Journal of Geochemical Exploration 158 (2015) 201–211

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Journal of Geochemical Exploration

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Mercury in stream water at five Czech catchments across a Hg and Sdeposition gradient

Tomáš Navrátil a,⁎, Jamie Shanley b, Jan Rohovec a, Filip Oulehle c, Pavel Krám c, Šárka Matoušková a,Miroslav Tesař d, Maria Hojdová a

a Institute of Geology of the CAS, v.v.i., Rozvojová 269, 165 00 Prague 6, Czech Republicb US Geological Survey, PO Box 628, Montpelier, VT 05601, USAc Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republicd Institute for Hydrodynamics of the CAS, v.v.i., Pod Patankou 5, 166 12 Prague 6, Czech Republic

⁎ Corresponding author.E-mail address: navratilt@gli.cas.cz (T. Navrátil).

http://dx.doi.org/10.1016/j.gexplo.2015.07.0160375-6742/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 November 2014Revised 2 June 2015Accepted 31 July 2015Available online 4 August 2015

Keywords:Filtered HgBlack TriangleRunoff fluxesSeasonal changesDOC qualityHg/DOC ratioSUVA

The Czech Republicwas heavily industrialized in the second half of the 20th century but the associated emissionsof Hg and S from coal burning were significantly reduced since the 1990s. We studied dissolved (filtered) streamwater mercury (Hg) and dissolved organic carbon (DOC) concentrations at five catchments with contrasting Hgand S deposition histories in the Bohemian part of the Czech Republic. The median filtered Hg concentrations ofstream water samples collected in hydrological years 2012 and 2013 from the five sites varied by an order ofmagnitude from 1.3 to 18.0 ng L−1. The Hg concentrations at individual catchments were strongly correlatedwith DOC concentrations r from 0.64 to 0.93 and with discharge r from 0.48 to 0.75. Annual export fluxes offiltered Hg from individual catchments ranged from 0.11 to 13.3 μg m−2 yr−1 and were highest at sites withthe highest DOC export fluxes. However, the amount of Hg exported per unit DOC varied widely; the meanHg/DOC ratio in stream water at the individual sites ranged from 0.28 to 0.90 ng mg−1. The highest streamHg/DOC ratios occurred at sites Pluhův Bor and Jezeří which both are in the heavily polluted Black Trianglearea. Stream Hg/DOC was inversely related to mineral and total soil pool Hg/C across the five sites. We explainthis pattern by greater soil Hg retention due to inhibition of soil organic matter decomposition at the siteswith low stream Hg/DOC and/or by precipitation of a metacinnabar (HgS) phase. Thus mobilization of Hg intostreams from forest soils likely depends on combined effects of organic matter decomposition dynamics andHgS-like phase precipitation, which were both affected by Hg and S deposition histories.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Mercury (Hg) is a neurotoxin, and one of the primary exposure risksto humans is through fish consumption (Carrington et al., 2004;Sunderland, 2007). Compared to many other countries, there has beenlittle public concern or awareness about the Hg hazard in the CzechRepublic. The European Commission Regulation number 1881/2006 ad-vises weekly consumption of no more than 1.6 μg of Hg per kg of bodyweight and Czech committee for food products recommends consump-tion of no more than 170 g of fish meat per week (Ruprich, 2006). FishHg concentrationsmeasured in larger rivers and fish farms in the CzechRepublic were relatively low. These include: chub, 0.12 to 0.24mg kg−1

in the Vltava River (Kružíková et al., 2008); carp, 0.02 to 0.06 mg kg−1

from ponds (Maršálek et al., 2007); and a variety of fish, 0.10 to0.90 mg kg−1 mostly from larger rivers (Kenšová et al., 2012).Kružíková et al. (2008) found 0.11 to 0.35 mg kg−1 of Hg in chub fromthe Bílina River, which drains part of the area known as the Black

Triangle where historic Hg deposition was likely high. The predatoryfish such as asp, pike and catfish in Czech rivers and water reservoirscontained frequently Hg concentrations in excess of maximum limitfor mercury in fish muscle tissue 0.5 mg kg−1 (Cerveny et al., 2014).

During the second half of the 20th century, the Czech Republicreceived extremely high loading of atmospheric S deposition fromcombustion of high-S coal for power production (Kopáček and Veselý,2005). The pollution was concentrated in the northwestern part of thecountry, where the resulting acid deposition caused widespread forestdieback (Ardö, 1998; Kandler and Innes, 1995). Other parts of theCzech Republic, such as the Bohemian Forest near the southern borderwith Germany and Austria, were relatively less affected by acidic pollu-tion (Fottová and Skořepová, 1998). The coal combustion belongs to themain anthropogenic sources of Hg emissions worldwide (Liu et al.,2012) and in Europe (Pacyna et al., 2009). Although therewere nomea-surements of Hg deposition during the period of elevated S loading inthe Czech Republic, Hg deposition was elevated as documented inpeat archives (Zuna et al., 2012).

Indirect estimates of Hg deposition over the past century are avail-able from two peat core studies. Zuna et al. (2012) reported a rate of

Fig. 1.Mapoffive study siteswithin theCzechRepublic and location of theCzechRepublic in Europe. Isolineswithin theperimeter of the Czech Republic denote the concentrations ofHg inforest humus modified from Suchara and Sucharová (2000). The gray triangle denotes the Black Triangle area.

202 T. Navrátil et al. / Journal of Geochemical Exploration 158 (2015) 201–211

50 μg m−2 yr−1 for the last 117 years, with considerably higher rates inthe late 20th century, based on a peat core from the Ore Mountains(Fig. 1). Ettler et al. (2008) reported an average Hg deposition rate of103 μg m−2 yr−1 for a similar period from the central area of Bohemiacontaminated by Pb-oremining and smelting (Fig. 1). The lowestHgde-position rate of 29 μgm−2 yr−1 over a similar time periodwas reportedin a peat core from the southern Bohemian Forest area (Zuna et al.,2012; Fig. 1). A survey of organic horizon soils at 219 sites across theCzech Republic revealed a pattern of Hg concentration that was in fairlyclose agreement with the pattern of S deposition (Suchara andSucharová, 2000). However, Navrátil et al. (2014) looking at Hg soilpools demonstrated that soil properties, rather than atmospheric depo-sition, controlled the size of Hg pools in soils.

We would expect that the history of elevated Hg deposition in theCzech Republic would result in elevated Hg concentrations in streamwater, but even less information is available. The only study of Hg instream water in the Czech Republic was by Navrátil et al. (2011), whofound elevated dissolved Hg concentrations (up to 15 ng L−1) in a for-ested headwater stream, in association with elevated DOC concentra-tion (up to 11 mg L−1). However, the Hg output flux was less than1 μg m−2 yr−1 due to a very low annual water yield of 47 mm.Schwesig and Matzner (2000, 2001) found higher Hg output fluxes of5 to 10 μg m−2 yr−1 at German catchments 30 and 130 km W of theBlack Triangle area with water yields of 176 and 774 mm yr−1. Thesesites were also impacted by high atmospheric S loading in the late20th century.

Since 1994, the long-termGEOMONprogramhas been investigatinghydrology and biogeochemical cycling at a network of 14 small forestedcatchments throughout the Czech Republic (Fottová and Skořepová,1998; Lamačová et al., 2014). The program is built on a foundation ofgauged streams for continuous flow records and monthly (or more fre-quent) sampling of bulk precipitation, throughfall and stream flow.GEOMON has focused on major ion chemistry and recovery from highacid loadings (e.g., Bohdálková et al., 2014; Novák et al., 2005; Oulehleet al., 2008), but has not addressed the mercury issue. Here, we reporton a 2-year study at 5 of the GEOMON sites where we sampled for Hg,DOC, and major ions in stream water on a monthly basis, as well as asmaller number of high-flow stream samples. We hypothesized thatthe streams at the sites subjected to historically high Hg depositionwould yield greater Hg output fluxes than those at less impacted sites.

Working at the GEOMON sites gives us the advantage of interpretingmercury dynamics in the context of the biogeochemical knowledgealready existing at these long-term study sites.

2. Methods

2.1. Study sites

The study sites include 5 of the 14 GEOMON catchments: Jezeří(JEZ), Lesní Potok (LES), Liz (LIZ), Lysina (LYS), and Pluhův Bor (PLB)(Fig. 1). The five sites differ in geology, climate and historical impactby acid deposition (Table 1). JEZ, in the heart of the Black Triangle, re-ceived the highest levels of acid deposition of all the GEOMON networksites in the 20th century (Table 1; Fottová and Skořepová, 1998). JEZ isin close proximity to themain industrial coal combustion basin, and hadextensive forest death in its watershed (Havel et al., 1999). JEZ had thehighest Hg deposition as demonstrated by nearby peat profiles that ac-cumulated an average 65 μg Hg m−2 yr−1 during the past 100 years,with a maximum Hg accumulation rate of up to 100 μg m−2 yr−1 inthe second half of 20th century when the emissions of S and Hg weregreatest (Zuna et al., 2012). Approximately 80 km to the SW and alsoin the Black Triangle, LYS and PLB in the Slavkov Forestwere extensivelystudied as end-members of acid precipitation buffering (e.g., HruškaandKrám, 2003; Krámet al., 1997, 2012)with LYS extremely vulnerableto acidification (Navrátil et al., 2007) and PLB extremely well-buffered(Krám et al., 2009). JEZ, LYS and PLB historically had high S depositionand fell in the average- to elevated-Hg areas in the soil Hg survey ofSucharová and Suchara (2004) (Fig. 1). Site LES in central Bohemia,30 km southeast of the capital Prague, typically has relatively low pre-cipitation and high evapotranspiration (Navrátil et al., 2003). The lowrunoff to precipitation ratio at LES (Table 1) could be in part due to effi-cient recharge of precipitation to groundwater because the lower part ofthe catchment is underlain by 4 to 5 m of sediments (Navrátil et al.,2003). LES is close to a contamination hot spot related to mining andsmelting activities (Ettler et al., 2007, 2008), thus historical Hg deposi-tionmay have been elevated. Finally, site LIZ in southern Bohemiawith-in the mountainous area of the Bohemian Forest was the catchmentleast impacted by anthropogenic deposition (Fottová and Skořepová,1998). A peat core near LIZ exhibited a significantly lower average Hgaccumulation rate of 28 μg Hg m−2 yr−1 in the past 100 years with

Table 1Basic information for the five Czech study catchments. Stream water annual mean discharge-weighted chemistry and runoff data originate from the GEOMON long-term monitoring da-tabase, with a timespan from1994 to 2012 (Fottová et al., 2013). The selected soil properties, bedrock and soil Hg concentrations and pools are fromNavrátil et al. (2014). For JEZwe usedsoil data from nearby (~15 km) Načetín (Oulehle et al., 2011). Data on S deposition in 1994 are from Fottová and Skořepová (1998).

Site Jezeří Lesní potok Liz Lysina Pluhův bor

Abbreviation JEZ LES LIZ LYS PLB

Location 50°32′N, 13°28′E 49°58′N, 14°49′E 49°04′N, 13°41′E 50°03′N, 12°40′E 50°04′N, 12°46′E

Catchment area (km2) 2.61 0.7 0.99 0.27 0.22Outlet elevation (m a.s.l.) 475 400 827 829 690Highest elevation (m a.s.l.) 924 495 1024 949 804Mean annual temperature (°C) 6.0 7.0 6.6 5.0 6.0Mean annual precipitation (mm) 773 625 894 1001 810Mean annual runoff (mm) 378 106 387 465 286Annual runoff/precipitation ratio 49% 17% 43% 46% 35%Stream water chemistry

Mean pH 5.50 5.05 6.42 4.06 7.16Mean alkalinity (μeq L−1) 19 32 143 −92 607Mean Cl− (mg L−1) 2.3 4.1 1.1 1.1 1.7Mean SO4

2− (mg L−1) 41.3 69.2 11.3 10.6 18.0Water type Ca–Na–SO4 Ca–Mg–SO4 Ca–Mg–SO4 Ca–Na–SO4 Mg–HCO3

Wetland area (%) 0 0 0 2 0.4Cover area (%)

Open 40 0 0 18 5Spruce 18 44 100 82 84Pine 0 0 0 0 11Beech 15 56 0 0 0Birch 27 0 0 0 0

Thr. N deposition in 1994 (kg ha−1) 26 13 7 10 8Thr. S deposition in 1994 (kg ha−1) 67 21 9 32 26Bedrock Gneiss Granite Paragneiss Granite SerpentiniteBedrock Hg conc. (μg kg−1) 0.8 1.5 0.7 0.6 6.5Soil Spodo-dystric cambisol Eutric cambisol Spodo-dystric cambisol Spodo-dystric cambisol Magnesic regosolOrg. soil mean Hg (μg kg−1) 392 393 277 375 287Min. soil mean Hg (μg kg−1) 100 54 98 79 24Org. pool Hg (mg m−2) 5.7 10.1 6.8 7.4 5.3Min. pool Hg (mg m−2) 26 37 130 63 16Total pool Hg (mg m−2) 32 47 137 70 22Org. pool C (g m−2) 4402 6855 4486 5884 5606Min. pool C (g m−2) 8508 4159 20,668 14,832 6020Total pool C (g m−2) 12,910 11,014 25,153 20,716 11,626

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respect to a peat profile near JEZ (Zuna et al., 2012). The Hg accumula-tion rate in peat near LIZ reached amaximumof 40 μgm−2 yr−1 duringthe 20th century.

In terms of vegetation cover, JEZ has forest cover of spruce, beechand birch (Table 1). A relatively large area (40%) of forest dieback oc-curred from the 1970s to 2000s caused by extreme levels of S deposition(Havel et al., 1999). These openings have since been successfullyreforested with a mixture of birch, alder, mountain ash and spruce.LES is 100% forested mostly by mixed spruce and beech forest. Theother sites are completely forested except for minor percentages ofopen area at LYS (18%) and PLB (5%) (Table 1). The dominant coniferousspecies at LYS and LIZ is spruce. The vegetation at PLB is dominated byspruce with admixture of pine.

The bedrock at all sites was low in Hg (Table 1; Navrátil et al., 2014).The Hg concentrations in bedrock ranged from 0.6 μg kg−1 at LYS to6.5 μg kg−1 at PLB. The mean concentration of Hg in mineral soils sam-pled to the depth of consolidated regolith at individual sites was at leastan order of magnitude greater than that in bedrock (Navrátil et al.,2014). The accumulation of Hg in soils was related to soil organic car-bon, which controls the Hg concentrations in mineral soils (Navrátilet al., 2014). The highest mean Hg concentration in mineral soil,100 μg kg−1, was at JEZ, while the lowest, 24 μg kg−1, was at PLB. LowHg concentrations in bedrock and correlation of Hg with C in mineralsoils (Navrátil et al., 2014) suggest that weathering at the studied sitesdoes not represent a significant Hg source. The concentration of Hg inorganic soilswith ahigh content of organicmatterwas at least twoordersofmagnitude greater than that in bedrock at all sites (Table 1). The highermean concentrations N 390 μg kg−1 occurred at JEZ and LES, whilelower mean Hg concentrations b 290 μg kg−1 occurred at LIZ and PLB.

2.2. Sampling and analysis

Stream stage wasmeasured at all five sites continuously with a floator ultra-sound recorders at V-notchweirs and recorded at 30-min inter-vals or more frequently. Stage was converted to discharge using thetheoretical rating verified by measurements.

The study streams were sampled at regular monthly intervals inaccordancewithGEOMONprotocols. Occasional samplingwas also con-ducted not limited to high-flowepisodes.Water samples for Hg analysiswere collected in glass bottles and transported to the laboratory.

Analytical procedures used for total mercury follow the recom-mended protocol US EPA 1631 (US EPA, 2002). All chemicals usedwere of special purity for mercury ultra-trace analysis, i.e., tin(II) chlo-ride dihydrate (Merck, b10 μg L−1 Hg), bromide/bromate solution(Merck, Titrisol), trace-metal reagent grade hydrochloric acid (Romil,b10 ng L−1 Hg). Argon and nitrogen of 5.0 gradeswere used for bubblerand drying operations.

Stream water mercury (Hg) concentrations reported in this paperare always filtered total Hg. Likemany authors, we interpret the filteredfraction to be dissolved, though it may contain colloidal Hg. Suspendedsediment is low at allfive sites, thus thefiltered fraction representsmostof the Hg. All samples were filtered prior to stabilization and analysisthrough a 0.4-μm glass fiber filter (Fisher Scientific). Glass filters wereused in order to avoid adsorption of mercury species on filtration disk.A filtered aliquot of 50 ml was stabilized with excess HCl. At least 24 hprior to analysis a mixture of HCl, KBrO3 and KBr was added to oxidizeall Hg species to Hg(II) and the samples were kept under laboratorytemperature. In this way, bromine chloride BrCl was generated in situ,imparting a yellow tint to the solution.

204 T. Navrátil et al. / Journal of Geochemical Exploration 158 (2015) 201–211

Total mercury analysis was performed by Cold Vapor-Atomic Fluo-rescence Spectroscopy (CV-AFS) on a Millennium Merlin instrument(PS Analytical, England) in Galahad setting, using a freshly preparedtin chloride solution. Briefly, in this setting mercury is trapped on goldamalgam, fromwhich it is expelled by heating pulse into the fluorimet-ric cell. The standard parameters of 120 s mercury collection time, 4 sheat pulse, and 15 sflush timewere used. The systemwas calibrated be-fore each measurement using freshly prepared calibration solutionswith concentrations 0.5–20 ng L−1 total Hg and blank. Calibration solu-tions were prepared by stepwise dilution of 1000mg L−1 certified stan-dard solution (1000ppmHg in chemical form of nitrate, Analytika s.r.o).Only linear calibration fits with r equal to 0.999 or better were used.

Quality assurance and quality control (QA/QC) procedures applied inthis paper are based on the US EPA 1631 method. Each experimentalbatch included a maximum of 30 samples. Before measurement ofeach batch, reagent blanks were checked and special attention waspaid to the tin chloride solution, which is prone to contamination withHg. A method blank was prepared together with calibration solutionsandwas runbefore calibration, after calibration, and at the endof exper-imental run (after every 30 samples). The mercury concentration in themethod blank was lower than the method detection limit. An ongoingprecision and recovery test was performed using a 5 ng L−1 Hg solution,which was inserted into each experimental run at least 3 times. Qualitycontrol was ensured by analysis of standard materials ORMS-4 (Elevat-ed mercury in river water, National Research Council Canada, 22.0 ±1.6 ng L−1) and BCR-579 (Coastal seawater, EC Joint Research Centre,1.90 ± 0.05 ng L−1). Measured recoveries were typically 93–115% forORMS-4 standard and 80–120% for BCR-579 standard, which was with-in the acceptable limits according to the EPA protocol.Method detectionlimit was 0.21 ng L−1 and the reporting limit was 0.5 ng L−1 in thisstudy.

DOC (dissolved organic carbon) concentration was determined on0.4-μm filtered samples using a Shimadzu TOC-V CPH analyzer. Sampleswere transferred onto platinum catalyst in the oven, where total com-bustion in a stream of CO2-free synthetic air proceeded. Gaseous CO2

was quantified on an IR detector. A primary dilution standard was pre-pared by dissolving oxalic acid dihydrate in C-free acidified water. Theinstrument was calibrated according to manufacturer recommenda-tions, in the range 0–50 mg C. Each sample measurement was per-formed at least three times, after statistical evaluation repetition wasadded as needed. QC/QA procedureswere taken fromUSEPA 415.3 pro-tocol (US EPA, 2005). Measured recoveries were typically 95–105%, andthe detection limit for organic carbonwas 0.25mg C L−1 (99% Student'sconfidence limit). An ongoing precision and recovery test was per-formed using 10 mg L−1 potassium hydrogenphthalate (KDP) solution.

UV–VIS absorbance wasmeasured on aliquots of streamwater sam-ples at 254 nm and 410 nm. Filtered samples (0.4-μm) were measuredat room temperature (25 °C) in a 1-cm quartz cell with distilled wateras the blank in the VIS region. Filtered samples for UV with absorbanceat 254 nm N 1.0 were measured using a 0.5-cm quartz cell. Sampleswere kept chilled (4 °C) and analyzed within 3 weeks after collectionfor DOC concentration and UV absorbance. QC for the UV absorbancemeasurements was ensured according to US EPA 415.3 (US EPA, 2005)by calibration of the system with KDP solutions. Measured UV absor-bance at 254 nm was corrected for Fe interference following Weishaaret al. (2003) assuming that measured Fe was present as Fe3+.

2.3. Statistical analysis

Stream chemistry (filtered Hg, DOC, Abs254, Abs410, SUVA254, con-ductivity, pH, Al, Ca, Fe, Mg,Mn, Na and S)was tested for site-to-site dif-ferences by one-way ANOVA. Stream chemistry and discharge from allsites were tested for skewness, kurtosis, and overall normality. Logtransformationswere appliedwhen datasets were not normally distrib-uted. For several variables the normality assumption was not met forlog-transformed data. In such cases the Kruskal–Wallis test, a non-

parametric version of one-way ANOVAwas used. Themultiple compar-ison post-hoc test procedure (Morris, 2012) was used for testing forsignificant differences among the sites after the Kruskal–Wallis test.The significance level for all statistical analyses was p b 0.05.

2.4. Calculation of Hg and DOC output fluxes

We calculated stream Hg and DOC fluxes by building concentrationmodels for each site based primarily on stream discharge, which wascontinuously measured at the five study streams. The concentrationmodels were applied to daily average streamflows to estimate meandaily Hg and DOC concentrations. These were then multiplied bymean daily discharges to compute daily Hg and DOC fluxes. All sitesexcept LES had a significant relation of filtered Hg to discharge (Supple-mentary data, Table S.1). At LES, Hg and DOC concentrations correlatedmore strongly to water temperature (r = 0.90 and 0.74, p b 0.001) socontinuously measured water temperature was used to build the Hgand DOC models for LES.

3. Results

3.1. Hydrology

During the 19-year GEOMON record (1994 to 2012), mean annualrunoff at all five catchments ranged by a factor of 4 with the lowest atLES (107 mm) and the highest at LYS (465 mm) (Table 1). Compared tothe 19-year averages, annual runoff for the hydrologic year (Novemberof prior year through October of designated year) at all five catchmentswas lower in 2012 and greater in 2013. At LES, hydrologic year 2013hadthe highest annual runoff for the entire 19-year record. The high runoffin 2013 at all catchments was due in part to a significant rain event inthe first days of June 2013, which caused severe flooding within theBohemia region of the Czech Republic. Runoff at all catchmentsresponded similarly to increased precipitation in 2013 except LES,which showed a greater relative increase to 2012. Stream samplescollected at each of the sites covered the wide spectrum of dischargeconditions representing flow exceedance levels ranging from 88% to0.1% (Supplementary data, Fig. S.1). The exception was JEZ, where nosamples were collected at the highest discharges (exceedance b1%).

3.2. Water quality

The count of samples taken during this study at each of the five sitesranged from 44 to 61 (Fig. 2a). The mean specific discharge during thesampling at JEZ, LES, LIZ, LYS and PLB was 0.099, 0.081, 0.071, 0.099and 0.070 mm h−1, respectively.

Filtered stream water Hg concentrations ranged over an order ofmagnitude within each site, and also by an order of magnitude (interms of median concentrations) across the five sites (Fig. 2a). Medianfiltered Hg concentrations ranged from 1.3 ng L−1 at the relativelyunimpacted LIZ to 18.0 ng L−1 at PLB. Based on an ANOVA, the medianfiltered Hg concentration at LYS of 5.0 ng L−1 was significantly lowerthan that at PLB but comparable to LES with a median 2.8 ng L−1. Fil-tered Hg concentrations at LIZ and JEZ had median values of 1.3 and2.3 ng L−1, respectively. These were not significantly different fromLES but were all significantly less than that at LYS (Fig. 2a).

The pattern of mean monthly Hg concentrations varied across sites.The highest monthlymean Hg concentration at PLB, LYS and LIZ was as-sociated with a major precipitation event in June 2013 (Fig. 3a). At LESthis event caused a dilution in Hg, resulting in a decrease in the Junemean monthly Hg concentration. At JEZ, the highest Hg concentrationoccurred inMarchdue to spring snowmelt. Relatively lowmeanmonth-ly Hg concentrationswere typical during thewarmmonths July to Octo-ber at PLB, LYS, JEZ and LIZ (Fig. 3a). The mean monthly concentrationsof Hg at LES were greatest in the summer due to the positive relation of

Fig. 2.Boxplots of (a)filteredHg, (b) DOC, (c) UVabsorbance at 254 nm, (d) absorbance at 410 nm(note log scale), (e) SUVA254, and (f)Hg/DOC ratio in streamwater samples from thefiveCzech study sites. The top and bottom of the box represent the 25th and 75th percentiles, the solid line in the box represents the median value, and the dashed line represents the meanvalue. Horizontal ticks outside the box indicate the 5th and 95th percentiles, and dots represent outliers. The capital letters denote homogenous groups of data determined by one-wayANOVA complemented with a multiple comparison post-hoc test. The number of samples (n) for each study site is denoted in panel (a).

205T. Navrátil et al. / Journal of Geochemical Exploration 158 (2015) 201–211

DOC concentrations to temperature. Mean monthly concentrations ofHg at all sites tracked those of DOC (Fig. 3b).

Filtered Hg concentrations correlated with DOC concentrations at allsites (Supplementary data, Table S.1). Similar to Hg, the highest medianDOC concentration of 20.3 mg L−1 was at site PLB (Fig. 2b). Streamwater DOC concentrations at PLB ranged over more than an order ofmagnitude from 4.8 to 52.0 mg L−1. The nearby LYS catchment hadthe second highest median DOC concentration at 16.8 mg L−1, with arange of 4.9 to 32.6 mg L−1 (Fig. 2b). The DOC concentrations at LYSwere not significantly lower than those at PLB but also not significantlyhigher than those at LES. The median DOC concentration at LES was7.9 mg L−1. The lowest median DOC concentrations were 3.1 mg L−1

at JEZ and 3.7 mg L−1 at LIZ. The DOC concentration ranges at JEZ, LESand LIZweremuch narrower than those at LYS and PLB (Fig. 2b). For ex-ample at JEZ, DOC concentration ranged only from 1.5 to 4.4 mg L−1.

UV absorbance at 254 nm (Abs254), which has been reported as agood proxy for DOC concentrations, followed similar patterns to DOCat all sites (Fig. 2c). The correlation of DOC and Abs254 was significantat all sites, with r from0.88 to 0.98 (p b 0.001). Similarly, DOC correlatedsignificantly with UV absorbance at 410 nm (Abs410) at all sites with r

from 0.51 to 0.99 (p b 0.001) (not shown). Abs410 relates strongly to ar-omatic DOC. The lowest correlation was at JEZ which had the lowestDOC concentrations, where the values of Abs410 were close to the detec-tion limit of the method, causing significant uncertainty.

SUVA254 expressed asAbs254/DOC, is an indicator of DOC aromaticity(Weishaar et al., 2003). The site-specific medians of SUVA254 correctedfor Fe increased from 2.1 at JEZ to 4.5 at PLB (Fig. 2d). Hg concentrationcorrelated with SUVA254 at sites LYS, JEZ and PLB (Supplementary data,Table S.1). The study sites could be separated based on analysis of vari-ance of SUVA254 (Fig. 2d). Sites JEZ and LES, which are dominated by de-ciduous forest, had the lowest SUVA254 values. Catchment LIZ, withconiferous vegetation, had higher median SUVA254. Finally PLB andLYS, also dominated by conifers, had the highest SUVA254 values(Fig. 2d).

3.3. Hg and DOC output fluxes

Site to site differences in Hg and DOC export result from both flowand concentration differences. Annual runoff variation among catch-ments arises fromdifferences in catchment size and annual precipitation

Fig. 3. Pattern of meanmonthly (a) Hg concentration, (b) DOC concentration, (c) Hg/DOC ratio and (d) SUVA254 in streamwater at the five study catchments for hydrological years 2012and 2013. Each monthly mean was calculated from all the samples available for a particular month from both hydrological years.

206 T. Navrátil et al. / Journal of Geochemical Exploration 158 (2015) 201–211

(Tables 1 and 2). Variation in streamHg andDOC concentrations amongcatchments is also considerable (Fig. 2a,b) and is explored below.

Annual stream water Hg output in hydrologic years 2012 and 2013ranged over two orders of magnitude, from 0.11 to 14.40 μg m−2

among the five catchments (Table 2). The Hg output flux at each sitein year 2012 was smaller than that in 2013 due to the markedly lowerwater output (Table 2). The lowest Hg output flux occurred in both hy-drologic years 2012 and 2013 at LES due to its low annual discharge(Table 2) and relatively low median Hg concentration (Fig. 2a). TheLES catchment had by far the lowest DOC output flux in hydrologicalyear 2012 (0.36 g m−2). In year 2013 due to the 5-fold greater waterdischarge of 276 mm, DOC output increased to a greater extent (9-fold) at LES than at the other sites.

PLB had the highest median Hg concentration (Fig. 2a) andthe highest annual Hg output fluxes (8.97 μg m−2 in 2012 and13.33 μg m−2 in 2013), despite its second lowest mean runoff(Table 1). The nearly 50% increase in Hg flux from 2012 to 2013corresponded to a N40% increase in runoff at PLB. The second greatestHg output flux (3.65 μgm−2 in 2012 and 5.42 μgm−2 in 2013) occurredat LYS. It is interesting to note the relatively low difference (11%) be-tween theDOC outputflux from LYS and PLB in contrast to the relatively

Table 2Annualwater output, calculated runofffluxes ofHg andDOC at thefive study catchments. Concely samples and episodic samples).

Site JEZ

2012 Water runoff mm 293Water runoff 103 m3 765Hg output flux μg m−2 0.55DOC output flux g m−2 0.89

2013 Water runoff mm 411Water runoff 103 m3 1072Hg output flux μg m−2 0.94DOC output flux g m−2 1.33

Increase in 2013 over 2012Water runoff Increase by % 40%Hg output flux 71%DOC output flux 49%

high differences (N140%) between Hg output fluxes (Table 2). DOC out-put fluxes at catchments LIZ and JEZ were comparable in hydrologicyear 2012 but the Hg output flux at JEZ was 60% greater. In hydrologicyear 2013 the increase in water discharge at JEZ resulted in increasesof Hg and DOC output fluxes by 71 and 40%, but at LIZ the output fluxesof both solutes increased by more than 120% (Table 2).

3.4. Stream Hg/DOC

The ratio of Hg/DOC in streamwater at a specific sitemay be a betterreflection of legacy Hg deposition because Hg transport may be limitedby DOC availability. Analysis of variance indicated that the siteswith thehighest median Hg/DOC ratio (ng mg−1), JEZ (0.73) and PLB (0.90)were not significantly different (Fig. 2f). LIZ (0.40) and LYS (0.34)were not significantly different from each other but were significantlyless than JEZ and PLB. Finally, the lowest median Hg/DOC ratio of 0.28was at LES. Hg/DOC ratios at LES were significantly less than those atLIZ but not significantly different from those at LYS (Fig. 2f).

MeanmonthlyHg/DOC ratios across thefive sites also varied season-ally. At JEZ, LYS, LIZ and PLB, the period from July to October typicallyhad low mean monthly Hg/DOC ratios (Fig. 3c). The highest mean

ntration-dischargemethod calculation uses all the samples for each catchment (12month-

LES LIZ LYS PLB

45 294 402 27231 291 110 590.11 0.33 3.65 8.970.36 1.03 8.86 9.70

276 495 544 389190 491 149 84

0.93 0.87 5.42 13.332.25 2.34 12.80 14.40

513% 68% 35% 43%720% 164% 49% 49%532% 127% 44% 48%

Fig. 4. Relationships of (a) Hg and (b) DOC concentrations to specific discharge (note log scale) at each of the five study sites with linear regression fits, correlation coefficient, and prob-ability level. On the left plot, the regression lines for LIZ and JEZ overlap. The number of samples in years 2012 and 2013 for each site is shown in Fig. 2a.

207T. Navrátil et al. / Journal of Geochemical Exploration 158 (2015) 201–211

monthly Hg/DOC values occurred typically in the colder period fromFebruary to April (Fig. 3c). LES behaved nearly oppositely, with thehighest stream Hg/DOC ratio from June to September (Fig. 3c) andlowest from December to March.

3.5. Episodic discharge dynamics

Both Hg and DOC had a significant positive correlation with dis-charge (Q) at all sites except LES (Fig. 4a,b). The lack of correlation atLES was due to increased concentrations of Hg and DOC during lowdischarge periods. Hg and DOC concentrations in stream water at LESwere positively correlated with water temperature (r = 0.90 and 0.74,respectively, p b 0.001, n = 44).

The Hg/DOC ratio increased with increasing flow at JEZ, LYS, LIZ andPLB (Supplementary data, Fig. S.2a). When LES samples were split intotwo groups based on discharge exceedance above and below 10%, thehigher flow samples had higher mean Hg/DOC ratios but the medianHg/DOC ratio was similar for the both discharge groups. Similarly tothe Hg/DOC ratio, SUVA254 values increased with increasing flow atJEZ, LYS and PLB but not at LES and LIZ (Supplementary data,Fig. S.2b). SUVA254 decreased with increasing flow at LIZ, and exhibitedno relation to flow at LES.

4. Discussion

4.1. Stream Hg concentrations and fluxes

Filtered Hg concentrations at the five sites ranged over an order ofmagnitude, from typical values for unpolluted temperate forestedstreams to among the highest reported in the literature (Shanley andBishop, 2012). Surprisingly, however, the pattern of stream Hg concen-trations did not simply follow the historical Hg deposition gradient. AtJEZ, the highest Hgdeposition site, themedian streamwater Hg concen-trationwas only 2.5 ng L−1, which is at the high end of background con-centrations for unpolluted streams (Shanley and Bishop, 2012). On theother hand, LYS and especially PLB, seven km apart in a somewhat lessimpacted setting but still within the Black Triangle, had strikingly highfiltered Hg concentrations: 5.0 ng L−1 at LYS and 18.0 ng L−1 at PLB,among the highest reported filtered median total Hg concentrations inheadwater forest streams worldwide (Shanley and Bishop, 2012). Theother two sites, LES (median Hg 2.8 ng L−1) and LIZ (median Hg1.3 ng L−1), are outside the Black Triangle and presumably had lowerHg deposition rates. They both had median stream Hg concentrations

much lower than PLB and LYS, but not significantly different thanthose at JEZ (Fig. 2a), the highest deposition site. LIZ, in the BohemianForest to the south, is the least impacted site and had the lowestmedianHg.

The high filtered Hg concentrations in stream water at LYS and PLBbracketed the median Hg concentration of 13 ng L−1 at the GermanLehstenbach catchment 60 km to the W (Schwesig and Matzner,2001), which was also subject to high Hg deposition from heavy indus-trialization in the 20th century. The other three sites had closer to back-ground Hg concentrations. For example, Riscassi and Scanlon (2011)reported median values of filtered Hg concentration ranging from 0.5to 1.0 ng L−1 from three undisturbed forested sites in Virginia, USA.Shanley et al. (2008) found 1.0 to 2.4 ng L−1 filtered Hg at five forestedheadwater sites across the USA. High maximum unfiltered Hg concen-trations up to 96 ng L−1 were reported from North American forestedcatchments but these were dominated by particulate Hg; Hg concentra-tions of their filtered counterparts were considerably less than the fil-tered Hg concentrations from LYS and PLB (Shanley and Bishop, 2012).

The filtered Hg output flux from the well-buffered serpentinitecatchment PLB in hydrological years 2012 and 2013 (9.0–13.3 μg m−2 yr−1, Table 2) ranks as the highest reported from a head-water forest ecosystem (Shanley and Bishop, 2012). These Hg outputfluxes are nearly half of the measured total deposition flux of Hg(throughfall + litterfall) at LES of 24.9 μg m−2 yr−1. Though thepresent-day deposition at PLB may be somewhat greater than at LES,these stream output fluxes clearly exceed the level that would be ex-pected after the typical 90% watershed retention and/or evasion(Shanley and Bishop, 2012). Although the Hg concentration in bedrockat PLB is greater than at the other sites, we suggest it does not representa significant source of Hg. This can be also supported by the lowestmean Hg concentrations in the mineral soils at PLB (Navrátil et al.,2014, Table 1). We submit that the high Hg stream flux at PLB in theBlack Triangle results primarily from release of legacy Hg from 20thcentury industrial Hg deposition.

The output flux of filtered Hg from the nearby acidic LYS site(3.65–5.42 μg m−2 yr−1, Table 2) was similar to the output fromthe similarly industrially impacted German Lehstenbach catchment of6.8 μg m−2 yr−1 (Schwesig and Matzner, 2001). The output fluxesfrom JEZ, LES, LIZ and LYS (Table 2) generally fell within the rangefrom 0.3 to 6.8 μg m−2 yr−1 summarized in the review by Shanleyand Bishop (2012) for European (Norway, Finland, Sweden,Germany), Canadian and USA sites. An exception was the tropical RíoIcacos (Puerto Rico) catchment with 54.4 μg m−2 yr−1 unfiltered Hg

208 T. Navrátil et al. / Journal of Geochemical Exploration 158 (2015) 201–211

output flux, but N90% of the output Hg was associated with suspendedsediment; the filtered Hg output flux was only 3.5 μg m−2 yr−1

(Shanley et al., 2008).

4.2. Role of stream discharge

High discharge periods at these Czech catchments are accompaniedby increased concentrations of Hg and DOC (Fig. 4a, b). Across the fivecatchments, discharge explained 26 to 42% of the variability in DOC con-centration, and 30 to 62% of the variability in Hg concentration (exclud-ing LES, which showed no correlation). Riscassi and Scanlon (2011)reported that discharge explained a similar level of Hg andDOCvarianceat USA sites that lacked wetlands.

Hg had a strong positive correlation to discharge at all sites exceptLES, but the slope of the relation varied greatly (Supplementary data,Fig. S.3a–e). Many studies have reported a positive relationship of Hgand DOC concentrations with discharge (e.g., Dittman et al., 2010;Riscassi and Scanlon, 2011; Shanley et al., 2008). Increased DOC andHg concentrations duringhigh discharge conditions result fromflushingof organic-rich soil horizons and lateral transport to the stream channel(Demers et al., 2010; Mitchell et al., 2008; Shanley et al., 2008). Thisscenario is consistentwith Czech forest soils, which have the highest or-ganic carbon and Hg concentrations in the organic-rich soil horizons(Navrátil et al., 2014), as do the corresponding soil solutions in these ho-rizons (unpublished data).

Annual runoff has a strong effect on Hg output fluxes, as illustratedby the year-to-year differences at the five catchments in the contrastinghydrological years 2012 and 2013. Due to the positive relationship of Hgand DOC with discharge (Fig. 4 and Supplementary data, Table S.1),years with greater runoff yield disproportionately greater output Hgfluxes. Relative to 2012, annual runoff in 2013 was 35 to 68% greaterat all sites except LES, where it was more than 500% greater (Table 2).Annual filtered Hg fluxes increased a greater amount in 2013, by 49 to164% at the first four sites and by 700% at LES. Similarly, annual DOCfluxes at JEZ, LIZ, LYS and PLB in 2013 were 44 to 127% greater and atLES more than 530% greater relative to 2012.

4.3. Role of stream temperature

Stream DOC and Hg concentrations at LES had a significant positivecorrelation with stream temperature (r = 0.74 and 0.90, respectively;p b 0.001, n= 38). In contrast to the other four sites, streamwater tem-perature at LES had more variability and a significant positive correla-tion with air temperature (r = 0.91, p b 0.001, n = 38). At the otherfour catchments, stream temperature fluctuated within a narrowrange close to mean annual air temperature. Thus we suggest that pro-cesses critical to the mobilization of DOC and Hg at LES are associatedwith biological processes stimulated by the increased ambient temper-atures in the warm months (July–September). Stream water tempera-ture tracked air temperature due to very low flow, stimulating anincreased level of organic material decomposition in the stream bedand/or riparian zones.

4.4. Role of DOC concentrations and quality

The Hg concentrations at PLB and LYS in the industrialized Black Tri-angle are well above background and seem clearly linked to high 20thcentury Hg deposition. But given that these two sites are only 7 kmapart and show a 3-fold difference in stream Hg suggests that otherfactors are also important. Underscoring this point, JEZ and LIZ are atopposite ends of the deposition gradient and each has low median Hgconcentrations that are not significantly different from each other.While stream Hg did not follow a deposition gradient, all five catch-ments showed a relatively strong relation betweenHg andDOC concen-trations (Supplementary data, Fig. S.3a–e). The strong relation betweenHg and DOC concentrations in stream water is well-established

(Åkerblom et al., 2008; Demers et al., 2010; Dittman et al., 2010;Grigal, 2002; Mitchell et al., 2008; Riscassi and Scanlon, 2011; Shanleyet al., 2002, 2008). At these five Czech catchments, the Hg-DOC relationis strongly influenced by DOC quality.

In general, Hg concentrations among the sites followed the expectedpatterns with respect to DOC. PLB and LYS had the highest Hg and thehighest DOC (Fig. 2b). JEZ, LIZ, and LES had comparably lowDOC and ac-cordingly low Hg. Hg and DOC were strongly correlated at all sites, butthere were clear differences in the slope of the relation, which we attri-bute to differences in DOC quality. For example, JEZ has a high percent-age of open land and deciduous forest, as well as steep slopes, leading tolow DOC with low SUVA. Thus, the paradox at JEZ of highest Hg deposi-tion but the second lowest streamHg concentrations (Fig. 2a) can be at-tributed to transport limitation, due to low DOC of “poor quality” for Hgmobilization. In contrast, LIZ had low deposition but DOC of greater aro-maticity due to coniferous forest, and relatively high SUVA. ComparingJEZ, LIZ, and LES, their similar Hg concentrations may be a result of off-setting of increasing SUVA254 with decreasing Hg deposition. Hg-DOCrelations at LES are confounded because LES had by far the lowestwater yield of the five sites, due to its relatively low precipitation andhigh evapotranspiration.

4.5. Stream Hg/DOC

To normalize the effect of different DOC concentrations at the fivesites we used the Hg/DOC ratio for site-to-site comparison. The Hg/DOCratio is a measure of how much Hg is transported per unit of DOC.Because DOC availability may limit the amount of Hg that can betransported, Hg/DOC may be a robust index of historical Hg depositionduring the 20th century. Hg/DOC was higher at all sites than the0.2 ng mg−1 reported by Grigal (2002) and most sites were greaterthan the 0.3–0.6 ng mg−1 range reported by Riscassi and Scanlon(2011). Although Hg/DOC was found in some studies to be relativelyconstant across diverse sites (Åkerblom et al., 2008; Grigal, 2002;Shanley et al., 2002), we observed significant site-to-site as well aswithin-site variability (Fig. 2f). Despite its low stream Hg, JEZ had thesecond highest mean value of Hg/DOC, and the highest discrete valuesof Hg/DOC during storms. The high Hg/DOC at JEZ despite lowSUVA254 (Fig. 2d) is indicative of the influence of the high legacy Hg de-position at that site in the heart of the industrialized region. PLB had ahigher median Hg/DOC than JEZ and a considerably narrower range(Fig. 2f). The higher Hg/DOC at PLB could again be explained by its high-ly aromatic DOC, as indicated by its highest Abs410 and SUVA254 values.

At PLB, LYS and JEZ the periods with low monthly Hg/DOC ratioswere coincident with periods of low SUVA254 (Fig. 3c,d). The interplaybetween periods with low Hg/DOC ratios and low SUVA254 might resultfrom decreased production of hydrophobic DOC in summer, which wasdocumented for forest soil solutions draining the organic horizons(Åkerblom et al., 2008; Kaiser et al., 2001). Summer base flow is alsodominated by groundwater that has lower DOC concentrations charac-terized by a low hydrophobic fraction because of physical adsorption onmineral surfaces as water passes through the soil profile. Since surficialsoil solution is clearly a source of DOC to forest streams (Hruška et al.,2014), this suggests either changes in composition or sources withinthe soil for stream water DOC.

4.6. Role of soils and legacy S

The pattern of median streamHg and DOC concentrations in streamwater at the five sites differed from that of median Hg and organic car-bon concentrations in either organic or mineral soils. We have alreadydiscussed that stream Hg is linked to stream DOC concentration andquality. Site-specific DOC concentration and quality depend, in turn,on the interacting processes of production, decomposition, sorptionand flushing. Therefore we attempted to link stream Hg and DOC tosoil parameters that may point at the processes that mobilize Hg and

209T. Navrátil et al. / Journal of Geochemical Exploration 158 (2015) 201–211

DOC in runoff. We also consider how legacy S from the heavy atmo-spheric deposition in the second half of the 20th century may affectstream Hg.

Mercury concentrations and pools in forest soils, representing theHg source for the stream water, have been primarily linked to soil or-ganic matter (Demers et al., 2013; Grigal, 2002) and by associationthus with soil pools of C, N and S (Navrátil et al., 2014). Riscassi andScanlon (2011) suggested that increasing soil organic carbon contentcaused a decrease in the Hg/DOC ratio in stream water. For the fivesites in this study, stream water Hg/DOC was not related to soil carbonconcentrations and pools at individual sites. Johansson and Iverfeldt(1994) reported no coupling between Hg/C in surface waters andHg/C in organic soils (mor layer) for a set of Swedish catchments withand without lakes and wetlands. Similarly, stream Hg/DOC was not re-lated to the organic soil Hg/C ratio at the five sites in this study (Fig. 5a),but the mean stream Hg/DOC ratio was inversely related to the Hg/Cratio of mineral and total soil pools (r= 0.98 and 0.92, p b 0.01; Fig. 5a).

While the relative partitioning of Hg to DOC and soil organic carbonis a primary control on streamHg inmost ecosystems, we cannot ignorethe potential role of legacy soil S from the high deposition in the 20thcentury. Sulfur functional groups arewell established preferential bond-ing sites for Hg (Skyllberg et al., 2003, 2006) thus soil organicmatter cansequester Hg. Microbially mediated retention of sulfur and oxidativelosses of carbon decrease the soil substrate C/S ratio (Demers et al.,2013). Navrátil et al. (2014) reported an exponential increase of thesoil Hg/C ratio with decreasing soil C/S ratio in forest soils of the CzechRepublic and attributed this increase to preferential loss of C (relativeto Hg and S) during the mineralization of soil organic matter. Anotherpossible mechanism that may cause differences of soil Hg/C and C/S ra-tios at the individual sites is Hg retention in soils due to the precipitationof HgS(s) compounds, i.e., a metacinnabar-like amorphous phase(Gerbig et al., 2011a,2011b; Skyllberg, 2011). This mechanism of Hg re-tentionwould tend to increase soil Hg/C, decrease soil C/S, and decreasethe streamHg/DOC ratio. Relatively stable HgS(s)-like phasesmay formin forest soils under seasonally anoxic conditions and its oxidation maybe kinetically limited (Gerbig et al., 2011a,2011b). It should be noted

Fig. 5. Relationship betweenmean streamwater Hg/C ratio and (a) mean soil pool Hg/C and (bratios at individual sites, and (d) relationship of total soil pool Hg/C and C/S ratios at individua

that direct observations of HgS(s)-like phases in soils have been limitedto heavily polluted sites or sites impacted bymining activities due to thenanocolloidal size of these aggregates (e.g., Han et al., 2008).

Another way that legacy S can affect Hg mobility is through itsinhibition of SOM decomposition. For mineral and total soil pools, LESand LIZ had low stream Hg/DOC, high soil Hg/C, and low C/S ratios(Fig. 5a,b,c,d). A possible explanation for these patterns is that soil or-ganic matter, especially in the mineral soil, at these two sites wasmore decomposed than the soil organic matter at the Black Trianglesites JEZ, LYS and PLB. This is in accordance with the results of Oulehleet al. (2011) who identified soil acidification caused by high S deposi-tion as an inhibitor of soil organic matter decomposition at Načetín, asite within the Black Triangle 15 km from JEZ. At the same time, the rel-atively high Hg/C and low C/S soil ratios at sites LIZ and LES could indi-cate retention of Hg by the precipitation of HgS-like phases mentionedearlier in this section.

An interesting example of contrasting sites arises from comparisonof alkaline PLB with nearby acidic LYS with similar deposition histories.The Hg/DOC ratio in stream water at PLB was significantly higher thanthat at LYS, while the soil Hg/C ratio was lower and soil C/S higher atPLB than at LYS (Fig. 5c,d). This pattern could be explained by favoredformation of more stable HgS-like phases under low pH andmore solu-bleHg–S complexes under high pH (Ravichandran et al., 1999). Alterna-tively, soils derived from the alkaline serpentinite bedrock at PLBmightinhibit organic matter decomposition. Soil organic matter decomposi-tion is a complex process determined by the interplay of soil pH, soiltexture, temperature and moisture conditions, litter quality, and otherfactors. Sites with the most favorable soil conditions for organic matterdecomposition (LIZ and LES) had the highestHg/C and lowest C/S ratios,whereas sites affected by unfavorable soil chemistry (PLB serpentinesoils) and by historical high soil acidification (LYS and JEZ) had lessdecomposed soil organic matter as revealed by lower soil Hg/C andhigher soil C/S ratios.

Wehypothesize that Hg immobilization inmineral soil ismore effec-tive at sites with high Hg/C and low C/S. Therefore, streamHg/DOCwasinversely related to the soil Hg/C ratio (Fig. 5a). Nevertheless recent

) C/S ratio at individual sites, (c) relationship of organic andmineral soil pool Hg/C and C/Sl sites, data on soil pools in panels c and d originate from Navrátil et al. (2014).

210 T. Navrátil et al. / Journal of Geochemical Exploration 158 (2015) 201–211

studies show that character of nanocolloidal HgS-like compounds possi-bly precipitating in soils is strongly affected by presence and propertiesof DOC (Gerbig et al., 2011a). This cross site synthesis suggests possiblesignificant role of soil organic matter dynamics, mineral precipitationreactions and mercury/sulfur deposition histories on differencesbetween site specific retention/release of Hg by forest soils.

5. Conclusions

The Czech Republic experienced a period of heavy industrializationin the 20th century, fueled by the combustion of high-sulfur coal. Theresulting high S deposition led to forest decline and even forest deathin some areas. We studied stream water Hg concentrations and fluxesat five sites along a gradient of industrial impact, from the heart of theBlack Triangle in the northwest to the relatively unimpacted BohemianForest to the south. This gradient of S depositionwas also assumed to re-flect a gradient of Hg deposition because of the close association of Hgand S in emissions from coal combustion. This assumptionwas support-ed by published analyses of Hg in peat cores.

Our hypothesis was that streamHg concentrations and fluxeswouldalign strongly with the gradient in Hg deposition. However, Hg deposi-tion history appeared to be only one of several factors affecting currentstream Hg dynamics. Two of the sites within the Black Triangle had fil-tered Hg concentrations and output fluxes that are among the highestreported in the literature. However, Hg concentrations at the most in-dustrialized site were near typical background levels for forestedstreams andwere indistinguishable from those at the lowest depositionsite.

As found in most stream Hg studies, Hg was strongly correlated tostreamDOC concentrations. However, the effectiveness of DOC inmobi-lizing Hg varied greatly among sites. We attribute these differences tovariations in DOC quality, in particular its aromaticity as expressed bySUVA. The most industrialized site had low DOC as well as the lowestSUVA of all sites, thus low capacity to mobilize stored Hg. The twohigh-Hg sites had the highest DOC as well as the highest SUVA, thusHg was readily mobilized. These two sites had similar DOC but theone site with significantly higher SUVA had threefold higher Hg,highlighting the importance of DOC quality to Hg transport.

In summary, the historical pattern of Hgdeposition is only one factorinfluencing current Hg mobilization from forest soils. Hg export wasprimarily linked to the quantity and quality of DOC export. The streamHg/DOC ratio is also linked to Hg/C and C/S stoichiometric ratios insoils. Stream Hg/DOC ratios were inversely related to soil Hg/C ratios,suggesting that catchments with the highest Hg/C soil ratio have thegreatest capacity to immobilize Hg.

Acknowledgments

The main financial support for this research has been provided bythe Czech Science Foundation Project No. P210-11-1369. The long-term research program at Lesní Potok catchment was provided by theinstitutional project RVO67985831 and the long-term GEOMON net-work was supported by the research plan of the Czech Geological Sur-vey (MZP0002579801). We are very thankful to Světlana Hubičkováand Irena Dobešová for sample processing and laboratory treatment.We appreciate the hard work of field observers and technicians OldřichMyška, Václav Kmínek, Jiří Václavek, Jan Čuřík and Vladimír Bláha. Theuse of trade names is for informational purposes only and does notimply endorsement by the U.S. Geological Survey.

Appendix A. Supplementary data

Supplementary data associated with this article can be found in theonline version, at http://dx.doi.org/10.1016/j.gexplo.2015.07.016.These data include the Google map of the most important areasdescribed in this article.

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