Environ. Sci. Technol. 1996, 30, 2572-2579
Atmospheric Mercury Concentrations Associated with Geologically and Anthropogenically Enriched Sites in Central Western Nevada MAE SEXAUER GUSTIN,* GEORGE E. TAYLOR, AND TODD L. LEONARD Department of Environmental and Resource Sciences, The University of Nevada, Reno, Nevada 89557
ROBERT E. KEISLAR Desert Research Institute, Stead, Nevada 89506
This paper documents the atmospheric mercury concentrations above anthropogenically contaminated and naturally enriched sites in central western Nevada. Atmospheric mercury concentrations were measured at five representative regional sites (1.27.5 ng/m3) and two anthropogenically contaminated areas (13-866 ng/m3) in the Carson River Superfund Site. The highest regional concentrations were measured at the Steamboat Geothermal area, where mercury mineralization occurs naturally. Concurrent with atmospheric sampling, environmental conditions were monitored to assess their covariance with mercury concentrations. Atmospheric mercury concentrations were influenced by multiple factors with dominance exerted by substrate mercury concentration, site surface characteristics, and local and synoptic scale air masses. A mercury flux of 5-125 ((50%) µg of mercury m-2 h-1 was estimated via modified K-theory for a contaminated location. This flux was scaled up to estimate the contribution of atmospheric mercury from mine wastes within the Carson River Superfund Site. The estimated annual flux (150-400 kg/yr) is comparable to that from a 1000 MW coal-fired power plant (300 kg/yr). The projected longevity of this diffuse source exceeds 104 years, so the cumulative contribution over time from this region far exceeds the corresponding contribution of a coal-fired power plant whose life time is measured in decades.
Introduction Elemental mercury vapor (Hg0v) has an atmospheric residence time of 1 yr allowing for global distribution, deposition, and potential contamination of areas remote * Corresponding author fax:
[email protected].
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from any Hg source (1-6). Emissions of mercury (Hg) from some anthropogenic point sources, such as sites of fossil fuel combustion, waste incineration, industrial processes (chloralkali plants), and metal ore-roasting and refining, are well characterized for developed countries (7, 8). Volatilization rates from diffuse terrestrial sources (e.g., spills, discarded mine wastes, naturally enriched areas) are poorly quantified (7). Constraining the contribution from diffuse sources is important for balancing the global Hg atmospheric budget and developing remediation strategies. Laboratory studies have demonstrated that Hg0v flux from substrate is strongly dependent on temperature, Hg concentration and speciation, and wind velocity (9-12). In order to predict Hg0v flux from diffuse sources, the relative importance of factors controlling evasion needs to be established in situ. This may be difficult given the multitude of factors affecting flux in the field (e.g., substrate physicochemical attributes, terrain features, temperature, humidity, wind velocity, and barometric pressure) (12-15). This paper documents atmospheric Hg0v concentrations associated with a broad area of natural enrichment (16, 17) and anthropogenic contamination in central western Nevada (Figure 1). Due to natural enrichment in the study area, Hg concentrations in substrate, air, water, and biota are elevated above global ambient background values (18). Mercury was naturally concentrated in several locations (Castle Peak Mine, Steamboat Springs Geothermal Area) and mined as cinnabar (HgS), calomel (HgCl2), and native Hg (17, 19, 20) (Figure 1). Natural enrichment of Hg occurs at low temperatures (100-200 °C) near the surface (21) in association with Tertiary and Quaternary hydrothermal and volcanic activity. Higher than ambient [1-3 ng/m3 (5)] atmospheric Hg0v concentrations reported for areas of geologic Hg enrichment [8-100 ng/m3 (22-27)] indicate that enriched areas constitute an atmospheric source term. Anthropogenically contaminated sites in the study area are a legacy of ore processing that occurred from 1860 to 1890 in the Comstock Mining District, Virginia City area (Figure 1). Mercury was used to amalgamate gold and silver at approximately 75 mills in the Carson River Drainage Basin; it is estimated that 5.5 × 109 g of Hg0 liquid were not recovered (28). Nriagu (29) suggested that 60% of Hg0 used in ore processing with crude techniques is commonly lost to the atmosphere. Based on his estimate, the amount of Hg in substrate and water distributed throughout the Basin during ore processing would have been on the order of 2.2 × 109 g. Presently, elevated levels of Hg contamination are found in mill tailings ( PBM > SBG g NBM > LC g DRI > LAH (Figure 2; Table 1). The mean ((SD) and median of Hg0v concentrations at regional sites were 3.4 ((1.9) and 3.1 ng/m3, respectively, both at the upper limit of ambient atmospheric concentrations (1-3 ng/m3) (5) (Figure 2). Based on ANOVA, there were statistically significant differences in atmospheric Hg0v concentrations. Duncans test demonstrated that atmospheric concentrations measured at PBM and BM were significantly different from all other sites (p < 0.05). On a monthly time step among the regional sites, mean atmospheric Hg0v at SBG was statistically different (p < 0.05) from all sites (except in September), and the mean concentration at NBM was different from other sites in January and April (p < 0.05).
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The highest regional concentrations (7.5 ng/m3) were measured at SBG, where the substrate is naturally Hgenriched, and fumaroles release Hg0v to the atmosphere (52). LAH had the lowest atmospheric Hg0v concentrations (1.2-3.0 ng/m3) despite its proximity to Hg-contaminated Lahontan Reservoir. These data suggest that either the Hg0v being evaded to the atmosphere from the water is diluted 100 m inland from the shore or that the flux from the water is low. Substrate at the site had a very low Hg concentration (0.02-1.9 µg/g) and would have contributed little Hg0v to the air (Table 1). Mercury flux has been shown to occur from both marine (53) and freshwaters (13, 54); however, little work has been done to characterize emission as a function of Hg concentration or physicochemical characteristics of a water body. Multiple measurements of atmospheric Hg0v were made at 1 m height at BM (mean 152, median 80, SD 180 ng/m3, n ) 46), with maximum and minimum concentrations of 866 ng/m3 (April 1995) and 13 ng/m3 (July 1994), respectively. No significant correlations were obtained between air concentrations and environmental factors. The lowest seasonal atmospheric Hg0v concentrations were measured at regional sites in April. This is inconsistent with the documented positive effect of temperature on Hg0v flux (9, 12) given that April was not the coldest month (Table 1). The consistently low air Hg0v concentrations measured during April may have been a product of a synoptic-scale air mass with low Hg0v concentration residing over the area (55, 56). The only environmental factor which showed a significant statistical correlation with seasonal atmospheric Hg0v concentrations for all sites was substrate Hg concentration (r ) +0.57, p < 0.05). This is consistent with results obtained from a laboratory study of flux from contaminated substrate cores from the same area (9). Atmospheric Hg0v concentrations measured at regional uncontaminated sites were positively correlated with substrate Hg concentration (r ) +0.62, p < 0.05) and negatively correlated with wind velocity (r ) -0.44, p < 0.05). Diel atmospheric Hg0v concentrations measured at the same location 1 m above the ground at BM in June and July 1994 and in March, April, and June 1995 were 13-43, 15135, 76-240, 145-866, and 65-698 ng/m3, in series (Figure 3; Table 2). Atmospheric Hg0v concentrations increased during the night and early morning, with the most dramatic increases during July 1994 and June 1995. Conditions in April were unusual compared to other sampling periods, with overcast skies, intermittent rain, and winds of 1-2.5 m/s (Table 2). Despite the fact that temperatures were coldest during diel data collection in March, atmospheric Hg0v concentrations were high (Table 2). No consistent statistically significant relationship was established for any environmental factor and atmospheric Hg0v concentrations on a diel time step. A negative correlation was obtained for soil and air temperature for all diel periods except June 1994 (Table 3). Wind velocity was positively correlated with Hg0v concentration for only the June 1995 diel (r ) +0.7, p ) 0.01). Both positive and negative correlations were established for relative humidity. Atmosphere Hg0v concentrations declined exponentially with height, and concentrations at the surface (0.43 m) during the day were >2 times those obtained at the highest sampling elevation (1.78 m) (Table 2). The difference in the air concentrations measured at 0.43 m versus 0.89 m during the June 1995 diel gives some indication of the
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FIGURE 3. Diel variation of atmospheric mercury concentrations (ng/m3) at the Bessels Mill site. Data are the mean of triplicate and duplicate samples (average coefficient of variation of 9%).
strength of the source term (Figure 4), with the greater the difference the more Hg0v being volatilized from the surface. The greatest difference was obtained in the morning (148 ng/m3), indicating higher rates of volatilization than during other periods during the day; this declined with nightfall until 200 h when the concentrations overlapped. Estimated fluxes ranged from 5 to 125 ((50%) µg m-2 h-1 (Table 2). In addition to the coefficient of variation due to sampling error, there is an additional uncertainty generated through the use of K-theory under non-ideal conditions. K-theory also disregards the effects of turbulent eddies larger than our sampling height. Given these sources of error, flux estimates could vary by an additional half order of magnitude. Fluxes were correlated with air temperature, wind velocity, incident radiation, and relative humidity for the April data and with incident radiation for June (Table 3). Fluxes in June declined at night with the decline in temperature and incident radiation (Figure 4). Xiao et al. (13) and Kim et al. (14) demonstrated that in situ Hg0v flux declined at night. Flux remained somewhat constant during the April diel after the initial sample (Figure 5). This is attributed to the fairly constant environmental conditions over the entire diel period. The highest estimated flux values were calculated for the morning sampling events in June and for the initial sample taken in April, which was the only time during the sampling period during which the ground surface received incident radiation. Preliminary research in our laboratory, using a gas exchange system to measure flux from tailings from the Carson River Superfund Site, has demonstrated that flux increases by 1-2 orders of magnitude with incident light. The higher flux values estimated for April relative to those for June may reflect the fact that air was continuously
TABLE 3
Pearson Correlation Coefficients (r) for the Relationship between Atmospheric Mercury Concentrations (ng/m3) Measured on Diel Time Step, Estimated Flux (ng m-2 h-1), and Several Environmental Factorsa environmental factor
June 94
July 94
March 95
April 95
June 95
soil temperature (°C)
+0.86 (0.006)
-0.65 (0.16)
-0.80 (0.05)
air temperature (°C)
+0.86 (0.006)
-0.53 (0.28)
-0.94 (0.005)
wind vel (m/s) at 1.8 m
+0.2 (0.63)
+0.31 (0.54)
-0.53 (0.27)
photon flux (µmol m-2 s-1)
+0.81 (0.26)
-0.46 (0.36)
-0.63 (0.17)
-0.36 (0.27) -0.59 (0.05) -0.70(0.02) +0.86 (0.001) +0.49 (0.12) +0.68 (0.02) +0.91 (0.001) +0.94 (0.001) +0.37 (0.26) +0.56 (0.07) -0.57 (0.06) -0.60 (0.05)
-0.70 (0.01) +0.37 (0.26) -0.62 (0.03) +0.22 (0.48) +0.72 (0.01) -0.36 (0.25) -0.28 (0.36) +0.96 (0.001) +0.74 (0.006) -0.4 (0.20) +0.78 (0.02) -0.15 (0.96)
wind vel (m/s) at 0.43 m relative humidity (%) a
Coefficients (r) calculated using flux are in italics, and P values are in parentheses.
FIGURE 4. Diel atmospheric mercury concentrations (ng/m3) and environmental parameters, June 1995. Closed symbols are atmospheric Hg0v concentrations measured concurrently at three heights.
moving over the site during sample collection. It has been demonstrated that the rate of diffusion of chemicals across the air-soil interface has a positive relationship with wind velocity (9, 58-60). The range in Hg0v flux (3-125 µg m-2 h-1) is comparable to that determined for cores of tailings collected from BM using a laboratory gas exchange chamber (0.1-63 µg m-2 h-1) (9). The flux to air Hg0v concentration ratio for June 1995 was approximately 50-100:1. Lindberg et al. (12) calculated a flux to concentration ratio of approximately 20:1 for HgScontaminated stream sediments at East Fork Popular Creek, TN. It is hypothesized that the higher ratio of flux to atmospheric concentrations for BM was due to the difference in climatic and substrate characteristics and Hg speciation between the two sites. The Tennessee site is in a mesic forest where the soil moisture was 7-27% (12) and the ground surface was covered with leaf litter; BM is a semi-arid, sparsely vegetated site with low soil moisture (56). One goal of this project was to determine those factors influencing atmospheric Hg0v concentrations at naturally enriched and contaminated sites. The poor correlation among environmental factors, atmospheric Hg0v concentrations, and estimated flux suggests a complex interaction of multiple factors. For example, despite the finding that substrate Hg concentration exerted a strong influence on atmospheric Hg0v concentrations, air concentrations were consistently higher at BM relative to PBM (Table 1, Figure
FIGURE 5. Diel atmospheric mercury concentrations (ng/m3) and environmental parameters, April 1995. Closed symbols are atmospheric Hg0v concentrations measured concurrently at three heights.
2). In this case, the influence of substrate Hg concentration was overwhelmed by the influence of substrate exposure since BM tailings are barren of vegetation and leaf litter, whereas tailings at PBM are shaded by cottonwoods and the ground surface is covered with leaf litter and grasses. Temporal changes in the volume of the atmospheric boundary layer would also affect Hg0v concentrations. The BM site is within Sixmile Canyon, which has local relief of 300 m. A scenario that would explain the observed nocturnal increases in atmospheric Hg0v concentrations and homogenization of atmospheric Hg0v concentrations measured simultaneously at different heights in June 1995 is that the atmosphere-surface boundary layer decreases
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in height at night (45) resulting in a smaller volume of air in which Hg0v could be diluted. The June 1994 and April 1995 diel data did not exhibit peak atmospheric Hg0v concentrations during the night or early morning. Wind velocities at night were high during these diel periods. Continuous air movement over the site would prohibit development of a stagnant air mass in which Hg0v could accumulate. Estimated fluxes of Hg0v for the afternoon in June are considered representative of the daily (12 h) contribution of atmospheric Hg0v from BM. Assuming flux was less by 50% at night and for half the year due to cooler temperatures, an estimated range of annual flux from the site would be 2-5 kg of Hg0v for an area of approximately 9 × 104 m2 (37). If residual tailings of an area comparable to Bessels Mill are associated with the other 75 working mills of the Carson River Superfund Site, the flux from the Basin would be 150400 kg of Hg0v/yr. This conservative estimate does not account for Hg volatilization from eolian and fluvial deposits throughout the Basin or from water. Miller et al. (30) concluded that eolian forces played an important role in dispersal of contaminated tailings on the Sixmile Canyon alluvial fan. A thin veneer of Hg-contaminated dust over the alluvial fan would provide a large surface area from which Hg0v flux could occur. A total of 130 yr have passed since the Comstock ores were mined. Using the above annual flux, Hg0v volatilized over the 13 decades from tailings is approximately 2% of the total Hg predicted to be lost to the Basin during ore processing. The estimated yearly contribution to the atmosphere from the site is greater than that from a municipal waste deposit in Sweden of 1-10 kg/yr (61), comparable to that from a representative 1000 MW coalfired power plant of 310 kg/yr (62), but significantly less than the amount estimated as discharged from a Hg distillation plant at Idrija, Slovenia of 7.3 × 103 kg/yr (63). Because the time period over which landscape-atmosphere flux of Hg0v from contaminated sites in the Carson River Superfund Site will occur (>10 000 yr) is much greater than the life span of most anthropogenic point sources, the cumulative contribution to the global atmospheric Hg budget from this diffuse source is greater than even the most prodigious point source. The site is highly contaminated and will continue to volatilize Hg0v to the atmosphere for thousands of years. Flux could also increase given the scenarios of global warming and the exponential relationship between Hg0v flux and temperature (9, 64).
Acknowledgments This work was supported by Grant P42EO5961-01 “Chemical Environmental Problems Associated with Historic and Current Precious Metal Mining” from the National Institute of Environmental and Health Sciences. The authors greatly acknowledge help from John Bowen and Steve Schmidt of Desert Research Institute, Reno, NV, in obtaining micrometeorological data.
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Received for review December 15, 1995. Revised manuscript received April 10, 1996. Accepted April 12, 1996.X ES950937D X
Abstract published in Advance ACS Abstracts, June 15, 1996.
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