Mercury deposition and methylmercury formation in ... - Core

Applied Geochemistry 50 (2014) 82–90

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Mercury deposition and methylmercury formation in Narraguinnep Reservoir, southwestern Colorado, USA John E. Gray a,⇑, Mark E. Hines b, Harland L. Goldstein c, Richard L. Reynolds c a

U.S. Geological Survey, P.O. Box 25046, MS 973, Denver, CO 80225, USA University of Massachusetts Lowell, Department of Biological Sciences, Lowell, MA 01854, USA c U.S. Geological Survey, P.O. Box 25046, MS 980, Denver, CO 80225, USA b

a r t i c l e

i n f o

Article history: Available online 16 September 2014 Editorial handling by M. Kersten

a b s t r a c t Narraguinnep Reservoir in southwestern Colorado is one of several water bodies in Colorado with a mercury (Hg) advisory as Hg in fish tissue exceed the 0.3 lg/g guideline to protect human health recommended by the State of Colorado. Concentrations of Hg and methyl-Hg were measured in reservoir bottom sediment and pore water extracted from this sediment. Rates of Hg methylation and methylHg demethylation were also measured in reservoir bottom sediment. The objective of this study was to evaluate potential sources of Hg in the region and evaluate the potential of reservoir sediment to generate methyl-Hg, a human neurotoxin and the dominant form of Hg in fish. Concentrations of Hg (ranged from 1.1 to 5.8 ng/L, n = 15) and methyl-Hg (ranged from 0.05 to 0.14 ng/L, n = 15) in pore water generally were highest at the sediment/water interface, and overall, Hg correlated with methyl-Hg in pore water (R2 = 0.60, p = 0007, n = 15). Net Hg methylation flux in the top 3 cm of reservoir bottom sediment varied from 0.08 to 0.56 ng/m2/day (mean = 0.28 ng/m2/day, n = 5), which corresponded to an overall methyl-Hg production for the entire reservoir of 0.53 g/year. No significant point sources of Hg contamination are known to this reservoir or its supply waters, although several coal-fired power plants in the region emit Hg-bearing particulates. Narraguinnep Reservoir is located about 80 km downwind from two of the largest power plants, which together emit about 950 kg-Hg/year. Magnetic minerals separated from reservoir sediment contained spherical magnetite-bearing particles characteristic of coal-fired electric power plant fly ash. The presence of fly-ash magnetite in post-1970 sediment from Narraguinnep Reservoir indicates that the likely source of Hg to the catchment basin for this reservoir has been from airborne emissions from power plants, most of which began operation in the late-1960s and early 1970s in this region. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction Mercury is a well-known and widespread environmental contaminant that affects land, lakes, reservoirs, and other water bodies in the USA and worldwide (NAS, 1978; Ullrich et al., 2001; USEPA, 1997). The U.S. Environmental Protection Agency (USEPA) indicated that, of the pollutants mentioned in the Clean Air Act, Hg has the greatest potential to affect human health (USEPA, 1997). All forms of Hg are potentially hazardous, have no biological function in humans, and any exposure to Hg is undesirable, but methyl-Hg (CH3Hg+) is potentially the most hazardous because it is a neurotoxin (Eisler, 1987; USEPA, 1997; WHO, 1990). Methylation of Hg is dominantly through the action of microorganisms, primarily by the action of sulfate reducing bacteria, which is most common in organic-rich sediment (Compeau and Bartha, 1985;

⇑ Corresponding author. Tel.: +1 303 236 2446.

Gilmour et al., 1992). Several studies have shown that Fe-reducing bacteria are also capable of methylating Hg (Alpers et al., 2013; Fleming et al., 2006; Kerin et al., 2006; Marvin-DiPasquale et al., 2014; Parks et al., 2013; Schaefer et al., 2014), although sulfate reducing bacteria are generally reported to be more dominant in the sediment column (Gilmour et al., 1992; Hammerschmidt and Fitzgerald, 2004; Marvin-DiPasquale and Agee, 2003). Methyl-Hg formation is most common in the sediment column, and because it is water soluble, it is transferred to the water column and then to biota, such as fish (NAS, 1978; USEPA, 1997). Studies of reservoirs have indicated that methylation of Hg is enhanced following the initial flooding of reservoirs, supply of nutrients and organic matter, reduction of oxygen supply producing anaerobic conditions, and supply and bioavailability of Hg (Bodaly et al., 1997; Bonzongo et al., 1996; Montgomery et al., 2000; TetraTech, 2001; Ullrich et al., 2001; Waldron et al., 2000). Human consumption of Hg contaminated fish is the most common pathway of methyl-Hg to humans (Clarkson, 1990) because generally more than 90%

E-mail address: [email protected] (J.E. Gray). http://dx.doi.org/10.1016/j.apgeochem.2014.09.001 0883-2927/Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

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J.E. Gray et al. / Applied Geochemistry 50 (2014) 82–90

of Hg in fish is methyl-Hg (Fitzgerald and Clarkson, 1991; Rimondi et al., 2012; Ullrich et al., 2001; USEPA, 1997). There are numerous natural and anthropogenic sources of Hg with the potential to contaminate air, land, and water, but of these, anthropogenic sources of Hg are the largest (Lamborg et al., 2002; Mason et al., 1994; NAS, 1978; USEPA, 1997). Burning of fossil fuels is the dominant anthropogenic source of Hg, and emissions from coal-fired power plants have been known as a significant source of atmospheric Hg for many years (USEPA, 1997). Deposition of Hg-bearing particulates emitted from various anthropogenic combustion point sources has been reported to lead to Hg contamination in local, regional, and remote lakes and other water bodies (Fitzgerald et al., 1998; Gray and Hines, 2006; Mason et al., 1994; USEPA, 1997). Several coal-fired power plants are located in the Four Corners region of the U.S. (Colorado, New Mexico, Utah, and Arizona) (Fig. 1). Two of the largest of these are the Four Corners (>11,000,000 MWH output) and San Juan (>14,000,000 MWH output) plants, both located about 80 km south of Narraguinnep Reservoir near Farmington, New Mexico (Fig. 1). Currently, there are 21 water bodies in Colorado with advisories recommending limited consumption of fish due to elevated concentration of Hg in fish tissue (CDPHE, 2012). Six of these advisories are located in the Four Corners area of Colorado including Narraguinnep, McPhee, Vallecito, Navajo, Echo Canyon, and Trotten Reservoirs. In addition, advisories for Hg in fish are widespread on water bodies in the Four Corners area such as Lake Powell in Arizona and Utah (AFGD, 2014; USEPA, 2014) and several lakes in northwest New Mexico, including Abiquiu Lake, Eagle Nest Lake, and El Vado Lake among others (NMDGF, 2012). Geochemical research of water ways and reservoirs in the Four Corners region have attempted to evaluate sources of various land and airborne contaminants (Butler et al., 1995, 1997; Gray et al., 2005; Malm et al., 1990; TetraTech, 2001). Several studies have indicated that coal-fired power plants in the Four Corners area 109° W

have likely emitted airborne particulates, which have been deposited to local land and water bodies (Gray et al., 2005; Malm et al., 1990; TetraTech, 2001; Wright, 2011; Wright and Nydick, 2010). Gray et al. (2005) studied historical deposition of Hg using sediment cores collected from Narraguinnep Reservoir and found an increase in Hg during the period that coal-fired power plants were in operation (about 1970 to present day) in the Four Corners region (Fig. 1). Wright and Nydick (2010) used wet deposition geochemical studies and back-trajectory analysis of climatic patterns that suggested that coal-fired power plants operating in the Four Corners region south of Mesa Verde National Park are likely an important source of Hg deposition in the park. However, these previous studies neither measured rates of Hg methylation in reservoirs with posted Hg advisories in the Four Corners area, nor attempted to identify the sources of particles in reservoir sediment, some of which may have been emitted from regional coal-fired power plants. Therefore, the objectives of this study were to determine (1) rates of Hg methylation and methyl-Hg demethylation in reservoir bottom sediment, (2) concentrations of Hg, methyl-Hg, organic carbon, and other trace elements in reservoir bottom sediment and pore water extracted from bottom sediment, and (3) the presence or absence of reservoir sediment particles that likely originated from coal-fired power plant emissions in the region.

2. Study area The area of study was Narraguinnep Reservoir, a small reservoir, located just west of McPhee Reservoir, with a surface area of about 2.53 km2 and a 23,000,000 m3 storage capacity. The primary water supply for both reservoirs is the Dolores River (Fig. 1). Water flows from the Dolores River into McPhee Reservoir and then into Narraguinnep Reservoir. McPhee Reservoir acts as a sink for coarse sediment flowing from the Dolores River, but Dolores River water 108° W

108° 30’ W

McPhee Reservoir Dolores

37° 30’ N

Narraguinnep Reservoir

Inset of Narraguinnep Reservoir

Cortez

Utah 37° N

Arizona

Colorado

B

New Mexico

San Juan power plant

Farmington Four Corners power plant

A

0

10

20 kilometers

Fig. 1. (A) Location of study area. (B) Inset shows the location of sample sites in Narraguinnep Reservoir.

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is turbid (up to 12 NTU, Nephelometric Turbidity Units) and suspended particulate matter flows into Narraguinnep Reservoir where considerable sediment is deposited (Gray et al., 2005). Small tributaries on the perimeter of Narraguinnep Reservoir also supply particulate runoff from local soil and bedrock into the reservoir. Narraguinnep Reservoir was originally built for agricultural irrigation, but present-day uses include recreational boating, fishing, and swimming. The area of the watershed upstream from the town of Dolores, Colorado, is approximately 1300 km2. (USBOR, 1992). The climate of the area is semiarid and precipitation averages about 32 cm/yr at Cortez and 46 cm/yr near the town of Dolores (Butler et al., 1995; USBOR, 1992). Precipitation generally increases with elevation and peaks in the upper Dolores basin reach about 4250 m and about 50–60% of the annual precipitation is from snowfall (USBOR, 1992). The mean discharge is about 13 m3/s and average annual yield of the Dolores River is about 390,000,000 m3 near Dolores (Butler et al., 1995; USBOR, 1992). Generally low carbon and nutrient supply suggest that Narraguinnep Reservoir is oligotrophic (Tables 1 and 2). During bottom sediment sample collection in 2008, the reservoir was rather shallow (up to 10.6 m deep), pH was generally consistent (7.8–8.4), dissolved oxygen indicated oxidized conditions (4.4–6.7 mg/L), and temperatures near the water surface were about 21 °C, but were as low as 15.4 °C near the sediment/water interface. The temperature decrease of about 6 °C from the water surface to the reservoir bottom indicates a weak stratification in Narraguinnep Reservoir, which is consistent with other studies of this reservoir (Gray et al., 2005; TetraTech, 2001). However, no attempt was made in this study to define a thermocline. A fish consumption advisory was posted on Narraguinnep Reservoir in 1991, remaining in place today, which advises limiting human consumption of Northern pike, Smallmouth bass, Channel catfish, and Walleye due to high concentrations of Hg in fish tissue (Butler et al., 1995; CDPHE, 2012). Average concentrations of Hg (n = 33) in these fish species (CDPHE, 2012) exceed the 0.30 lg/g

methyl-Hg (wet weight) fish-tissue guideline recommended by the U.S. Environmental Protection Agency (USEPA) to protect human health (USEPA, 2001). Northern Pike fish tissue in Narraguinnep Reservoir has been reported to contain Hg concentrations as high as 1.5 lg/g (average = 0.56 lg/g, n = 4) (CDPHE, 2012) and Hg in Walleye (n = 4) was reported to average 1.11 lg/g (Butler et al., 1995).

3. Materials and methods 3.1. Sample collection 3.1.1. Reservoir bottom sediment and pore water Bottom sediment was collected from five sites (Table 1) throughout Narraguinnep Reservoir using a 23 23 23 cm Ekman™ grab sampler. The collected sediment was taken to a laboratory within an hour of collection and placed in an oxygen-free glove bag filled with nitrogen gas. All samples were sectioned and processed in oxygen-free air in order to avoid oxidation of sediment or water. The focus of this study was on the sediment/water interface, and thus, the collected sediment samples were sectioned at 0–3 cm, 3–6 cm, and 6–9 cm depths. A split of each sediment sample was sectioned in the glove bag and then placed in a precleaned 500-ml Teflon™ container, frozen, and shipped via express delivery to Battelle Marine Sciences, Sequim, WA. The frozen sediment samples were then thawed and separation of pore water by oxygen-free centrifugation was carried out for analysis of Hg and methyl-Hg, and sediment splits were obtained that were analyzed for Hg and methyl-Hg concentrations. Other sediment aliquots were obtained for Hg methylation, methyl-Hg demethylation, methane (CH4), and carbon dioxide (CO2) production rate measurements at the University of Massachusetts Lowell. These sediment samples were separated into glass jars with Teflon™-lined lids and these samples were kept

Table 1 Geochemical data for bottom sediment collected from Narraguinnep Reservoir. Sample

NAR1 A NAR1 B NAR1 C NAR2 A NAR2 B NAR2 C NAR3 A NAR3 B NAR3 C NAR4 A NAR4 B NAR4 C NAR5 A NAR5 B NAR5 C

Location

37.49282° N 108.62701° W 37.49282° N 108.62701° W 37.49282° N 108.62701° W 37.49393° N 108.62366° W 37.49393° N 108.62366° W 37.49393° N 108.62366° W 37.49273° N 108.62550° W 37.49273° N 108.62550° W 37.49273° N 108.62550° W 37.49121° N 108.62337° W 37.49121° N 108.62337° W 37.49121° N 108.62337° W 37.49553° N 108.61428° W 37.49553° N 108.61428° W 37.49553° N 108.61428° W

Depth

Hg

Methyl-Hg

Methyl-Hg/Hg

TOC

V

Co

Methylation

(cm)

(ng/g)

(ng/g)

(%)

(%)

(lg/g)

(lg/g)

(%/day)

CH4 (%/day)

Demethylation CO2 (%/day)

Total (%/day)

0–3

44

0.09

0.20

1.3

110

10

0.74

0.02

2.6

2.6

3–6

41

0.13

0.32

1.2

110

8.9

0.90

0.07

1.0

1.1

6–9

44

0.11

0.25

1.1

109

9.3

0.74

0.04

1.0

1.0

0–3

37

0.09

0.24

1.5

100

9.0

0.36

0.01

6.9

6.9

3–6

38

0.12

0.32

1.1

104

8.4

1.0

0.11

3.4

3.5

6–9

36

0.11

0.31

1.0

100

8.6

0.83

0.17

1.4

1.6

0–3

42

0.08

0.19

1.2

114

9.4

0.58

0.13

2.1

2.2

3–6

40

0.11

0.28

2.0

115

9.5

0.82

0.17

1.2

1.4

6–9

46

0.11

0.24

1.2

118

9.3

0.60

0.03

1.0

1.0

0–3

39

0.10

0.26

1.2

111

9.5

0.77

0.07

2.0

2.1

3–6

39

0.12

0.31

1.2

109

8.9

0.78

0.20

1.3

1.5

6–9

46

0.11

0.24

1.1

112

9.3

0.72

0.09

1.6

1.7

0–3

39

0.10

0.26

1.2

113

9.4

0.66

0.00

5.9

5.9

3–6

31

0.09

0.29

1.1

102

8.6

0.67

0.09

1.7

1.8

6–9

29

0.09

0.31

1.1

100

8.3

0.65

0.26

0.91

1.2

85


Table 2 Geochemical data for pore water extracted from sediment collected from Narraguinnep Reservoir. Kmeth are the methylation rate constants and Kdeg are the methyl-Hg demethylation rate constants. Sample NAR1 NAR1 NAR1 NAR2 NAR2 NAR2 NAR3 NAR3 NAR3 NAR4 NAR4 NAR4 NAR5 NAR5 NAR5

A B C A B C A B C A B C A B C

Depth (cm)

pH

Hg (ng/L)

Methyl-Hg (ng/L)

Methyl-Hg/Hg (%)

DOC (mg/L)

Fe (mg/L)

Fe2+ (mg/L)

Kmeth (%/day)

Kdeg (%/day)

Net Hg methylation (ng/m2/day)

0–3 3–6 6–9 0–3 3–6 6–9 0–3 3–6 6–9 0–3 3–6 6–9 0–3 3–6 6–9

7.3 7.3 7.4 7.4 7.2 7.2 7.2 7.2 7.3 7.2 7.2 7.2 7.3 7.3 7.1

5.8 3.6 2.4 3.7 2.3 1.1 3.5 2.0 1.6 2.4 1.4 1.4 3.6 2.0 4.0

0.14 0.11 0.09 0.11 0.08 0.07 0.10 0.06 0.05 0.11 0.06 0.08 0.07 0.05 0.09

2.4 3.1 3.6 3.1 3.3 6.3 2.9 3.3 3.3 4.6 4.5 6.1 2.0 2.4 2.4

9.7 9.2 9.3 12 11 11 11 9.6 8.9 7.9 9.3 12 7.4 9.8 10

3.4 5.5 7.0 4.3 13 20 5.3 8.5 8.9 2.7 6.8 14 3.0 7.4 8.1

1.0 2.0 5.0 2.0 8.0 10 4.0 7.0 8.0 2.0 6.0 8.0 2.0 6.0 5.0

0.74 0.89 0.74 0.35 1.02 0.83 0.58 0.82 0.60 0.76 0.78 0.72 0.66 0.67 0.65

2.6 1.1 1.1 6.9 3.5 1.6 2.3 1.3 0.98 2.1 1.5 1.6 5.9 1.8 1.2

0.56

refrigerated. Additional sediment splits, also collected in glass jars with Teflon-lined lids, were frozen, later freeze dried, pulverized, and analyzed for major and trace element concentrations using various analytical techniques at the U.S. Geological Survey (USGS), Denver, Colorado. The final split of the sediment was centrifuged at about 12,000 rpm to extract sediment pore water. Following centrifugation, a portion of the pore water was filtered using 0.45lm nitrocellulose filters in the glove bag. An aliquot of this pore water was acidified using ultrapure nitric acid (HNO3, 0.5% v/v) and used for cation analysis by inductively coupled plasma-mass spectrometry (ICP-MS). A second pore water aliquot was also filtered using a 0.45-lm polyethersulfone filter and preserved by freezing for anion analysis by ion chromatography (IC). A third aliquot was obtained for determination of several parameters using field test kits or portable meters of which pH, total Fe, and ferrous iron (Fe2+) are reported here. Pore water was also collected for dissolved organic carbon (DOC) determinations and these samples were filtered with C-free 0.7 lm borosilicate glass filters into Cfree glass vessels and immediately acidified with ultra-pure hydrochloric acid (HCl, 0.5% v/v). 3.2. Analytical methods 3.2.1. Sediment The concentration of Hg was determined in the reservoir sediment samples using an aqua-regia digestion following USEPA method 1631 and cold-vapor atomic fluorescence spectrometry (CVAFS) following USEPA method 1631e (Bloom and Fitzgerald, 1988; USEPA, 2002). Analysis of methyl-Hg followed USEPA method 1630 using CVAFS (Bloom, 1989; USEPA, 1998) and previously described extraction procedures (Bloom et al., 1997). Quality control for Hg and methyl-Hg analysis was established using method blanks, blank spikes, matrix spikes, standard reference materials (SRMs), and sample replicates. Recoveries on blank and matrix spikes were 89–102% for Hg and 89–100% for methyl-Hg. The relative percent difference in sediment sample replicates was 68% for Hg and 612% for methyl-Hg. For the SRM, IAEA-405 (Estuarine sediment, certified Hg value = 0.81 lg/g) analyzed in this study, Hg recoveries ranged from 90% to 103% of certified value. For methyl-Hg, IAEA405 (certified methyl-Hg value = 5.5 ng/g) was also analyzed and recoveries ranged from 87% to 94% of certified value. Method blanks were below the limits of determination of 0.50 ng/g for Hg and 0.03 ng/g for methyl-Hg. The sediment samples were also analyzed for several elements using inductively coupled plasma-mass spectrometry (ICP-MS) following digestion with a mixture of HCl, HNO3, HClO4 and HF

0.08

0.26

0.23

0.28

(Taggart, 2003), but only cobalt (Co) and vanadium (V) were evaluated in this study. Results for Co and V were deemed acceptable if recovery was ±15% at five times the lower limit of determination and the calculated relative standard deviation (RSD) of replicates was no greater than 15%. Lower limits of determination were 0.10 lg/g for Co and 1.0 lg/g for V. Total organic carbon (TOC) was also determined in sediment samples by subtracting carbonate C from total C concentrations. Total C was measured using an automated C-analyzer with an infrared detector that measures CO2 gas liberated as the sample is combusted at 1370 °C. Carbonate C was determined by liberating CO2 following treatment with 2N HClO4; this CO2 was collected in a solution of monoethanolamine that was coulometrically titrated using platinum and silver/potassium-iodide electrodes. The relative percent difference of TOC in sediment sample duplicates was 615% and the lower limit of determination was 0.05%. 3.2.2. Pore water Concentrations of Hg were determined in the extracted pore water samples using cold-vapor atomic fluorescence spectrometry (CVAFS) following USEPA method 1631e (USEPA, 2002). For methyl-Hg analysis, the water samples were processed following a distillation and ethylating method (Horvat et al., 1993) and analyzed by CVAFS (Bloom, 1989; USEPA, 1998). Recoveries for Hg on blank and matrix spikes were 80–111% and for methyl-Hg were 86–97%. The relative percent difference in water sample replicates was 67% for Hg and 620% for methyl-Hg. The SRM, NIST 1641d (water solution, certified Hg value = 1.56 lg/g) was analyzed with the samples and Hg was within 1–4% of the certified value. Although no SRM for methyl-Hg is available for water, the SRM DORM-2 (Dogfish muscle, certified methyl-Hg value = 4.5 lg/g) was analyzed along with the water samples to evaluate method accuracy and methyl-Hg recovery was 2% of the certified value. Method blanks were below the lower limit of determination of 0.15 ng/L for Hg and 0.01 ng/L for methyl-Hg. The pore water samples were also analyzed for several elements by ICP-MS, but only Fe was considered in this study (Table 2). Data for Fe were deemed acceptable if recovery was ±10% at five times the lower limit of determination and the calculated RSD of replicates was no greater than 10% (Taggart, 2003). Field and method blanks were below the lower limit of determination for Fe in water of 0.03 mg/L. Dissolved organic carbon (DOC) also was determined in pore water using a Shimadzu TOC-VCSH instrument™ at the USGS, Denver, Colorado. Organic carbon was oxidized to CO2 by high temperature catalytic oxidation and CO2 was measured by a non-dispersive infrared detector. Precision of the method for the

86


determination of DOC was ±10%. Field and method blanks contained DOC below the lower limit of determination of 0.3 mg/L. Dissolved anions were measured by injecting 25-lL samples into an automated Dionex 2000 ion chromatograph (IC) fitted with an AS14-4 mm ion exclusion column with suppression and a sodium hydroxide gradient eluent generator, and conductivity detection. The relative percent difference of IC determinations in duplicate pore water samples indicated a precision of 67.5%. 3.2.3. Methylation and demethylation rates Potential rate constants of methyl-Hg production and destruction (%/day) were determined using radioisotopic methods (Hines et al., 2006, 2000). Sample slurries consisting of 3.0 mL of wet sediment diluted with 3.0 mL of anoxic water were injected (2.0 lL) with the radiotracers 203HgCl2 (0.5 lCi equivalent to 170 ng Hg) or 14C-methyl-Hg chloride (0.1 lCi equivalent to 334 ng Hg) for methylation and demethylation, respectively, and incubated at room temperature in darkness for 18 ± 2 h. The 14C-methyl-Hg chloride was extracted with methylene chloride before use to remove impurities (Hines et al., 2012). Slurries were also used to ensure that radiotracers were evenly mixed in samples. Although this technique may have influenced microbial processes, the uniformity in the incubation conditions allowed for a more thorough comparison of data among sites. The amount of Hg added as 203Hg was 4–6 times greater than ambient HgT in samples. The 14C-methyl-Hg added was 2500–4000 times higher than ambient methyl-Hg. Acid (HCl, 6N) was added following incubation to stop the methylation reaction. Alkali (NaOH, 3N) was added following incubation to stop the demethylation reaction and to sequester 14CO2 in the liquid phase. For methylation assays, radiolabeled 203Hg-methyl-Hg was extracted twice into toluene following treatment with CuSO4 and KCl in H2SO4. Pooled toluene extracts were dehydrated using anhydrous NaSO4 and radioactivity determined by scintillation counting. Methylation activity, determined from conversion of 203HgCl2 to methyl-203Hg, yielded a first order rate constant that is a measure of the percentage of inorganic 203Hg converted to methyl-203Hg per time (day). Demethylation was determined by measuring 14C in CO2 and CH4 using a gas-stripping and trapping system similar to that previously described (Marvin-DiPasquale and Oremland, 1998). Methane was flushed from the vial with air (30 mL/min for 15 min) and combusted to CO2 in a CuO-packed quartz column at 850 °C. The resulting CO2 was trapped in phenethylamine, methanol, and a toluene-based scintillation fluid and 1.0 mL of CH4 was injected into each vial to facilitate removal of CH4. Following 14CH4 measurements, samples in vials were acidified with 1.0 ml of 6.0 N HCl and after 24 h, 14CO2 was stripped via a stream of N2 for 15 min and trapped as described above. Radioactivity was determined by scintillation counting. Demethylation activity, determined from the conversion of 14C-methyl-Hg to 14CO2 and/or 14CH4, yielded a first-order rate constant that is a measure of the percentage of 14 C-methyl-Hg degraded per time (day). 3.2.4. Sediment respiration rates To determine rates of sediment respiration, vertical sections of sediment were incubated and gases were measured over time. Duplicate 20 ml portions of sediment were added to 60 ml serum vials, in a N2-filled glove bag. Sediment samples were diluted with an equal volume of anoxic reservoir water (de-aerated with N2) and slurries incubated in the dark for seven days. Samples of the headspace were removed periodically over a seven-day period and CH4 and CO2 concentrations determined using gas chromatography with flame ionization and thermal conductivity detection, respectively (Gray and Hines, 2009). Regression analyses of time-course data were used to calculate rates of gas formation.

3.2.5. Petrography Petrographic studies were carried out on sediment samples obtained from four dated sediment cores collected from Narraguinnep Reservoir that were archived from a previous study (Gray et al., 2005). Reflected-light microscopy was undertaken to determine the types of magnetic minerals in reservoir sediment samples, thus, providing reliable identification of different Fe-oxide minerals on grains larger than about 3 lm in diameter and the presence of Fe oxide in grains as small as about 1–2 lm. The grains were prepared as polished grain mounts after isolation from the bulk sediment in a pumped-slurry magnetic separator (Reynolds et al., 2001). From the four cores, 13 samples of about 10 g each were examined from depths of 3–66 cm. Sediment samples representing the pre- and post-power plant operation periods (before and after 1970) were examined from each core.

4. Results and discussion 4.1. Sediment Concentrations of Hg and methyl-Hg in reservoir bottom sediment collected from the five sites studied varied within a narrow range, 29–46 ng/g and 0.08–0.13 ng/g, respectively (Table 1). Concentrations of Hg in surrounding bedrock in the Dolores River basin are not well known, but soil collected proximal to Narraguinnep Reservoir (Boerngen and Shacklette, 1981) was somewhat lower in Hg (mean = 30 ng/g, n = 4) than Hg in bottom sediment in this study (mean = 39 ng/g, n = 15). The higher Hg concentrations in reservoir bottom sediment compared to surrounding soil likely indicates an additional land or airborne source of Hg in this area. Concentrations of methyl-Hg in the reservoir bottom sediment were generally low and the proportion of methyl-Hg/Hg was also found to be low (0.19–0.32%, Table 1) compared to other reservoir Hg studies where methyl-Hg/Hg varied from 0.50% to 8.1% (Gray and Hines, 2009; He et al., 2007; Ullrich et al., 2001; Yan et al., 2013). In addition to Hg, previous studies have suggested that particulates emitted from coal-fired power plants can be elevated in several trace elements including V and Co (Ito et al., 2006). Concentrations of Hg in sediment were found to correlate positively with V (R2 = 0.59, p = 0.0008) and Co (R2 = 0.53, p = 0.002) in sediment (Fig. 2). Concentrations of V (mean = 44 lg/g, n = 8) and Co (mean = 5.9 lg/g, n = 8) in soil collected in the Four Corners region (Boerngen and Shacklette, 1981) were lower than V (mean = 108 lg/g, n = 15) and Co (mean = 9.1 lg/g, n = 15) found in reservoir bottom sediment collected in this study (Table 1). An additional comparison is soil collected throughout the western United States that contained concentrations of V (mean = 70 lg/g, n = 516) and Co (mean = 7.1 lg/g, n = 403) (Shacklette and Boerngen, 1984), which were also generally lower than V and Co in Narraguinnep Reservoir bottom sediment. The correlations of Hg with V and Co, and elevated concentrations of V and Co compared to that in soil in the area, may suggest an additional airborne source of V and Co to Narraguinnep Reservoir. However, the Mancos Shale is found along the perimeter of Narraguinnep Reservoir and in upstream basins and is also a potential source of considerable V and Co to the reservoir. Samples of Mancos Shale were reported to contain V that averaged about 155 lg/g (n = 10) and Co that averaged 8.1 lg/g (n = 10) (Tuttle et al., 2014), which are similar in concentration to Narraguinnep Reservoir bottom sediment (Table 1). It is likely that Narraguinnep Reservoir receives sediment from a combination of local and regional soil, bedrock, and airborne sources. Soil is known to be an important reservoir of atmospheric Hg, which shows regional and local deposition to the terrestrial environments, in some cases (Fitzgerald et al., 1998; Nater and Grigal, 1992). Several studies have reported that

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variations (Table 1). These rates are similar to those reported for other depositional environments (Hammerschmidt and Fitzgerald, 2004; Hines et al., 2006; Marvin-DiPasquale and Agee, 2003), but less than in other reservoirs studied, especially the eutrophic Salmon Falls Creek Reservoir in Idaho (Gray and Hines, 2009) that exhibited methylation rate constants over 10% per day. In that study, sediment also displayed respiration rates that were much higher than sediment in the Narraguinnep Reservoir (2–50 lg/mL day), which underscores the importance of microbial activity in controlling the methylation of Hg in sediment (Meng et al., 2010; Yan et al., 2013). Demethylation activities were more rapid than methylation in Narraguinnep Reservoir at 1% to 7% per day (Table 1), and these values were similar to those noted in other reservoir/freshwater sediment (Gray and Hines, 2009; Hines et al., 2006; MarvinDiPasquale and Agee, 2003). Unlike the methylation rate constants that did not fluctuate appreciably with depth, the demethylation values generally decreased with depth at all sites suggesting that at least a portion of the methyl-Hg produced in the sediment was degraded near the surface. The majority of the 14C degraded was recovered as 14CO2 (Table 1) indicating that methyl-Hg was degraded primarily by an oxidative process (Barkay et al., 2003). The formation of CH4 from methyl-Hg can occur during the reductive demethylation process catalyzed by enzymes produced by the mer genetic system (Barkay et al., 2003), or during the degradation of methyl-Hg by methane-producing bacteria. In the Narraguinnep Reservoir sediment samples, C respiration produced CO2 with little to no CH4 (Table 1) and the C in methyl-Hg was also degraded to CO2, indicating that demethylation was conducted by bacteria other than methanogenic bacteria. Methanogenesis was likely restricted mostly to sediment deeper than the upper 9 cm studied.

V in sediment (µg/g)

A R2 = 0.59, p = 0.0008

Co in sediment (µg/g)

Hg in sediment (ng/g)

B R2 = 0.53, p = 0.002

Hg in sediment (ng/g) Fig. 2. (A) Concentrations of V versus Hg in sediment. (B) Concentrations of Co versus Hg in sediment.

atmospheric deposition of Hg has affected the geochemistry of lakes throughout North America (Driscoll et al., 2007; Engstrom and Swain, 1997; Fitzgerald et al., 1998; Swain et al., 1992). Sediment collected from Narraguinnep Reservoir were examined microscopically and found to contain spherical magnetitebearing particles characteristic of power plant fly ash (Locke and Bertine, 1986) (Fig. 3). The fly-ash particles are less than about 15 lm in diameter and were identified in post-1970 Narraguinnep Reservoir sediment, but not found in pre-1970 reservoir sediment. The size and presence of power plant fly-ash particles in post-1970 Narraguinnep Reservoir sediment most likely suggests that the source of these particulates was from emissions from the coal-fired electric power plants in the Four Corners area, the largest of which began operation in this region in the late-1960s and early 1970s. Hg methylation activity in Narraguinnep Reservoir sediment was similar at all sites with rate constants varying between approximately 0.36–1.0% per day with no distinctive depth

4.2. Pore water Similar to bottom sediment, concentrations of Hg and methylHg in pore water extracted from Narraguinnep Reservoir sediment varied within a narrow range 1.1–5.8 ng/L and 0.05–0.14 ng/L, respectively (Table 2). Pore water concentrations of Hg and methyl-Hg generally were higher at the sediment–water interface at the five sites studied in Narraguinnep Reservoir (Fig. 4). Concentrations of Hg and methyl-Hg in pore water correlate positively (R2 = 0.60, p = 0.0007, Fig. 5), which likely indicate transference of Hg and methyl-Hg from the sediment interstitial pore water. Also similar to reservoir bottom sediment, the proportion of methyl-Hg/ Hg in pore water was generally low (2.0–6.3%, Table 2) and was lower than that reported in other reservoir studies, which ranged from 1.3% to 65% (Gray and Hines, 2009; He et al., 2007; Yan et al., 2013). Other studies of reservoir pore water have shown

A

B

Fig. 3. Spherical fly-ash particles observed in post-1970 Narraguinnep Reservoir sediment. Particles consist mostly of magnetite (gray) on the basis of optical properties in reflected light under immersion oil.


Sample depth (cm)

88

Pore water concentration (ng/L)

R2 = 0.60, p = 0.0007

Hg in pore water (ng/L) Fig. 5. Concentrations of Hg versus methyl-Hg in pore water.

correlations between methyl-Hg and DOC (Gray and Hines, 2009), but there was no significant correlation found between pore water methyl-Hg and DOC in this study. A negative correlation was found between concentrations of methyl-Hg and Fe2+ in pore water (R2 = 0.52, p = 0.002, Fig. 6). Results from other studies (Rytuba, 2000) also reveal oxidation of Fe2+ to Fe3+ and consequent precipitation of Fe-bearing oxyhydroxides. Mercury and methyl-Hg are then strongly sorbed onto these Fe-oxides and carried with these precipitates, effectively depleting Hg and methyl-Hg in pore water. However, no attempt was made to evaluate the proportion of Hg or methyl-Hg carried during precipitation of Fe-oxides. Pore water Hg and methyl-Hg concentrations and Hg transformation activities were used to estimate the production rate of methyl-Hg in Narraguinnep Reservoir bottom sediment (Table 1). We assumed that dissolved Hg and methyl-Hg best represented the species most likely to be methylated and demethylated, respectively. These data were multiplied by methylation and demethylation rate constants and were used to convert to rates based on sediment dry weight. Demethylation rates were subtracted from methylation rates to generate net methylation. The resulting net methyl-Hg rates varied from 0.08 to 0.56 ng/m2/day (Table 2). When compared to the total pool of methyl-Hg in the sediment samples, it would take about 15 years to accumulate

Fe2+ in pore water (mg/L)

Methyl- Hg in pore water ( ng/L)

Fig. 4. Pore water concentrations of Hg and methyl-Hg versus sample depth.

R2 = 0.52, p = 0.002

Methyl-Hg in pore water (ng/L) Fig. 6. Concentrations of methyl-Hg versus Fe2+ in pore water.

the amount of methyl-Hg present in the bottom sediment, i.e., the residence time of methyl-Hg would be 15 years. Geochemical data were used to estimate the formation and the flux of methyl-Hg diffused into the water column from pore water in reservoir bottom sediment. The methyl-Hg pore water concentration used was the mean of the top 3 cm samples obtained from all five sediment collection sites. Diffusional flux was calculated using Fick’s first law as previously described (Covelli et al., 2008).

F ¼ ðUDW =h2 ÞdC=dx where F is the flux of a solute with concentration C at depth x, U is the sediment porosity that was determined from weight loss following drying of sediment samples, DW is the diffusion coefficient of the solute in water in the absence of the sediment matrix, and h is the tortuosity (dimensionless) that was estimated from porosity as h = 1 ln(U2). It was assumed that methyl-Hg was as CH3HgSH with an estimated DW of 1.2 105 cm2 s1 as previously described (Covelli et al., 2008). Using this approach, the methyl-Hg load delivered to reservoir water was 0.57 ng/m2/day or about 0.53 g/yr for the entire reservoir. When the net methyl-Hg production rate calculated above is applied to the upper 3 cm of sediment, the average production rate is equal to 0.28 ng/m2/day, which is close to the 0.57 ng/m2/day calculated from diffusion alone. Thus, a steady state production of methyl-Hg in the upper 3 cm is sufficient to supply all


of the methyl-Hg to the water column. This calculated methyl-Hg flux is lower than that reported in other studies of peat, flooded soils, and wetlands (3.6–44 ng/m2/day) (Porvari and Verta, 1995), but within the range of that reported in lakes and reservoirs with fish containing elevated Hg (0.17–28 ng/m2/day) (Hines et al., 2004; Rogers et al., 1995). 4.3. Source of Hg in Narraguinnep Reservoir Research dating back to 1990 has indicated that coal-fired electric power plants operating in the Four Corners have likely deposited airborne particulates to land and water bodies in the region (Gray et al., 2005; Malm et al., 1990; TetraTech, 2001). Malm et al. (1990) used residence time of back trajectory endpoints and principal component analysis to conclude that emissions from coal-fired power plants in the Four Corners region had deposited particulates in the area. TetraTech (2001) presented numerous geochemical data for Hg in water, air, sediment, and fish, and a review of previous research in the region, and concluded that atmospheric Hg loads were derived from distant and local sources and appeared to be enhanced by emissions from coal-fired power plants in the Four Corners region. More recently, several studies also have suggested that an important source of Hg deposition in the Four Corners region has been from particulates emitted from coal-fired electric power plants located in this area (Gray et al., 2005; Wright, 2011; Wright and Nydick, 2010). Using dated sediment cores collected from Narraguinnep Reservoir, Gray et al. (2005) indicated an increase in the historical deposition of Hg in the reservoir after 1970, which was consistent with the time that power plants began operation or were expanded in the Four Corners region, particularly the San Juan and Four Corners plants near Farmington, New Mexico (Fig. 1). In addition, petrographic analysis of sediment cores presented here support this conclusion as post1970 sediment from Narraguinnep Reservoir contain fly-ash magnetite indicative of particulates emitted from coal-fired power plants (Fig. 3), whereas fly-ash magnetite was absent in sediment older than about 1970. Fly-ash particles in Narraguinnep Reservoir were likely derived from soil runoff in the upstream catchment basin or were directly deposited into the reservoir. Furthermore, Gray et al. (2005) concluded that historical patterns of Hg deposition in Narraguinnep Reservoir did not support significant Hg derivation from surrounding bedrocks or upstream mining sources. In addition, Wright and Nydick (2010) used wet deposition geochemical studies and back-trajectory analysis of climatic patterns to conclude that ‘‘coal-fired power plants south of Mesa Verde National Park are likely an important source of Hg in wet deposition to the Park’’. Using principal component analysis, Wright and Nydick (2010) also found an association among Hg, sulfate, and nitrate in Mesa Verde precipitation, which they indicated was likely derived from coal-fired power plants. In effect, the conclusions of these studies are consistent with the much earlier work of Malm et al. (1990). 5. Conclusions Concentrations of Hg and methyl-Hg in bottom sediment and pore water extracted from this sediment were relatively low in Narraguinnep Reservoir. Similarly, rates of Hg methylation in Narraguinnep Reservoir bottom sediment were lower than those found in studies of other reservoirs. It is likely that the oligotrophic conditions (relatively low carbon and nutrient supply) impede methyl-Hg production in this reservoir and highlight the importance of primary production and microbial degradation activity in controlling the rate of methyl-Hg formation. Conversely, although the total production of methyl-Hg was only 0.53 g/yr

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for the entire reservoir, methyl-Hg produced in Narraguinnep Reservoir has resulted in elevated methyl-Hg in fish as several fish species in this water body contain methyl-Hg exceeding the 0.3 lg/g fish tissue guideline recommended to protect human health. Therefore, the potential for methyl-Hg contamination in reservoirs is not restricted to eutrophic water bodies, but can occur readily in various lakes and reservoirs. There are no significant point sources of Hg to Narraguinnep Reservoir, although several previous studies have suggested that emissions from coal-fired power plants in the Four Corners area may contribute Hg-bearing particulates to water bodies in this region. In this study, petrographic studies of sediment collected from Narraguinnep Reservoir identified spherical, magnetitebearing particles typical of fly ash emitted from coal-fired power plants. These fly-ash particles were found in sediment deposited after about 1970, the time that power plants began operation in this region. It is most likely that power plant emissions have deposited some Hg-bearing particles in the upstream catchment basin for the reservoir or directly into Narraguinnep Reservoir, or both. Acknowledgments This study was funded by the U.S. Geological Survey (Mineral Resources Program) and the University of Massachusetts Lowell. We thank Jiang Xiao for magnetic mineral separations. Greg Lee (USGS) provided assistance with graphics. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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