Hydrologic controls on water chemistry and mercury biotransformation in a closed river system: The Carson River, Nevada

Applied Geochemistry Applied Geochemistry 21 (2006) 1999–2009 www.elsevier.com/locate/apgeochem

Hydrologic controls on water chemistry and mercury biotransformation in a closed river system: The Carson River, Nevada Jean-Claude J. Bonzongo a

a,*

, Bassel W. Nemer a, W. Berry Lyons

b

Department of Environmental Engineering Sciences, P.O. Box 116450, University of Florida, Gainesville, FL 32611-6450, USA b Byrd Polar Research Center, Ohio State University, Columbus, OH 43210-1002, USA Available online 12 October 2006

Abstract The Carson River flows in a closed basin system and the total flow of the river water decreases downstream due to both evaporation and consumptive uses. This river system is fed primarily by snow pack in the Sierra Nevada during the winter, which flows down gradient following melting in spring and summer. Water loss through evaporation in the Carson River results in a downstream buildup of conservative elements such as Cl and certain oxyanion forming elements including Se, Mo and W, which are known to interfere with the transformation of Hg within the S cycle. In addition to these naturally occurring hydrologic processes and the resulting affects on water chemistry, the Carson River Basin has been historically impacted by Au and Ag mining that used Hg amalgamation techniques. Contamination of Hg in the Carson River system is now well documented and published Hg concentrations in different environmental compartments are extremely high. In this study, hydrologically driven changes in water chemistry of the river system and the resulting effects on Hg cycling were examined. Results show that periods of low water flow correspond to high water pH (up to 8.3), relatively high concentrations of oxyanion forming elements (e.g., As, Se, Mo and W), and low Hg methylation potential in sediment. In contrast, periods of high flow bring about dilution, which results in lower pH (7), lower concentrations of oxyanion forming elements, but higher Hg methylation potential. Overall, changes in flow regimes likely affect rates of methyl-Hg (MeHg) production through a combination of factors such as high pH, which favors MeHg demethylation, and the occurrence of relatively high concentrations of Group VI oxyanions that could interfere with microbial SO4 reduction and MeHg production. Ó 2006 Elsevier Ltd. All rights reserved.

1. Introduction The bioaccumulation of Hg in fish tissue has become an increasingly problematic issue throughout the world and poses a health hazard for humans *

Corresponding author. E-mail address: bonzongo@ufl.edu (J.-C.J. Bonzongo).

and ecosystems in many localities. In Au mining impacted aquatic environments, a significant fraction of metallic Hg (Hg0) used in Au extraction by amalgamation techniques becomes oxidized and ultimately transformed into highly toxic MeHg by both biotic and abiotic pathways. This aspect of environmental Hg cycling has driven most of the studies on its fate and transport, as well as on its

0883-2927/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2006.08.010

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J.-C.J. Bonzongo et al. / Applied Geochemistry 21 (2006) 1999–2009

impact on human health and ecosystem functions. The investigation of environmental cycling of Hg in the past two decades has identified many physicochemical and biological parameters that affect MeHg production and accumulation. However, very little is known on the indirect effect of changes in hydrology on MeHg production, particularly in semi-arid environments, where aquatic systems undergo extreme changes in hydrological conditions. River systems located in temperate arid and semi-arid environments are subject to extreme climatic changes, resulting in large fluctuations of water flow between dry and wet seasons. The Carson River, in central western Nevada is a riverine system flowing in a closed basin (Fig. 1), which in addition to being subject to dramatic changes in water flow regimes (Fig. 2), is heavily contaminated with Hg from historic mining of Au and Ag. In 1859, the Comstock lode was discovered in Nevada, and this discovery was followed by an intensive use of Hg0 for over 50a in the amalgamation process to

extract Au and Ag from ore and fine alluvial material. Unfortunately, the lack of Hg0 containment practices at the time led to its dispersal throughout the drainage basin. Mercury contamination of the Carson River has been well studied and several papers have described the geomorphology and geochemistry (Gustin et al., 1994; Miller et al., 1995, 1998, 1999; Bonzongo et al., 1996a; Wayne et al., 1996; Waltman and Warwick, 1999), biogeochemistry (Oremland et al., 1995; Bonzongo et al., 1996b; Chen et al., 1996, 1997; Marvin-Dipasquale and Oremland, 1999), and modeling (Heim and Warwick, 1997; Waltman and Warwick, 1999; Diamond et al., 2000; Carroll and Warwick, 2001; Carroll et al., 2000, 2004). Besides the anthropogenically-derived Hg of this river basin, the Carson River flows over geologic materials that are naturally enriched in both Hg (Gustin et al., 1994) and Group VI element-containing minerals (Johannesson et al., 2000). Therefore, water in the Carson River tends to be naturally

Nevada

Watershed boundary

N

California

40°00 |

Carson Playa 119°00|

0 0

10 10

20 20

Truckee Canal

30 miles

30

Lahontan Valley

Km

Fallon

3

Lahontan Dam Virginia City

Six Mile Canyon

Lahontan Reservoir Carson Lake

120°00 |

Stillwater Wildlife Refuge

2 Carson City

39°00 |

1

Sample Sites West Fork Carson River East Fork Carson River

NEVADA

1. Genoa 2. Fort Churchill 3. Stillwater

CALIFORNIA

Fig. 1. Map of the Carson River Drainage and selected sampling sites.

Jean-Claude J. Bonzongo et al. / Applied Geochemistry 21 (2006) 1999–2009

2001

700

1997 Flood

500

3

Daily mean streamflow (m /sec)

600

400

300

Sample collection during high flow

200

Sample collection during low flow

100

0 1/1/95

1/1/96

1/1/97

1/1/98

1/1/99

1/1/00

1/1/01

1/1/02

1/1/03

1/1/04

Date of daily mean streamflow measurements Fig. 2. Trend of water discharge at Fort Churchill (sampling site #2) from 1995 to 2004 (US Geological Survey – National Water Information System; http://waterdata.usgs.gov/nwis).

enriched with these elements, which fluctuate with changes in water flow regimes (Doyle et al., 1995; Johannesson et al., 1997, 2000). The Carson River is fed primarily by snow pack in the Sierra Nevada during winter, which flows down gradient following melting in spring and summer. Total flow of the Carson River decreases downstream due to a combination of water evaporation and agriculture uses (Bonzongo et al., 2002). Extreme seasonal shifts between high and low flow in this river system can range from peak discharge of >560 m3/s as reported by the US Geological Survey (USGS) in 1997, to little or simply no flow in dry seasons (Fig. 2). In the Carson River, water quality parameters such as pH, total suspended solids (TSS), dissolved organic matter, and major ions vary with seasonal shifts in water levels and flow (Bonzongo et al., 1996a). In addition, the reported evapoconcentration of the above mentioned Group VI elements in this river system could be important with regard to Hg cycling as laboratory experiments have shown that they can inhibit the production of MeHg by SO4- reducing bacteria (SRB) in sedimentary environments (Bonzongo et al., 1996b; Chen et al., 1997). This study combines field observational data and laboratory experiments to assess the impact of hydrologic changes on Hg transformation in the Carson River system.

2. Materials and methods 2.1. Study area The Carson River flows eastward out of the Sierra Nevada Mountains south of Lake Tahoe until reaching the Carson Playa, a large hydrologically closed lake in the Carson Desert (Fig. 1). The area under investigation is a section of the Carson River basin in western Nevada, which has been described in detail in many papers (Cooper et al., 1985; Gustin et al., 1994; Bonzongo et al., 1996a, 2002; Chen et al., 1996). Three sample sites (Fig. 1) were selected based on the reported distribution of Hg along the river (Wayne et al., 1996; Bonzongo et al., 2002). The first sample site, Genoa, is located upstream of the contaminated river section, whereas the remaining sample sites are located downstream and within the contaminated region. As mentioned earlier, water flow in the Carson River is sustained primarily by snowmelt from the Sierra Nevada in the mid-spring to early summer. Periodically, catastrophic floods occur that are generated by ‘‘rain-on-snow’’ events during winter months. The summer months have low flow due to lack of precipitation and typical annual precipitation is about 12 cm. Fig. 2 shows both the trend of the water discharge measured by USGS at Fort

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Churchill (sampling location #2, Fig. 1) and the time of collection of samples used in this study. 2.2. Sample collection Surface water and surface sediment samples were collected from three locations along a longitudinal transect starting from near the river headwater to one of its natural terminal basins, Stillwater Wildlife Refuge (Fig. 1). Samples were collected in 2001 during high flow (31 m3/s) in May and again during the low flow season (<0.03 m3/s) based on water flow measured by the USGS at Fort Churchill (Fig. 2). Water samples were collected in acid pre-cleaned Teflon bottles for the determination of Hg species and the selected oxyanion forming elements using ultra clean sampling techniques as previously described (Bonzongo et al., 1996a, 2002; Bonzongo and Lyons, 2004). Additionally, samples for determination of major ions were collected in Nanopure pre-cleaned high density polyethylene (HDPE) bottles. In the laboratory, water sample aliquots were filtered (0.45 lm). Both the filtered and unfiltered samples for trace metal analysis were acidified with ultra pure HCl to a final concentration of 1% (v/v). Samples were kept refrigerated and were analyzed within one week of collection. Samples for the determination of major ions were also filtered (0.45 lm) and kept refrigerated without acidification until analysis. Surface sediment samples (5–10 cm) were collected using acid pre-cleaned polyethylene scoops and transferred into acid pre-cleaned wide mouth HDPE bottles. Samples were kept on ice in the dark during sampling and transportation. In the laboratory, samples were refrigerated at 4 °C until analysis and used in laboratory experiments conducted within two weeks of the time of collection. 2.3. Sample analysis In the aqueous phase, total-Hg (THg) concentrations were determined after subjecting whole water samples to BrCl/SnCl2, followed by gas-phase purging with Hg-free N2 and trapping of Hg0 onto Aucoated sand (Bloom and Crecelius, 1983). The Hg0 was then thermally desorbed from the Au-trap and released into a Hg-free He stream and quantified by cold vapor atomic fluorescence spectrometry (CV-AFS). Methyl-Hg was first separated from its original matrix by distillation or extraction in an organic solvent (Bloom, 1989; Horvat et al., 1993)

and then ethylated using sodium tetraethylborate (Bloom, 1989), followed by CV-AFS detection after GC-separation and thermal decomposition of alkylHg compounds. The concentrations of total As, Se, Mo and W were determined on filtered samples by inductively coupled plasma mass spectrometry (ICP-MS). For THg, MeHg, and the oxyanion forming elements, QA/QC requirements were met by running calibration curves, sample spikes, and duplicate analyses. Major ions were determined by ion chromatography (Welch et al., 1996). The concentration of TSS, was determined on a 1-L sample volume, filtered through a 0.45-lm pre-weighed filter, and dried at 60 °C to a constant weight. Water temperature, dissolved O2, and pH were measured in situ. In sediments, MeHg was determined using a method adapted from Bloom (1989). MeHg was first released from the sample matrix by solvent extraction, followed by ethylation, TenaxÒ trapping, GC-separation, thermo-decomposition, and detection by CV-AFS. In contrast, samples for THg were first digested in acid-cleaned volumetric flasks with a 7:3 mixture of HNO3/H2SO4 and analyzed by a SnCl2-reduction technique with detection by CV-AFS. For both MeHg and THg, in addition to reagent blanks and liquid standard solutions, a certified reference material (IAEA-405, estuarine sediments containing an average THg of 0.81 mg kg1 and MeHg of 5.49 ng g1) was run with all digestions. The percent recovery on the IAEA-405 averaged 95 ± 11% (n = 10), and 93 ± 8% (n = 10) for THg and MeHg, respectively. 2.4. Hg methylation and methyl-Hg demethylation experiments Potential rates of Hg methylation and MeHg degradation were measured according to the method of Pak and Bartha (1998). This method involves the use of cold spikes of inorganic Hg and MeHg, and has been employed successfully in other recent studies of microbial Hg transformation (King et al., 1999, 2000, 2001; Warner et al., 2003). Surface sediments from each site were first wetsieved (1 mm mesh) under a N2 atmosphere in a glove box (815-PGB) to produce homogenous material, then mixed with deionized water (1:1; vol/vol), and bubbled with N2:CO2 (95%:5%) for 1 h. For each site, sediment samples were homogenized and apportioned into replicate serum vials (2–3 mL sediment in 10 mL vials). Vials were capped with thick

Jean-Claude J. Bonzongo et al. / Applied Geochemistry 21 (2006) 1999–2009

rubber stoppers, and bubbled (via incurrent and exit needles) for 1 h with N2:CO2 (95%:5%) to accelerate the development of anaerobic conditions that favor Hg biotransformation. Individual vials were then spiked with either HgCl2 (for methylation experiments) or CH3HgCl (for MeHg demethylation experiments) to a final concentration of 1.0 and 0.006 mg L1, respectively. The addition of inorganic Hg to sediment slurries raised total-Hg concentrations by an order of magnitude, while CH3HgCl additions to slurries in demethylation experiments raised MeHg concentrations by about three orders of magnitude. Vials were incubated statically for 5 days at room temperature (22 °C) and in the dark. At the end of the incubation period, Hg biotransformation in the vials was stopped by the addition of 10% (vol/vol) 1 N HCl (Optima) through the stopper and the samples were then stored at 18 °C prior to extraction and analysis. The M/D ratios of determined potential transformation rates (M = methylation and D = demethylation) were then used as parameter to assess the potential of each site to produce and accumulate MeHg as the concentrations of oxyanion forming elements changed in the overlying water.

2003

inhibition of MeHg production by SRB as they interfere with the metabolism of SO2 4 . To identify the predominant species of Se, Mo and W in analyzed samples, the authors used mineral equilibrium modeling (MINEQL+ 4.5; Allison et al., 1991; Schecher and McAvoy, 1992), to predict the speciation of the elements under the two hydrologic conditions. This approach allowed the determination of the fraction of total concentrations that occurred as oxyanions in the aqueous phase and the comparison of speciation to ambient MeHg concentrations and M/D ratios determined in sediments collected during high and low flow regimes. 3. Results and discussion 3.1. Dissolved concentrations of oxyanion forming elements in water Measured total concentrations of As, Se, Mo and W are presented in Table 1a. Concentrations of As vary from 210 to 890 nmol L1 during the low flow season, and decrease during high flow (44–550 nmol L1). Similar trends were observed for all of the other oxyanion forming elements. These results show that: (1) hydrologic changes affect water concentrations of the studied elements; and (2) downstream evapoconcentration was found regardless of the season. The latter observation is explained by water loss by evaporation, which takes place along the path of the river and becomes more pronounced in the more arid downstream reaches. Concentrations of As and the Group VI elements determined in this study were often higher than those reported by Doyle et al. (1995) and Johannesson et al. (1997), except for W. This is primarily due to the fact that samples in these three different studies were collected under different flow regimes.

2.5. Aqueous speciation of oxyanion forming elements using MINEQL+ In general, the toxicity of elements such as As, Se, Mo, W, and Hg is a function of the aqueous activities of the metal ions and their complexes and not of total concentrations (Morel and Hering, 1993). In this study, the analytically determined concentrations of Se, W and Mo in water samples are total concentrations. However, from laboratory experiments (Bonzongo et al., 1996b; Chen et al., 1997), oxyanions of these elements, namely, 2 2 SeO2 4 , MoO4 and WO4 , are associated with the

Table 1a Concentrations of oxyanion forming elements (As, Se, Mo, W) and Hg speciation in water samples collected from the three study sites along the Carson River System Site samples

1 2 3

Genoa Fort Churchill Stillwater

As

Se

Mo

W

THg (unfiltered)

MeHg (unfiltered)

T-DHg (filtered)

DMeHg (filtered)

LF

HF

LF

HF

LF

HF

LF

HF

LF

HF

LF

HF

LF

HF

LF

HF

210 130 890

44 60 550

0.0 5.7 55

0.05 0.11 0.94

90 200 670

21 22 170

6.9 29 38

0.85 7.4 33

14 930 1200

55 6600 220

2.2 8.3 18

1.5 nd 30

1.3 170 47

0.13 nd 46

nd 0.44 4.7

0.45 nd 29

LF = low flow (May 2001); HF = high flow (August 2001); and nd = not determined. Concentrations in nmol L1.

2004

J.-C.J. Bonzongo et al. / Applied Geochemistry 21 (2006) 1999–2009

column in Lahontan Reservoir, a man-made lake located between sites 2 and 3. In contrast, the finest grained particles, which have high adsorption capacity for trace metals, tend to remain in suspension longer and become dispersed over long distances. Also, in the low flow season, Hg shows a better relationship with DOC in the Carson River (Bonzongo et al., 1996a), which would favor long distance transport of Hg in the aqueous phase and possible accumulation in the river terminal basins. This is illustrated by the measured high concentrations of dissolved THg in samples collected from Stillwater. Methyl-Hg concentrations in aquatic systems are controlled by both river transport mechanisms and on-site production capacity. Whereas transport of MeHg is directly affected by changes in hydrologic regime, the production of MeHg at a given site is affected primarily by site-specific conditions. The objective of this study with regard to MeHg is to link major trends in the biogeochemistry of Hg to hydrology and water chemistry. This stems from the natural occurrence of high concentration of Group VI oxyanion forming elements as well as changes in pH and Hg speciation, which may interfere with microbial Hg transformation. Therefore, trends of MeHg concentrations are discussed later in Section 3.4 to take into account the effect of Group VI oxyanion on MeHg production.

3.2. Mercury concentrations in filtered and unfiltered water The temporal and spatial distribution of Hg as well as concentrations of Hg species in surface water of the Carson River system have previously been reported (Gustin et al., 1994; Bonzongo et al., 1996a,b; Wayne et al., 1996). In this study, the emphasis is on the shift of Hg concentrations and Hg speciation versus hydrologic change (Table 1a). Unlike the oxyanion forming elements described earlier, Hg is a highly particle reactive element, which does not show conservative behavior in aquatic systems. On the other hand, the high affinity of Hg for organic C leads to a high Hg fraction bound to dissolved organic matter in organic-rich water. Data for THg obtained for unfiltered water samples show an increase in concentration from low to high flow, but only for the first two riverine sites. In Genoa (Site 1, Fig. 1), located upstream from historic mining sites, Hg concentrations are generally low during both low and high water flow. In contrast to Genoa, Fort Churchill (site 2, Fig. 1) and Stillwater (site 3, Fig. 1) are located in the portion of the river system that is affected by Hg contamination from upstream mine tailings spatially distributed in the floodplain, channel banks and sediment. As reported previously (Bonzongo et al., 1996a), concentrations of THg in water appear to increase with increasing input of Hg-contaminated particles from the watershed during spring snow melt. However, unlike the first two sites, which are on the riverine section of this aquatic system, Stillwater is a wetland that is fed by the river water and behaves as a terminal basin with no water outflow besides losses by infiltration and evaporation. This difference in hydrology affects the trend of TSS (Table 1b) and any associated Hg. In fact, this terminal wetland is not reached by Hg-contaminated coarse particles as they become removed from the water

3.3. Major ions, pH, and total suspended solids (TSS) Concentrations of the major anions (F, Cl, 2 Na+, NO 3 and SO4 ; see Table 1b) tend to follow that of the oxyanion forming elements (Section 3.1) in that low flow season concentrations are generally higher than those measured during high flows. An evapoconcentration signature tends to move downstream (Wayne et al., 1996; Bonzongo et al., 2002). Conversely, measured TSS concentrations

Table 1b Major anions, pH, and total suspended solids (TSS) measured in water samples collected during low and high flow regimes in the three study sites along the Carson River System Site samples

1 2 3

Genoa Fort Churchill Stillwater

F (lmol L1)

Cl (lmol L1)

Na+ (lmol L1)

NO 3 (lmol L1)

SO2 4 (lmol L1)

pH

LF

HF

LF

HF

LF

HF

LF

HF

LF

HF

LF

HF

LF

HF

9.2 25 33

2.5 5.8 26

190 430 3800

32 76 430

920 2000 8400

200 410 720

1.7 0.15 1.7

0.15 0.15 0.15

280 1100 1400

49 120 1100

8.1 8.3 8.3

7.3 7.5 7.5

4.0 2.0 240

120 110 24

LF = low flow (May 2001); HF = high flow (August 2001).

TSS (mg L1)

Jean-Claude J. Bonzongo et al. / Applied Geochemistry 21 (2006) 1999–2009

and pH behave differently. Concentrations of TSS increase with increasing water flow due to both soil erosion (from the river banks and the flood plain) and sediment resuspension. An opposite trend is observed in the terminal wetland where very fine particles reach the wetland at the end of the watershed, whereas coarse materials are deposited in Lahonton Reservoir farther upstream. With regard to pH, periods of low water flow lead to high pH (8.1–8.3), whereas periods of high water flow correspond to lower circumneutral pH (7.3–7.5). Overall, these data confirm that changes in hydrology affect the water chemistry by changing the ionic strength of the water and physical parameters such as turbidity. 3.4. Linking water chemistry to Hg transformation in sediment In the past two decades, laboratory experiments and field measurements have identified SRB as important in the production of MeHg in most sedimentary environments (Compeau and Bartha, 1984, 1985; Gilmour and Henry, 1991; Gilmour et al., 1992; Bonzongo et al., 1996b; Chen et al., 1996; King et al., 1999, 2001, 2002). Generally, SRB use SO2 4 as a terminal electron acceptor during the oxidation of organic matter, and consequently, convert SO2 4 to reduced sulfur species. Group VI 2 2 oxyanions such as SeO2 4 , MoO4 , and WO4 closely 2 resemble SO4 in terms of size, charge, and stereochemistry. Consequently, these oxyanions can interfere with metabolic processes involving SO2 4 reduction (Piscator, 1986). Since SRB have been identified as principal methylators of Hg in Carson River sediment (Chen et al., 1997), it is possible that the presence of such oxyanions could limit MeHg production in this system. To verify this hypothesis, in addition to measuring the concentration of MeHg

2005

in sediment, laboratory experiments were conducted to determine the potential rates of Hg methylation (M) and MeHg demethylation (D) in sediment samples collected during the two different flow regimes (Table 2; Fig. 3a and b). Regardless of the flow regime, concentrations of MeHg in sediment samples do not parallel the trend of MeHg concentrations observed in the aqueous phase, as the latter increases from upstream to downstream. This aqueous trend of MeHg concentration suggests that a significant fraction of MeHg found in downstream sites is likely produced in upstream reaches of the river. Generally, Hg-rich river bank materials tend to become sites of Hg methylation as the water content of these sediment materials increases with river water contact during high flow (Carroll et al., 2004). Accordingly, these river banks would account for MeHg input into the aqueous phase, which is then dispersed over long distances by fluvial processes. In contrast, ambient concentrations of MeHg in sediment show a trend that is completely opposite to the trend of aqueous Group VI elements and any other elements with conservative behavior. The authors suggest that the observed trend of decreasing MeHg concentrations in sediment (Fig. 3a) is partly due to the inhibition of MeHg production in sediments by Group VI elements within the S cycle. However, this explanation would be relevant only if the determined Group VI elements are found primarily as oxyanions (Bonzongo et al., 1996b; Chen et al., 1997). To verify this, equilibrium modeling (MINEQL+ 4.5) was used to predict the speciation of analyzed elements for the prevalent conditions in the river system during the two flow regimes. The results indicated that Se, Mo and W in the Carson River occur predomi2 2 nantly as SeO2 4 , MoO4 and WO4 (Tables 3a and 3b). For example, during low flow, these oxyanions comprised 100% of the Se and W and >96% of the Mo. At high flow, 100% of the Se and W were again

Table 2 Total-Hg and methyl-Hg concentrations and potential biotransformation rates of Hg in sediments collected from the three study sites THg (ng g1)

1. Genoa 2. Fort Churchill 3. Stillwater

MeHg (ng g1)

Potential rates of Hg2+ methylation (M) in ng g1 day1

Potential rates of MeHg demethylation (D) in ng as Hg g1 day1

M/D ratios

LF

HF

LF

HF

LF

HF

LF

HF

LF

HF

410 14,000 830

0.33 11 1.9

6.0 0.40 0.01

2.3 1.7 1.5

0.86 0.22 0.080

1.6 1.2 0.96

0.086 0.33 0.40

0.22 0.14 0.16

10 0.65 0.20

7.0 8.5 6.0

LF = low flow (May 2001); HF = high flow (August 2001).

J.-C.J. Bonzongo et al. / Applied Geochemistry 21 (2006) 1999–2009

MeHg (ng/g), GroupVI elements/10 (nmol/L), M/D ratios

2006

12

a 10

8

MeHg M/Dr atios Group VI elements

6

4

2

0 0.191

0.432

3.831

Chloride (mmol/L)

MeHg (ng/g), GroupVI elements/10 (nmol/L), M/D ratios

9

b MeHg

8

M/D ratios Group VI elements

7

6

5

4

3

2

1

0 0.0316

0.0762

0.4354

Chloride (mmol/L) Fig. 3. Longitudinal distribution of ambient concentrations of MeHg in sediments, sum of the concentrations of the three Group VI P 2 2 ; MoO elements ð ðSeO2 4 4 ; WO4 ÞÞ in water, and M/D ratios determined on samples collected during low flow (a) and high flow (b) regimes. Chloride is used in the x-axis as a tracer for downstream evapoconcentration in the aqueous phase. To fit the concentrations of Group VI elements in the same graph as MeHg and M/D ratios, the above calculated sums have been divided by 10.

Jean-Claude J. Bonzongo et al. / Applied Geochemistry 21 (2006) 1999–2009

2007

Table 3a MINEQL+ output of the speciation of Se, Mo, and W for sample collected during low flow (LF) season

1. Genoa 2. Fort Churchill 3. Stillwater a

% Of total Se conc. present as SeO2 4

% Of total Mo conc. present as MoO2 4

% Of total Mo conc. present as HMoO 4

% Of total W conc. present as WO2 4

–a 100 100

97.5 97.7 96.1

2.5 2.3 3.9

100 100 100

Analyzed concentration was below analytical detection limit.

Table 3b MINEQL+ output of the speciation of Se, Mo, and W for sample collected during high flow (HF) season

1. Genoa 2. Fort Churchill 3. Stillwater

% Of total Se conc. present as SeO2 4

% Of total Mo conc. present as MoO2 4

% Of total Mo conc. present as HMoO 4

% Of total W conc. present as WO2 4

100 100 100

82.1 88.8 87.9

17.8 11.2 12.0

100 100 100

present as these oxyanions, whereas >82% of the Mo was found as MoO2 with 11–18% present as 4 HMoO . Accordingly, there is some potential for 4 these oxyanions to limit MeHg production by SRB. Using Cl as an example of a tracer for conservative behavior (Fig. 3a and b), the sum of the concentrations of oxyanions generally behave conservatively, with the highest concentrations observed in the terminal basin located in the most arid portion of the river system. When considered individually, the concentration of each Group VI element determined in this study may not be high enough to produce a significant inhibition of Hg methylation (Chen et al., 1997). However, under natural conditions these elements are generally found together, but unfortunately, the combined effect of these Group VI elements on Hg biotransformation has not been studied. Data also show the trends of methylation to demethylation (M/D) ratios, indicating that increasing concentrations of the Group VI oxyanions correspond with decreasing M/D ratios (Fig. 3a and b). Interestingly, ambient sediment MeHg concentrations follow the trend of M/D ratios. The observed longitudinal decreasing trends in ambient MeHg concentrations in sediment, as well as the trends in calculated M/D ratios, are suggestive of a potential role of Group VI oxyanions in SRB driven MeHg production in the Carson River. If true, this observation would then be in agreement with experimental data reported previously (Bonzongo et al., 1996b; Chen et al., 1997). The peculiar water geochemistry of the Carson River helps maintain MeHg concentrations in this aquatic system below those expected given the sediment concentrations of THg. However, it is impor-

tant to point out that Group VI anions are not the only parameter that may interfere with MeHg production in the Carson River system. The presence of ligands such as S2 and Cl can also reduce the bioavailability of Hg in anoxic sediments and in evapoconcentrated terminal basins, respectively. Additionally, the accumulation of MeHg can also be affected by several other parameters, such as pH and nitrate, that favor MeHg degradation as their concentrations increase with decreasing water flow. This makes it difficult to quantitatively assess the potential in situ contribution of Group VI elements in the observed trends of MeHg concentrations. 4. Conclusions This study has shown that in the Carson River system ambient concentrations of MeHg are controlled, at least partly, by the river’s peculiar water chemistry, which includes the presence of high concentrations of Group VI elements such as Se, Mo and W, known to interfere with Hg methylation by SRB. The use of MINEQL+ showed that up to 100% of the total concentrations of Se, Mo and W (of +6 oxidation state) in Carson River water are found primarily as oxyanions. In natural systems, these elements would probably act simultaneously to interfere with microbial Hg transformation by SRB. Based on data presented in this paper, the authors suggest a potential role of these Group VI elements in the inhibition of Hg methylation by SRB. This is because several other parameters known to affect the net balance of methylation and demethylation respond to hydrologic changes in a similar manner as the three studied Group VI

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