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Mercury and Organic Carbon Dynamics During Runoff Episodes from a Northeastern USA Watershed

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Abstract

Mercury and organic carbon concentrations vary dynamically in streamwater at the Sleepers River Research Watershed in Vermont, USA. Total mercury (THg) concentrations ranged from 0.53 to 93.8 ng/L during a 3-year period of study. The highest mercury (Hg) concentrations occurred slightly before peak flows and were associated with the highest organic carbon (OC) concentrations. Dissolved Hg (DHg) was the dominant form in the upland catchments; particulate Hg (PHg) dominated in the lowland catchments. The concentration of hydrophobic acid (HPOA), the major component of dissolved organic carbon (DOC), explained 41–98% of the variability of DHg concentration while DOC flux explained 68–85% of the variability in DHg flux, indicating both quality and quantity of the DOC substantially influenced the transport and fate of DHg. Particulate organic carbon (POC) concentrations explained 50% of the PHg variability, indicating that POC is an important transport mechanism for PHg. Despite available sources of DHg and wetlands in the upland catchments, dissolved methylmercury (DmeHg) concentrations in streamwaters were below detection limit (0.04 ng/L). PHg and particulate methylmercury (PmeHg) had a strong positive correlation (r 2 = 0.84, p < 0.0001), suggesting a common source; likely in-stream or near-stream POC eroded or re-suspended during spring snowmelt and summer storms. Ratios of PmeHg to THg were low and fairly constant despite an apparent higher methylmercury (meHg) production potential in the summer. Methylmercury production in soils and stream sediments was below detection during snowmelt in April and highest in stream sediments (compared to forest and wetland soils) sampled in July. Using the watershed approach, the correlation of the percent of wetland cover to TmeHg concentrations in streamwater indicates that poorly drained wetland soils are a source of meHg and the relatively high concentrations found in stream surface sediments in July indicate these zones are a meHg sink.

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Notes

  1. The use of brand, trade™, or firm names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.

  2. The use of brand, trade™, or firm names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.

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Acknowledgements

We would like to thank Doug Halm and Ann Chalmers of the USGS for their assistance in the field and Kenna Butler, Jennifer L. Agee, Le Kieu, and Hillary A. Harms for their analytical assistance. We would also like to thank Don Campbell and Steffanie Keefe of the USGS for their comments and suggestions.

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Correspondence to P. F. Schuster.

Appendix

Appendix

Water Sample Processing and Analysis

Major anions, DOC, and SUVA

Samples for major anion and DOC analyses were filtered from the 1-L amber bottles using a 60 mL Becton DickinsonFootnote 2 plastic syringe with Luer-lok fitting and a 25 mm Gelman Acrodisk filter with 0.45 μm SUPOR™ membrane (Babiarz et al. 2000) into 40 mL baked, amber glass vials with Teflon lined caps (DOC) and clean 125 mL high density polyethylene bottles. All samples were collected in duplicate. The samples were shipped chilled to the USGS research laboratories in Boulder, Colorado.

Major anions were determined by ion chromatography using a Dionex DX120 ion chromatograph™ (Fishman and Freidman 1989). DOC measurements using the Pt catalysed persulfate wet oxidation method were made on an O.I. Corporation Analytical Model 700 TOC Analyzer™. A detailed description of the analytical method for DOC is given in Aiken et al. (1992). SUVA, defined as the UV absorbance of a sample measured at a given wavelength divided by the DOC concentration, is an “average” molar absorptivity for all the molecules that comprise the DOC in a water sample. SUVA is, therefore, a parameter that indicates the nature or “quality” of DOC in a given sample and has been used as a surrogate measurement of DOC aromaticity (Chin et al. 1994; Weishaar et al. 2003). SUVA values at 254 nm (SUVA254) are reported in the current work because natural organic matter absorbs strongly at this wavelength, thereby giving increased sensitivity, and because of the strong correlation with the aromatic carbon content of natural organic matter at this wavelength. SUVA254 was determined for each sample in attachment to manuscript this study by dividing the UV absorbance determined at λ = 254 nm by the DOC concentration of the sample. SUVA values are reported in units of L/(mg carbon × m) and have a standard deviation of ±0.1 L/(mg carbon × cm).

DOC Fractionation

For DOC fractionation analyses, 3 L of sample were filtered in the field with Gelman AquaPrep 600 capsule filters™ (0.45 um) into 1-L, burned, amber glass pesticide bottles with Teflon lined caps. These samples were immediately refrigerated. Samples were shipped chilled overnight to the USGS Boulder laboratory. Samples were refrigerated at the lab and analyzed within 2-weeks of arrival.

DOC fractionation analyses were performed utilizing Amberlite™ XAD resins. The DOC present in select samples was chromatographically fractionated into five fractions (hydrophobic neutrals, hydrophobic acids, transphilic neutrals, transphilic acids and hydrophilic organic matter) using XAD-8 and XAD-4 resins. The distribution of organic matter in these fractions is a “fingerprint” of DOM in the system and can provide information about how various processes affect the chemistry of the DOM. DOC fractionation is utilized to break the DOC down into groups and determine what mass percent of each group is contained in the overall DOC pool. The XAD-8 resin retains the hydrophobic acid fraction, which is the most aromatic fraction containing the humic and fulvic acids, and the hydrophobic neutrals. The XAD-4 resin retains the transphilic acids and transphilic neutrals, while the XAD-4 effluent contains the low molecular weight hydrophilic acids as well as any other compounds that are not retained by either resin.

The method is a modified version of the XAD-8/XAD-4 methods used to isolate organic matter from water samples (Aiken et al. 1992).

Particulate organic carbon (POC)

This analysis does not distinguish between particulate organic and particulate inorganic carbon of a sample. Many studies have shown that the carbon and nitrogen particulate component is almost entirely organic in natural waters (Keefe 1994). Therefore, it is assumed that the total particulate carbon (TPC) measured in this study occurs as POC. For TPC determinations, filters were sent in duplicate to the Nutrient Analytical Services Laboratory (NASL) at the Chesapeake Bay Laboratories (CBL) for analysis using high temperature combustion where the combustion products (water vapor, carbon dioxide and nitrogen gas) are analyzed via a series of thermal conductivity cells and compared to a known standard.

Using a vacuum pump connected to an acrylic filter funnel holder, well-mixed water from a Teflon sample bottle was filtered through a 25 mm glass fiber filter (GFF) with a nominal pore size of 0.7 um. A sufficient volume (50 to 300 mL) was filtered to acquire enough material on the filter surface for PC analysis. All samples were collected in duplicate. The filters were folded into aluminum foil and kept chilled until analysis. A complete discussion of quality assurance/quality control procedures can be found at http://cbl.umces.edu/nasl/index.htm.

Dissolved mercury (DHg) and particulate mercury (PHg)

Field seasons 2000 and 2001, collection and processing

The well-mixed sample was filtered through a 0.40 μm pore-size Nuclepore polycarbonate membrane filter™, using a vacuum filter apparatus made from polytetrafloro-ethylene (PTFE) (Kelly and Taylor 1996). The protocol used for the filtration to minimize artifactual contamination included: (1) the filter apparatus was thoroughly cleaned with 1% HNO3 and rinsed with deionized water; (2) a new 0.40-μm pore size, 47-mm-diameter polycarbonate-membrane filter was placed on the filter support and precleaned by drawing 50 mL of 0.1% (volume/volume) ultrapure nitric acid rinse solution through the filter into a waste bottle; (3) the filter was then rinsed by drawing 100 mL of deionized water through it into a waste bottle; (4) about 25 mL of a subsample of the composite was then filtered to prerinse the 125 mL acid-rinsed (aquaregia) glass sample receiving bottle and discarded (this step also effectively preloads the filter with particulates); (5) the balance of the subsample was filtered into the sample bottle; and (6) the filtered sample was preserved with the addition of 5 mL of concentrated ultrapure nitric acid–potassium dichromate solution resulting in a solution of approximately 0.04% nitric acid–potassium dichromate/4% nitric acid. Using Teflon™-coated tweezers, the filter were placed in acid-rinsed (aquaregia) 60 mL glass bottles. The same nitric acid–potassium dichromate solution (2.5 mL) was added to leach and preserve the filter sample for analysis of particulate mercury (DHg). Only one filter membrane per sample was used for the entire filtration process. Both DHg and PHg samples were collected in duplicate.

Field seasons 2000 and 2001, Laboratory Analysis

DHg and PHg for the field season 2000 and 2001 were determined at the USGS Research lab in Boulder, Colorado, using an automated cold-vapor atomic fluorescence spectrometer manufactured by PS Analytical™. Details of the method have been previously described by Roth (1994). Elemental mercury vapor from the sample was produced by chemically reducing mercury in the sample with excess stannous chloride. The resulting vapor was transported to the detector in a stream of argon gas. Peak height intensities of unknown samples were compared to a six-point calibration curve prepared from aqueous standards ranging in concentration from 0 to 50 ng/L. The calibration standards were preserved with the same concentration of K2Cr2O7/HNO3 as the samples.

DHg, PHg, dissolved methylmercury (DmeHg) and particulate methylmercury (PmeHg)

Field season 2002, collection and processing

An acid-cleaned 1-L Teflon bottle was immersed below the stream–water surface in the centroid of flow, rinsed several times, and filled as described above. Samples were placed on ice and shipped by overnight carrier to the Wisconsin District Mercury Lab (WDML) for processing. Upon receipt at the WDML, samples were well-mixed and filtered through a 47 mm diameter quartz fiber filter (QFF) with a nominal pore size of 0.7 um using a vacuum filtration chamber. From each 1-L whole water sample, four samples were collected: filtered total mercury, filtered methyl mercury, particulate total mercury, and particulate methyl mercury. Approximately 500 mL of sample was filtered, the filtrate collected into a 500 mL acid-cleaned Teflon bottle, and the filter saved in a Teflon petri dish for subsequent analyses for DHg and PHg, respectively. The remainder of the sample was filtered and 250 mL of filtrate was collected into a 250 mL acid-cleaned Teflon bottle and the filter saved in a Teflon petri dish for subsequent analyses for DmeHg and PmeHg, respectively. Each Teflon bottle was triple rinsed with small amounts (∼10–20 mL) of filtrate before filling the bottle to the shoulder. The DHg and DmeHg samples were preserved with 5 and 2.5 mL of low-mercury 12 N HCl, respectively. The particulate samples were frozen until analysis. All sample processing and preservation were performed in the WDML clean lab observing clean hands/dirty hands protocols.

Field season 2002, Laboratory Analysis

For the field season 2002, DHg, DmeHg, PHg, and PmeHg were determined at the WDML (USEPA Method 1631 1996; DeWild et al. 2002, 2004; Olson and DeWild 1999; Olund et al. 2004). Analyses for total Hg were performed with Dual Amalgamation Cold Vapor Atomic Fluorescence Spectrometry (USEPA Method 1631 1996) with slight modifications for particulate analyses http://infotrek.er.usgs.gov/doc/mercury/methods.html). A bromine monochloride (BrCl) solution were added to the water samples to oxidize all mercury in the sample to Hg(II). After the addition of the BrCl, the samples were placed in a 50°C oven for a minimum of 3 days to insure complete oxidation. When samples were highly stained or contain large amounts of organic carbon they were first placed near a UV source until they became colorless. Just prior to analysis, a small amount of NH2OH–HCl was added to the sample to destroy free halogens. An aliquot of sample was then reduced with stannous chloride (SnCl2) to convert Hg(II) to volatile Hg(0). The Hg(0) was purged from the sample aliquot with N2 gas onto a gold coated glass bead trap. Mercury was thermally desorbed from the sample trap in an Argon gas stream and collected on a second gold trap (analytical trap), thermally desorbed from the analytical trap in the same gas stream and carried into the cell of a cold vapor atomic fluorescence spectrometer (CVAFS) for detection. The modification of this method for particulate samples consisted of transferring the filter from the petri dish to a Teflon bottle and adding 100 mL of 5% BrCl in reagent water. The particulate samples were then treated as water samples. This modification can be used to determine THg concentrations in particulate samples with a method detection limit (MDL) of 0.060 ng of mercury on a filter. The amount of mercury on a filter is dependant on the sediment load in the sample and volume of sample filtered. Methylmercury analyses were performed using USGS method numbers I-1045-02 (filtered water) and I-3045-02 (unfiltered water) (DeWild et al. 2002) with slight modifications for particulate samples (http://pubs.water.usgs.gov/tm5A7/). Water samples were distilled to remove potential matrix interferences. The pH of the distillate was adjusted to 4.9 (to maximize ethylation potential) using acetate buffer. The distillate then was ethylated using sodium tetraethyl borate (NaBEt4) and allowed to react for 15 min. After reaction with NaBEt4, the distillate was purged with nitrogen gas (N2) for 20 min and the ethylated mercury species were collected on a sample trap containing Carbotrap. These ethylated mercury species were desorbed thermally from the sample trap, separated using a gas chromatographic (GC) column, reduced using a pyrolytic column, and detected using a cold vapor atomic fluorescence spectrometry (CVAFS) detector. Particulate samples were distilled by adding 50 mL of reagent grade water and 2 mL of a combined reagent consisting of 20% KCl, 50% H2SO4, and 1 M CuSO4 in a ratio of 1:2:2. The distillates were then treated as water samples for detection. This modification can be used to determine methyl mercury concentrations in particulate samples with a method detection limit of 0.010 ng of mercury on a filter. The amount of mercury on a filter is dependant on the sediment load in the sample and volume of sample filtered.

Methylmercury Production/Degradation Studies

In situ measurements of pH, redox, and temperature were made at the sample location as described in Marvin-DiPasquale et al. (2003) (see further readings). Clean hands protocols and acid-cleaned stainless steel tools were used during sample collection. MeHg production potentials are defined in this study as the product of [the total 203Hg(II) radiotracer amendment added x fraction of 203Hg(II) radiotracer converted to me 203Hg], normalized per day and per gram of wet sediment. Similarly, MeHg degradation potentials are defined as the product of [the total 14CH3Hg radiotracer amendment added x fraction of the 14CH3Hg radiotracer degraded to 14CH4 + 14CO2], normalized per day and per gram of wet sediment. Descriptions of meHg production and degradation potential rate assays are given elsewhere (Marvin-DiPasquale and Agee 2003; Marvin-DiPasquale et al. 2003) (see further readings).

Briefly, all sediment was initially homogenized and sub-sampled in an anoxic (N2 flushed) glove bag. Rates of microbial SR, Hg(II)-methylation and MeHg degradation were measured in parallel sample sets using the 203HgCl2, and 14CH3HgCl radiolabel techniques, respectively. Specific activities of the injection solutions [April 2002–July 2003] were 0.70–0.84 and 0.30 mCi/mg (as Hg), respectively. Radiotracer solution amendments of 1,000–1,225 nCi (per 0.1 mL) of 203HgCl2 and 9.8–10.1 nCi (per 0.1 mL) of 14CH3HgCl, into 3.0 g of sediment per sample, resulted in final amendment concentrations of 475–489 ng Hg(II) per g wet sediment and 10.9–11.2 ng CH3Hg+ (as Hg) per g wet sediment, respectively. These values generally exceed the in situ sediment HgT (8–100 ng/g wet sediment) and meHg (0.08–3.2 ng/g wet sediment) concentrations, and thus cannot be considered true tracer assays. Rate constants for each MeHg production were calculated as the fraction of the 203Hg(II) isotope converted to Me203Hg during the 23–24 h incubation period. Radioactivity in incubated samples was corrected for carry-over of unreacted 203Hg(II) in the organic extraction of Me203Hg, by subtracting the activity in killed controls that were similarly amended but not incubated. Potential rates of meHg production and degradation were then calculated as a function of the radiotracer derived rate constants and the original amendment concentration, assuming psuedo-first order kinetics (Marvin-DiPasquale et al. 2003) (see further readings).

Quality Assurance

Concentrations of major ions, DOC, POC, DHg, PHg and other trace metals in blanks processed at the on-site lab were below detection limits, indicating a clean sample-processing environment. Differences in concentrations measured in field replicates were less than 10% for DOC and DHg, and less then 20% for POC and PHg indicating acceptable reproducibility. Differences in concentrations measured in laboratory duplicates were less than 10% indicating high analytical precision. An inter-laboratory comparison for DHg and PHg showed that the difference between replicates was about the same as the difference between laboratories (approximately 0.3 ng/L). Moreover, a paired T-test indicated no significant difference in Hg concentrations between laboratories (p < 0.05). The method for determination of PmeHg produced detection limits as low as 0.001 ng/L, where as the detection limit for DmeHg was much greater (0.04 ng/L) (Olson and DeWild 1999). Because of these substantial differences in detection limits, only PmeHg could be used in ratios to THg to avoid large errors in uncertainty.

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Schuster, P.F., Shanley, J.B., Marvin-Dipasquale, M. et al. Mercury and Organic Carbon Dynamics During Runoff Episodes from a Northeastern USA Watershed. Water Air Soil Pollut 187, 89–108 (2008). https://doi.org/10.1007/s11270-007-9500-3

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