Environ. Sci. Technol. 2005, 39, 1679-1687
Determination of Mercury Evasion in a Contaminated Headwater Stream A N T U C . M A P R A N I , † T O M A . A L , * ,† KERRY T. MACQUARRIE,‡ JOHN A. DALZIEL,§ SEAN A. SHAW,† AND PHILLIP A. YEATS§ Department of Geology and Department of Civil Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada, and Fisheries and Oceans Canada, Marine Chemistry Section, Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada
Evasion from first- and second-order streams in a watershed may be a significant factor in the atmospheric recycling of volatile pollutants such as mercury; however, methods developed for the determination of Hg evasion rates from larger water bodies are not expected to provide satisfactory results in highly turbulent and morphologically complex first- and second-order streams. A new method for determining the Hg evasion rates from these streams, involving laboratory gas-indexing experiments and field tracer tests, was developed in this study to estimate the evasion rate of Hg from Gossan Creek, a first-order stream in the Upsalquitch River watershed in northern New Brunswick, Canada. Gossan Creek receives Hg-contaminated groundwater discharge from a gold mine tailings pile. Laboratory gas-indexing experiments provided the ratio of gasexchange coefficients for zero-valent Hg to propane (tracer gas) of 0.81 ( 0.16, suggesting that the evasion mechanism in highly turbulent systems can be described by the surface renewal model with an additional component of enhanced gas evasion probably related to the formation of bubbles. Deliberate field tracer tests with propane and chloride tracers were found to be a reliable and practical method for the determination of gas-exchange coefficients for small streams. Estimation of Hg evasion from the first 1 km of Gossan Creek indicates that about 6.4 kg of Hg per year is entering the atmosphere, which is a significant fraction of the regional sources of Hg to the atmosphere.
Introduction The rates and mechanisms of air-water exchange of oxygen, greenhouse gases, and volatile pollutants such as mercury (Hg) are of considerable interest among environmental scientists. In their global model of atmospheric Hg cycling, Mason et al. (1) estimate that the flux of Hg from the ocean to the atmosphere is about half the total annual anthropogenic emissions and twice the natural flux of Hg to the atmosphere from all terrestrial sources. However, the relative importance of Hg emitted from freshwater systems as compared to evasion from soils or other terrestrial emission sources is still largely unknown but may constitute a * Corresponding author phone: (506)447-3189; fax: (506)447-5055; e-mail:
[email protected]. † Department of Geology, University of New Brunswick. ‡ Department of Civil Engineering, University of New Brunswick. § Fisheries and Oceans Canada. 10.1021/es048962j CCC: $30.25 Published on Web 02/08/2005
2005 American Chemical Society
significant contribution to Hg budgets and the cycling of Hg on regional and/or global scales (2). The role of evasion from first- and second-order streams in a watershed may be of particular importance to the atmospheric recycling of Hg (and volatile pollutants in general) in terrestrial environments since (i) these streams constitute the major portion of the total stream length of any watershed, (ii) turbulence in these streams is much higher than in most higher order streams or stationary water bodies, and (iii) headwater streams are the principal collectors of rainfall, and in many cases the Hg concentration in rainwater is much higher than in groundwater, the other major source of water in stream systems. Air-water gas transfer rates in oceans and lakes are commonly estimated by applying Whitman’s stagnant film model (3-5) using a measured concentration gradient across the air-water interface, the Henry’s law constant, and the gas transfer velocity, which is assumed proportional to the molecular diffusion coefficient of the gas. Alternatively, empirical and semi-empirical equations have been developed to estimate the gas transfer velocity for oceans or lakes based on correlation of field data and wind velocity (6-8). The surface renewal model (9, 10) is commonly applied for flowing water bodies. This model assumes that the gas transfer velocity is proportional to the square root of the molecular diffusion coefficient and the surface renewal rate. In the context of stream reaeration studies, several empirical equations have been developed based on correlation of field data and the hydraulic characteristics of the water body (11). However, a recent investigation of these equations using a large set of field data on re-aeration rates (12) demonstrated that use of empirical equations outside of the stream systems for which they were derived generally results in a poor fit with field data. A similar observation was reported when empirical equations are used for estuarine systems (13). Other field estimation techniques include natural (14, 15) or deliberate (11, 16) tracer methods and flux chamber methods (17-21), which are suitable for both stagnant and flowing water bodies, as well as micro-meteorological methods (22, 23) for larger water bodies. Mercury evasion rates from small headwater streams have not been reported to date, and the methods developed for large water bodies are not expected to provide satisfactory results in highly turbulent and morphologically complex first- and second-order streams. Therefore, a new approach for assessing Hg evasion from these streams involving field-tracer tests and laboratory gasindexing experiments is developed in this study. Study Area. The Murray Brook volcanogenic massive sulfide deposit consists mostly of pyrite and copper-zinclead sulfides at depth. It is overlain by a Cu-rich supergeneenrichment zone and a surficial gossan (24, 25). The gossan is composed primarily of goethite, quartz, beaudantite, jarosite, plumbojarosite, bindheimite, and scorodite with accessory cinnabar, native Ag, Au, and cassiterite (25). The gossan was mined by open pit methods between 1989 and 1992. The ore was crushed to 1.9 cm and treated with cement (5 kg/ton) and hydrated lime (8 kg/ton) producing an agglomerate material that was placed in concrete vats. The agglomerate was sprayed with calcium cyanide solution (prepared at 500-700 mg/L free cyanide) for Au/Ag extraction, and a Merryl-Crowe zinc precipitation process was used to liberate the precious metals from the cyanide (CN) solution. The CN leaching process also extracts Hg from the ore, and a total of 1000 kg of Hg was recovered during the Au production process. The tailings, which contained residual Hg and CN, were stored in a pile on the site (Figure 1). VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Location of Gossan Creek, the multi-level piezometer, and the sampling stations for the tracer experiments. Contours on the stream location map are meters above sea level with a contour interval of 10 m. Gossan Creek, a first-order stream in the Upsalquitch River watershed in northern New Brunswick, Canada, has elevated concentrations of total dissolved Hg (HgT up to 60 µg/L) as a result of leaching from the tailings disposal site (Figure 1). In April 1991, before mining ended, Boyle and Smith (24) recorded HgT concentrations as high as 7200 µg/L in the groundwater downgradient from the tailings. The principal form of Hg in the gossan ore is cinnabar (25), and Boyle and Smith (24) suggested that residual CN in the tailings caused leaching of Hg from cinnabar, leading to the highly elevated aqueous Hg concentrations. Since the end of mining operations in 1992, elevated HgT and Au concentrations have been observed in the groundwater and surface water downgradient from the tailings (24, 26, 27). Measurements of HgT mass load at various control points along the creek show a 9599% reduction in HgT mass load within 4 km of the headwaters of Gossan Creek (27). The loss of HgT may be a result of reaction with the creek bed sediments, uptake by the biota, and evasive losses to the atmosphere as Hg0 is formed in the stream by reduction of Hg(II) (28, 29). Objectives. The objectives of this work are to further demonstrate the link between the mine tailings and Hg contamination in Gossan Creek, to determine the extent to which Hg in Gossan Creek is transformed to the volatile Hg0 form, and to develop a technique to estimate the magnitude of the Hg0 evasion flux from Gossan Creek.
Materials and Methods Mercury and Cyanide in Groundwater and Surface Water. The ground surface slopes downward from the tailings toward the valley where Gossan Creek occurs (Figure 1). There is a similar gradient on the groundwater table that results in groundwater flow from the tailings toward the headwaters 1680
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of the creek. To demonstrate that the Hg contamination in the creek originates from the tailings and that the elevated Hg concentrations are related to residual CN in the tailings, groundwater samples were collected from a multi-level piezometer located a short distance downgradient from the tailings, and surface water samples were collected from Gossan Creek. The multi-level piezometer was constructed in bedrock to a total depth of 17.5 m within a 10 cm diameter diamond-drill hole. A total of 11 individual 1.6 mm (i.d.) polyethylene tubes were completed at different depths with 10 cm long screens that were wrapped with fine nylon mesh. The screened sample points were separated by distances ranging between 1 and 1.5 m. The annular space around the screens was filled with coarse-grained silica sand (0.8-1.0 mm), and the annular space between sample points was filled with fine grained silica sand (0.2-0.3 mm) to prevent cross-flow between sample intervals. All water samples were filtered in the field using 0.45 µm cellulose-acetate membranes. Samples for Hg analysis were preserved with 0.5% BrCl, and samples for CN analysis were preserved with NaOH pellets to an approximate concentration of 0.1 M. The water samples were analyzed for HgT by oxidation, purge-and-trap, and CVAAS detection following U.S. EPA Method 1631 (30). U.S. EPA Method 1631 is intended for CVAFS detection of Hg; however, the method explicitly allows for the use of CVAAS detection in cases where lower sensitivity is required (Method 1631 section 9.1.2). The precision on sample replicates was 2.9% (rsd). Total CN (CNT) analyses were conducted by aqueous micro-distillation coupled with spectrophotometric determination following the method outlined by Volmer and Giesselmann (31) with the following modifications: (i) distillation time was reduced from 1.5 to 1.0 h; (ii) 9 M H2SO4 was used instead of a citrate/phosphoric
acid buffer; and (iii) spectrophotometric determination (Hach 2010 spectrophotometer) was performed at 578 nm instead of 600 nm to improve the sensitivity. The precision on sample replicates was 8.75% (rsd). Approach to Mercury Evasion Measurement. To estimate the rate of evasion from a stream, the gas-exchange coefficient for Hg (kHg) must be determined for the stream reach in question. The coefficient may be obtained for a single, extensive stream reach or a number of values may be determined for smaller sub-reaches with distinct morphological and turbulence characteristics. It is not practical to measure kHg directly in the field because of the high background Hg concentrations in the streamwater and because of the difficulty related to obtaining permits that would allow for the release of Hg to a stream. As an alternative, we have derived kHg from field measurements of the gasexchange coefficient for propane (kpr), combined with determinations of kHg/kpr ratios from controlled experiments in the laboratory. The approach is based on the assumption that the ratio of exchange coefficients for two gases in the same water body is independent of mixing conditions (32). As noted by Rathbun et al. (32), the air-water transfer of slightly soluble gases may be described by the first-order kinetic expression:
dC/dt ) k(Ceq - C)
(1)
where C is the concentration of gas in the water at time t; Ceq is the concentration of gas in the water in equilibrium with the atmosphere, and Ceq ) CA/H, where CA and H are the gas concentration in the air and Henry’s constant, respectively; and k is the gas-exchange rate constant. By integrating and rearranging, eq 1 gives:
Ceq - C ) (Ceq - C0)e
-kt
(2)
where C0 is the concentration of gas in water at t ) 0. In controlled experiments, the concentration of gas in the air is kept equal to zero by maintaining continuous airflow over the water surface. Thus, the equilibrium gas concentration in water (Ceq) will be zero, and eq 2 becomes:
C ) C0e-kt
(3)
The rate constant k (T-1) can be obtained from the linear least-squares regression analysis of ln(C/C0) versus time. The rate constant changes with temperature and turbulence; consequently, the rate constants are herein referred to as gas-exchange coefficients. In a stream system that receives Hg contamination at a point source, the Hg evasion flux, F [M/T], for a given stream reach can be calculated as follows:
F ) [(Ceq - Cu) - (Ceq - Cu)e-(kHg‚t)]Q
(4)
where Cu is the Hg0 concentration at the upstream end of the stream reach, t is the residence time of the water in the stream reach, and Q is the average stream discharge rate in the reach [L3/T]. Laboratory Gas-Indexing Experiments. The ratio of water-air gas-exchange coefficients for Hg and propane has been determined by laboratory experiments. A clean plastic water bath having dimensions 0.7 m (L) × 0.48 m (W) × 0.31 m (D) was filled with 64 L of distilled water and placed in a fume hood. The water was saturated with propane by bubbling propane gas into the water through glass frits for 100 min. The initial concentration of propane was approximately 20 mg/L. The initial concentration of Hg0 in the water was fixed at around 700 ng/L by adding a standard Hg(II) solution converted to Hg0 by successive reduction with
hydroxylamine hydrochloride and stannous chloride solutions. An excess amount of stannous chloride was added to the bath to ensure all Hg was in the Hg0 state. The Hg and propane experiments were conducted separately because initial attempts to combine the measurement of kHg and kpr indicated that Hg evasion did not behave according to firstorder kinetics as expected. The reason for the deviation from first-order kinetics is uncertain, but implicit in the experimental procedure is the assumption that HgT measurements represent Hg0. In the case of combined Hg and propane experiments this assumption may be invalidated by the formation of aqueous Hg(II)-organic complexes or solid compounds (a cloudy white precipitate was observed in the reaction vessel following all experiments involving propane). The implications of these observations are discussed further below. The water in the bath was circulated at a constant rate using an electric mixer. The mixer propeller was immersed to the bottom of the water bath to achieve uniform mixing. The mixer was stopped for a short period (45 s) during sampling; otherwise, the mixing rate was constant for the duration of each experiment. Samples were obtained for Hg and propane by submerging the sample bottles about middepth in the water bath. A 500 mL, acid-washed fluoropolymer bottle was used for Hg sampling, and the samples were immediately preserved by adding 0.5% BrCl solution. Samples for propane were collected in 40 mL glass vials with fluoropolymer-lined rubber septa, leaving no headspace. The experiments continued for 80 min, and duplicate samples for propane and Hg were obtained at 5, 10, 20, 30, 50, and 80 min. The temperature of the bath was continuously monitored and recorded at the time of sample collection. The turbulence of water circulation varied according to the settings on the mixer; the settings are referred to as stage 1 (low), stage 2 (medium), and stage 3 (high). The turbulence intensity at the three stages of mixing was measured at three locations (mid-depth) in the bath using a SonTek 10 MHz Micro Acoustic Doppler Velocimeter (Series A548). The intensity of turbulence is expressed in terms of the rootmean-square value of the time series of the individual x, y, and z velocity magnitude values (RMSV). The turbulence intensity values obtained in this manner provide comparisons of turbulence at each stage of mixing in the laboratory experiments; however, there is no satisfactory way to compare these values to the natural turbulence in Gossan Creek. The experiments were conducted with constant turbulence (stage 1) at three temperatures (8.3 ( 1.0, 11.4 ( 1.0, and 21.0 ( 1.0 °C) to establish the dependence of gas-exchange coefficients on temperature. Similarly, experiments with the three different turbulence conditions and constant temperature (21 °C) were conducted to establish the influence of turbulence on the gas-exchange coefficients. The experiments were also replicated three times at constant temperature (21 °C) and constant turbulence (stage 1) in order to determine the experimental precision. The concentration of Hg in each sample was determined by oxidation, purge-and-trap, and analysis by CVAAS following U.S. EPA Method 1631 (30). The propane concentrations were determined by a headspace analysis method (33) on a SRI GC/FID. The analytical precision (expressed as rsd) for Hg measurements in the experiments was
