Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Total Mercury and Methylmercury in Lake Water of Canada’s Oil Sands Region Craig A. Emmerton,*,† Colin A. Cooke,†,‡ Gregory R. Wentworth,† Jennifer A. Graydon,§ Andrei Ryjkov,∥ and Ashu Dastoor∥ †
Environmental Monitoring and Science Division, Alberta Environment and Parks, Edmonton, Alberta T5J 5C6, Canada Department of Earth and Atmospheric Sciences and §Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2R3, Canada ∥ Air Quality Research Division, Environment and Climate Change Canada, Dorval, Quebec H9P 1J3, Canada Downloaded via KAOHSIUNG MEDICAL UNIV on September 19, 2018 at 12:47:31 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Increased delivery of mercury to ecosystems is a common consequence of industrialization, including in the Athabasca Oil Sands Region (AOSR) of Canada. Atmospheric mercury deposition has been studied previously in the AOSR; however, less is known about the impact of regional industry on toxic methylmercury (MeHg) concentrations in lake ecosystems. We measured total mercury (THg) and MeHg concentrations for five years from 50 lakes throughout the AOSR. Mean lake water concentrations of THg (0.4−5.3 ng L−1) and MeHg (0.01−0.34 ng L−1) were similar to those of other boreal lakes and 100 km northwest of oil sands mines and received runoff from geological formations high in metals concentrations. MeHg concentrations were highest in those lakes, and in smaller productive lakes closer to oil sands mines. Simulated annual average direct deposition of THg to sampled lakes using an atmospheric chemical transport model showed 1 ha in surface area45 occur radially within a few hundred kilometers of oil sands facilities (median lake area ± SE: 4.2 ± 12 ha). In this study, we monitored the 50 diverse, well-mixed lakes of the Acid Sensitive Lakes Monitoring Program.46 These lakes were originally selected based on several factors including (1) an analysis of potentially acid sensitive lake systems in the AOSR,47 (2) increasing distance from open-pit mining operations, (3) accessibility, primarily by float plane (sampled median lake area: 131 ± 92 ha; range: 3−4349 ha), and (4) differences in geographical and environmental conditions48 (“lake clusters”; Table 1; Figure 1). Lakes within the Caribou and Birch mountain clusters are primarily at high elevations, have bog-rich watersheds underlain by organic cryosol soils and shale and siltstone bedrock, and have typically stained and moderately productive surface waters. Canadian Shield lakes are large, clear, and unproductive, with watersheds covered by upland forests, thin soils, granitic outcrops, and fens. The NE/ W Fort McMurray and Stony Mountain lake watersheds are dominated by fen cover but are slightly more developed than other watersheds. The surface waters of these lakes are productive, moderately stained, and underlain by mesisolic soils and shale, sandstone, and siltstone rock. Lake Water Quality Sampling and Chemical Analyses. All 50 remote Acid Sensitive Lakes were sampled once annually during the peak boreal growing season in July and August. Most lakes were accessed by float plane, except for nine lakes which were sampled by a float-equipped helicopter (Figure 1). Near the center of each lake, a multiprobe instrument (Hydrolab Minisonde, MS-5) was used to measure the water column for several physical parameters (water temperature, pH, specific conductivity, dissolved oxygen, redox potential, and turbidity). At the same location, unfiltered THg and unfiltered MeHg samples were directly collected into new, acid-rinsed amber glass bottles at 30 cm depth using ultraclean protocols.49 Duplicate samples and field and trip blanks for THg and MeHg analysis were collected periodically throughout the program for quality control purposes (Table S1). All THg and MeHg samples were preserved immediately after collection with trace-metal-grade HCl at 0.4% and 0.5% of the sample volumes, respectively. For additional chemical analyses, multiple integrated water samples were collected from the entire (well-mixed) measured euphotic zone of each lake using
concentrations in aquatic ecosystems. Landscape disturbances within watersheds have also been shown to affect the mobility, methylation, and concentration of Hg,40 including in the AOSR.41 Though findings from previous studies have improved our understanding of Hg cycling in the AOSR, we comparatively know little about Hg concentrations within important methylating environments such as lake ecosystems. Further, understanding how Hg concentrations in AOSR lakes vary relative to patterns of atmospheric Hg deposition and sitespecific conditions is important, but unknown, for all users of aquatic resources in the AOSR. Here, we present five years of midsummer unfiltered THg and unfiltered MeHg concentrations from the surface waters of 50 lakes within the AOSR. The majority of these lakes rest within undisturbed catchments and they are located at varying distances from oil sands facilities. Our specific objectives were to (1) compare THg and MeHg concentrations in lakes against relevant surface water quality guidelines, (2) determine how THg and MeHg concentrations in lakes vary in space relative to oil sands facilities, and (3) identify the physical and chemical conditions within the lakes and their watersheds that associate closest with their THg and MeHg concentrations.
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MATERIALS AND METHODS Regional Description and Study Lakes. The AOSR (Figure 1) resides mostly within the Boreal Plains ecozone of western Canada, which is characterized by extensive coverage of low-relief forests (Picea and Pinus spp.), bog and fen lowlands, and glacially derived lakes. Boreal Shield covers a northeast portion of the region and is dominated by granite outcrops, Picea and Pinus forests, fen wetlands, and numerous large glacial lakes. The climate in northeastern Alberta is continental with warm to cold summers.42 Daily mean air temperature at Fort McMurray is 1.0 °C (range: −5.1 to 7.0 °C) with mean annual precipitation of 419 mm (32% as snow43). Human activities and resulting land disturbances across the AOSR are primarily related to oil and gas exploration and recovery. However, these activities are most visibly concentrated north of Fort McMurray, and for the purposes of this paper, we will use site AR644 (Figure 1) (herein “oil sands facilities”) as a general locator of these features. C
DOI: 10.1021/acs.est.8b01680 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
port) to sampled lake watersheds. Spatially distributed THg deposition rates (including particle-bound Hg) to ecosystems for 2011 to 2015 in Canada were simulated by the Hg version of Environment and Climate Change Canada’s operational airquality forecast model GEM-MACH (Global Environmental Multiscale, Modeling Air Quality and Chemistry model), GEM-MACH-Hg (see Supporting Information for additional detail). Unconstrained Principal Components Analysis (PCA) with supplementary variables was used to analyze associations among Hg species, chemistry, and watershed characteristics data of the 50 lakes (Canoco v. 5.03; Biometris, The Netherlands; Table S2). This PCA approach defines ordination axes ad-hoc (THg, MeHg, %MeHg) and they represent linear combinations of other explanatory parameters. Individual lakes were also underlain onto the axes to identify lakes most responsible for strong associations between Hg and explanatory variables.
a Tygon sampling tube with a one-way valve. These samples were pooled in a plastic carboy, manually mixed, and poured into plastic bottles for chemical analyses. All water samples were sent for chemical analyses the same day as collection. All chemical analyses were performed at accredited laboratories within acceptable holding times and using standard methods and internal quality control practices. THg samples were analyzed at Innotech Alberta (2012−14) and the University of Alberta Biogeochemical Analytical Service Laboratory (BASL; 2015−16) using EPA method 163150 with minor modifications (see Supporting Information). Reported detection limits for THg were 0.035 to 0.080 ng L−1 for Innotech and 0.06 ng L−1 for BASL. THg laboratory duplicates had relative percent differences within 15% (mean: 1%). All MeHg samples were analyzed at the BASL using isotope dilution and EPA method 163051 with minor modifications (see Supporting Information). Reported detection limits of MeHg were 0.010 to 0.016 ng L−1 and laboratory duplicates had relative percent differences within 24% (mean: 2%). Using THg and MeHg results of all samples, we calculated the proportion of THg as MeHg (%MeHg) to use as a proxy for MeHg production in aquatic ecosystems.27 The BASL was also used to analyze samples for additional chemical parameters including physical chemistry (color, total alkalinity), major ions (Na+, K+, Ca2+, Mg2+, HCO3−, Cl−, SO42−), nutrients (total nitrogen (TN), total dissolved N (TDN), dissolved inorganic N (NO 3 − , NH 4 + ), total phosphorus (TP), total dissolved P (TDP)), carbon (DOC), and chlorophyll-a concentration. Total and dissolved metals (e.g., Al, B, Fe, Mn, Ni, Pb, V, Zn and others; see Table S2) were measured at Innotech Alberta. For dissolved measurements, samples were passed through filters with a 0.45-μm pore-size before analysis. All analyses were performed using standard methods and sample handling protocols. Numerical and Spatial Analyses. Most measured parameters had fewer than 10% of samples below method detection limits, including 0% for THg samples and 7% for MeHg samples. Therefore censored values were replaced with one-half the method detection limit.52 Samples with nondetects comprising more than 20% of all sample data were not included in any analyses. THg and MeHg concentrations of all samples were directly compared with Federal and Provincial surface water quality guidelines for protection of aquatic life.53,54 Spatial distribution of mean THg, MeHg concentrations, and mean %MeHg of the 50 lakes across all years was visualized using an inverse-distance weighting algorithm between lakes and creation of a contour surface in ArcMap (ArcGIS v. 10.3.1; ESRI, Redlands, CA, USA). A power of two and a 15-lake search neighborhood was defined in the algorithm. The location of higher Hg concentration contours relative to oil sands surface-mining facilities was used as a qualitative assessment of potential depositional impacts of oil sands emissions on lakes. Spatial hot-spot analysis (Getis-Ord Gi* statistic55) was used to identify lakes with locally high THg and MeHg concentrations and %MeHg. This analysis was performed using an inverse distance algorithm and Euclidean distances between lakes in ArcMap. As an additional quantitative assessment of the potential impact of oil sands activities on THg concentrations in sampled lakes, we used a regional chemical transport model to simulate the emissions, transport, and direct deposition of THg from oil sands activities, wildfires, and other sources (i.e., long-range trans-
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RESULTS AND DISCUSSION Hg in AOSR Lakes Relative to Regulatory Guidelines and Other Northern Lakes. THg and MeHg concentrations, and %MeHg in northern lakes is underreported in the literature relative to Hg body burdens in biota directly or indirectly consumed by humans.56,57 However, sampling surface waters for THg and MeHg concentrations represents a potential exposure of MeHg to food webs without timeconsuming, destructive sampling of biota. Mean (±SE) THg concentrations of the 50 sampled lakes was 2.05 ± 0.17 ng L−1 (median: 1.79 ng L−1) and ranged from 0.36 (Lake S1) to 5.33 (Lake BM6) ng L−1 with a right-skewed distribution. Over the five years of monitoring (245 samples), the Provincial chronic water quality guideline for THg (5 ng L−1) was exceeded 10 times, most predominantly in three lakes (BM5, BM6, and BM8) located in the Birch Mountains (Figure 2). Exceedances were distributed across all sampling years, but occurred more frequently during 2012, 2014, and 2016 (Figure S1). WF1 was the only lake outside of the Birch Mountains to exceed guidelines, which occurred only in 2012. Other lakes showed little temporal variation in THg concentrations. Acute guidelines for THg were not exceeded during any year in any lake. All MeHg concentrations were well below water quality guidelines (Figure 2). Mean MeHg concentration across all lakes was 0.097 ± 0.011 ng L−1 (median: 0.086 ng L−1) with a larger relative range of concentrations from
