Mercury distribution in water, sediment and soil in the Idrijca and Socariver systems

M. Horvat1, V. Jereb1, V. Fajon1, M. Logar1, J. Kotnik1, J. Faganeli2, M. E. Hines3 &J.-C. Bonzongo4

1Department of Environmental Sciences, Institute Jožef Stefan Jamova 39, 1000 Ljubljana, Slovenia(e-mail: Milena.Horvat@ijs.si)

2Marine Biological Station, National Institute of Biology, Fornace 41, Piran, Slovenia3Department of Biological Sciences, University of Alaska Anchorage, Anchorage, AK 99508, USA

(Present address: Department of Biological Sciences, University of Massachusetts Lowell, One University Ave,Lowell, MA 01854, USA)

4Department of Environmental and Engineering Science, P.O. Box 116450, University of Florida, Gainesville,FL 32611-6540, USA

ABSTRACT: Although the production of mercury (Hg) at the Idrija mercurymine, Slovenia, stopped in 1994, the tailings and contaminated soils in themining region are continuously eroded and serve as a permanent source of Hgto the downstream rivers, flood plains and the Gulf of Trieste. The presentarticle describes measurements of total Hg and monomethylmercury (MeHg) invarious environmental compartments in the rivers Idrijca and Soca and the Gulfof Trieste during the period 1998–2000. Total Hg in the Idrijca river waterupstream of the mercury mine varied between 2.8 and 6.9 ng l�1 and increasedto 197 ng l�1 downstream of the mercury mine and remained elevated furtherdownstream along the rivers Idrijca and Soca (from c. 10 to 59 ng l�1), withlowest concentrations found in marine waters (from 0.2 to 2.0 ng l�1). Theconcentrations of total Hg were higher after the rain, which caused erosion andtransport of particles enriched with Hg. Concentrations of MeHg were quitevariable and did not follow the same trend as total Hg. The concentrations ofMeHg in August 1999 were significantly higher than in June 1998, indicating theimportance of seasonal effects, as well as hydrological conditions, on theproduction of MeHg. Above the mercury mine, MeHg was 0.1–0.2 ng l�1 anddoubled below the mercury mine. Downstream, in the rivers Idrijca and Soca,MeHg varied from 0.05 to 0.53 ng l�1 and in marine waters from 0.06 to0.08 ng l�1. Concentrations of Hg in sediments also increased downstreamseveral fold with a value of 5.39 µg g�1 above the mercury mine to a maximumof 727 µg g�1 below the mine in the silt and clay fraction. The coarser-grainedfraction (0.063 < d < 1.4 mm) frequently exceeded the value of total Hg in thesilt and clay fraction due to the presence of particles of cinnabar. Theconcentrations of MeHg in sediments did not follow the same trend as totalmercury. Highest concentrations of 14.1 and 10.0 ng g�1 were found down-stream of the peak total Hg concentration in sediments. In general, higher MeHgconcentrations were associated with the silt and clay fraction, and accounted for0.001–0.144% of total Hg. Total Hg in flood plain surficial soils varied from 60to 150 µg g�1 with less than 0.01% of mercury as MeHg. The results confirmedthe fact that beside soils that are naturally elevated in Hg due to the high levelsof Hg in geological materials in the Idrija district, there are also Hg-ladenmaterials and tailings that are continuously eroded and deposited in flood plainsdownstream and serve as an important additional source of Hg to the rivers andthe Gulf of Trieste. Important transformation mechanisms take place duringfluvial transport, evidenced by the presence of MeHg in the various samplesanalyzed.

KEYWORDS: mercury, Idrija mine, sediment, soil, water, methylmercury

Geochemistry: Exploration, Environment, Analysis, Vol. 2 2002, pp. 287–296 1467-7873/02/$15.00 � 2002 AEG/Geological Society, London

INTRODUCTION

The geological structure of the Idrija area is complex. The Idrijamercury (Hg) deposit was discovered around 1490. It extendsNW–SE for 1500 m, is 300–600 m wide and 450 m thick. Intotal, 156 ore bodies were found, 15 in carboniferous shales and141 in Permian and Lower and Middle Triassic beds. Theprincipal ore mineral is cinnabar, but native mercury also occursin quite significant amounts. Among the total of 28 minerals inthe deposits, matacinnabarite occurs in traces and quite fre-quently the barren minerals pyrite, marcasite, dolomite, calcite,kaolinite, epsomite and melanterite also occur (C{ar 1996; Pirc2001). Mining activities, ore processing and disposal of oreresidues and mining waste have enhanced the dispersal of Hg inthe town of Idrija and its surroundings.

More than 5 Mt of Hg ore were mined and it is estimated thatthe average Hg recovery from the ore in the past 500 years wasonly 73% (Miklavcic 1999). In Idrija, a total of about 147 000 tof Hg were produced till 1977, which amounts to about 13%of the world mercury production until then. The remainder(c. 40 000 t of Hg) was dispersed into the environment. Mercuryproduction peaked in the 1970s. Exhausts from the smelterstack and ventilation of the mineshafts periodically caused highlevels of airborne Hg (up to several mg m�3). Most of theairborne Hg was redeposited in the Idrijca basin and causedelevated Hg levels in surface soil layers. Weathering of Hg-bearing bedrock, and the erosion of contaminated soils anddumps of ore residues and mining wastes, contaminate theIdrijca–Soca river system and ultimately the Gulf of Trieste withHg (Gosar et al. 1996; Palinkaš et al. 1995; Gnamuš & Horvat1999). Although mining operations in Idrija ceased in 1994 andthe mine is currently in a phase of gradual total shutdown, aconsiderable amount of Hg is still being transported fromcontaminated sites to the Gulf of Trieste (Horvat et al 1999;Biester et al. 2000; Hines et al. 2000). The information presentedin this article summarizes the results from the samplings inAugust 1999 and June 2000. The purpose was to determine Hgspecies in different environmental compartments (water, sedi-ment, soil) in the Idrijca–Soca–Gulf of Trieste system. Thestudy was directed towards better understanding of the fate ofHg; its transport, transformation, and remobilization; accumu-lation in the flood plains and its final output to the marineenvironment. These data complement a 1998 study of mercuryspeciation in riverine water samples (Hines et al. 2000).

EXPERIMENTAL

Sampling sites

The study area, location and names of sample sites are given inFigure 1. The river Idrijca flows through the mercury town ofIdrija located in western Slovenia and merges with the riverSoca about 40 km downstream from Idrija. The river Soca hasthree dams associated with hydroelectric power plants. Theupstream stretch of the river Soca is mountainous (Julian Alps,highest mountain 2863 m) and the middle stretch is mostly hilly.After crossing the Slovene–Italian border, the river flows overthe low fertile plains to the Gulf of Trieste. The river Idrijcajoins the Soca River in the middle stretch at the village of Mostna Soci. Both rivers have torrential characteristics. As themountains block air circulation from the northern Adriatic Seato the north, annual precipitation is high and ranges from 2400to 5200 mm. Severe erosion occurs due to the steep mountainslopes, especially in the upper stretches of both the rivers, andresults in high sediment transport. However, this sedimenttransport is greatly diminished by the three dams (Ajba, Plave

and Solkan at sampling sites 9, 11 and 13, respectively) used forhydroelectric power production. Before the river reaches thesea, the bed load is practically eliminated in the reservoirsbehind the dams, while transport of suspended sedimentremains high, especially during high water discharge. Thehydrology of the river Soca is well known in the cross-sectionat Solkan in Slovenia (station 13). At this point, the mean annualdischarge equals 94 m3 s�1 with monthly variations from 60 to70 m3 s�1 in February and August to 110 to 120 m3 s�1 in Mayand June, and up to 140 m3 s�1 in November. During theyear, there are typically two flow extremes: the longer springmaximum from March to June (due to snowmelt); and theshorter, but more intensive autumn maximum in October andNovember.

The hydrology of the Italian part of the watershed of theriver Soca (called Isonzo in Italy) is less known as thecontinuous measurements are not available downstream fromSolkan. It is estimated that the mean discharge of the river Socaat its mouth is about 170 m3 s�1 (Mosseti 1983). In extremesituations, such as in November 1997, the peak discharge at themouth was 2500 m3 s�1 (Rajar 2001).

Samples were first collected on 18 August 1999, 2 days afterheavy rain that occurs frequently during this time of the year.Water, sediment and soil samples were collected along the riversIdrijca and Soca and in the Gulf of Trieste. In the Idrijca–Socasystem, 14 water samples were collected starting with twosampling locations upstream of the mine and 12 sites down-stream. Water samples from the Soca were collected upstreamof the reservoirs (sites 9, 11 and 13) and downstream from thedams (sites 10, 12 and 14) to determine any effects of theimpoundments on Hg retention and speciation. Sedimentsamples were also collected at the same sites (except sites 2, 9,11 and 13) where available. Three soil samples from flood plains(sites 3, 5 and 7) were also collected. In the Gulf of Trieste,water and sediment samples were collected during March1999 to represent marine waters (Faganeli et al. 2001). Weincorporated these data in the report to compare riverine andmarine Hg analysis and speciation. Water and sediment sampleswere also collected from the Isonzo in Italy on 15 June 2000,although under different hydrometeorological conditions.Sampling sites 15 and 16 are located below the dams, and site17 is on the estuarine part of the river Isonzo. The hydrologicalconditions in August 1999 were typical of high water flow afterheavy rain events, with the total suspended solids (TSS)concentrations reaching 115 mg l�1. In June 2000, when waterdischarge was much lower, TSS was only up to 3 mg l�1.Samples were also collected from Lake Doberdob (site 18), akarstic lake influenced by the river Soca inflow. Except atsampling site 15, where smaller grain-sized sediment wasunavailable, water and sediment samples were taken at all foursites in Italy.

Sampling and sample preparation

Ultraclean protocols were employed during sampling andsample pre-treatment. Water samples were collected by hand atthe surface in 1 l acid-cleaned Teflon bottles. Unacidified watersamples were stored at 4�C until further processing in thelaboratory, which was within a maximum of 2 days aftersampling. Water samples were filtered (pre-cleaned WhatmanGFC (glass microfibre filters) with a pore size of 0.75 µm) inorder to determine ‘dissolved’) and particulate-bound total Hgand MeHg. Filtered and non-filtered water samples were storedin a freezer (c. �20�C) until analysis was performed in thelaboratory. The content of TSS was determined after filtrationof a 2 l water sample through a 0.45 µm filter, and drying andweighing of the filter.

M. Horvat et al.288

At three locations, five depth-profile samples down to 25 cm(5 cm each sample) were collected. Sediment and soil sampleswere placed in plastic containers and stored frozen until furtherprocessing. After removal of gravel, stones and plant residues,the soil and sediment samples were further processed asfollows. Sediment samples were wet sieved and two fractions ofthe sediment were analyzed: (1)<0.63 mm; and (2)>0.63 to<1.4 mm. Samples were then freeze dried at �52�C and apressure of 0.05 bar (Christ, Loc-1, Alpha 1-4) to constantweight (for about 48 h) and then homogenized in zirconium(Zr) containers with one Zr ball (Fritsch, Laborgeratebau). Thewater content in the samples was recorded. Residual water insediment and soils was determined in a separate aliquot by ovendrying at 105�C until constant weight. All results for sedimentsand soils were then expressed on a dry weight basis.

Determination of total mercury in soil and sedimentsamples

About 100–200 mg of sample was weighed directly in a Teflondigestion vessel. After addition of 4 ml of concentrated HNO3

and 2 ml of concentrated H2SO4, the vessel was closed and themixture was left to react at room temperature overnight.Digestion was finished by heating in an Al block at 70�C for12 h on a hot plate. The digest was diluted with doubly distilledwater to the mark (26.8 ml). An aliquot of the digest was addedto the reduction vessel and after reduction with SnCl2, mercurywas swept from the solution by aeration and concentrated on agold trap. Mercury was then released from the gold trap byheating and measured on an LDC Milton Roy instrument bycold vapour atomic absorption spectrophotometry (CVAAS)(Horvat et al. 1991). The detection limit of the procedure was0.2 ng ml�1. The precision varied from 2 to 5%. Calibrationwas performed by a Hg standard solution in 5% HNO3,prepared from pure elemental mercury. The accuracy of thisstandard solution was verified by calibration against saturatedmercury vapour at a known temperature.

Determination of monomethylmercury (MeHg) in soiland water samples

Approximately 200 mg of sample was weighed into a Tefloncentrifuge tube, and 5 ml of acid solution (18% KBr and 5%

Fig. 1. Sampling locations in the study area. Sampling locations are identified by a number and arrow: 1, Podroteja – upstream from Idrija; 2,centre of the town of Idrija; 3, under smelter chimney; 4, Sp. Idrija; 5, Travnik; 6, Stopnik; 7, above river Baca inflow; 8, Most na Soca (Lake);9, above the Plave dam; 10, Avce, railway station; 11, Ajba, above the dam; 12, Ajba, below dam; 13, Solkan above the dam; 14, Solkan, belowthe dam; 15, Majnica/Mainizze; 16, Zagraj/Sagrado; 17, Gradež/Grado under bridge; 18, Lake Doberdob; D6, Mouth of the river Soca/Isonzo;AA, from the mouth; CZ, centre of the Gulf. The symbol – indicates dams of Plave (between stations 9 and 10), Avce (between stations 11 and12) and Solkan (between stations 13 and 14).

Hg distribution in Idrijca and Soca rivers 289

sulphuric acid, 1:1) and 1 ml 1 M CuSO4 was added. Thecontents were shaken for 15 min and MeHgBr was extractedinto 10 ml CH2Cl2. After centrifugation at 300 rev min�1, theaqueous phase was discarded. MeHgBr was then back-extractedinto 20 ml of water by evaporation of CH2Cl2 at about 80�C.An aliquot of the aqueous sample was then submitted toderivatization by adding 0.2 ml of 1% Na-tetraethylborate andleft to react for 15 min. Ethylated Hg species were purged ontoa Tenax trap at room temperature and then thermally desorbedonto an isothermal GC column (60�C) and then pyroliticallydecomposed (600�C) and detected by a cold vapour atomicfluorescence detector (Tekran 2600). The method is based onliterature references (Horvat et al. 1989, 1993; Liang et al. 1994).The reproducibility of the method assessed from quality controlcharts was 5%. The calibration solution of methylmercury wasprepared from crystalline CH3HgCl (Merck) in aqueous media.From a stock solution of 1 mg ml�1, a working standardsolution was prepared daily at the concentration of 1 ng ml�1.

Determination of reactive Hg in water samples

Immediately after sampling, an aliquot of 250 ml of the watersample was transferred into a 500 ml reduction vessel filled withSnCl2, to convert ‘reducible’ (or) ‘free’ inorganic Hg2+ to Hg0.Reduced Hg0 was swept from the solution by aeration with N2and concentrated on a gold trap. Mercury was then releasedfrom the gold trap by heating and measured on an LDC MiltonRoy instrument by CVAAS. A detailed description of themethod is given elsewhere (Horvat et al. 1987, 1991). The limitof detection (LOD), expressed as three standard deviations ofthe blanks, varied from 0.01 to 0.05 ng l�1, dependent on therepeatability of the blank. It should be noted, however, thatreactive Hg is a method-dependent parameter. Some authorsmeasure it by reduction of acidified water samples (Gill &Fitzgerald 1987), while others use non-acidified samples. Thelength of purging and exposure to light may also affect theresults (Horvat 1996).

Determination of total mercury in water samples

Sea-water samples were thawed in the laboratory and an aliquotof 200 ml of water sample was placed into a pre-cleaned 250 mlTeflon bottle. The sample was acidified with 1 ml of concen-trated HC1. The oxidation of all Hg compounds to Hg2+ wasachieved after adding 2 ml of BrCl solution and by additionalexposure of the sample to ultraviolet radiation for 3 h. Justbefore measurement, the sample was pre-reduced with 120 µ lNH2OH · HCl to remove the excess of bromide/chloride,which may otherwise cause problems during analysis. Analiquot of the sample was then added to the reduction vesselfilled with SnCl2 to convert Hg2+ to Hg0 Reduced Hg0 wasswept from the solution by aeration and concentrated on a goldtrap. Mercury was then released from the gold trap by heatingand measured on an LDC Milton Roy instrument by CVAAS.The LOD was 0.1 ng l�1, calculated on the basis of threestandard deviations of the reagent blank. The repeatability andreproducibility of the method was 5 and 10%, respectively(Horvat et al. 1987, 1991). Recovery of spiked Hg was quanti-tative, so no recovery factors were used for the final calculation.

Determination of MeHg in water samples

An aliquot of 70 ml of water sample was put into a carefullypre-cleaned 125 ml Teflon bottle and 5 ml of concentrated HCland 30 ml of CH2Cl2 were added. MeHgCl was extracted intothe organic solvent by shaking the sample overnight. The upper

aqueous layer was discarded. MeHgCl was then back-extractedinto 40 ml of MilliQ water by evaporation of CH2Cl2 at about80�C. Once the CH2Cl2 was visibly evaporated, the sample waspurged for 5 min with Hg-free nitrogen to quantitativelyremove CH2Cl2 The whole amount of sample was transferredto a Teflon reaction vessel, buffered with 200 µ l of acetatebuffer to reach a pH of 4.9, as necessary for the ethylationprocess, and 50 µ l of 1% of NaBEt4 solution was added. Thevessel was immediately closed, and the mixture allowed to reactwithout bubbling for 15 min. Ethylated MeHg as ethylmethyl-mercury was purged onto a Tenax trap for 15 min with Hg-freenitrogen and thermally desorbed (200�C) onto an isothermalGC column at 80�C. Under a flow of argon, the eluted Hgspecies were converted into Hg0 by pyrolitic decomposition at600�C and then detected by a cold vapour atomic fluorescencedetector. The LOD, calculated on the basis of three times thestandard deviation of the blanks, was about 30 pg MeHg l�1

when 70 ml of sample was analyzed. The repeatability andreproducibility of the method was 5 and 10% (Horvat et al.1993b; Liang et al. 1994, 1996). Spike recovery was performed ineach batch of analysis and it ranged from 80 to 90%. The resultswere corrected for the recovery factors obtained for each batch.

It should also be mentioned that strict quality controlprotocols (the use of validated analytical methods, regular use ofcertified reference materials, quality control charts, participationin interlaboratory comparison exercises, qualified and well-trained staff) were implemented in order to guarantee the qualityof data.

RESULTS AND DISCUSSION

Mercury speciation in water

The results obtained for non-filtered water samples taken in August1999 are presented graphically in Figure 2. Total Hg concen-trations increased over 20-fold, from about 10 ng l�1 upstreamto almost 200 ng l�1, just downstream of the mining district(site 3), but remained elevated until the confluence of the riverIdrijca with the river Soca. One sample collected at site 8 on theSoca above the confluence of the Idrijca had the same total Hgconcentration as sites upstream of the mine. Total Hg concen-trations in the river Soca downstream from its confluence withthe Idrijca increased by a factor of 4, but two small decreaseswere observed below the dams (sites 12 and 14). This is relatedto the increased concentration TSS below the confluence of therivers Soca and Idrijca as far as Solkan (sites 13 and 14). In Italy,total Hg concentrations decreased from about 40 ng l�1 at site15 to about 4 ng l�1 in the estuarine part of the river Isonzo(site 17). It should be noted, however, that these samples werenot collected on the same day. Therefore, the total Hgconcentrations measured may reflect different hydrological andphysicochemical characteristics of the river, represented by adecrease in TSS. The concentrations of TSS in the rivers Idrijcaand Soca vary, reflecting differences in the geological andtopographical characteristics of the catchment areas.

The concentrations of total Hg and MeHg in Lake Doberdob(station 18), which partially receives water from the river Soca,are relatively low (2.75 ng l�1) with MeHg representing 5.4% ofthe total Hg, comparable to the values found in alkaline lakes inremote areas. In the marine environment, the total Hg concen-tration at the mouth of the river was 20.1 ng l�1 with MeHgaccounting for only 0.2% of total Hg present. With distancefrom the river mouth to the central part of the Gulf (stationsD6–CZ), a steady decrease was observed in total Hg, while theconcentration of MeHg was the same throughout the Gulf(from 0.030 to 0.045 ng l�1). The data for Hg in marine watersare comparable to those measured in 1992 and 1995 (Horvat

M. Horvat et al.290

et al. 1999), indicating that the river Soca continuously suppliesHg to the Gulf of Trieste, although active Hg mining stoppedin 1994.

Comparisons of data for total Hg and MeHg concentrationsin non-filtered samples collected in June 1998 and August 1999are presented in Figure 3. The differences observed are mainlyrelated to different hydrological conditions, where heavy rainand storm conditions caused leaching, erosion and transport ofHg-enriched particles, in particular in August 1999. The greatestdifference was observed at station 3 below the former smeltingfacilities, where under normal weather conditions, Hg concen-trations are about 10 ng l�1, while after heavy rain in August1999, the concentration increased almost 20 times. Total Hgdownstream in the river Idrijca could only be compared for twomore stations before its confluence with the river Soca, wherehigher values were observed in June 1998, and at the stationabove the dam at Podsela (station 9). These higher values weremainly related to the amount of TSS, which was higher duringJune 1998 compared to August 1999. Further downstream,total Hg concentrations in the river Soca were much higherand correlated well with the amount of TSS resulting from‘after-storm’ conditions.

Results for MeHg in non-filtered water samples (Fig. 3) donot follow the same trend as for total Hg. At station 3, whichis located downstream of the smelting facilities, higher values(0.6 ng l�1) were found in June compared to August 1999. Thismay be due to the fact that higher inorganic mercury concen-trations in water samples can induce the activity of organo-mercurial lyase, causing decomposition of MeHg in watersamples (Barkay 2001). At most stations further downstream onthe rivers Idrijca and Soca, as well as in the marine environment,higher values for MeHg were found in August 1999, probablyas a result of additional influx from contaminated soil andtransport downstream. Worth mentioning is the fact that asimilar deviation as for total Hg was again observed at stations6 and 7 (stations on the Idrijca before its confluence with theSoca), where values for MeHg in June 1998 are comparable toand/or exceed the values of August 1999. These variationsmake it necessary to design a sampling plan for water analysiscarefully in order to study trends in mercury transport fromcontaminated areas to the Gulf of Trieste.

Reactive Hg in non-filtered water samples was determined im-mediately after sampling by transfer of an aliquot of about200 ml of water sample into a reduction vessel filled withacidified SnCl2 solution. After purging, volatile Hg species andreduced inorganic Hg were collected on a gold trap, which washeated and the released Hg was then swept into a CVAAS.Reactive Hg, therefore, represents that portion of Hg thatconsists of dissolved gaseous Hg (and dimethyl Hg, if present)and easily reducible (by SnCl2) inorganic Hg. There is no formaldefinition of what ‘reactive Hg’ is. This parameter is, thus, amethodologically defined value, but can be useful for interpret-ing data. Reactive Hg concentrations in unfiltered water samplesincreased from <1 ng l�1 just upstream of the mining area to amaximum of 16.7 ng l�1 at site 3 within the town of Idrija.Concentrations of reactive Hg decreased downstream in theriver Idrijca and reached 3.7 ng l�1 at site 3 just before theBaca inflow at site 7. Site 8 is located on the northern part ofa lake next to the town of Most na Soci, just upstream of theconfluence of the Soca with the river Idrijca. The concen-tration of reactive Hg was 10 times lower (0.33 ng l�1) at site8. This concentration then increased downstream and reached0.80 ng l�1 site 14. Concentrations of reactive Hg in theIsonzo River in Italy in June 2000 were between 0.5 and1.0 ng l�1. The concentration of reactive Hg correlatedwell with that of dissolved Hg (r2 = 0.899), although theconcentration of reactive Hg was lower by about a factor of3 (ranging from a factor of 1.4 to c. 8). This may suggestthat one portion of the dissolved Hg is the one responsiblefor further transformation of Hg in the water. The absenceof a correlation between reactive Hg and Hg bound toparticles probably indicates the low reactivity of the Hgbound to the particles. The correlation between reactive Hgand dissolved MeHg is not as strong as with dissolved Hg,but is statistically significant (r2 = 0.40, p < 0.01). Furtherspeciation of methodologically defined ‘reactive Hg’ is neces-sary in order to distinguish between dissolved elemental Hgand Hg(II) in water, and then to correlate the Hg(II) witheither MeHg or dissolved Hg. Reactive Hg in the marineenvironment accounted for only 5.5% of total Hg close tothe river mouth and reached 18.7% in the central part of theGulf.

Fig. 2. Distribution of total Hg, reactive Hg and MeHg in non-filtered water samples. Note that bar graphs represent the concentration of TSS.

Hg distribution in Idrijca and Soca rivers 291

Analyses of filtered water samples revealed that 1–68% of thetotal Hg passed through a Whatman GC/F glass filter (Fig. 4).Dissolved Hg was much higher in the Idrijca, ranging from<30% upstream of the mine and before the confluence with theriver Soca to 68% at the most contaminated site. Alternatively,the average value of dissolved Hg in the river Soca (Isonzo) wasjust around 10% of total Hg, indicating that most Hg is boundto particulate matter. The percentage of dissolved Hg in theSoca in Italy slowly increased to a maximum of 42% at Grado(station 17) and then significantly decreased in the estuarineportion of the river and the Gulf of Trieste, indicating that mostmercury is bound to particles with less than 20% in thedissolved form. The dissolved mercury fraction in LakeDoberdob represented 42% of the total Hg.

MeHg concentrations in non-filtered water samples revealed aslightly different picture compared to the total Hg. Although a

similar trend was observed (Fig. 2), the concentration of MeHgin the river Idrijca increased by only a factor of 2–3 below themine, and interestingly, reached its first maxima a little furtherdownstream at site 4 (0.45 ng l�1). This concentration thendecreased downstream and reached its minimum (0.05 ng l�1)before the confluence with the Soca. In the lake next to Mostna Soci (site 8), the concentration of MeHg was again elevated(up to 0.30 ng l�1) and downstream reached the next twomaxima above the first two dams at sites 9 and 11 (0.45 and0.42 ng l�1, respectively). Below these two dams, a decrease intotal MeHg was observed. An increase of total MeHg was againdetected downstream of the third dam. These results suggestthat reservoirs affect mercury transformation processes.

Dissolved MeHg represents 30–80% of total MeHg in the riverIdrijca and about 3–20% in the river Soca. The percentage ofdissolved MeHg was always higher (as much as three-fold

Fig. 3. Comparison of total Hg and MeHg in non-filtered water samples analysed in June 1998 and August 1999.

M. Horvat et al.292

higher) below an impoundment than above the impoundment,which again indicates that dams may influence Hg transfor-mation mechanisms. In the river Isonzo in Italy, the percentageof MeHg in filtered water samples increased and reached itsmaximum (80%) in the estuarine environment.

The distribution coefficient (Kd) for river water, defined asthe ratio between Hg bound to TSS and dissolved Hg (l kg�1),is in the range of 105–107 for total Hg (THg) and 104–105 forMeHg. This indicates stronger binding of THg to suspendedsolids compared to MeHg. Large differences in Kd values aredue to variabilities in the concentration of TSS. There is anegative correlation between TSS and Kd for THg and MeHg,which is consistent with the other studies performed in streamwaters impacted by Hg mining activities (Gray et al. 2000),where it was observed that with increasing content of TSS, totalHg increases and MeHg decreases. In the marine environment,more uniform values for Kd values were obtained and thestronger binding of THg on TSS compared to MeHg was alsoconfirmed.

The ratio of MeHg to Hg in non-filtered water samplesincreased upstream of the mine and in the river system as onemoved downstream in the Soca. Upstream of the mine, MeHgwas nearly 2% of total Hg and then decreased to 0.17% at site3. Along the Idrijca downstream, MeHg remained less than0.7% until the Idrijca merges with the Soca. In the first tworeservoirs (sites 9 and 11), MeHg accounted for 2–3% of totalHg, but in the third reservoir (site 13) it was less than 1%. Afterthe first dam, the ratio of total MeHg to Hg decreased, but afterthe next two dams this ratio increased. In the river Isonzo, inItaly, the ratio of total MeHg to total Hg increased from 1% atsite 15 to more than 3% in the estuarine area.

Lake Doberdob is constantly recharged with water from theriver Soca. The total Hg concentration in water of LakeDoberdob was 2.75 ng l�1, with dissolved Hg accounting for42%. The concentration of total MeHg was 0.15 ng l�1, whichrepresents more than 5% of the total Hg. The ratio of dissolvedMeHg to total MeHg was high, reaching almost 84%. This mayindicate the important influence of lake biogeochemistry on Hgtransformation mechanisms.

In general, it can be concluded that the results from thisstudy show similar trends as those observed during an earlier

sampling programme in 1998 (Hines et al. 2000). Although theconcentrations of Hg reflect different hydrological conditions,the trend remains similar, indicating resuspension of Hg inthe river waters, erosion and transport of Hg-contaminatedparticles and the role of hydroelectric power reservoirs on Hgtransformations.

Hg speciation in sediments

Concentrations of total Hg and MeHg in river sediment arepresented in Figure 5. Two sediment fractions were analysed:the silt and clay size fraction with diameter less than 0.063 mmand a coarser-grained fraction (referred to as sand) between0.063 and 1.4 mm. Stream sediments were not collected at allsites due to the absence of sediment in the river bed.

Analysis of the silt and clay size fraction of stream sedimentsgave total Hg concentrations from 5 µg g�1 above Idrija to727 µg g�1 downstream of the mercury mine. Mercury concen-trations rapidly decreased downstream to about 60 µg g�1 atsites 4 and 5. A further 10-fold decrease was detected at site 6.Before confluence of the Idrijca with the Soca, the concentra-tion increased to about 25 µg g�1. Concentrations of total Hgin Soca sediments were lower than in the Idrijca, starting with1.3 µg g�1 at site 8. Total Hg concentrations in sedimentsdownstream of the dams were slightly higher and slowlydecreased from 4.9 to 2.0 µg g�1. MeHg concentrations wereabout 3 ng g�1 upstream of Idrija, about 7 ng g�1 at site 3,reached a maximum at site 4 with 14 ng g�1 and decreased to10 ng g�1 before the confluence with the Soca at site 7.Concentrations of MeHg in Soca sediments ranged from 0.8 to3 ng g�1.

The distribution of Hg between different size fractions is notuniform along the two rivers. In most cases, much higherconcentrations of Hg were associated with smaller particle sizes,but at stations 5, 6, 12 and 14, higher concentrations were foundin the coarser fraction. At station 7, a similar concentrationof Hg in both fractions was measured. This indicates thatthe sources and transport of Hg are strongly influenced byinhomogeneities along the river systems that include particles ofcinnabar originating from the mine. The same applies topartitioning of MeHg between the different size fractions.Comparison of these results with data from 1991 and 1995

Fig. 4. Total Hg and MeHg in filtered water samples.

Hg distribution in Idrijca and Soca rivers 293

(Gosar et al. 1996) shows that Hg concentrations in streamsediments have significantly decreased since then, probably as aconsequence of complete termination of Hg mining andsmelting activities in Idrija.

The concentration of total Hg in Lake Doberdob is similar tothe concentrations in the central part of the Gulf of Trieste andclearly indicates the influx of mercury by the river Soca,which partly recharges the water in this lake. This concentrationdoes not represent natural background, which has been esti-mated to be below 0.1 µg g�1 in the area outside Idrija (Covelliet al. 2001). The concentration of MeHg in Lake Doberdobsediment (0.32 ng g�1) is also similar to the concentrationfound in the middle of the Gulf of Trieste at station CZ. Itshould be stressed, however, that the concentration of MeHg insediment reflects the amount of MeHg bound to particles. It isof much greater importance to estimate the potential of thesesediments to transform inorganic Hg to MeHg and to estimatethe flux of MeHg produced in the water column. So far, such

studies have only been performed in the marine environment ofthe Gulf of Trieste (Covelli et al. 1999; Hines et al. 2000) andindicated that the major source of MeHg in the Gulf of Triesteis the marine sediment rather than the inflow from the riverSoca.

Mercury speciation in soil

Concentrations of total Hg and MeHg in soil samples fromalluvial ground and agricultural land along the river systemswere determined at several sites: station 3 close to the smeltingfacilities; station 5 from alluvial ground and a meadow; andstation 7 from a meadow. At all sites, a soil core was analysedto a depth of 25 cm (Fig. 6).

The sampling stations were selected based on the expecteddeposition of Hg-enriched particles during frequent floodingevents. The profile close to the smelting facilities showed highvalues of total Hg ranging from 43 to over 100 µg g�1 with a

Fig. 5. Total Hg and MeHg in river sediments and the Gulf of Trieste.

M. Horvat et al.294

tendency to decreasing values with depth. This indicates theimportance of deposition of Hg-enriched particles duringflooding and constant deposition from the atmosphere in recenttimes. The time scale of Hg deposition in the flood plainsis difficult to estimate, as the cores were not dated. Althougha decrease of Hg in stream water has been observed as aconsequence of a less active anthropogenic input in recentyears, the data from the soil profiles indicate that Hg accumu-lated in soil has a longer residence time and continues to supplymercury to the river during flood events. These data comparewell with those obtained during a 5-year monitoring programme(1990–1995) of soil samples collected close to the smeltingfacilities (Gnamuš & Horvat 1999; Gnamuš et al. 2000). Soilprofiles were taken from two sites at site 5 (Travnik). Thealluvial ground profile had high concentrations of Hg at thesurface (106 and 95 µg g�1) followed by a sharp increase to820 µg g�1. These very high Hg concentrations probably reflectactive Hg transport from mining sites and recent and pastdeposition of Hg-enriched particles close to turbulent waterflow. Another soil profile was collected in a meadow about100 m from the first location in a direction away from the river.The meadow is located a few metres above the water flow andis flooded only in extreme events. The depth-profile showed aslight increase in Hg from 106 µg g�1 at the surface to122 µg g�1 at 20 cm depth. This could be explained by thedeposition of fine particles that had been less contaminated inrecent years. At station 7, located close to the confluence of therivers Soca and Baca, Hg concentrations were lower at thesurface, but with a significant increase of concentration withdepth from about 31 to 104 µg g�1 at 25 cm. Again, this profileindicates less transport and deposition of Hg in recent years.These data cannot be directly compared to those analysed in1990 (Gosar et al. 1996) because the samples were collected atdifferent locations and different size fractions were used foranalysis.

Concentrations of MeHg in soils are lower than in riversediments and show irregular concentration profiles with depth(Fig. 6). This indicates that Hg transformation in soils results inrelatively low net Hg methylation. It is interesting to note thatMeHg concentrations in forest soil close to the smeltingfacilities, about 50 m above the river bank, are much higher(65–97 ng g�1) (Gnamuš et al. 2000) compared to alluvial soil

(2–3 ng g�1). This fact may be related to the different origin ofthe mercury. Mercury in forest soil close to the smelting facilitymainly originates from atmospheric deposition of Hg, while inthe alluvial ground, Hg originates from particles in which Hg ispresent as cinnabar (Biester et al. 2000). The reactivity ofmercury deposited from the atmosphere seems to be muchhigher than that of unreactive cinnabar particles, resulting inabout 20 times higher concentrations of MeHg in soil samplesfrom near the smelter. This is important because during heavyrain, Hg and MeHg may be released from contaminated forestsoils, which have less binding capacity than for inorganic Hg.Whether this represents a significant source of MeHg in riverwater during flood events will require more careful sampling.

CONCLUSIONS

The results of this study clearly show that sediments and soilnear the Idrija mine continue to supply Hg to the Idrijca andSoca river system, which empties into the Gulf of Trieste some100 km downstream. Beside soils that are naturally elevated inHg, Hg-laden material and tailings from the mine are con-tinuously eroded and serve as an important additional source ofHg for the river, flood plains and the Gulf of Trieste. Duringsuch transport, important transformation mechanisms takeplace, evidenced by the presence of MeHg in the varioussamples analysed. The percentage of Hg as MeHg is highest inwater samples, followed by sediments and soil (Fig. 7).

The results suggest the importance of the water compart-ment in Hg transformation processes because water suppliesreactive Hg that triggers further transformations of Hg(methylation, demethylation and reduction). In the town ofIdrija, which is highly contaminated with Hg, atmospheric Hgdeposition may still be very important. Future studies shouldcarefully address fluxes of Hg from contaminated soil into theatmosphere, its redeposition on soil and water and its furthertransformation.

The results also show that reservoirs play an important rolein mercury transformation and transport processes. Furtherstudies should be conducted to understand the biogeochemistryof Hg in these artificial lakes.

The results also suggest that mercury transport in the studyarea is mainly driven by hydrometeorological conditions and,therefore, a careful sampling programme is necessary to accu-rately assess Hg transport for mass-balance calculations and toestimate the temporal variability of Hg transport since the endof active Hg mining.

Fig. 6. Distribution of total Hg and MeHg with depth in soil samplescollected downstream from the Idrija mine.

Fig. 7. Relative percentage of Hg as MeHg in water, stream sedimentand riverbank soil in area. Results are presented as mean values of allmeasured data and the error bar represents the range (min–max).

Hg distribution in Idrijca and Soca rivers 295

This work was conducted within the programme P531 Biological andGeochemical Cycles funded by the Ministry of Education, Science andSport (MESS) of the Republic of Slovenia, through a SLO–USAbilateral agreement funded by the MESS and NSF, USA, and theIAEA CRP entitled Health Impacts of Mercury Cycling in ContaminatedEnvironments Studies by Nuclear Techniques. Dr. A. R. Byrne is thankedfor his editorial expertise.

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