Journal of Geochemical Exploration 149 (2015) 97–105

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Journal of Geochemical Exploration

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Contaminated sediment loads from ancient mercury ore roasting sites,Idrija area, Slovenia

Mateja Gosar, Tamara Teršič ⁎Geological Survey of Slovenia, Dimičeva ulica 14, SI-1000 Ljubljana, Slovenia

⁎ Corresponding author. Tel.: +386 1 2809 760.E-mail address: tamara.tersic@geo-zs.si (T. Teršič).

http://dx.doi.org/10.1016/j.gexplo.2014.11.0120375-6742/© 2014 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 19 June 2014Accepted 26 November 2014Available online 4 December 2014

Keywords:Mercury contaminationOre roastingStream sedimentIdrija Hg mine

The distribution and transport of Hg and other elements were studied in aquatic systems draining contaminatedancient Hg ore roasting sites Pšenk and Frbejžene trate in Idrija surroundings. Hg concentrations in sediments atthe studied roasting site areas were found extremely high, ranging from 18 to 1240 mg/kg (average 387 mg/kg)in the b0.063 mm grain-size fraction, and from 48 to 1080 (average 456 mg/kg) in the b0.125mm fraction. Fur-ther downstream along the Padarjeva grapa Stream, which drains both contaminated areas, Hg concentrationswere 330–2040 (average 721 mg/kg; for b0.125 mm) and 380–840 (average 531 mg/kg; for b0.04 mm). In-creased Hg concentrations were found in the Idrijca River sediments downstream of the Padarjeva grapa Streamconfluence. The results of our investigation have proved that Hg-loadedmaterials are not only still present at theroasting sites, but are intensively eroded and transported downstream during high waters. Consequently theyhave significant influence on the Idrijca River sediment contamination. Delivery of Hg contaminants from histor-ically contaminated roasting sites via Padarjeva grapa Streamduring heavy rainfalls wasmost intense during op-eration of the roasting sites, but remains an ongoing issue.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

During the past centuries, anthropogenic activities have altered thenatural distribution of mercury (Hg) in Idrija environments where Hgmining and ore processing activities lasted for nearly 500 years, until1995. The only outcrops of Hg-ore-containing rocks are situated alongthe Nikova River in the western part of Idrija City (Čar, 1998). Thereare no natural sources of Hg upstream of the city.

Aquatic sediments of the Idrija area were investigated by many au-thors to determine the burden of Hg caused by mining and ore process-ing activities. Many studies identified high Hg loads in the Idrijca andSoča sediments (Bavec at al., 2014; Biester et al., 2000; Gosar, 2008;Gosar et al., 1997; Hines et al., 2006; Horvat et al., 2002; Kocman et al.,2011). Besides the expected elevated Hg content in sediments down-stream of Idrija, increased Hg concentrations were also measured up-stream (Gosar, 2008; Gosar and Žibret, 2011). At first there was noexplanation for these increased concentrations; however, detailedstudy of the different ore roasting techniques used in the first 150years of Hg production (Čar and Terpin, 2005) revealed 21 locations ofancient roasting sites in the woods around Idrija. The significance ofthese sites for Hg load in soils and sediments was as anticipated. From2006 onwards, intensive studies were performed at the historicalroasting sites Pšenk and Frbejžene trate to define the environmental im-pacts. It was estimated that about 1.4 t of Hg are still present at the

Pšenk locality, and about 40 t of Hg at all established roasting sites(Gosar and Čar, 2006). At both sites, extremely high Hg concentrationswere identified in soils. The determined Hg medians at Pšenk were20 mg/kg (max. 4200 mg/kg) in SOM (soil organic matter) and 68(max. 8600) mg/kg in top soil (0–15 cm). At the Frbejžene trate theHg medians were 96 (max 37,000) mg/kg in SOM and 370 (max20,000) mg/kg in top soil (Teršič, 2010a; Teršič et al., 2011a,b). In bothareas, Hg binding forms were also investigated using the solid phasethermo desorption technique. Two different releases of Hg were deter-mined in fresh soil and SOM samples: fromHg bound to the soil-matrixcomponents and from cinnabar. Cinnabar and non-cinnabar Hg com-pounds were almost equally distributed in most of the SOM and soilsamples (Teršič et al., 2011a,b). Total water-soluble Hg at those roastingsites was determined by Teršič et al. (2014). The total water-soluble Hgwas further separated to easily reducible (reactive) soluble Hg and com-plex bound (non-reactive) Hg. Total leachable Hg in the soil samplesranged between 16 and 18,000 μg/kg, representing 0.0002 to 1.1% ofthe aqua regia soluble Hg. In SOM samples leachable Hg concentrationsranged between 13 and 6000 μg/kg, representing 0.0017–0.074% of theaqua regia soluble Hg. In the soil profiles investigated at the Pšenklocality, Hg total concentrations gradually decreased with depth,although elevated Hg concentrations (N100 mg/kg) were measuredeven in the deepest layers (75–95 cm). Leachable Hg was the highestin the 30–50 cm soil layer (up to 18,000 μg/kg, 1.7% of the aqua regiasoluble Hg). The concentrations sharply decreased in the bottom layer(75–95 cm, 730 μg/kg, 0.52% of the aqua regia soluble Hg). On anaverage, more than 90% of the leachable Hg occurred as non-reactive

98 M. Gosar, T. Teršič / Journal of Geochemical Exploration 149 (2015) 97–105

complex bound Hg. The portion of reactive, easily reducible Hg wastherefore much lower, and varied between 0.03 and 8.6%. It was esti-mated that there is still about 10 kg of soluble Hg stored in soils of theinvestigated roasting sites. This Hg continuously leaches into surfacewaters and deeper into the soil.

The sediment load transported from the extremely contaminatedold roasting sites Pšenk and Frbejžene trate has not received consider-able attention in past studies of Hg contamination of the Idrija area.Accordingly, the aims of this study were to estimate past, present andfuture influence of ancient, highly contaminated Hg ore roasting siteson the Hg loads in the stream sediments in the upper Idrijca River Valleyand further downstream, and to estimate the risk potential of the Hgstored in sediments.

1.1. Site description

The roasting site areas Pšenk and Frbejžene trate are among the larg-est sites of Hg ore roasting, and many roasting-vessel fragments can befound there. The sites are located about 2 km south-west from Idrija.Pšenk lies on flattened surface along the Lačna voda Stream, above itsconfluence with the Padarjeva grapa Stream (Fig. 1). Single potteryfragments can be found along the entire gravelly bed of the Lačnavoda Stream. Frbejžene trate lies along two streams that are dry mostof the time. At the margins of the roasting site, the streams combineinto Frbejžene Stream, which flows into the Padarjeva grapa Stream.In the area between streams (~30 m wide), many fragments of brokenpottery can be found. In the natural banks of the streams, they occuras much as 1 m deep. In places a layer of black, organic-matter-richsoil, up to 45 cm thick, mixed with pieces of charcoal, was uncoveredin the stream-bank where numerous pottery fragments can also be

Fig. 1. Study area with stream-se

seen. This contaminated material is partly transported by the Padarjevagrapa Stream,whichflows into the Idrijca River about 2 kmupstreamofIdrija town (Fig. 1). Single fragments of roasting vessels can also befound along the Padarjeva grapa Stream.

1.2. Sampling

Stream sediment sampling at the roasting site localities Pšenk andFrbejžene trate was performed in November 2008. The weather wasdry and cloudy, and the streams were mostly dry. A total of five streamsediment sampleswere collected. At Pšenk, two sediment sampleswerecollected from the Lačna voda Stream. Sample S1 was taken upstream,and sample S2 about 100 m downstream of the roasting site area.Three sediment samples were collected at the Frbejžene trate. SamplesS3 and S4were taken above the roasting site area, one sample fromeachof the streams. Sample S5 was collected about 50 m below the conflu-ence of the two streams (Fig. 1).

Stream sediment samples from the Padarjeva grapa Stream werecollected in May 2009 at low water. Along the stream bed, from belowthe confluence of the Frbejžene Stream to the Idrijca River, 10 sampleswere collected (PG01–PG10). From the Idrijca River, two sedimentsamples were collected: PG11 approximately 150 m upstream andPG12 approximately 80 m downstream of the Padarjeva grapa conflu-ence (Fig. 1). At each sampling location, a composite samplewas collect-ed from at least three locations within 10 m distance.

1.3. Sample preparation

Approximately 1 kg of sediment was collected at each site with ascoop and stored in polyethylene bags. The samples were oven-dried

diment sampling locations.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

PG01 PG02 PG03 PG04 PG05 PG06 PG07 PG08 PG09 PG10 PG11 PG12

> 2 mm

0.63-2mm

0.2-0.63mm

0.063-0.2mm

< 0.063mm

Fig. 2. Grain-size distribution in sediments from the Padarjeva grapa Stream (PG01 toPG10) and from the Idrijca River (PG11 and PG12).

99M. Gosar, T. Teršič / Journal of Geochemical Exploration 149 (2015) 97–105

below 35 °C to prevent excessive evaporation of Hg. Dry sieving wasperformed to determine the particle-size distribution according tostandard (EN ISO, 14688-1:2002). Two fractions were prepared formulti-elemental concentration analysis (b0.125 and b0.063 mm forsamples from roasting sites, and b0.125 and b0.04 mm for samplesfrom Padarjeva grapa Stream).

1.4. Multi-elemental analysis

The powdered sediment samples were submitted for chemical anal-ysis to ACME Analytical Laboratories in Vancouver (Canada) accreditedunder ISO 9001:2008. A total of 36 elements (Ag, Al, As, Au, B, Ba, Bi, Ca,Cd, Co, Cr, Cu, Fe, Ga, Hg, K, La, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Sc, Se, Sr,Th, Ti, Tl, U, V, W, and Zn) were analyzed with inductively coupledplasmamass spectrometry (ICP-MS) after aqua regia digestion. Samplesexceeding the upper detection limit for Hg (100 mg/kg) were analyzedusing ICP emission spectrometry (ICP-ES).

The precision of the analytical method was controlled by analysis ofduplicate samples. The shipment of samples (n = 34) and replicates(n= 4) to the laboratorywas carried out in a random succession to dis-tribute evenly any errors due to laboratory performance. This procedureensured an unbiased treatment of samples and a random distribution ofpossible drift of analytical conditions over all of the randomly selectedneutral samples. Objectivity was assured by using neutral laboratorynumbers. The accuracy of the analytical method was verified againststandard materials (Acme internal standard DS7, OREAS45PA andR4A). The average percent recovery was between 85 and 125%; thehighest (125%) was obtained for Cr. The precision, measured as relativepercentage difference (RPD), was b10% for most of the elements, onlyforMo (17.14%) and As (11.84%)was the RPD slightly higher. Accordingto the results obtained, the quality of the analytical determinationwas considered acceptable and adequate for usage in further statisticalanalyses.

The detection limits were 1 mg/kg for Ba, Cr, Ga, La, Mn, Sr and Zn;0.5 mg/kg for As and Se; 0.1 mg/kg for Ag, Bi, Cd, Co, Cu, Mo, Ni, Pb,Sb, Sc, Th, Tl, U, and W; 0.01 mg/kg for Hg; 0.01% for Al, Ca, Fe, K andMg; 0.05% for S; 0.001% for Na and P; 2 mg/kg for V; 20 mg/kg for B;and 0.5 μg/kg for Au.

Preparation of the raw data proceeded by excluding elements forwhich more than 30% of samples containing these elements were re-ported below the detection limit. For other elements at concentrationsbelow the detection limit, their concentrations were assumed to be50% of the detection limit. In total, the data for 23 elements wereretained and used for the presentation of results.

2. Results and discussion

2.1. Grain size distribution

Basic descriptive statistics of particle size distribution are presentedas percentages of different fractions in Fig. 2. Themajority (65 to 90%) ofparticles made up sand-size fractions and the rest were gravel (2.3 to33%) and silt + clay particles (1 to 8%). Sand was further distributedin particles of coarse,medium, and fine sizes, wheremedium and coarsesand was more common than fine sand.

2.2. Elemental concentrations

Basic statistics for the elemental content parameters in the studiedstream sediments are presented in Table 1. The average concentrationsobtainedwere compared to themedian values for European stream andoverbank sediments (Salminen et al., 2005), and with the action valuesin the New Dutch List (MHSPE, 2013). Elemental concentrations in thestudied sediments were generally similar to the averages determinedfor European stream- and overbank sediment (Table 1). The contentsof metals were far below the action values defined in the New Dutch

List with the exception of Hg. The Hg concentration highly exceededtheNewDutch List action value (10mg/kg), and is highly enriched com-pared to European median values in stream and overbank sediments.Concentrations of total Hg in the studied sediments are much higherthan the uncontaminated worldwide baseline (0.18 mg/kg; Turekianand Wedepohl, 1961), average stream sediment for Europe(0.038 mg/kg; Salminen et al., 2005), average stream (0.06 mg/kg;Sotlar, 1995) and overbank (0.09 mg/kg; Bidovec et al.,1998) sedimentfor Slovenia. It is also higher than the estimated geochemical back-ground for Hg in the Idrijca River stream sediments (2 mg/kg; Gosaret al., 1997) and overbank sediment (0.75 mg/kg; Gosar and Žibret,2011).

The arsenic content was also slightly elevated in sediments adjacentto the studied roasting site areas. There it ranged from 4.2 to 19 mg/kg(average 9.1 mg/kg) and from 4.5 to 14 (average 8.5 mg/kg) forthe b0.125 mm and b0.063 mm grain-size fractions, respectively(Table 1). Similar concentrations were also measured in the Padarjevagrapa sediments;while in the Idrijca River sedimentswere lower (aver-age 3.4 mg/kg). Elevated As concentrations in the soil of the Idrija sur-roundings (90 km2) had already been reported (Gosar and Šajn,2005). No correlation between As and Hg, as a result of long-lastingHg production in Idrija, was found (Gosar and Šajn, 2005). In anotherstudy (Teršič, 2010b) high concentrations of As in soils from theroasting site Pšenk were reported, reaching as much as 122 mg/kgat the depth of 5–20 cm (average 44.3 mg/kg). It is interesting thatthe spatial distribution of As in the soil (5–20 cm) from the studiedroasting site shows a pattern opposite to that of the Hg distribution:the highest concentration appeared in the northern and southernparts of the investigated area, while the lowest values occurred in thecentral part (Teršič, 2010b). It was assessed that, at the investigatedroasting sites, the As concentrations appeared to be higher than inother Idrija areas. To establish firmly the connection between Hg andAs at the roasting sites, further investigation is required.

2.3. Hg in sediments from roasting sites

The Hg concentrations in sample S1 were 360 mg/kg in theb0.125 mm fraction and 220 mg/kg in the b0.063 mm fraction. InS2, the contents were 730 mg/kg in the b0.125 mm fraction, and400 mg/kg in the b0.063 mm fraction (Fig. 3). In both sediment sam-ples from the Pšenk roasting site, the Hg concentration was higher inthe coarser fraction. The influence of past roasting activities can be seenin the enriched Hg downstream of the roasting site (sample S2), wherethe determined Hg concentration was approximately twice as much asin sample S1. Sample S1 is also not free of Hg contamination. This isprobably due to the washing out of material from another roasting

Table 1Basic statistics of elemental contents in stream sediments from the Pšenk and Frbejžene trate roasting sites, in sediments from Padarjeva grapa Stream and Idrijca River, and comparison to the European averages for overbank- and stream-sediments(Salminen et al., 2005) and to the action values for sediments regarding the New Dutch List (MHSPE, 2013).

Element Unit Sediments from roasting site areas Pšenk and Frbejžene trate (n= 5)

Sediments from Padarjeva grapa Stream (n = 10) Idrijca abovethe confluence(PG11)

Idrijca belowthe confluence(PG12)

Europe (Salminenet al, 2005)

Action valuesaccording to theNew Dutch List(MHSPE, 2013)

Fraction (mm)

b0.125 b0.063 b0.125 b0.04 b0.125 b0.04 b0.125 b0.04 Overbanksedimentb0.063 mm

Streamsedimentb0.15 mm

Min Max Arithmeticmean

Min Max Arithmeticmean

Min Max Arithmeticmean

Min Max Arithmeticmean

Al % 0.47 1.6 0.86 0.6 1.3 0.93 0.33 0.75 0.51 0.32 0.83 0.61 0.73 0.79 0.66 0.88 5.51⁎ 5.46⁎As mg/kg 4.2 19 9.1 4.5 14 8.5 / / / 3.7 12 7.9 1.7 2 5.5 4.5 6 6Ba mg/kg 37 46 41 35 48 44 22 52 31 27 48 37 80 76 36 46 82 86 625Ca % 7.7 12 9.7 6.9 12 9.2 8.1 11.5 9.9 6.3 11.5 9.2 5.7 4.4 11 8.2 1.48⁎ 1.66⁎Co mg/kg 2.2 8 4.3 2.4 5.7 4.1 0.05 2.8 1.3 0.05 7.2 2.3 3.2 3.6 6.5 7.5 8 8 240Cr mg/kg 4 19 8 6 18 10 0.5 13 4.2 0.5 21 7.9 3 6 15 19 23 21 380Cu mg/kg 3.4 12 7.9 4.5 13 8.5 0.05 7.1 2.5 0.05 17 5.9 3.8 5.5 7.6 11 16 14 190Fe % 0.83 1.7 1.2 0.84 1.34 1.1 0.37 1.1 0.78 0.36 1.71 0.88 1.3 1.2 1.3 1.4 1.95 1.97Ga mg/kg 1 5 2.4 2 4 2.8 0.5 2 1.3 0.5 3 1.8 2 2 2 2 11⁎ 12⁎Hg mg/kg 48 1080 456 18 1240 387 330 2040 721 380 840 531 27 2.3 14 9.5 0.045⁎ 0.038⁎ 10K % 0.08 0.25 0.15 0.1 0.22 0.17 0.005 0.13 0.078 0.02 0.13 0.082 0.13 0.13 0.07 0.08 1.66⁎ 1.67⁎La mg/kg 14 24 19 14 24 17 7 19 15 8 18 14 29 23 13 13 24.9⁎ 26.7⁎Mg % 4.5 7.9 6 4.2 7 5.6 4.7 6.7 5.6 4.4 6.6 5.4 2.95 2.2 4.9 3.4 0.72⁎ 0.72⁎Mn mg/kg 181 642 371 187 514 348 16 418 252 0.5 412 261 138 148 406 448 446 452Mo mg/kg 0.6 3.7 1.8 1 3.9 1.9 0.05 1.6 1 0.05 1.8 1.1 0.3 0.4 1.4 1.2 0.62⁎ 0.63⁎Na % 0.007 0.011 0.01 0.006 0.012 0.008 0.0005 0.107 0.031 0.007 0.21 0.072 0.009 0.008 0.007 0.007 0.59⁎ 0.67⁎Pb mg/kg 15 34 23 19 27 23 0.05 10 5.8 0.05 33 16 21 26 19 30 16 14 530P % 0.012 0.042 0.02 0.017 0.036 0.026 0.008 0.028 0.016 0.006 0.043 0.023 0.009 0.012 0.023 0.032 0.048⁎ 0.076⁎Sc mg/kg 1 2.1 1.6 1.2 2.2 1.7 0.05 1.3 0.59 0.05 2.2 0.83 2.1 2.3 1.4 1.7Sr mg/kg 49 54 51 46 56 51 15 45 39 30 61 46 40 43 57 55 131⁎ 126⁎Th mg/kg 2.5 4.1 3.2 2.2 4.2 3.2 0.05 4.1 2.4 0.05 6.1 2.7 6.6 5.5 2.8 2.5 8 10U mg/kg 1 2.6 1.5 1.2 2.4 1.6 0.05 1.1 0.72 0.05 2.5 1.2 1.1 1 1.2 1 2⁎ 2⁎Zn mg/kg 26 56 38 31 51 42 0.5 25 15 0.5 45 20 26 31 29 45 56 60 720

⁎ No data for aqua regia digestion.

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149(2015)

97–105

0

500

1000

1500

S1 S2 S3 S4 S5

Hg

(mg/

kg)

<0.125mm

<0.063mm

Fig. 3. Hg in Lačna voda sediments from the Pšenk roasting site (S1 and S2) and in sedi-ments from the Frbejžene trate roasting site (S3, S4, S5).

101M. Gosar, T. Teršič / Journal of Geochemical Exploration 149 (2015) 97–105

site upstream. A roasting site named Lačna voda is situated about 500mupstream (Čar and Terpin, 2005).

At the Frbejžene trate locality, elevated Hg concentrations were de-termined in all three samples investigated. Upstream of the roasting site,Hg concentrations in sample S3 were 63 mg/kg in the b0.125 mm frac-tion, and 57 mg/kg in the b0.063 mm fraction, while in sample S4 theywere 48 mg/kg in the coarser and 18 mg/kg in the finer fractions(Fig. 3). Hg is highly enriched downstream from the roasting area,where the Hg concentrations in S5 were 1080 (b0.125 mm) and1240 mg/kg (b0.063 mm). This is also the only sampling location whereHg concentration was higher in the finer fraction. Higher Hg concentra-tions in the coarser fraction indicate that finer particles were mostlytransported downstream during periodic rainfall while coarser particleswere able to linger at the same place for longer periods. Regarding thespatial distributions of Hg in the soil (Teršič et al., 2011b), sample S4 islocated in the area where the soils are most contaminated; therefore,we expected even higher Hg concentrations in sediments from this loca-tion. It is possible that the majority of mercury-bearing waste material(broken earthen vessels with HgS coatings and unroasted ore fragments)were dumped downstream from this sampling location.

2.4. Hg in Padarjeva grapa sediments

In the sediments from Padarjeva grapa Stream downstream of thestudied roasting localities, highly elevated Hg concentrations were

PG02

PG03

PG04

PG05

PG06

PG07

PG01

0

500

1000

1500

2000

0 200 400 600 800 1

Hg(

mg/

kg)

dista

Fig. 4. Hg in sediments from Padarjeva grapa Stream and Idrijca Rive

found (330–2040, average of 721 mg/kg in the b0.125 mm; 380–840,average of 531 mg/kg in the b0.04 mm fraction) (Table 1; Fig. 4). Inmost samples, there was only a small difference in Hg concentrationsbetween the analyzed fractions, with the exception of samplinglocations PG05 and especially PG06, where in the coarser fraction thedetermined Hg was 2-times and 5-times higher, respectively. In theb0.04 mm fraction, Hg concentrations were relatively constant overthe whole Padarjeva grapa Stream Valley, with the maximum(840mg/kg) determined just below the confluence of Frbejžene Stream(sample PG01; Fig. 4), about 50m downstreamof the sampling locationS5 (Fig. 1). Higher mean concentrations of Hg in the coarser fractionobserved in the studied stream sediments were mainly the conse-quence of highly elevated Hg concentrations in the samples PG05 andPG06 (b0.125 mm). These spikes are attributed to individual Hg-bearing particles in the analyzedmaterial. These highHg concentrationsare in accordance with reports of many authors indicating that coarserparticles of the sediment can be retained for a longer time at the samesite, often in shallow places of rivers and streams. For this reason, theywould be better to document the anthropogenic inflow of heavymetalsfrom point sources (Boszke et al., 2004; Salomons and Förstner, 1984).Coarser particles thus have longer on-site-time for development ofmer-cury adsorbing coatings on their surface (e.g., an organic layer and/or alayer of manganese and iron hydroxides), than would finer particles(Boszke et al., 2004). Enrichment of Hg in the coarser-grained fraction(b0.125 mm) rather than the finer one (b0.04 mm) was also observedin previous studies of the Idrijca River sediments (Biester et al., 2000;Gosar, 2008; Gosar et al., 1997).

2.5. Hg enrichment in the Idrijca River sediments

In the sediments of the Idrijca River, much lower but still elevated,Hg concentrationswere determined. In PG11 upstreamof the Padarjevagrapa 27 mg/kg in the b0.125 mm fraction, and 10-times lower(2 mg/kgHg) in the b0.04mm fractionwas determined, In PG12, down-stream of the Padarjeva grapa, there was less difference in the twoanalyzed fractions (14 mg/kg in the b0.125 mm, and 9 mg/kg in theb0.04 mm fraction) (Table 1). The 2 mg/kg Hg in the finer fraction ofPG11 is in accordance with the estimated geochemical background forHg in the Idrijca River (2mg/kg; Gosar et al., 1997). The elevatedHg con-centration in the coarser fraction of PG11 could be an outlier. It could alsoindicate that it is possible that the sources of Hg contaminated materialmight exist upstream of the Padarjeva grapa Stream confluence, andthat this material is also being eroded and transported downstream.

PG08PG09

PG10

PG11 PG12

000 1200 1400 1600 1800 2000

nce (m)

<0.125mm

<0.04mm

r upstream (PG11) and downstream (PG12) of Padarjeva-grapa.

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Better insight into the Hg concentrations in the upper Idrijca Valleysediments is given by Gosar (2008) because she has monitored Hg con-centration in the stream sediments of the Idrijca River systematicallyevery five years since 1991. Hg concentrationsmeasured at the locationI-2 upstream of the Padarjeva grapa (Fig. 1) varied from 2.0 to 4.7 withthe average of 3.5mg/kg in the b0.04 fraction, and from 1.9 to 10.2 withthe average of 5.7 mg/kg (Table 2; data after Gosar, 2008 and unpub-lished for 2010). In our sampling campaign, sediment samples fromthe Idrijca River were only taken once at two locations, one upstreamand the other one downstream of the Padarjeva grapa confluence.These only showed Hg concentrations under the conditions existing atthe time of sampling. Data published in Gosar (2008) and unpublishedfor 2010 (analyzed using the same methodology as in Gosar, 2008)(Table 2) certainly better reflect overall conditions, because they giveaverages throughmany years and evidently show that the high Hg con-centrations have not decreased considerably over this interval. The data(Table 2) show low concentrations upstream of the Padarjeva grapaStream confluence, and elevated Hg concentrations at the location I-3downstream of the Padarjeva grapa and upstream of Idrija and themain source of the pollution (Fig. 1). At I-3, Hg concentrations variedfrom 5.4 to 245 with the average of 74.2 mg/kg in the b0.125 mm frac-tion, and from 4.8 to 62.3 with the average of 25.1 mg/kg in theb0.04 mm fraction. In the coarser fraction, great variability in the mea-sured Hg concentration can be observed in different years. The greatestincrease at the I-3 location could be seen from 2001 to 2005, and thegreatest decline from2005 to 2010, while at the I-2 location the concen-trations were very constant through the years. The increase in Hgconcentration at I-3 compared to I-2 is most probably related to the in-fluence of historical roasting sites, from which the Hg-contaminatedmaterial is being eroded, and transported down the Padarjeva grapaduring periodic heavy rainfall. The impact of Hg-loaded sedimentsfrom the Padarjeva grapa on Hg concentrations in the Idrijca River isshown in Fig. 5. Padarjeva grapa Stream sediments are undoubtedlythe source of the Hg-enriched material. The variability of Hg at I-3could be partly attributed to the hydrological conditions; the torrentialnature of the Padarjeva grapa Stream and of the Idrijca River, meansthat a great quantity of polluted sediment is being transported down-stream during periods of higher discharge. In contrast, the constant,although slightly elevated Hg concentrations, at the I-2 location proba-bly mean that there is no great input of Hg-contaminated material up-stream of this location. Slightly increased Hg values upstream of thePadarjeva grapa have already been observed in floodplain sedimentsinvestigated by Gosar and Žibret (2011), and were explained as the in-fluence of some ancient ore roasting sites. The results of our study clear-ly show the significance of the input of Hg-enriched material fromthe Padarjeva grapa Stream, indicating that the highly contaminated

Table 2Hg concentrations in sediments fromsampling locations I-2 and I-3 (data frommonitoringin 5-year intervals from 1991 until 2010 (one determination/year); data after Gosar, 2008and unpublished for 2010).

Year Sampling location

I-2(Idrijca upstreamPadarjeva grapa)

I-3(Idrijca downstreamPadarjeva grapa)

Fraction (mm) Fraction (mm)

b0.125 b0.04 b0.125 b0.04

1991 1.9 3.6 5.4 18.51995 8.1 2.0 91.0 25.02001 10.2 2.9 19.6 14.92005 4.5 4.2 245.0 62.32010 3.9 4.7 9.9 4.8Min 1.9 2.0 5.4 4.8

Max 10.2 4.7 245.0 62.3Arithmetic mean 5.7 3.5 74.2 25.1

roasting sites studied are constantly contributing to the stream sedi-ment load of Hg.

The studied stream sediment samples collected at the Pšenk andFrbejžene trate roasting sites, and from the Padarjeva grapa, containedup to 2000 mg/kg Hg. These high concentrations are probably dueprimarily to the presence of the mercury sulfide, HgS, either cinnabaror meta-cinnabar, both highly resistant to physical and chemicalweathering. Broken pottery fragments with red and/or black coatingsare clearly visible in stream detritus at and below the roasting sites,and along the whole Padarjeva grapa Stream bed as well. The highlyelevated Hg concentrations determined in the sediment samples arethe consequence of human activities in the past, and indicate that alarge amount of Hg-enriched material remains at the historical roastingsites, as reported by Teršič et al. (2011a,b). Thismaterial is consequentlybeing eroded and transported downstream. In periods of low to meanwater discharge, Hg-bearing particles concentrate in the streambedsediments. Most of the time no or very little water can be observed inthe streambeds at the studied locations; however, during occasionalstorms or longer periods of rainfall, significant rises in water level canoccur and rapidly wash exposed material downstream to the Padarjevagrapa Stream and further to the Idrijca River.

The influences of ore-processing in the woods from the very begin-ning of Hgmining in Idrija, are recorded also in the floodplain sedimentsupstream of Idrija. In the upper Idrijca Valley, upstream of Idrija town,three floodplain profiles were sampled (Gosar and Žibret, 2011). Theywere mostly sampled upstream from the main contamination sources(Idrija town area), but the most interesting profile (ZAG) was sampleddownstream of the Padarjeva grapa Stream confluence. The Hg valuesexceeded the expected background values (Fig. 6, data from Gosar andŽibret, 2011). In the upper 100 cm of the profile, Hg content varied be-tween 5 and 20 mg/kg, but at a depth of 130 cm, the Hg concentrationwas above 40 mg/kg and reached its maximum (70.5 mg/kg) at160 cm (Fig. 6). This increase was explained by Gosar and Žibret(2011) as an influence of small ore roasting sites operating in the 16thand 17th centuries, when these sites were active. Above this level(from 0 to 100 cm), Hg content can be regarded as a consequence ofthe erosion of contaminated material from ore roasting localities afterthey were abandoned, and possibly also from some atmosphericsources (Gosar and Žibret, 2011). This explanation can be confirmedby the data gathered in our present investigation.

We assume that in the 16th and 17th centuries, when roasting of Hgore was performed, probably high amounts of Hg-rich material werereleased to the stream, due to the high ore grade and the low efficiencyof the roasting process, resulting in high Hg concentrations in thesediments. During higher stream discharges, this material was carrieddownstreamby the river, partly deposited on thefloodplains of the Idrijcaand Soča River valleys (Gosar, 2008; Gosar and Žibret, 2011), and partlytransported further towards the Gulf of Trieste. The deposition of Hgdownstream of the source could result in the remobilization of Hg thatis available for methylation. Hence, secondary sources of toxic forms ofHg can be produced at great distances from the original source (Hineset al., 2000). It was shown in several studies that the Soča River is themost important source of dissolved and total Hg in the Gulf of Trieste(Covelli et al., 2006, 2007; Faganeli et al., 2003; Hines et al., 2000).

With the present study, we have also proved that high Hg loadingsare still present in the sediments collected from streams draining runofffrom the two most important historical roasting sites (Pšenk andFrbejžene trate). The strong influence of other identified ancientroasting sites on stream-sediment-Hg contamination of the IdrijcaRiver drainage system was noticed also by Bavec et al. (2014), who in-vestigated road-sediment and stream-sediment contamination in theurban area of Idrija City.

Regarding Hg concentrations in the Padarjeva grapa Stream sedi-ments, and fromothermining contaminated areas (Table 3), the studiedsediments are highly loaded with Hg. The maximum determined Hgconcentration is close to those of the most contaminated sediments

Fig. 5. Box plot of Hg concentrations in Idrijca sediments upstream of Padarjeva grapa Stream (I-2), in Padarjeva grapa Stream sediments (PG), and in Idrijca sediments downstream ofPadarjeva grapa Stream (I-3).

103M. Gosar, T. Teršič / Journal of Geochemical Exploration 149 (2015) 97–105

reported (16,000 mg/kg at the Almadén district, Higueras et al., 2006).This is despite the fact that about 400 years have already passed sincethe time of roasting mercury-ore at the studied localities, and that

Fig. 6. Hg concentrations in the overbank sediment profile (ZA

probably large amounts of mercury-rich waste material, including con-taminated soils and sediments, have already been transported and de-posited downstream.

G) upstream of Idrija (data after Gosar and Žibret, 2011).

Table 3Hg concentrations in stream and river sediments collected from mining-contaminated areas.

Locality n Hg (mg/kg) Reference

Pšenk Above roasting site 1 360 This studyDownstream from roasting site 1 730

Frbejžene trate Above roasting site 2 48–63 This studyDownstream from roasting site 1 1080

Padarjeva grapa Downstream from historical roasting sites 10 330–2040 This studyIdrija Above mine 16 0.9–10.2 Gosar (2008)

Below mine 40 32–4121Almadén district, Spain 6061 0.5–16,000 Higueras et al. (2006)Azogado Creek, Almadén, Spain Downstream from mine 770–2300 Gray et al. (2004)Humboldt River, Nevada, USA 29 0.008–170 Gray et al. (2002)Carson River, Nevada, USA Above mine 0.03–6.1 Gray et al. (2002)

Below mine 0.05–881Punitaqui Cu–Au–Hg mining district (Chile) 0.3–5.3 Higueras et al. (2004)Kuskokwin River, South-west Alaska Above mine 0.02–0.78 Gray et al. (2000)

Below mine 0.9–5500Mercury mine, Palawan, Philippines Downstream from mines 3.7–15 Gray et al. (2003)

Mine-pit lake 6.9–400Local background value Idrija 2 Gosar et al. (1997)Average stream sediment Slovenia 0.09 Bidovec et al., 1998Average overbank sediment Slovenia 0.06 Sotlar, 1995Stream sediment Europe 0.038 Salminen et al. (2005)Average shale World 0.18 Turekian and Wedepohl (1961)

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3. Conclusions

Stream sediment Hg contamination was studied in an area affectedby historical Hg ore roasting. Highly elevated Hg concentrations weredetermined at the ancient roasting site areas, and in the Padarjevagrapa Stream, which drains the investigated roasting sites and flowsinto the Idrijca River. Investigation of the Hg load in the Idrijca Riversediments evidently shows the influence of the old roasting sites.Moreover, the concentrations downstream from the confluence of thePadarjeva grapa Stream are much higher than upstream, indicatingthat Hg-contaminated material is being eroded and transported downthe Padarjeva grapa during periodic heavy rainfall and into the IdrijcaRiver. We assume that at the time of roasting in the 16th and 17th cen-turies even larger amounts of Hg-enriched material were constantlywashed away. This was partly deposited on the Idrijca River floodplainand partly transported further to the Soča River. We should emphasizethat despitemore than 350 years have already passed since ore roastingin the surroundings of Idrija, the ancient roasting sites still remain animportant source of Hg contaminated material and one of the primaryconcerns for persistent Hg release into the aquatic ecosystem, especiallybecause the area is mostly karstic.

Acknowledgments

The present study was financially supported by the Slovenian Re-search Agency (ARRS) in the framework of research programGroundwa-ter and Geochemistry (P1-0020), which is performed by the GeologicalSurvey of Slovenia.

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