year oscillated between emission (81% of samples with positive flux) and deposition (19% of samples with negative flux). A seasonal trend of lower emission in the spring and summer (closed canopy) relative to the fall and winter (open canopy) was observed. Correlations were found between the air/surface exchange and certain environmental factors on specific days sampled but not collectively over the entire year. The very low magnitude of Hg air/surface exchange as observed in this study suggests that an improved methodology for determining and reporting emission fluxes is needed when the values of fluxes and chamber blanks are both very low and comparable. This study raises questions and points to a need for more research regarding how to scale the Hg air/surface exchange for surfaces with very low emissions.
Mercury emission from terrestrial background surfaces in the eastern USA. II: Air/surface exchange of mercury within forests from South Carolina to New England by Todd Kuiken; Mae Gustin; Hong Zhang; Steve Lindberg; Ben Sedinger (356-368).
Mercury air/surface exchange was measured over litter-covered soils with low Hg concentrations within various types of forests along the eastern seaboard of the USA. The fieldwork was conducted at six forested sites in state parks in South Carolina, North Carolina, New Jersey, Pennsylvania, New York and Maine from mid-May to early June 2005. The study showed that the Hg air/surface exchange was consistently very low and similar (overall daytime mean flux = 0.2 ± 0.9 ng m−2 h−1, n = 310, for all six sites monitored) with the various forest types. These flux values are comparable with those found in a year-long study in Tennessee (yearly daytime mean = 0.4 ± 0.5 ng m−2 h−1), but lower than many previous flux results reported for background soils. The Hg fluxes at all sites oscillated around zero, with many episodes of deposition (negative fluxes) occurring in both daytime and nighttime. While there were particular days showing significant correlations among the Hg air/surface exchange and certain environmental parameters, perhaps because of the low fluxes encountered, few significant correlations were found for any particular day of sampling between the Hg flux and environmental parameters such as solar radiation, soil temperature, air temperature (little variability seen), relative humidity, and ambient air Hg concentrations. Factors driving the Hg exchange as previously found for enriched soils may not hold for these background litter-covered forest soils. The results suggest that spatial variations of the Hg air/surface exchange were small among these different forest types for this particular time of year.
Gaseous mercury emissions from urban surfaces: Controls and spatiotemporal trends by Chris S. Eckley; Brian Branfireun (369-383).
The spatial and temporal variability of Hg emissions from urban paved surfaces was assessed through repeated measurements under varying environmental conditions at six sample sites in Toronto, Ontario, Canada. The results show significant spatial variability of the Hg emissions with median values ranging from below detection limit to 5.2 ng/m2/h. Two of the sites consistently had higher Hg emissions (on several occasions >20 ng/m2/h) than the other 4, which were equivalently low (maximum emission: 2.1 ng/m2/h). A surrogate measure of the pavement Hg concentrations was obtained during each day of sampling through the collection of street dust. The median street dust concentration also showed significant spatial variability (ranging from 9.6 to 44.5 ng/g). Regression analysis showed that the spatial variability of the Hg emissions was significantly related to the street dust concentrations. Controlled experiments using Hg amended street dust confirmed the relationship between Hg surface concentration and emission magnitude. Within a given sample site, Hg emissions varied temporally and multiple regression analysis showed that within-site variability was significantly influenced by changes in solar radiation with only a minor effect from surface temperature. Controlled experiments using shade cloths confirmed that solar radiation can have a large influence on the magnitude of Hg emissions within a given site. The emissions measured in Toronto were contextualized through comparison sampling in Austin, Texas. The Hg emissions measured in Austin were within the range detected in Toronto and also showed significant correlation with Hg street dust concentrations between sites. To provide a holistic assessment of Hg emissions from urban environments, samples were also collected from other common urban surfaces (soil, roofs, and windows). Soils consistently had higher emissions than all the other surfaces (7.3 ng/m2/h, n = 39).
A comparison of winter mercury accumulation at forested and no-canopy sites measured with different snow sampling techniques by Sarah J. Nelson; Kenneth B. Johnson; Kathleen C. Weathers; Cynthia S. Loftin; Ivan J. Fernandez; Jeffrey S. Kahl; David P. Krabbenhoft (384-398).
Atmospheric mercury (Hg) is delivered to ecosystems via rain, snow, cloud/fog, and dry deposition. The importance of snow, especially snow that has passed through the forest canopy (throughfall), in delivering Hg to terrestrial ecosystems has received little attention in the literature. The snowpack is a dynamic system that links atmospheric deposition and ecosystem cycling through deposition and emission of deposited Hg. To examine the magnitude of Hg delivery via snowfall, and to illuminate processes affecting Hg flux to catchments during winter (cold season), Hg in snow in no-canopy areas and under forest canopies measured with four collection methods were compared: (1) Hg in wet precipitation as measured by the Mercury Deposition Network (MDN) for the site in Acadia National Park, Maine, USA, (2) event throughfall (collected after snowfall cessation for accumulations of >8 cm), (3) season-long throughfall collected using the same apparatus for event sampling but deployed for the entire cold season, and (4) snowpack sampling. Estimates (mean ± SE) of Hg deposition using these methods during the 91-day cold season in 2004–2005 at conifer sites showed that season-long throughfall Hg flux (1.80 μg/m2) < snowpack Hg (2.38 ± 0.68 μg/m2) < event throughfall flux (5.63 ± 0.38 μg/m2). Mercury deposition at the MDN site (0.91 μg/m2) was similar to that measured at other no-canopy sites in the area using the other methods, but was 3.4 times less than was measured under conifer canopies using the event sampling regime. This indicates that snow accumulated under the forest canopy received Hg from the overstory or exhibited less re-emission of Hg deposited in snow relative to open areas. The soil surface of field-scale plots were sprayed with a natural rain water sample that contained an Hg tracer (202Hg) just prior to the first snowfall to explore whether some snowpack Hg might be explained from soil emissions. The appearance of the 202Hg tracer in the snowpack (0–64% of the total Hg mass in the snowpack) suggests that movement of Hg from the soil into the snowpack is possible. However, as with any tracer study the 202Hg tracer may not precisely represent the reactivity and mobility of natural Hg in soils.
Recent increase in atmospheric deposition of mercury to California aquatic systems inferred from a 300-year geochronological assessment of lake sediments by Rhea D. Sanders; Kenneth H. Coale; Gary A. Gill; Allen H. Andrews; Mark Stephenson (399-407).
Age-dated sediment cores from 4 remote lakes across California were analyzed for total Hg (HgT) concentration as a function of pre- and post-industrialization. Particle size, magnetic susceptibility and organic C and N, were measured to determine if the Hg concentration in sediment cores could be related to atmospheric deposition and/or watershed processes. Results indicate that (a) for each lake modern (1970–2004) HgT lake sediment concentrations have increased by an average factor of 5 times more than historic (pre-1850) HgT concentrations; (b) the ratio of modern to pre-industrial lake sediment HgT for these lakes are higher than estimated for other locations where atmospheric deposition is presumed to be the main source of Hg; (c) 2 of the 4 studied lakes demonstrated significant relationships between HgT concentrations and percentage organic material (r 2 = 0.68 and p < 0.01; r 2 = 0.67 and p < 0.01) whereas the other two indicated no significant relationship (r 2 = 0.05 and p = 0.51; r 2 = 0.12 and p = 0.36).
Total particulate and reactive gaseous mercury in ambient air on the eastern slope of the Mt. Gongga area, China by Xuewu Fu; Xinbin Feng; Wanze Zhu; Wei Zheng; Shaofeng Wang; Julia Y. Lu (408-418).
Total particulate mercury (TPM) and reactive gaseous mercury (RGM) concentrations in ambient air on the eastern slope of the Mt. Gongga area, Sichuan Province, Southwestern China were monitored from 25 May, 2005 to 29 April, 2006. Simultaneously, Hg concentrations in rain samples were measured from January to December, 2006. The average TPM and RGM concentrations in the study site were 30.7 and 6.2 pg m−3, which are comparable to values observed in remote areas in Northern America and Europe, but much lower than those reported in some urban areas in China. The mean seasonal RGM concentration was slightly higher in spring (8.0 pg m−3) while the minimum mean concentration was observed in winter (4.0 pg m−3). TPM concentrations ranged across two orders of magnitude from 5.2 to 135.7 pg m−3 and had a clear seasonal variation: winter (74.1 pg m−3), autumn (22.5 pg m−3), spring (15.3 pg m−3) and summer (10.8 pg m−3), listed in decreasing order. The annual wet deposition was 9.1 μg m−2 and wet deposition in the rainy season (May–October) represented over 80% of the annual total. The temporal distribution of TPM and RGM suggested distinguishable dispersion characteristics of these Hg species on a regional scale. Elevated TPM concentration in winter was probably due to regional and local enhanced coal burning and low wet deposition velocity. The RGM distribution pattern is closely related to daily variation in UV radiation observed during the winter sampling period indicating that photo-oxidation processes and diurnal changes in meteorology play an important role in RGM generation.
Characterization and cycling of atmospheric mercury along the central US Gulf Coast by Mark A. Engle; Michael T. Tate; David P. Krabbenhoft; Allan Kolker; Mark L. Olson; Eric S. Edgerton; John F. DeWild; Ann K. McPherson (419-437).
Concentrations of atmospheric Hg species, elemental Hg (Hg∘), reactive gaseous Hg (RGM), and fine particulate Hg (Hg-PM2.5) were measured at a coastal site near Weeks Bay, Alabama from April to August, 2005 and January to May, 2006. Mean concentrations of the species were 1.6 ± 0.3 ng m−3, 4.0 ± 7.5 pg m−3 and 2.7 ± 3.4 pg m−3, respectively. A strong diel pattern was observed for RGM (midday maximum concentrations were up to 92.7 pg m−3), but not for Hg∘ or Hg-PM2.5. Elevated RGM concentrations (>25 pg m−3) in April and May of 2005 correlated with elevated average daytime O3 concentrations (>55 ppbv) and high light intensity (>500 W m−2). These conditions generally corresponded with mixed continental-Gulf and exclusively continental air mass trajectories. Generally lower, but still elevated, RGM peaks observed in August, 2005 and January–March, 2006 correlated significantly (p < 0.05) with peaks in SO2 concentration and corresponded to periods of high light intensity and lower average daytime O3 concentrations. During these times air masses were dominated by trajectories that originated over the continent. Elevated RGM concentrations likely resulted from photochemical oxidation of Hg∘ by atmospheric oxidants. This process may have been enhanced in and by the near-shore environment relative to inland sites. The marine boundary layer itself was not found to be a significant source of RGM.Size segregation determination, using a limited dataset from two different methods, suggested that a significant fraction of particulate Hg was bound to coarse particles (>2.5 μm). A potential source of the large fraction of coarse particulate Hg in the study area is sequestration of RGM within sea salt aerosols. The presence of rapidly depositing RGM and coarse particulate Hg may be important sources of Hg input along the Gulf Coast. However, the impact of these species on deposition rates is yet to be determined.
Atmospheric mercury near Salmon Falls Creek Reservoir in southern Idaho by Michael L. Abbott; Che-Jen Lin; Peter Martian; Jeffrey J. Einerson (438-453).
Gaseous elemental mercury (GEM) and reactive gaseous mercury (RGM) were measured over 2-week seasonal field campaigns near Salmon Falls Creek Reservoir in south-central Idaho from the summer of 2005 through the fall of 2006 and over the entire summer of 2006 using automated Tekran Hg analyzers. GEM, RGM, and particulate Hg (HgP) were also measured at a secondary site 90 km to the west in southwestern Idaho during the summer of 2006. The study was performed to characterize Hg air concentrations in the southern Idaho area for the first time, estimate Hg dry deposition rates, and investigate the source of observed elevated concentrations. High seasonal variability was observed with the highest GEM (1.91 ± 0.9 ng m−3) and RGM (8.1 ± 5.6 pg m−3) concentrations occurring in the summer and lower values in the winter (1.32 ± 0.3 ng m−3, 3.2 ± 2.9 pg m−3 for GEM, RGM, respectively). The summer-average HgP concentrations were generally below detection limit (0.6 ± 1 pg m−3). Seasonally averaged deposition velocities calculated using a resistance model were 0.034 ± 0.032, 0.043 ± 0.040, 0.00084 ± 0.0017 and 0.00036 ± 0.0011 cm s−1 for GEM (spring, summer, fall and winter, respectively) and 0.50 ± 0.39, 0.40 ± 0.31, 0.51 ± 0.43 and 0.76 ± 0.57 cm s−1 for RGM. The total annual RGM + GEM dry deposition estimate was calculated to be 11.9 ± 3.3 μg m−2, or about 2/3 of the total (wet + dry) deposition estimate for the area. Periodic elevated short-term GEM (2.2–12 ng m−3) and RGM (50–150 pg m−3) events were observed primarily during the warm seasons. Back-trajectory modeling and PSCF analysis indicate predominant source directions to the SE (western Utah, northeastern Nevada) and SW (north-central Nevada) with fewer inputs from the NW (southeastern Oregon and southwestern Idaho).
Sensitivity of the global atmospheric cycle of mercury to emissions by Kristen Lohman; Christian Seigneur; Mae Gustin; Steve Lindberg (454-466).
A systematic investigation of the impact of current uncertainties in Hg emissions from specific source categories on global air Hg concentrations is presented. First, the uncertainties in different emission source categories are discussed and then the results of a base simulation and three sensitivity simulations conducted with a global chemical transport model for mercury (CTM-Hg) are presented. The total Hg emissions in the four scenarios range from 6600 to 9400 Mg/a. The sensitivity studies investigate the impact of the range in uncertainty in natural emissions, emissions of previously deposited Hg, and anthropogenic emissions both in China and worldwide, while taking into account constraints imposed by available data (current/pre-industrial emission ratio of 2–4). In one case, natural emissions and emissions of previously deposited Hg were changed to represent a mid point of the range of values found in the literature. This lead to a 16% increase in background emissions, i.e., natural emissions and emissions of previously deposited Hg combined. Increasing natural emissions by 16% or Chinese anthropogenic emissions by 100% yielded atmospheric Hg concentrations comparable with those measured across the globe without any changes to the atmospheric chemistry. Increasing natural emissions and emissions of previously deposited Hg by 16% and all anthropogenic emissions by 100% as compared to the base scenario yielded atmospheric Hg concentrations that were not compatible with measurements and changes in the chemical behavior of Hg in the atmosphere would be required to yield results that are consistent with observed Hg concentrations. The current uncertainty in total Hg emissions at the global scale is placed at about a factor of two.
The development and application of a mass balance model for mercury (total, elemental and methyl) using data from a remote lake (Big Dam West, Nova Scotia, Canada) and the multi-species multiplier method by A.L.M. Ethier; D. Mackay; L.K. Toose-Reid; N.J. O’Driscoll; A.M. Scheuhammer; D.R.S. Lean (467-481).
Big Dam West, located in Kejimkujik Park, Nova Scotia, Canada, is a remote lake with elevated Hg concentrations in fish due in part to low pH and high dissolved organic carbon (DOC) concentrations. These features reflect the poor buffering capacity of peat lands in the flat drainage basin. To address the multiple species of Hg, a model was developed by coupling the “multiplier method” for multi-species chemicals with average concentration ratios to the Quantitative Water Air Sediment Interaction (QWASI) model. Elemental Hg was the “key species” modeled. Total Hg fluxes and concentrations were computed using concentration ratios. Many of the concentrations and Hg flux processes within the Big Dam West catchment had been previously measured with concentration ratios that were either computed or taken from the literature when measured data were not available. Measured values for total Hg concentration in each compartment (air, water, sediment) and Hg fluxes (e.g., precipitation, sediment deposition) enable verification of model concentration and flux estimates for total Hg in each environmental compartment. The model was also run with and without the inclusion of upward Hg fluxes from the underlying sediment to determine the significance of this controversial flux. The Hg QWASI model presented in this paper could be a valuable screening tool, especially for remote lakes, due to its ability to provide reasonable Hg estimates with a limited amount of data (water inflow rate, suspended particulate matter, sediment deposition velocity, and concentration of Hg in atmosphere and inflow water).
An update on the natural sources and sinks of atmospheric mercury by Mae Sexauer Gustin; Steven E. Lindberg; Peter J. Weisberg (482-493).
This paper summarizes recent advances in the understanding of the exchange of Hg between the atmosphere and natural terrestrial surfaces including substrates (soil, rocks, litter-covered surfaces and weathered lithological material) and foliage. Terrestrial landscapes may act as new sources of atmospheric Hg, and as repositories or temporary residences for anthropogenically and naturally derived atmospheric Hg. The role of terrestrial surfaces as sources and sinks of atmospheric Hg must be quantified in order to develop regional and global Hg mass balances, and to assess the efficacy of regulatory controls on anthropogenic point sources in reduction of human Hg exposure.Continued field research has allowed for refinement of emission estimates for geothermal and volcanic, and Hg mineralized areas in the western USA to ∼1.2–3.0, and 10–20 Mg/a, respectively. The emission estimate for areas of Hg mineralization in the western USA includes only identified Hg deposits and occurrences, and since other areas of geologic Hg enrichment such as Au and Ag deposits are not considered, the range in values is most likely an underestimate. Laboratory and field measurements have improved understanding of air–surface Hg exchange associated with soils with low or natural background concentrations of Hg (<100 ppb), litter-covered forest floors, and foliar surfaces, all of which have large spatial coverage. Deposition of atmospheric Hg and re-emission are important processes occurring at these surfaces on diel and seasonal time scales. Foliage is a significant sink for atmospheric elemental Hg, however, the net flux associated with low Hg containing soils is uncertain. Mass balances developed for soil–air exchange using measured fluxes and estimated deposition indicate that over a year background soils may exhibit no net flux. This suggests that the residence time for elemental Hg in the air is on the order of hours to weeks. Short term exchange would result in a homogenous air Hg concentration due to constant mixing and in an apparent calculated residence time that is most likely too long (one year). Recycling of atmospheric Hg between natural background soils and foliar surfaces also provides a mechanism for long-term atmospheric contamination and continued deposition in pristine ecosystems well after anthropogenic sources are controlled.
Phytoreduction and volatilization of mercury by ascorbate in Arabidopsis thaliana, European beech and Norway spruce by Florian Battke; Dieter Ernst; Frank Fleischmann; Stefan Halbach (494-502).
Mercury vapor (Hg0) emission from plants contributes to the atmospheric Hg cycle. Young barley (Hordeum vulgare L.) plants grown on a hydroponic cultivation medium containing Hg(II) have previously been shown to increase their Hg0 emission significantly by reduction of Hg(II) with endogenous ascorbic acid. Regarding the potential contribution to the Hg cycle from the vast forest-covered areas, it was important to investigate this mechanism in trees. The increase in Hg0 emission from young European beech plants cultivated on a HgCl2 medium exceeded that from controls by ca. tenfold and was proportional to the Hg(II) concentration. From these experiments, a flux of 12.8 μg Hg0/h/m2 was estimated at an exposure of the roots to 20 μM Hg(II). Mercury vapor release from homogenates of Norway spruce needles exceeded that from European beech leaves by a factor of 2.3–4, i.e. in proportion to the reported AA concentrations; the reduction was maximal at alkaline pH which is typical for AA. The 8.4-fold difference in Hg0 release between homogenates from wild-type Arabidopsis thaliana and from its AA-deficient mutant vtc 1-1 also paralleled the reported difference in AA levels of both species. It is concluded that the phytoreduction and vaporization of Hg by AA is an important mechanism as much for Hg detoxification in trees as for Hg emission to the atmosphere. The efficiency of this process seems to result from the optimal coordination of transfer and biochemical transformation of mercuric ions and Hg vapor. There is no evidence for a relevant difference in the mechanisms of biogenic Hg(II) reduction between grass plants and trees.
Assessing sulfate and carbon controls on net methylmercury production in peatlands: An in situ mesocosm approach by Carl P.J. Mitchell; Brian A. Branfireun; Randall K. Kolka (503-518).
The transformation of atmospherically deposited inorganic Hg to the toxic, organic form methylmercury (MeHg) is of serious ecological concern because MeHg accumulates in aquatic biota, including fish. Research has shown that the Hg methylation reaction is dependent on the availability of SO4 (as an electron acceptor) because SO4-reducing bacteria (SRB) mediate the biotic methylation of Hg. Much less research has investigated the possible organic C limitations to Hg methylation (i.e. from the perspective of the electron donor). Although peatlands are long-term stores of organic C, the C derived from peatland vegetation is of questionable microbial lability. This research investigated how both SO4 and organic C control net MeHg production using a controlled factorial addition design in 44 in situ peatland mesocosms. Two levels of SO4 addition and energetic-equivalent additions (i.e. same number of electrons) of a number of organic C sources were used including glucose, acetate, lactate, coniferous litter leachate, and deciduous litter leachate. This study supports previous research demonstrating the stimulation of MeHg production from SO4 input alone (∼200 pg/L/day). None of the additions of organic C alone resulted in significant MeHg production. The combined addition of SO4 and some organic C sources resulted in considerably more MeHg production (∼500 pg/L/day) than did the addition of SO4 alone, demonstrating that the highest levels of MeHg production can be expected only where fluxes of both SO4 and organic C are delivered concurrently. When compared to a number of pore water samples taken from two nearby peatlands, MeHg concentrations resulting from the combined addition of SO4 and organic C in this study were similar to MeHg “hot spots” found near the upland–peatland interface. The formation of MeHg “hot spots” at the upland–peatland interface may be dependent on concurrent inputs of SO4 and organic C in runoff from the adjacent upland hillslopes.
The pathway of mercury in contaminated waters determined by association with organic carbon (Tagus Estuary, Portugal) by João Canário; Carlos Vale; Marta Nogueira (519-528).
Reactive dissolved Hg (HgR), non-reactive dissolved Hg (HgNR), particulate Hg (HgP), dissolved organic C (DOC), particulate organic C (POC), salinity and other interpretative parameters were determined in water samples collected in the North Channel and in adjacent areas of the Tagus estuary (Portugal). Higher concentrations of both dissolved and particulate Hg in the North Channel indicate a pollution source and raise the possibility of Hg escaping to adjacent areas by tidal action. This transport was confirmed by the increase of HgR with salinity and HgNR with DOC, along a longitudinal axis paralleling the North Channel. Apparently, Hg leaving this channel is progressively complexed by inorganic and organic ligands. Near the mouth of the estuary, values decreased reflecting dilution with seawater. Moreover the HgP:POC ratio also increased seaward, suggesting mixing with Hg enriched particles that escaped the North Channel, or incorporation of dissolved Hg species in river-derived particles. These results suggest that the pathway of anthropogenic Hg in contaminated waters may be identified by their enrichment in organic matter, both in the dissolved and particulate fraction.
Benthic fluxes of mercury species in a lagoon environment (Grado Lagoon, Northern Adriatic Sea, Italy) by Stefano Covelli; Jadran Faganeli; Cinzia De Vittor; Sergio Predonzani; Alessandro Acquavita; Milena Horvat (529-546).
The role of the major biogeochemical processes in Hg cycling at the sediment–water interface was investigated in the Grado Lagoon (Northern Adriatic Sea). This wetland system has been extensively contaminated from the Idrija Hg Mine (Slovenia) through the Isonzo River suspended load carried by tidal fluxes. Three approaches were used to study the sediment–water exchange of total Hg (THg), methylmercury (MeHg), reactive Hg (RHg) and dissolved gaseous Hg (DGHg): (1) estimation of diffusive fluxes from porewater and overlying water concentrations, (2) measurements of benthic fluxes using a deployed light benthic chamber in situ and (3) measurements of benthic fluxes during oxic–anoxic transition with a laboratory incubation experiment. The THg solid phase, ranging between 9.5 and 14.4 μg g−1, showed slight variability with depth and time. Conversely, MeHg contents were highest (up to 21.9 ng g−1) at the surface; they tended to decrease to nearly zero concentration with depth, thus suggesting that MeHg production and accumulation occur predominantly just below the sediment–water interface. Porewater MeHg concentrations (0.9–7.9 ng L−1, 0.15–15% of THg) varied seasonally; higher contents were observed in the warmer period. The MeHg diffusive fluxes (up to 17 ng m−2 day−1) were similar to those in the nearby Gulf of Trieste [Covelli, S., Horvat, M., Faganeli, J., Brambati, A., 1999. Porewater distribution and benthic flux of mercury and methylmercury in the Gulf of Trieste (Northern Adriatic Sea). Estuar. Coast. Shelf Sci. 48, 415–428], although the lagoon sediments contained four-fold higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (up to 110 ng m−2 day−1) were one- to two-fold higher than those previously estimated for the Gulf of Trieste. The diurnal MeHg benthic fluxes were highest in summer at both sites (41,000 and 33,000 ng m−2 day−1 at the fishfarm and in the open lagoon, respectively), thus indicating the influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with O2 and inversely with DIC, suggesting an important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through the production of organic matter promptly available for methylation. The results from the dark chamber incubated in the laboratory showed that the regeneration of dissolved THg was slightly affected by the oxic–anoxic transition. Conversely, the benthic flux of MeHg was up to 15-fold higher in sediments overlain by O2 depleted waters. In the anoxic phase, the MeHg fluxes proceeded in parallel with Fe fluxes and the methylated form reached approximately 100% of dissolved THg. The MeHg is mostly released into overlying water (mean recycling efficiency of 89%) until the occurrence of sulphide inhibition, due to scavenging of the available Hg substrate for methylation. The results suggest that sediments in the Grado Lagoon, especially during anoxic events, should be considered as a primary source of MeHg for the water column.
Evidence for mass-independent and mass-dependent fractionation of the stable isotopes of mercury by natural processes in aquatic ecosystems by Togwell A. Jackson; D. Michael Whittle; Marlene S. Evans; Derek C.G. Muir (547-571).
Isotopic and chemical analyses were performed on crustaceans, forage fish, top predator fish, and sediment cores from Lake Ontario and two boreal forest lakes to investigate fractionation of the stable isotopes of Hg in aquatic ecosystems. Multicollector inductively coupled mass spectrometry was used to determine Hg isotope abundances. The Hg isotope data for all three lakes showed mass-independent variation in the organisms but only mass-dependent variation in the sediments. The mass-independent isotope effect was characterised by (1) selective enrichment in isotopes of odd mass number (199Hg and 201Hg), (2) enrichment in 201Hg relative to 199Hg, (3) an inverse relationship between isotopes of odd and even mass number in fish, and (4) a positive correlation with methylHg (CH3Hg+) concentration, and hence with trophic level (although lake whitefish were consistently anomalous, possibly owing to biochemical demethylation). Isotope signatures of species at the same trophic level varied with habitat and diet, differentiating between planktonic and benthic crustaceans and their predators, and between fish that frequent deep, cold water and fish of similar diet that prefer warmer, shallower water, because of corresponding differences in CH3Hg+ and inorganic Hg content. Isotopic analysis of CH3Hg+ and inorganic Hg extracted from lake trout proved that the mass-independent isotope effect was due to anomalously high abundances of 199Hg and 201Hg in CH3Hg+, as implied by the data for whole organisms, suggesting mass-independent fractionation during microbial methylation of Hg. The purely mass-dependent variation in the sediments is attributable to the fact that Hg in sediments is mostly inorganic. The mass-independent fractionation of Hg isotopes can be explained by effects of nuclear spin or nuclear field shift, or both, and penetration of the inner electron shells of Hg by valence electrons of Hg-binding ligands. The results of the research demonstrate that isotopic analysis of Hg could yield valuable information about the biogeochemical cycling of Hg.
Transport of elemental mercury in the unsaturated zone from a waste disposal site in an arid region by Michelle A. Walvoord; Brian J. Andraski; David P. Krabbenhoft; Robert G. Striegl (572-583).
Mercury contained in buried landfill waste may be released via upward emission to the atmosphere or downward leaching to groundwater. Data from the US Geological Survey’s Amargosa Desert Research Site (ADRS) in arid southwestern Nevada reveal another potential pathway of Hg release: long-distance (102 m) lateral migration of elemental Hg (Hg0) through the unsaturated zone. Gas collected from multiple depths from two instrumented boreholes that sample the entire 110-m unsaturated zone thickness and are located 100 and 160 m away from the closest waste burial trench exhibit gaseous Hg concentrations of up to 33 and 11 ng m−3, respectively. The vertical distribution of gaseous Hg in the borehole closest to the disposal site shows distinct subsurface peaks in concentration at depths of 1.5 and 24 m that cannot be explained by radial diffusive transport through a heterogeneous layered unsaturated zone. The inability of current models to explain gaseous Hg distribution at the ADRS highlights the need to advance the understanding of gas-phase contaminant transport in unsaturated zones to attain a comprehensive model of landfill Hg release.
Impact of mercury emissions from incineration of automobile shredder residue in Japan by Fumitake Takahashi; Mitsuo Yamagata; Kenji Yasuda; Akiko Kida (584-593).
Mercury emissions from the incineration of automobile shredder residues (ASRs) were investigated. Continuous monitoring of elemental and reactive gaseous Hg in flue gas was performed in lab-scale and plant-scale ASR incineration. Results of continuous monitoring agreed with those obtained using the JIS K0222 method and Ontario-Hydro method. Before cleaning by air pollutant control devices (APCDs), reactive Hg was the dominant form of that element in both lab-scale and plant-scale results. Emission factors of reactive Hg before APCDs estimated from monitoring results showed large differences between plant-scale and lab-scale emissions. The emission factor in the plant scale was more than 10 times larger than that in the lab-scale, which is explainable by the different Hg contents of ASR. Based on plant-scale monitoring at the stack, emission factors after APCDs were estimated as 0.79 mg-Hg/Mg-ASR for elemental Hg and 6.8 mg-Hg/Mg-ASR for reactive Hg. Using these emission factors, total Hg emissions from ASR incineration were estimated as 2.2 kg/a. An ASR incineration plant investigated in this study used highly effective APCDs. Consequently, these emission factors might result in underestimation of national Hg emissions from ASR incineration. Emission factors estimated from lab-scale monitoring at a fabric filter outlet side might be more appropriate. However, even if emission factors calculated from plant-scale or the lab-scale monitoring are used, estimated emissions are still less than 1.0% of total Hg emissions in Japan. Therefore, Hg emissions from ASR incineration can be evaluated as insignificant. Unless Hg contents of ASR increase extremely, ASR incineration would be a minor source of Hg atmospheric emission in Japan, even if all ASRs were incinerated.
Quantifying the effect of humic matter on the emission of mercury from artificial soil surfaces by Conrad Mauclair; Julie Layshock; Anthony Carpi (594-601).
A set of surface samples was created using purified laboratory grade sand treated with 0.05 μg/g Hg as the HgCl2 salt and various concentrations of purified humic and fulvic acids. Emissions of elemental Hg from these substrates to the atmosphere were inversely correlated with the organic content of the samples (99% confidence level). The greatest differences in Hg emissions were found between samples containing the lowest concentrations of humic matter (0% versus 0.01% humic, and 0.01% versus 0.1% humic), only small differences in Hg flux were found to exist for samples with higher concentrations of humic acid (1%, 5%, and 100%). This effect was independent of the type of humic substance used, with both humic and fulvic acids showing an inhibitory effect on surface Hg emissions.