Measurement of surface mercury fluxes at active industrial gold mines in Nevada (USA)

Science of the Total Environment 409 (2011) 514–522

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Science of the Total Environment j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s c i t o t e n v

Measurement of surface mercury fluxes at active industrial gold mines in Nevada (USA) C.S. Eckley a,1, M. Gustin a,⁎, F. Marsik b, M.B. Miller a a b

Department of Natural Resources & Environmental Science, University of Nevada, Reno, NV 89557, USA University of Michigan, Department of Atmospheric, Oceanic and Space Sciences, USA

a r t i c l e

i n f o

Article history: Received 22 June 2010 Received in revised form 12 October 2010 Accepted 13 October 2010 Keywords: Mercury flux Open pit gold mining Atmospheric mercury Nonpoint source

a b s t r a c t Mercury (Hg) may be naturally associated with the rock units hosting precious and base metal deposits. Active gold mines are known to have point source releases of Hg associated with ore processing facilities. The nonpoint source release of Hg to the air from the large area (hundreds to thousands of hectares) of disturbed and processed material at industrial open pit gold mines has not been quantified. This paper describes the field data collected as part of a project focused on estimating nonpoint source emissions of Hg from two active mines in Nevada, USA. In situ Hg flux data were collected on diel and seasonal time steps using a dynamic flux chamber from representative mine surfaces. Hg fluxes ranged from b 1500 ng m−2 day−1 for waste rock piles (0.6–3.5 μg g−1) to 684,000 ng m−2 day−1 for tailings (2.8–58 μg g−1). Releases were positively correlated with material Hg concentrations, surface grain size, and moisture content. Highest Hg releases occurred from materials under active cyanide leaching and from tailings impoundments containing processed high-grade ore. Data collected indicate that as mine sites are reclaimed and material disturbance ceases, emissions will decline. Additionally local cycling of atmospheric Hg (deposition and re-emission) was found to occur. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Mineral deposits enriched in gold are often naturally enriched in mercury (Hg) (Barnes and Seward, 1997). The state of Nevada is the largest gold producer in the USA and one of the top gold producing regions in the world. Industrial gold mines have known point sources of atmospheric Hg associated with ore processing facilities (i.e. mills, autoclaves, roasters) (TRI, 2007). As of 2008, there were 20 operating precious metal mines in Nevada releasing approximately 2000 kg of Hg year−1 from point source stack emissions at ore processing facilities (http://ndep.nv.gov/mercury/). At these facilities there is also the potential for significant nonpoint source release of Hg since Hg can volatilize from surfaces under ambient conditions and the mining process brings large quantities o subterranean Hg-enriched material to the surface that is distributed over hundreds to thousands of hectares. While considerable effort has been directed towards monitoring as well as reducing point source Hg emissions (http:// ndep.nv.gov/mercury/), the non point source release of Hg from active gold mines has not been quantified. The main form of Hg volatizing from natural surfaces is gaseous elemental Hg (Engle et al., 2005). This form of Hg is known to exhibit bidirectional exchange with natural surfaces and has a long atmospheric lifetime (Zhang et al., 2009). Thus ⁎ Corresponding author. E-mail address: [email protected] (M. Gustin). 1 Present address: Canadian Government Laboratory Visiting Fellow, Environment Canada, 201-401 Burrard Street, Vancouver (BC) V6C 3S5 604-664-9380, Canada. 0048-9697/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2010.10.024

emissions of Hg from surfaces at mines are expected to be subject to long range transport and contribute to the global atmospheric Hg pool before eventual deposition (Kolb et al., 2009). It is well established that under ambient conditions Hg can be emitted to the air from natural Hg-enriched surfaces (Gustin et al., 1999; Rasmussen et al., 2006; Schroeder et al., 2005). The surfaces at industrial gold mines are heterogeneous (i.e. variable Hg concentrations, disturbance, moisture, etc) and understanding the factors controlling Hg release is important for developing means of emission reduction. The exposed surfaces change over the course of a day and from year to year as mining operations evolve. Blasting of rock within active open pit mines occurs almost daily and the fragmented rock is transported to specific locations depending on the gold content. The material containing no economical concentration of gold is placed in waste rock dumps/overburden piles. Low-grade ore (i.e. low concentrations of economical gold) is transported to heap leach pads, where the material is stacked in ~ 30 m high layers on an impermeable surface. This low-grade ore is irrigated with a dilute cyanide solution that percolates through the heap to extract the gold. At any given time, there are sections of leach pads that are awaiting cyanide addition (pre-leach materials), sections where cyanide is being applied (active leach materials) and sections where gold has been extracted (post-leach). The leach solution also accumulates Hg as it is recycled through the ore processing system (Tassel et al., 1997; Matlock et al., 2002). High-grade ore (i.e. high concentration of economical gold) is placed in stockpiles and eventually transferred to a mill where it is crushed and processed to extract the gold. The

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procedure followed depends on the chemical characteristics of the ore. Milled and processed (vat leached, autoclaved or roasted) material is transported to tailings impoundments via slurries containing cyanide solution. As such, tailings impoundments have a large range of surface moistures with some sections being inundated with solution and others being dry. The overall objective of this project was to estimate the total nonpoint source Hg emissions from two active industrial gold mines in Nevada using data collected in both a field and laboratory setting. In this manuscript we describe the data collected in the field. A second paper describes fluxes measured in a laboratory setting from mesocosms of material from each mine used to develop functional relationships between Hg fluxes and environmental variables (see Eckley et al., submitted for publication). Since the mesocosms were of limited area and depth, field data was used to verify that laboratory results were representative of field conditions. Additionally, some mine surfaces could not be replicated in the laboratory, such as the tailings impoundments inundated with cyanide solution, heap leach materials being actively processed, and older reclaimed materials. Measurements from these surfaces along with the equations developed from the mesocosms were used for spatiotemporal scaling to obtain annual nonpoint source emission estimates for both mines. The results of the scaling exercise identified that the two mines released ~ 20 and 110 kg Hg year−1 representing 56 and 14% of each mine's total (point + nonpoint) emissions, respectively (based on conditions in 2008: Eckley et al., submitted for publication). Previous studies have measured and estimated Hg emissions from historic/abandoned Hg mines (Ferrara et al., 1998; Gronlund et al., 2005; Gustin et al., 2003; Wang et al., 2005), historic/abandoned gold mines (Beauchamp et al., 2002; Gustin et al., 2003), and contemporary artisanal gold mines in the developing world (Garcia-Sánchez et al., 2006). While these studies found elevated Hg emissions from mining impacted landscapes, the results from these studies cannot be extrapolated to active industrial gold mines. This is due to the fact that many historic gold mines and contemporary artisanal gold mines utilized imported elemental Hg in an amalgamation processes to extract gold from the ore. In contrast, contemporary industrial gold mines do not use Hg in the extraction process; instead Hg in the rock material is derived from geologic enrichment associated with formation of the ore body. Furthermore, site disturbance, active leaching, young exposed surfaces and fresh tailings associated with active mines may have a different rate of Hg release relative to historic operations. 2. Materials and methods 2.1. Site description Two mines located in Nevada, USA were selected for this project: Cortez-Pipeline mine (40° 4′ North, 116° 42′ West, 1570 m) operated by Barrick Gold Corporation and Twin Creeks mine (41° 15′ North, 117° 9′ West, 1560 m) operated by Newmont Mining Corporation. These two mines have different degrees of Hg enrichment, ore types, processing techniques, and active as well as older inactive/reclaimed surfaces. Currently at the Twin Creeks mine ~30% of the material processed at the mill and input to the tailings originates off site, while at Cortez-Pipeline the high-grade carbonaceous-ore mined is stored on site but processed at an off-site roasting facility. Both mines have been in operation for over a decade and are expected to be actively mined for the next 5–10 years (see ref Miller 2010 for geologic description). Both mines are located in a high desert environment with low annual precipitation (20.4 cm year−1), warm summers (mean high: 35 °C) and cold winters (mean low: −9 °C). Flux measurements were made from the major types of mine surfaces during each season between February 2008 and March 2009 (Table 1). At Twin Creeks and Cortez-Pipeline mines these surfaces

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include waste rock piles (covering 45% and 24 % of the mine's surface respectively), leach pads (9% and 20% respectively), tailings impoundments (5 and 11% respectively), and reclaimed areas (15 and 7% respectively). In situ flux measurements were not obtained from high-grade ore stockpiles (≤4% of each mine's surface); however the material from these surfaces were collected for the flux measurements as part of the laboratory-based mesocosm component of this project (see Eckley et al., submitted for publication). Fluxes from leach pads during cyanide addition were measured at Twin Creeks where drip irrigation was used, but were not obtained at Cortez-Pipeline mine where the cyanide was applied using a sprinkler system. Fluxes were measured at a tailings impoundment at Twin Creeks along the perimeter (n = 23); while at Cortez-Pipeline measurements were obtained from only one area where the material was several years old and dry. Data was not collected from roads, parking areas, and leach solution storage ponds (b5% of the total surface of each mine). 2.2. Surface Hg concentrations A minimum of 10 substrate samples were collected from all major surface types to capture the spatial variability of Hg concentrations across each mine. Samples of each surface were collected using a clean trowel and the location was recorded using a GPS (Garmin GPSMAP 60; n = 401, Fig. 1). Samples were sieved into four size fractions– pebbles (N13 mm), gravel (13–2.4 mm), coarse sand (2.4–0.6 mm), and fine sand (b0.6 mm) and each size class was weighed. The b0.6 mm fractions were lyophilized and stored in clean glass vials in a −20 °C freezer before analyzing for total Hg (other size fractions were not analyzed for Hg). Some areas of the mine contained large cobbles and boulders not included in the surface grain size classification. Sieves were thoroughly cleaned with DI water between collection of different surface materials and a different set of sieves were used for each mine. The surface Hg content was analyzed using a Milestone Direct Mercury Analyzer following EPA Method 7473. Standard reference material (NIST 2711 Montana Soil II: 6.25 μg g−1 and NIST 2709 San Joaquin Soil 1.4 μg g−1) and blanks were analyzed every 10 samples and sample triplicates every five samples. Reference material concentrations were 102 ± 7% of the mean reported value (n = 76), blanks were 0.003 ± 0.005 μg of Hg (n = 57), and the difference between triplicates was 16 ± 10% (n = 71). 2.3. Hg flux measurements Hg fluxes were measured from each surface type using the dynamic flux chamber (DFC) method applied in other studies (see summary in ref Eckley et al., 2010). The DFCs were made of thin Teflon® material with a 2.0 L volume and 0.036 m2 footprint (see Eckley et al., 2010 for a detailed description of the chambers). DFCs were cleaned with 20% nitric acid (24 h) and then 10% hydrochloric acid (24 h) before each field sampling campaign and blank checked outside for ~ 24 h by placing the DFC over clean polyvinyl film. Several DFCs were brought into the field and a new/cleaned DFC was used after sampling at locations with high emissions. The chamber blank fluxes were low (15 ± 27 ng m−2 day−1, n = 586 hourly fluxes) relative to those measured in situ and not subtracted from the flux measurements. Most Hg air concentrations were measured using Tekran® 2537A Total Gaseous Mercury Analyzers; however some data at the Twin Creeks tailings were collected using a Lumex® Mercury Analyzer (RA-915+) due to air concentrations above the working range of the Tekran® 2537A. Data collected using these two analyzers were compared by simultaneously measuring air concentrations in the University of Nevada, Reno (UNR) greenhouse (Lumex® median: 6.1 ng m−3, range: 3.2 to14 ng m−3, n = 28,840 one second average data; Tekran median: 5.5 ng m−3, range: 4.3 to7.0 ng m−3, n =48 three hundred second

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Table 1 Summary of mean (standard deviation) daily Hg flux data and associated ancillary parameters. An asterisk (*) next to the Hg flux data indicates that at least one of the averaged fluxes was significantly different than the others (Mann–Whitney p b 0.05). In parenthesis the number of sample locations and hourly fluxes (based on 13-minute measurements) are shown (n = locations; hourly measurements). Surface Mine: Cortez-Pipeline Waste rock

Leach pad (pre-cyanide)

Leach pad (post-cyanide)

Leach pad (old/inactive) Tailings

a

b

Open pit Mine: Twin Creeks Waste rock c

Leach pad (pre-cyanide)

Leach pad (active cyanide)

Season

Daily Hg flux (ng m−2 day−1)

Substrate Hg (μg g−1)

Substrate Moisture (%)

Solar (W m−2)

Winter Spring Summer Fall Winter Summer Fall Winter Spring Summer Fall Winter Spring Spring Summer Fall Summer

80 (n = 1;19) 44 ± 23 (n = 3; 54) 105 ± 67* (n = 7; 240) 41 (n = 1; 34) 2260 (n = 1; 48) 13,000 ± 18,000* (n = 3; 263) 3310 ± 2200* (n = 3; 43) 2310 (n = 1; 59) 2290 ± 2290* (n = 6; 332) 783 ± 58 (n = 3; 113) −1650 ± 525* (n = 3; 66) 1210 (n = 1; 47) 162 ± 52 (n = 3; 192) 5280 ± 1030 (n = 3; 80) 4120 ± 2750 (n = 3; 107) 52,300 ± 17,800* (n = 3; 170) 1256 ± 1590* (n = 3; 216)

0.63 0.73 ± 0.70 0.92 ± 0.10 0.53 1.2 4.5 ± 5.5 0.26 ± 0.03 0.92 0.98 ± 0.72 1.0 ± 0.69 3.9 ± 1.3 4.2 5.2 ± 0.91 0.62 ± 0.11 0.85 ± 0.47 0.47 ± 0.05 0.87 ± 0.91

10.3 0.6 ± 0.7 0.4 ± 0.5 0.6 12.0 10.1 ± 3.1 9.8 ± 4.5 12.8 9.5 ± 1.7 0.3 ± 0.4 2.7 ± 0.6 10.3 0.2 ± 0.1 2.2 ± 1.0 0.3 ± 0.1 3.6 0 (solid rock)

119 304 313 147 96 292 146 139 302 301 154 139 326 258 276 89 338

5 16 22 8 3 28 10 4 23 22 11 4 25 24 25 9 26

64 44 22 35 85 11 48 71 16 24 38 71 17 12 20 43 40

Winter Spring Summer Fall Spring Summer Fall Winter

908 ± 779* (n = 2; 99) 1160 ± 508* (n = 3; 154) 327 ± 202* (n = 5; 377) 140 ± 105* (n = 2; 76) 2280 ± 1470* (n = 3; 100) 1810 ± 842* (n = 3; 65) 4130 (n = 1; 53) Ridge: 21,200 (n = 1; 45) Valley: 53,200 ± 24,400* (n = 2; 107) 101,300 ± 53,400 (n = 3; 147) Ridge: 11,800 (n = 1; 56) Valley: 322,000 (n = 1; 90) Ridge: 27,500 (n = 1; 59) Valley: 283,000 ± 320,000* (n = 2; 130) 2970 (n = 1; 46) 1750 ± 572 (n = 3; 46) 4530 ± 3230* (n = 3; 131) 11,680 ± 8680 (n = 2; 37) 9150 (n = 1; 71)_ 949 ± 738* (n = 3; 54) 1800 ± 739 (n = 3; 170) 9000 ± 1540* (n = 3; 123) 27,600 ± 32,800* (n = 3; 72) 201,000 ± 286,000* (n = 7; 168) 12,200 ± 22,900* (n = 10; 192) 36 ± 6 (n = 2; 144)

0.79 ± 0.65 0.15 ± 0.03 0.44 ± 0.38 0.62 ± 0.57 2.7 ± 0.29 22 ± 7.3 12 7.9 10.3 ± 0.60 2.7 ± 1.0 10 12 14 8.4 ± 2.7 5.6 5.7 ± 2.3 13 ± 15 6.5 ± 2.2 61 15 ± 3.3 49 ± 19 21 ± 2.7 85 ± 77 46 ± 24 69 ± 58 0.76 ± 0.67

4.5 ± 2.6 11.8 ± 2.2 0.1 ± 0.0 1.2 ± 0.3 10.6 ± 2.4 1.5 ± 1.8 2.6 8.7 20.3 ± 0.5 14.9 ± 2.3 0.9 19.5 7.0 21.4 ± 2.0 7.6 7.2 ± 4.1 1.1 ± 0.9 1.2 ± 0.2 4.1 15.3 ± 1.8 0.1 ± 0.1 24.6 ± 7.4 15.8 ± 5.6 25.5 ± 40.2 10.5 ± 16.2 0 (solid rock)

197 140 319 106 56 467 83 174 “” 125 223 “” 105 “” 199 106 300 98 216 139 272 68 120 207 61 215

1 10 27 7 4 29 4 6 “” 4 23 “” 4 “” 4 4 26 3 −1 11 25 8 7 18 11 25

48 81 18 44 79 12 60 54 “” 80 19 “” 66 “” 44 68 4 46 62 62 16 60 49 20 64 19

Spring Summer Fall Leach pad (post-cyanide)

Leach pad (old/inactive)d

Tailings

Open pit

Winter Spring Summer Fall Winter Spring Summer Winter Spring Summer Fall Summer

Temp. (°C)

RH

a

It was determined that this old leach pad still contained recoverable gold ore, and so the material was moved to an active leach pad. Therefore, this site no longer existed when sampling in the summer and fall. b Due to safety concerns, the mine did not allow us to sample the tailings during the winter. c Due to safety concerns regarding high activity at this site, the mine did not allow us to sample this waste rock dump during the spring. As an alternative location, sampling was conducted at the capped tailings, which had a similar substrate Hg concentration as the waste rock dump. d In the fall, this site was directly downwind from the mill's point source emissions. Due to difficulties obtaining reliable flux data under these conditions, the site was not sampled.

average data; Mann–Whitney: p b 0.001) and at the Twin Creeks tailings (Lumex® median: 273 ng m−3, range: 81 to greater than 2400 ng m−3, n =24,961 one second average data; Tekran median: 277 ng m−3, range: 100 to greater than 2400 ng m−3, n =72 two hundred second average data ; Mann–Whitney: p= 0.63). Overall, the instruments showed good agreement (within 0.6 ng m−3 at low air concentrations—though these values were significantly different; and within 4 ng m−3 at the high air concentrations—which were not significantly different). Before and after each field campaign, Tekran® analyzers were calibrated using an internal permeation source and checked using injections of gaseous elemental Hg in ambient air using a Tekran® 2505 Mercury Vapor Primary Calibration Unit with acceptable error being ± 5%. The Lumex® instrument was calibrated by the manufacturer and baseline checks/corrections performed hourly during sampling. When using the Tekran® analyzer to measure flux, a Tekran® Model 1110 Two Port Synchronized Sampler was used to sequentially sample

the air at the DFC inlet and outlet in 6.6-minute intervals (two 3.3minute samples). The same sampling scheme and times were used with the Lumex®, but switching between the inlet and outlet was performed manually. Chambers were continuously flushed with ambient air at similar rates while the air at the sample inlet was collected. The inlet and outlet air was sampled through acid cleaned Teflon® tubing with a disposable PTFE syringe filter (0.22 micron Cole-Parmer®) at the inlet. Flow (Q) through the chamber was maintained at a constant rate and checked daily using a Sierra Instrument® flow meter. The Hg flux was calculated as: F=Q

C0 −Ci A

ð1Þ

where F is the Hg flux (ng m−2 h−1), Q is the chamber flushing flow rate (m3 h−1), C0 is the mean of two repeated air Hg concentration

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Fig. 1. Location of the two gold mines within the state of Nevada, USA. The symbols on the aerial photos (NAIP, 2006) show the locations where surface Hg samples were collected and the type of surface.

samples measured at the outlet (ng m−3), Ci is the mean of two air Hg concentration samples measured at the inlet (immediately before and after the C0 samples), and A is the footprint area of the chamber (m2). Q averaged 2.5 ± 0.3 L min−1 (n = 148) corresponding to a chamber turnover time of 0.8 min. Based on the work in Eckley et al. (2010) the flow rate applied was optimal for measuring flux from waste rock and low-grade ore (fluxes b1900 ng m−2 day−1) which covered N85% of the surface area of the mines. Flux calculation relies on the assumption that the average Ci value (measured for two 3.3-minute periods before and after the 6.6minute outlet sample collection) is representative of the air concentration flowing into the chamber while Co is measured. However, this assumption was not always valid while sampling at the mines, for at times air concentrations could vary by 300 ng m−3 in b10 min due to advection of air from outside of the flux measurement area. The impact of the changing air concentrations on the ability to calculate a flux was dependent upon the magnitude of the surface flux being measured. As such, all data were evaluated using the following criterion: if |ΔCoi| N |ΔCii| flux was assumed to be valid; however if vice versa, the flux was not used. ΔCoi is the difference between the mean of the two Co and two Ci samples, and ΔCii is the difference the two Ci samples. This criterion ensured that all flux measurements were obtained during relatively stable air concentrations.

Two Tekran® DFC systems were used to measure flux simultaneously at each site (with a few exceptions). DFC-1 would measure flux at a single location for ~ 24 h; while DFC-2 would measure flux at two locations ~10 m apart for 12 h each. The trends in flux between the two DFCs over the same time periods were significantly correlated (mean r2 = 0.59 and p = 0.016). The 24 h measurements from DFC-1 were used to create a ratio of its 24 h average flux to its 12 h average flux (corresponding to the specific 12 h period for each DFC-2 measurement) and using the following equation to obtain an estimated 24 h flux for each DFC-2 measurement: DFC124hour flux = DFC112hour flux × DFC212hour flux = DFC224hour flux ð2Þ

As such, most sites had triplicate daily flux measurements during each sampling campaign, which allowed for an assessment of smallscale spatial variability within a particular mine surface. A positive flux represents net emission, while a negative flux is net deposition. At the end of each flux measurement, the material under the chamber was collected and analyzed for total Hg content and percent moisture (determined gravimetrically). During the summer and fall sampling, the material under the chamber was also sieved using the same size classifications as described above. Surface meteorological data collected on site and averaged over 5-minute intervals (Campbell

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Scientific CR10x data logger) included: air temperature, relative humidity (Campbell Scientific HMP45C-L), solar radiation (Li-Cor LI200X-L), precipitation (Texas Electronics — Tipping Bucket Rainfall Sensor) and wind speed (R.M. Young 05103-5 Wind Monitor).

Table 2 Summary of grain size distributions from surfaces at each mine. Analysis of variance and the post-hoc Tukey test were used to identify homogenous groups (indicated with superscripts). Grain size:

Percent b 0.6 mm

Percent 0.6–2.4 mm

Percent 2.4–13 mm

Percent N 13 mm

18 ± 61 22 ± 81 11 ± 42 15 ± 91,2 11 ± 32 100 ± 03

28 ± 61 25 ± 61 19 ± 71,2 18 ± 62,3 17 ± 33 0 ± 04

32 ± 51 31 ± 61 34 ± 101 29 ± 81 29 ± 41 0 ± 02

22 ± 111 23 ± 111 36 ± 122 38 ± 162 44 ± 92 0 ± 03

Mine: Cortez-Pipeline Reclaimed 27 ± 51 Waste rock 18 ± 82 Active leach 12 ± 62 Ore-stockpiles 17 ± 102 Tailings 100 ± 03

22 ± 31 21 ± 91 14 ± 61 20 ± 111 0 ± 02

28 ± 51 27 ± 91 23 ± 81 28 ± 71 0 ± 02

23 ± 91 34 ± 231,2 52 ± 192 37 ± 211,2 0 ± 03

2.4. Data analysis Daily Hg flux was calculated by integrating the area under the empirical 24-h dataset. Statistical analysis was performed using Statgraphics (version XV). The Shapirpo–Wilks test was used to determine if data was normally distributed and where large departures from normality existed, nonparametric statistical tests were used. The Mann–Whitney test was used to identify differences between two independent measurements and the Wilcoxon test was used if the data were paired. The Kruskal–Wallis test was used for testing differences between more than two samples. Fisher's least significant difference test was used during post-hoc analysis to identify homogenous groups. Multivariate regression analysis was used to describe the impact of two or more quantitative factors on a dependent variable and the resulting T-statistics (with their associated p values) for each independent variable were used to identify and rank the relative importance of the variables influencing fluxes (the higher the value of the T-statistics the greater the influence of the parameter). Simple linear regression analysis was used to relate fluxes and surface concentrations and differences between regression slopes were determined using the analysis of covariance test (ANCOVA). 3. Results and discussion 3.1. Surface Hg concentrations and grain size The surface Hg concentrations for Twin Creeks materials were higher (on average 4.5-fold) than for similar materials from CortezPipeline (Mann–Whitney: p b 0.001, n = 401), except for the active leach pads which were similar (Table 1). Within each mine, there were significant differences in concentrations between the surface types (Kruskal–Wallis: Cortez: p b 0.001, n = 142; Twin Creeks: p b 0.001, n = 259). Tailings collected at Twin Creeks had higher concentrations than the high-grade ore stockpiles (which were the source of tailings material). This reflects the fact that the tailings impoundment includes multiple years of accumulation, while the stockpiles represent contemporary inputs. The lower Hg concentrations in the Cortez-Pipeline tailings material relative to Twin Creeks is due to the fact that only low Hg content oxide ore (median: 2.8 μg g−1) was processed at the on-site mill. For comparison, the median Hg concentrations from undisturbed materials located outside of each mine were 14-fold lower than the median mine concentrations (median for all undisturbed materials b 0.3 µg g− 1; Mann–Whitney, p b 0.001, n = 216 to 333; Miller 2010). Waste rock and heap leach materials are crushed by blasting and had similar grain size distributions while the tailings materials have been further processed in a ball mill and are finer grained (b0.6 mm; Table 2). 3.2. Hg fluxes Hg fluxes at the mines were measured from surfaces with a large range in material Hg concentrations (0.2 to 85 μg g−1) and under a variety of meteorological conditions (Table 1). There was significant spatial variability in the magnitude of Hg fluxes across each mine—from relatively low values for the waste rock materials (b1500 ng m−2 day−1) to high release from the active leach pads and tailings (N500,000 ng m−2 day−1) . To contextualize the magnitude of these fluxes, emissions from natural undisturbed areas outside the mines were typically b200 ng m−2 day−1 (Miller 2010). At Twin Creeks, the fluxes from the tailings and leach pads (during cyanide addition) were significantly higher than the other mine surfaces (i.e. waste rock, pre-leach materials, etc), and at Cortez-Pipeline

Mine: Twin Creeks Reclaimed Waste rock Active leach Inactive leach Ore-stockpiles Tailings

the fluxes from the tailings were higher than the other mine surfaces (Kruskal–Wallis: Twin Creeks pb 0.001, n=74; Cortez-Pipeline pb 0.001, n=45). Emissions measured at these gold mines are on the higher end of measurements made from other Hg contaminated sites (e.g. maximum daily fluxes ranging from 3400 to 662,000 ng m−2 day−1—Beauchamp et al., 2002; Wang et al., 2007; Gustin et al., 2003; Carpi and Lindberg, 1997). Comparison of proximal simultaneous DFC measurements showed significant small-scale spatial variability in Hg fluxes from within a given type of mine surface. This was more pronounced at the Twin Creeks mine than at the Cortez-Pipeline mine (71% and 32% of withinsite fluxes were significantly different, respectively; Mann–Whitney p b 0.05; Table 1). Multivariate regression analysis of the relative standard deviation of the three replicate surface measurements from both mines showed that the variability of the within-site fluxes was correlated with the variability of the surface grain size, moisture, and Hg concentration (R2 = 0.58, p = 0.015). 3.2.1. Influential variables The strength of correlations of environmental parameters with Hg flux can be affected by the time resolution used in analysis and Stamenkovic et al. (2008) suggested that analyzing hourly data best elucidates parameters that are important on a diel time-step, while averaged daily values can be used to identify environmental factors correlated with seasonal trends in fluxes, as well as the influence of the material physicochemical properties. Multivariate regression analysis using hourly flux data showed that all measured parameters (Table 1) were correlated with Hg flux to some degree (overall R2 = 0.21, p b 0.001 for all variables, n = 1429). The best correlated parameters were surface moisture content (T = 9.3) and Hg concentration (T = 8.4). The three meteorological parameters (solar radiation, temperature, RH) were all significantly correlated with each other (Pearson p b 0.001 for all combinations) and of these multivariate regression analysis showed that solar was best correlated with Hg flux (T = 7.4). Fluxes were typically highest during the day and lower at night (e.g. Fig. 2) and similar to values reported for a variety of other surfaces (Stamenkovic et al., 2008; Carpi and Lindberg, 1997; Ericksen et al., 2006; Wang et al., 2007). The daily Hg flux from each surface type varied between seasons (Table 1), however there was not a consistent seasonal response of fluxes between all surfaces due to inconsistent weather within a given season (e.g. spring—Cortez-Pipeline: dry and 22 °C; Twin Creeks: rainy and 6 °C sampled during consecutive weeks). Multivariate regression analysis of the daily fluxes (R2 = 0.22, p b 0.001, n = 92) and co-occurring parameters in Table 1 found that only the material Hg concentration (T = 2.6, p = 0.01) and moisture (T = 2.0, p = 0.05) were significantly correlated. Additional statistical analysis performed

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Fig. 2. Example of a typical diel trend in Hg fluxes from mine surfaces, with higher emissions during the day than at night. This example is from the old/inactive leach pad at Twin Creeks mine sampled during the winter, 2008. Other examples of diel trends in Hg fluxes from other mine surfaces can be found in Figs. 4 and 5.

using only Hg fluxes associated with dry materials did not show a significant relationship with any of the meteorological variables. These results indicate that material characteristics were important in determining the magnitude of emissions while light conditions were important in driving daily and seasonal trends. The ambient Hg air concentrations at each mine typically ranged from 10 to 66 ng m−3 (Cortez-Pipeline) and 18 to 124 ng m−3 (Twin Creeks) (interquartile range—encompassing the middle 50% of the data). However at times, values were much higher (N800 ng m−3) due to advection of Hg from point and nonpoint sources (e.g. Fig. 3). During these periods, the flux associated with some surfaces changed from net emission to net deposition (Fig. 3). Air concentrations during these periods were variable making quantification of fluxes difficult (see the discussion in the Materials and methods section). For these periods, as air concentration increased Hg deposition was measured (Cortez-Pipeline: r2 = 0.73, p b 0.001, n = 110; Twin Creeks r2 = 0.27, p b 0.001, n = 277). Similarly, net deposition during periods of elevated air concentrations was observed at natural undisturbed locations just outside of the mine (Miller 2010) and has also been observed in other studies (Engle et al., 2001; Nacht et al., 2004). Periods of deposition were interspersed with measured emissions that were significantly greater than those occurring during lower air Hg concentrations. This suggests that a component of the deposited Hg was rapidly emitted back to the air. For example, while sampling flux over waste rock at the Cortez-Pipeline mine, the air concentrations during the day were relatively low and stable (median 6.0 ng m−3; range: 3.5 to 7.8 ng m−3) as were the Hg fluxes, which were all net emission (median: 2.5 ng m−2 h−1; range: 0.6 to 4.5 ng m−2 h−1) (Fig. 3). In the early evening air concentrations increased and were variable through the night (median: 15 ng m−3; range: 3.3 to 73 ng m−3) as were the measured fluxes (median: −10 ng m−2 h−1; range: −40 to 32 ng m−2 h−1). Hg emissions measured during the period of variable nighttime air concentrations were 13-fold higher than those recorded during the daytime periods when the air concentration was lower and less variable. Based on these observations we suggest that the high nighttime emissions reflect the re-emission of recently deposited Hg and that atmospheric Hg emitted from point and nonpoint sources at the mines may be locally recycled between the air and surfaces at the mine. Since Hg flux was correlated with surface Hg concentrations and similar to that measured from materials in a controlled laboratory setting (at nearbackground atmospheric Hg concentrations—see Eckley et al., submitted for publication), it appears that most Hg emissions measured at the mines is indigenous to the material but some component could be from previous atmospheric deposition. This is important to consider when assessing the total point and nonpoint source releases from the mines.

Surface disturbance varied across each mine and has been shown to influence emissions (Gustin et al., 2003; Zhang et al., 2002; Nacht et al., 2004). The results from laboratory experiments using the mine materials clearly showed elevated emissions following surface disturbance (Eckley et al., submitted for publication). During the spring sampling at Cortez-Pipeline, the Hg flux was measured from proximate post-leached materials—one compacted and one disturbed by a bulldozer. The concentration of these materials were similar (disturbed: 0.9 ± 0.0 × μg g−1; compacted: 0.7 ± 0.1 μg g−1), however the flux from the disturbed material was 3-fold higher than the compacted material (2397 ± 92 ng m−2 day−1 compared to 857 ± 682 ng m−2 day−1: Mann–Whitney, p = 0.004, n = 59). Further evidence of the influence of surface disturbance comes from measurements obtained from an older/inactive leach pad at the Cortez-Pipeline mine that had not been disturbed for N4 years. Comparing in situ fluxes from this surface versus those predicted using mesocosm derived equations for more recently disturbed materials of the same concentration showed a flux 3 to 17-fold lower than expected (Eckley et al., submitted for publication, Fig. 4). These examples, although limited, suggest that over time emissions may decrease due to surface compaction and/or due to a decrease in a labile surface pool of Hg. 3.2.2. Tailings The tailings impoundment at Twin Creeks was heterogeneous in terms of Hg concentrations (19 to 177 μg g−1), surface moisture (0.1% to 100% liquid), and Hg emissions (889 to 684,000 ng m−2 day−1). The magnitude of the emissions were positively correlated with soil moisture (r2 = 55, p = 0.001, n = 16), but not with surface Hg concentration (r2 = 0.03, p = 0.6, n = 15). The Hg concentration of unfiltered liquid tailings solution was 496 ± 27 μg L−1 (n= 4 unfiltered samples obtained during the fall and winter sampling) and fluxes measured from the liquid surface ranged from 529 to 3810 ng m−2 h−1 (n= 12 hourly fluxes collected in the fall during daylight). At Cortez-Pipeline only one section of the tailings that consisted of older (N1 year) drier materials was accessible for flux measurements. During the fall sampling, the material in this section of the tailings had a marginal increase in moisture (from 1 to 4%) and the Hg emissions were an order of magnitude higher than measured from the drier materials during the other seasons (Table 1). Since temperature and solar radiation were reduced during this time, it is possible that the slight difference in soil moisture was influencing flux. This suggests that the emissions from the sections of Cortez-Pipeline tailings that have greater year-round moisture content (or are liquid) may be higher than we measured from the relatively dry section. To verify

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Time of Day Fig. 3. Example of Hg deposition occurring during the periods of elevated air Hg concentrations from (A) the capped tailings at Twin Creeks mine from June 17th–21st and (B) the waste rock dump at Cortez-Pipeline from August 15th–16th. The air Hg concentration is shown in the solid black line, all Hg flux data is shown in a dashed line, and the triangles represent the flux data that passed the acceptance criteria.

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Time of Day Fig. 4. Example of the Cortez-Pipeline inactive leach surface that showed significant differences between in situ field and laboratory flux measurements. Field flux: 221 ng m−2 day−1; substrate Hg: 6.2 μg g−1; solar radiation 353 W m−2; air temperature 26 °C. Laboratory flux: 2690 ng m−2 day−1; substrate Hg: 6.7 μg g−1; solar radiation: 317 W m−2; air temperature: 17 °C.

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that the Cortez-Pipeline tailings emissions increase with moisture content, dry tailings material from this site were transported to the University of Nevada Reno (UNR) laboratory where controlled wetting experiments were performed with low Hg concentration water (2.3 ng L−1; described in ref Eckley et al., submitted for publication). The results confirmed that emissions from the tailings materials increased ~3-fold at moisture contents between 2 to 8%. Since the field flux measurements at Cortez-Pipeline were confined to a dry section of the impoundment, the field data is not considered representative of the entire tailings area which had large sections of moist and liquid tailings (30 to 80% of the surface area depending on the season). Factors that could have contributed to the higher Hg emissions from the tailings compared to other mine surfaces were: 1) high Hg concentration; 2) high moisture content; 3) small grain size; and 4) material/solution Hg chemistry. At Twin Creeks mine, material concentrations were significantly higher than the other surfaces (median: 40 μg g−1; Mann–Whitney p b 0.001, n = 72) and parts of the surface were inundated with solution. It is also possible that the ore processing that included crushing (reducing gain size/increasing surface area) and oxidizing the sulfur and carbon in the ore made Hg more available for release from this material. 3.2.3. Active cyanide addition Since soil moisture is a factor enhancing Hg release from soil (Gustin and Stamenkovic, 2005; Lindberg et al., 1999; Song and Van Heyst, 2005) the potential for active cyanide heap leaching to influence Hg release was investigated. On average, the daily emissions from the materials during cyanide addition to the heap leach pads were 35-fold higher than those measured from materials before and after leaching (during the same seasons) even though there was not a significant difference in the material Hg concentration (Kruskal– Wallis p = 0.77, n = 27; Table 1; Fig. 5). Additionally the increase in flux observed during cyanide application was significantly higher than that during mesocosm flux measurements after addition of rain water (Eckley et al., submitted for publication—5 to 6-fold increase). The magnitude of increase in fluxes in response to rain is similar to that observed by others (Lindberg et al., 1999; Gustin and Stamenkovic, 2005). Thus fluxes from the active heap leaches during cyanide solution addition are much larger than would be expected due to surface wetting alone, suggesting that the application of the leach solution affects emissions. Cyanide solution has been shown to solubilize Hg associated with gold ore (Matlock et al., 2002) and the

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Hg concentration of the leach solution collected at the point of application (470 ± 33 μg L−1 n = 10 unfiltered samples collected during seasonal measurements) was more than four orders of magnitude greater than average regional precipitation (0.013 μg L−1, MDN website). The high Hg concentration is due to subterranean Hg being dissolved in solution as it migrates through the heap and to the continued recycling of this liquid after being stripped of gold. Assuming that the cyanide solution was applied to the heap leach surface at a rate of 12 L m−2 h−1 over a 42ha area (information provided by Newmont Mining Co. for Twin Creeks) 58 kg day−1 of Hg would be added to the heap leach surface. However, the scaled Hg emissions measured during cyanide addition were substantially lower (b0.1 kg day−1), suggesting that only a small amount (b0.25%) of the Hg in the cyanide solution applied to the heap leach surface is released to the air. While the amount of Hg released from solution is small, this could account for a substantial component of the Hg emitted from actively leached materials. An alternate hypothesis is that Hg released during cyanide addition is derived from sub-surface leach solution/ore/soil gas interactions. Prior to irrigation, heap leach surfaces are mechanically plowed to increase infiltration, which creates a series of valleys and ridges (similar to a plowed agricultural field). With solution application the valleys become saturated with leach solution, while the ridges remain drier (Table 1). Flux measurements from adjacent ridges and valleys showed that the wetter valleys had fluxes that were 3 to 30-fold higher than the ridges. We suggest that the differences in emissions from proximate (b1 m) locations indicate that the cyanide solution is contributing to the Hg release because processes occurring at depth would result in more homogenous emissions across the surface. Additionally, the fact that during cyanide addition the amount of Hg released during the day is significantly higher than that observed at night (Fig. 5) suggests that the emissions from the leach pads are influenced by incident solar radiation.

3.2.4. Remediated sites Measurements from an inactive tailings impoundment that had been reclaimed by capping the surface with ~1 m of alluvium material showed that the surface Hg concentrations (0.2 ± 0.03 μg g−1 n =8) and fluxes (spring: 1159 ± 508 ng m−2 day−1 n = 154; summer: 309 ± 81 ng m−2 day−1 n = 411) were 24 to 650-fold lower than measured from the active tailings (Table 1). These findings are in agreement with

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other work that has shown that Hg release from contaminated materials decreases after disturbance ceases (Gustin et al., 2004). 4. Conclusions Nonpoint source Hg releases from active industrial gold mines are spatially and temporally variable, and influenced by substrate characteristics (e.g. Hg concentration, moisture content, and grain size), ore processing (e.g. cyanide leaching, autoclaving, etc), and environmental conditions (e.g. solar radiation, temperature, etc) (see also Eckley et al., submitted for publication). Overall, surfaces associated with active processing of the ore (i.e. wet tailings and heap leach pads) exhibited the highest emissions. This is a function of both the high Hg concentration in the material and their wetting with cyanide solution. Fluxes from these materials were several orders of magnitude higher than natural emissions from undisturbed surfaces in the surrounding area (typically b200 ng m−2 day−1 Miller 2010). At these mines the highest degree of natural Hg enrichment is associated with the highest grade ore. This material is typically processed by milling and oxidizing the ore prior to cyanide addition and then storing the waste in a tailings impoundment. The processing facilities are point sources of Hg and the impoundments are significant nonpoint sources of Hg. As tailings dry and are capped emissions from these materials will decline. The process of heap leaching also results in enhanced Hg release. Given that the cyanide solution acts a pathway for transport of Hg from within the ore to the surface, measures to reduce Hg in the cyanide solution or immobilize it and reduce exposure of the solution to the air could be important for mitigating nonpoint source Hg emissions. The life of a mine will depend upon the economic value of the resource, the technology to obtain the ore and the costs of the infrastructure needed to support the mine. As a mine comes to closure, surfaces that are no longer active are often reclaimed by capping and/or revegetating. Our results suggest that these actions will result in significant reductions in surface Hg emissions. Acknowledgements This research was funded by a grant from the Nevada Division of Environmental Protection, with contributions from Nevada's gold mining companies. We would like to thank the Barrick and Newmont mining corporations for their project participation and mine personnel for their help during this project. We also thank, UNR students — JJ Sanders, C Woodward and C Weaver. Thanks also to the four anonymous reviewers for their time and effort. References Barnes HL, Seward TM. Geothermal systems and mercury deposits. In: Barnes HL, editor. Geochemistry of hydrothermal ore deposits. New York: John Wiley and Sons; 1997. p. 669–736. Beauchamp S, Tordon R, Phinney L, Abraham K, Pinette A, MacIntosh A, et al. Air– surface exchange of mercury in natural and anthropogenically impacted landscapes in Atlantic Canada. Geochem Explor Environ Anal 2002;2:157–65. Carpi A, Lindberg SE. Sunlight-mediated emission of elemental mercury from soil amended with municipal sewage sludge. Environ Sci Technol 1997;31:2085–91. Eckley C.S., Gustin M.S., Miller M.B., Marsik F. Nonpoint source Hg emissions from active industrial gold mines—influential variables and annual emission estimates. Environ Sci Technol (submitted for publication). Eckley CS, Gustin M, Lin C-J, Li X, Miller MB. The influence of dynamic chamber design and operating parameters on calculated surface-to-air mercury fluxes. Atmos Environ 2010;44:194–203.

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