Article pubs.acs.org/est
Soil−Air Mercury Flux near a Large Industrial Emission Source before and after Closure (Flin Flon, Manitoba, Canada) Chris S. Eckley,*,†,∥ Pierrette Blanchard,† Daniel McLennan,‡ Rachel Mintz,‡ and Mark Sekela§ †
Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario M3H 5T4, Canada ‡ Meteorological Service of Canada, Environment Canada, 9250-49 Street NW, Edmonton, Alberta T6B 1K54, Canada § Pacific and Yukon Regional Office, Environment Canada, 201-401 Burrard Street, Vancouver, British Columbus V6C 3S5, Canada S Supporting Information *
ABSTRACT: Prior to its closure, the base-metal smelter in Flin Flon, Manitoba, Canada was one of the North America’s largest mercury (Hg) emission sources. Our project objective was to understand the exchange of Hg between the soil and the air before and after the smelter closure. Field and laboratory Hg flux measurements were conducted to identify the controlling variables and used for spatial and temporal scaling. Study results showed that deposition from the smelter resulted in the surrounding soil being enriched in Hg (up to 99 μg g−1) as well as other metals. During the period of smelter operation, air concentrations were elevated (30 ± 19 ng m−3), and the soil was a net Hg sink (daily flux: −3.8 ng m−2 h−1). Following the smelter closure, air Hg0 concentrations were reduced, and the soils had large emissions (daily flux: 108 ng m−2 h−1). The annual scaling of soil Hg emissions following the smelter closure indicated that the landscape impacted by smelter deposition emitted or re-emitted almost 100 kg per year. Elevated soil Hg concentrations and emissions are predicted to continue for hundreds of years before background concentrations are re-established. Overall, the results indicate that legacy Hg deposition will continue to cycle in the environment long after point-source reductions.
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metabolism.16,17 As such, the Hg pool available for emission may be limited to shallow surface soil exposed to light.7 However, if Hg deeper in the soil column could be released by nonphoto-related processes or indirectly by hydrological translocation to the surface, the pool of Hg available for emission and re-emission could be substantially larger.18 The Hudson Bay Mining and Smelting company (HBMS) base-metal smelter in Flin Flon, Manitoba, Canada began operating in the 1930s and was one of the largest Hg emitters in North America.19,20 In 2010, the smelter was permanently closed, allowing a rare opportunity to measure the before and after effects of a large reduction in atmospheric Hg emissions.21 In the decade prior to its closure, the smelter released approximately 1000 kg Hg per year (Figure S1 in the Supporting Information); however, historic emissions prior to the 1990s have been estimated to be over an order of magnitude larger.22 Recent measurements of Hg wet-deposition loads in Flin Flon (2009−2010) were 3-fold greater than the North American average.21 Historic deposition was also higher prior to the construction of a taller stack in 1974 that helped
INTRODUCTION Mercury (Hg) released to the atmosphere will eventually be deposited via wet and dry mechanisms.1 Increased anthropogenic Hg emissions over the last century have resulted in elevated soil and sediment concentrations several-fold above background.2,3 Although the degree of enrichment is most pronounced nearer industrial sources,3,4 some Hg species are relatively stable (e.g., gaseous elemental Hg or Hg0) and can be subjected to long-range transport and remote region contamination.1 The Hg flux between the atmosphere and soils is not onedirectional because soil Hg can be emitted to the air under ambient environmental conditions.5−7 The magnitude of surface Hg fluxes varies between regions and landscapes, with much of this spatial variability resulting from differences in soil Hg concentrations.5,6,8,9 Fluxes also vary temporally and are influenced by air Hg concentrations as well as several meteorological and soil variables.6,10,11 The potential for previously deposited Hg to be re-emitted to the atmosphere has important implications for the long-term impact of legacy Hg continuing to cycle in the environment. Most soil Hg is in an oxidized form (e.g., Hg2+) bound to the functional groups of soil organic matter.2,12,13 For emission to occur, Hg2+ must be reduced to the volatile form Hg0, which can occur via photoreductive processes7,14,15 and microbial © XXXX American Chemical Society
Received: April 20, 2015 Revised: July 9, 2015 Accepted: July 18, 2015
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DOI: 10.1021/acs.est.5b01995 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
region, with no other industries within 450 km reporting Hg emissions on the National Pollution Release Inventory.19 Field Flux Measurements. Dynamic flux chambers (DFCs) made of thin Teflon material were used in this study and described elsewhere.28 DFC blank fluxes obtained by placing the chamber over clean polyvinyl film were relatively low (0.3 ± 0.9 ng m−2 h−1, n = 186 hourly fluxes) compared to the magnitude of fluxes measured in this study and were not subtracted from the measured values. Gaseous elemental Hg concentrations were measured using a Tekran 2537A total gaseous mercury analyzer at flow rate of 1.5 L min−1. The analyzer was calibrated before and after each field and laboratory sampling campaign using an internal permeation source and checked using injections of Hg0 in ambient air using a Tekran 2505 mercury vapor primary calibration unit, with acceptable error being ±5%. A Tekran Model 1110 two-port synchronized sampler was used to sequentially sample the air at the DFC inlet and outlet in 10 min intervals (two 5 min samples). Measurements at the sample inlet (approximately 2 cm above the sediment) provided a measure of near-soil ambient Hg0 concentrations. A vacuum pump with a built-in regulator (KNF Laboport minipump) was set to the same flow rate as the Tekran and was used to flush air through the sample lines between the sequential measurements. The inlet and outlet air was sampled through acid-cleaned Teflon tubing with a disposable PTFE syringe filter (0.22 μm Cole-Parmer) at the inlet. Surface meteorological data (air temperature, percent humidity, wind speed and direction, precipitation, and solar radiation) was collected at 5 min average intervals alongside the DFC measurements. Flux calculations are reported elsewhere, with negative values representing net deposition and positive values representing emission.28 All field flux measurements were conducted in 2010, a few weeks prior to the smelter closure (June) and three months following the closure (September and October). Flux locations were chosen to be representative of a gradient of soil Hg concentrations to facilitate spatial scaling (Figure 1). The Douglas Lake sample location was later identified to contain historic mine materials that had elevated Hg levels but were not associated with atmospheric deposition from the HBMS smelter. Therefore, the data from the Douglas Lake location was not included in the analysis focusing on understanding the re-emission of atmospherically deposited Hg. Fluxes were also measured at a background reference location approximately 70 km southeast of the smelter in Grass River Provincial Park. Given the potential for the long-range atmospheric Hg transport, it is possible that the soils at this site are not entirely free from the Hg deposition associated with the smelter; however, the air and soil concentrations are similar to the observations from other remote regions,29 indicating that any impact of the smelter at this site is subtle. At all sample locations, fluxes were measured for approximately 24 h in one location; the chamber was then moved approximately 10 m, and the flux continued to be measured for approximately 5 h more. At some sample areas, this was repeated again, resulting in triplicate measures of the small-scale spatial variability of Hg fluxes (Table S1 in the Supporting Information). Surface soil samples were collected from the area directly under the DFC at the end of the flux measurements. In addition, a soil depth profile (up to 20 cm) was collected using a corer and divided into 2 cm increments, and a separate sample was collected for bulk density determination. Soil samples were sieved to 2 mm and analyzed at Environment
better disperse emissions into the free troposphere. The impact of decades of enhanced Hg deposition have been welldocumented in soils and sediments around Flin Flon.22−25 The most dramatic effects of smelter deposition are observed close to the source (approximately 3 km), where ridges are devoid of vegetation due to sulfur dioxide (SO2) and metal deposition.26,27 Our study objective was to understand how surface−air Hg0 fluxes change in response to a large decrease in atmospheric Hg concentrations. We also wanted to identify the variables controlling Hg0 fluxes, such that emission estimates could be scaled spatially and temporally. In a related study, measurements from an air monitoring station in Flin Flon showed that prior to the smelter closure, air Hg concentrations were significantly elevated (4.1 ± 3.7 ng m−3) and decreased only 20% during the year following its closure. The new lower concentrations were still approximately 2-fold greater than background levels.21 These results suggested that the reemission of legacy soil Hg may be contributing to the continuation of elevated air Hg concentrations.19
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MATERIALS AND METHODS Area Description. Flin Flon, Manitoba, Canada (54.77◦ N; −101.88◦ W; elevation: 304 m; 2011 population: 5363) is located in the northern boreal ecozone (Figure 1). The climate borders on humid continental and subarctic, experiencing warm summers (mean high of 23.7 ◦C) and cold winters (mean low of −24.5◦C). HBMS was the largest and only major industrial employer in the community located in an otherwise remote
Figure 1. Study area map showing the locations of the four Hg0 flux sampling areas in Flin Flon, Manitoba, as well as the background reference sample area in the Grass River Provincial Park shown on the inserted map. The dashed black-and-white outline shows the area of the HBMS smelter facility and associated tailing pond. B
DOI: 10.1021/acs.est.5b01995 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
the measurements were within a 50 km radius of the HBMS stack (Figure S2 in the Supporting Information). The annual amount of Hg emitted over this areas was estimated using a GIS approach to create a layer of spatially interpolated THg concentrations and using the regression equation (described in the Results section) to create an emission layer. The emissions were scaled over a year, assuming that no fluxes occurred during periods of snow cover (2009 to 2011 had an average of 142 days of snow cover).5,21,32 The controlled lab experiments in this study, as well as the results from several other studies,33−36 clearly indicate that emissions would increase in response to precipitation. A 4-fold multiplier37,38 was applied to 33 days per year on the basis of the average number of rainfall events in 2009−2011 that occurred during snow-free conditions. The sources of uncertainty and error in the scaled flux estimates include the lack of seasonal variability in solar radiation and other meteorological variables and the simplified flux response to precipitation, which does not take into consideration the antecedent moisture conditions.18,36 Also, the field measurements used for annual scaling were based on a pair of one-week field campaigns and do not represent the full range of meteorological and surface conditions that can influence fluxes.
Canada’s National Laboratory for Environmental Testing (NLET) for total metals (NLET method 2403; total recoverable metals by strong acid microwave digestion followed by ICP-MS or ICP-OES detection), Hg (NLET method 2602; total Hg by strong acid microwave digestion followed by CVAAS detection), organic carbon and nitrogen (NLET method 1090), and pH. All sample quality assurance and quality control (QA−QC) was performed by NLET and met their method’s criteria. Soil results are presented on a dry-weight basis. The soil beneath the DFCs were analyzed for percent moisture (determined gravimetrically). Laboratory Flux Measurements. Over 100 kg of soil were obtained from sample location near the HBMS boundary (Figure 1). The soil was brought to Environment Canada’s Pacific Environmental Science Centre in Vancouver, BC where four mesocosms were constructed to allow flux measurement under controlled conditions. All mesocosms had the same surface area (2320 cm2), but two had a shallow depth (3 cm), and two were deeper (15 cm). Prior to mesocosm construction, the soil was homogenized through mechanical mixing (Hg: 28 ± 4 μg g‑1; sulfur: 329 ± 30 μg g‑1; pH: 4.4 ± 0). The mesocosms were housed indoors with constant light exposure (Multiple Vapor 1000 W; 930 ± 17 W m−2 measured at soil surface) for 6 h per day (annual average sunlight hours in Flin Flon), followed by 18 h of low-light and low-dark conditions (mean solar radiation: 1.4 ± 5.6 W m−1). Mesocosm fluxes were measured for 51 days under dry conditions, followed by wetting with Milli-Q deionized water applied from a misting spray bottle to reach 15% moisture content in the top soil layer. A Tekran 1115 synchronized multiport sampling system was used to sequentially sample DFCs on shallow and deep mesocosms at rotating 20 min intervals (two 5 min measurements of inlet air; two 5 min intervals of outlet air per chamber) switching after approximately 3 days to the replicate mesocosms. A total of four new (not utilized on any previous projects) and identical DFCs were used for this sampling (one per mesocosm) to avoid any disturbance to the soil when changing sampling between replicates and to remove any chamber-induced biases between treatments. Air temperature, relative humidity, solar radiation, and soil temperature were measured at 5 min average intervals using a Campbell Scientific CR1000 data logger. Soil moisture was measured gravimetrically every 3 days. Spatial and Temporal Scaling. Solar radiation has a large influence on emissions and varies spatially (topography, vegetation, etc.) and temporally (cloud cover, day-length, etc.).7,9 Therefore, all flux measurements were standardized to the same level of solar radiation. This was accomplished by fitting a 2-order polynomial equation relating flux and solar radiation for each sample location and day (mean r2 = 0.79 ± 0.14). A polynomial equation was chosen due to its flexibility in capturing nonlinear relationships between the parameters. These equations were then used to calculate solar normalized fluxes by using the same 24 diel curve of solar radiation. A diel curve with a mean of 65 W m−2, based on a daily and seasonal average for the Flin Flon area, was used in this study. The average daily flux (presented on an hourly basis) was calculated by integrating the area under the diel curve. Georeferenced surface-soil Hg concentrations were obtained from a Geological Survey of Canada and Manitoba Energy and Mines geochemical mapping project23,30 and a Flin Flon soil survey by Manitoba Conservation.31 Combined, these data sets include 1630 surface-soil THg concentrations, of which 939 of
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RESULTS AND DISCUSSION Soil Mercury Concentrations. Hg concentrations in surface soils in the area near the smelter were several orders of magnitude above background levels, as shown by the results at the reference location 70 km away (Figure 2). Soil-depth
Figure 2. Total Hg concentration with soil depth at the flux sample location distances from the smelter. Error bars represent the standard deviation of duplicate samples that were collected at some of the sites. Note the log-scale of the x-axis. Only a surface sample (0−2 cm) was collected at the 9 km location. Soil THg data from this figure available in Table S2 in Supporting Information.
profiles showed that although the top few cm of soils could be very enriched (up to 99 μg/g), there was not significant vertical transport of deposited Hg within the soil profile. For example, at all sample locations, soil concentrations below depths of 10 cm were
