Pre-mining acid rock drainage in the Talvivaara Ni–Cu–Zn–Co deposit (Finland): Natural peat layers as a natural analog to constructed wetlands

Journal of Geochemical Exploration xxx (2014) xxx–xxx

GEXPLO-05362; No of Pages 12

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Pre-mining acid rock drainage in the Talvivaara Ni–Cu–Zn–Co deposit (Finland):Natural peat layers as a natural analog to constructed wetlands

Annika Parviainen a,⁎, Markku Mäkilä b, Kirsti Loukola-Ruskeeniemi b

a Department of Mineralogy and Petrology, University of Granada, Avda. Fuente Nueva s/n, E-18002 Granada, Spainb Geological Survey of Finland, Betonimiehenkuja 4, FI-02151 ESPOO, Finland

⁎ Corresponding author. Fax: +34 958243368.E-mail addresses: [email protected] (A. Parviainen),

(M. Mäkilä), [email protected] (K. Louko

http://dx.doi.org/10.1016/j.gexplo.2014.03.0240375-6742/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Parviainen, A., et al.,as a natural analog to construc..., J. Geochem

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Article history:Received 23 July 2013Accepted 18 March 2014Available online xxxx

Keywords:PeatPre-mining conditionsAcid rock drainageElectric conductivitySulfide depositTalvivaara

Altogether 62 peat, 411 bedrock, and 1163 glacial till sampleswere selected in pre-mining conditions around thepresent Talvivaara Ni–Cu–Zn–Co mine in Finland. The samples were collected from different lithological areasrepresenting mineralized (N0.07% Ni) and non-mineralized bedrock. The influence of bedrock and till composi-tion, and subsequent groundwater and surface water quality, on the metal and sulfur concentrations in the peatdeposits was evaluated. The peat samples at the non-mineralizedmica schist study siteM1 showedmoderate in-crease in element concentrations (Cu, Ni, S) and decreasing electric conductivity (EC) as a function of depth,whereas at themineralized black schist study sitesM2 andM3 the heavymetal (Fe, Ni, Zn) and sulfur concentra-tions and EC exhibited a significant increase towards the Carex-dominated bottom layers. Besides anomalousmetal (up to 12.0wt.% Fe, 758 mg/kgNi, 5270 mg/kgZn) and S (13.6 wt.%) concentrations, the siteM2presentedanomalously high ash content (median 35%) and low pH (3.4), which were attributed to the acid rock drainagederived from an adjacent mineralized black schist outcrop. Samples from site M2 were subjected to sequentialextractions to study elemental distribution. The studied peatland is a natural analog to constructed wetlands inpassive treatment of acid mine drainage waters, though the retention capacity is lower due to low pH and de-creased buffering capacity.

© 2014 Elsevier B.V. All rights reserved.

Introduction

Peatlands are a sink for potentially harmful elements which recordatmospheric, hydrospheric and lithospheric signals of both natural andanthropogenic origins. Elements and more complex compounds aretransported in groundwater through capillary flow in minerogenicpeat deposits. They can be mobilized from the underlying mineral sub-strate into peat via plant uptake in the form of soluble ions. Ombrogenicpeat deposits are principally fed by rainwater and precipitation ofairborne particles. Heavy metal retention in peat layers results fromcomplex interaction of processes depending on pH, redox potential,ion exchange capacity, bacterial activity, and organic matter. Theseprocesses are settling, sedimentation, sorption, co-precipitation, cationexchange, photodegradation, phytoaccumulation, biodegradation,microbial activity and plant uptake (Sheoran and Sheoran, 2006;Sobolewski, 1999). At oxygenated surface layers, precipitated Fe(III) hy-droxides may retain trace elements and sulfate by adsorption and co-precipitation, however this retention mechanism is highly pH and Ehsensitive. Metal retention by sulfide minerals in reductive conditionsmay supply a more long-term deposition of metals (Sobolewski,

[email protected]).

Pre-mining acid rock drainag. Explor. (2014), http://dx.do

1996). The precipitation of metal sulfides persists as long as reductiveconditions prevail and wetland plants provide organic substrate forgrowth of sulfate reducing bacteria (Sobolewski, 1996). According toSobolewski (1999), sorption onto organic matter is important in metalremoval, particularly for copper, nickel, and uranium.

Metal accumulation in peat derived from anthropogenic sources isdescribed in areas with mining, smelting, and industrial activities(Connor and Thomas, 2003; Martínez-Cortizas et al., 2002; Mighallet al., 2002; Rausch et al., 2005; Schöner et al., 2009; Ukonmaanahoet al., 2004; Vleeschouwer et al., 2009). Acid mine drainage (AMD) is apersistent environmental problem and its treatment and remediationrequire a lot of economical and maintenance efforts. Knowing the ca-pacity of natural peatlands to retain potentially toxic elements, overthe past decades constructed wetlands have been used as cost-effective passive treatment systems of AMD waters (Blowes et al.,2003; Gazea et al., 1996; Matthies et al., 2010; Mays and Edwards,2001; Mitsch and Wise, 1998).

In many mineralized areas, acid rock drainage (ARD) is describedunder natural conditions (Nordstrom, 2011). Black shales (here referredto as black schist as they have undergone medium grade metamor-phism at the study site) containing sulfides, mainly pyrite [FeS2] andpyrrhotite [Fe(1 − x)S], and graphite are prone to weathering andknown to produce ARD (Gustavsson et al., 2012; Kwong et al., 2009;Loukola-Ruskeeniemi et al., 1998; Mäkilä et al., 2012; Yu et al., 2012).

e in the TalvivaaraNi–Cu–Zn–Co deposit (Finland): Natural peat layersi.org/10.1016/j.gexplo.2014.03.024

http://dx.doi.org/10.1016/j.gexplo.2014.03.024

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Establishing background threshold values in areas with potential forARD is crucial when evaluating the impact of mining operations to thesurrounding environment. In this study, the aim was to investigatepre-mining conditions and metal accumulation in three peat coresrepresenting different peat formations from different geological contextin an area where Talvivaara Ni–Cu–Zn–Co mine is operating at present.The secondaimwas to evaluate the impact of ARD onpeatlands as a nat-ural analog to constructed remediation wetlands.

Material and methods

Study area

Since 2008, Talvivaara Ni–Cu–Zn–Comine has been operating in thestudy area, in the Sotkamomunicipality in Eastern Finland. The mineralresources of the Talvivaara deposit are estimated to be 2053 Mt (0.22%

Fig. 1. Geological map of the Talvivaara area showing the relation of the bedrock context with tlogical map was modified from Kontinen (2012). The outlines of the Kuusilampi and Kolmisocomm.).

Please cite this article as: Parviainen, A., et al., Pre-mining acid rock drainagas a natural analog to construc..., J. Geochem. Explor. (2014), http://dx.do

Ni, 0.13% Cu, 0.50% Zn, and 0.02% Co) (Talvivaara Mining Company,2013). The ore deposit is hosted by graphite- and sulfide-rich blackschist and the deposit is divided into two polymetallic ore bodies:Kuusilampi and Kolmisoppi (Loukola-Ruskeeniemi and Heino, 1996;Loukola-Ruskeeniemi and Lahtinen, 2013; Fig. 1). The black schists con-taining abundant carbon (originally present as organic carbon, now asgraphite) and sulfur (mainly bound to sulfides) contentswere originallydeposited on the bottom of the sea floor under anoxic conditions(Loukola-Ruskeeniemi, 1999). The black schists are composed ofquartz, mica, graphite, and sulfides as the main minerals, with rutile,apatite, zircon, feldspar, and garnet as common accessory minerals. Py-rite and pyrrhotite are the dominant sulfide minerals, whereas chalco-pyrite [FeCuS2], sphalerite [ZnS], alabandite [MnS], and pentlandite[(Fe,Ni)9S8] occur in minor concentrations.

The bedrock in the Sotkamo area is typically covered by glacial tillvarying in thickness from a few centimeters to over onemeter. Ablation

he locations of glacial till sampling lines and peat sampling sites M1, M2, andM3. The geo-ppi ore bodies were modified by the Talvivaara Exploration Ltd. (Hannu Lahtinen, pers.



3A. Parviainen et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

till, characterized by sand and gravel, is encountered, especially, in thewestern part of the study area. The transportation distance of the fineparticles of the till (b60 μm) was less than 200 m (Gustavsson et al.,2012; and reference therein Murtoniemi, 1984). The topographyis dominated by elevated moraine ridges, and peatlands are restrictedto low-lying areas. The mean thickness of peat in the Sotkamo area is1.5 m, including the slightly humified Sphagnum dominated surfacelayer, which averages 0.3 m in thickness (Häikiö et al., 1997). Thewater table of the peatlands in boreal vegetation zone lies close to thepeat surface. Hence, discrete sample cores represent vast peatlandareas overlying certain rock types. The paludification commencedroughly 10,000 years ago after withdrawal of the continental ice sheetand the discharge of the Sotkamo Ice Lake (Saarelainen and Vanne,1997). Wetland drainage was efficient in Finland in the 1960s and1970s for forestry purposes, and vast areas of peatlands in the Sotkamoarea were impacted by this activity.

Fig. 2. A) Soil stratigraphy exposed at study site M2 (Fig. 1) in 2011. At the time of sampling inGrundström, Geological Survey of Finland); and B) schematic presentation of acid rock drainagaddition to bedrock, also sandy till contains greater than average concentrations of S, Fe, Ni, Cu


Sampling

Glacial till sampling was carried out in 1981 using percussion drill. Atotal of 1163 till samples were collected from 261 discrete sites at inter-vals of approx. 100 m in seven lines located 400 to 800 m apart (Fig. 1).The length of the sampling lines ranged from approx. 0.5 to 3 km, andthey crosscut the different bedrock types of the study area includinggneiss granite, quartzite, black schist, and mica schist. The till sampleswere collected from surface to the bedrock surface with varying thick-ness of till cover from a few centimeters to approx. 1.4 m. Weatheredbedrock samples (305) and bedrock samples (106) were also collectedduring till sampling.

The first peat sampling was carried out at Talvivaara in 2005 beforethe Talvivaara Ni–Cu–Zn–Co mine commenced large-scale mining ac-tivities. Three discrete sampling sites were chosen: M1 representingnon-mineralized mica schist area, and M2 and M3 representing

2005, the site was forested and represented a transformed spruce peatland (Photo by Alee (ARD) from sulfide-rich outcrops to the sloping peat formation at the study site M2. In, Zn, and Co.




mineralized black schist areas near theKuusilampi ore body (Fig. 1). Thesampling site M2 was located in a slope within the watershed affectedby the Kuusilampi ore deposit, while the other two sites were notunder the influence of this watershed (Figs. 1 and 2). The peatland atthe site M2 was formed on a sloping terrain roughly 100 m from ablack schist outcrop, and the site was impacted by the extensivedrainage activities of the 1960s (Fig. 2). The peat samples werecollected from the surface until reaching mineral subsoil at the depthsof 2.0, 2.6, and 1.2 m, respectively, using a 0.5 m-long Russian core sam-pler (∅ 5 cm). The peat coreswere sliced into 0.1m sections in the field,and a total of 58 samples were collected. In 2011, three years after themining activities had initiated and six years after the initial peat sam-pling, a second sampling was performed at site M2, roughly 50 mfrom themining premises. The peat core was divided into four compos-ite samples at half a meter intervals (0.0–0.5 m; 0.5–1.0 m; 1.0–1.5 m;1.5–1.8 m; new samples are called M2*), and the core was considerablyshorter due to the compression of the peatlands provoked by the defor-estation carried out in 2008.

The main peat type and the proportions of minor constituents weredetermined in the field by visual inspection of the peat cores accordingto Lappalainen et al. (1984), and humification (H) was estimated ac-cording to the 10-degree scale of von Post (1922). Additionally, thetype of subsoil was recorded.

Analysis

The till samples were sieved to a fraction b0.064 mm, whereas rocksamples were ground as whole. The samples were digested using aquaregia leach (7MHNO3 and 6MHCl, 5:1), and analyzed by flame atomicadsorption spectroscopy (AAS) at the chemical laboratory at the Geo-logical Survey of Finland.

The peat samples were dried at 40 °C and homogenized at the labo-ratory before chemical analysis. The samples collected in 2005, weredigested with microwave assisted HNO3 extraction (EPA 3051), andthe chemical composition was analyzed using inductively coupledplasma mass spectroscopy (ICP-MS; PerkinElmer ELAN 6000, ThermoX7) and inductively coupled plasma atomic emission spectroscopy(ICP-AES) at the accredited chemical laboratory at theGeological Surveyof Finland.

The peat samples were also analyzed for pH, electric conductivity,total organic carbon, water, and ash content, and for dry bulk density.The pH was gauged for wet peat using Mettler Toledo S20 SevenEasypH meter. The electric conductivity was measured at the field fromthe surface to the bottom of the cored profiles at intervals of 25 cmusing an electric conductivity probe (Puranen et al., 1997, 1999). Theprobe sensor was attached to the core sampler and inserted into thepeat next to the whole where the peat cores (samples) were taken.The four Wenner electrodes (with a spacing of 1.5 cm) comprising theconductivity sensor operated at a high frequency (500Hz) to avoid elec-trode polarization. Conductivity was corrected to a temperature of18 °C. The total organic carbon content was gauged by carbon/nitrogenanalyzer (Elementar vario MAX CN), the water content was calculatedas a percentage of the difference of wet and dry weight (drying at105 °C), the ash content was measured by loss on ignition (LOI) at815 °C, and the dry bulk density per cubic meter of peat in situ was cal-culated on the basis of the dry weight for a known sample volume.There is a highly significant negative correlation between dry bulk den-sity and water content of peats (−0.96).

For the 14C dating, thin slices (3–5 mm) of peat samples were col-lected in order to obtain high-resolution records (Mäkilä and Goslar,2008). Dating of the bottom peat layer (initiation) was determinedusing accelerator mass spectrometry, AMS, 14C dating at the Poznanlaboratory (Poz) in Poland (Czernik and Goslar, 2001; Goslar et al.,2004). The 14C ages were converted to calendar years by a computerprogram (Stuiver et al., 2005).


The peat samples collected in 2011, were subjected to a six-step se-quential extraction procedure consecutively attacking water-soluble,exchangeable, Fe(oxy)hydroxide, organic matter, sulfide, and residualfraction. The exchangeable fraction includes adsorbed elements, car-bonates, and monosulfides, though carbonates are not expected to bepresent because of the acidic conditions prevailing in the studiedpeats. The extraction sequence was adapted from Dold (2003) andTorres and Auleda (2013) using 1 g of sample. (1) Water-soluble min-erals were extracted at room temperature (RT) shaking for 1 h withdeionized water; (2) exchangeable fraction at RT and pH 4.5 shakingfor 2 h with 1 M NH4-acetate; (3) Fe(III)hydroxides in water bath at80° at pH 3 shaking for 2 h 0.2 M NH4-oxalate; (4) organic fraction atRT shaking for 16 h with 0.5 M NaOH; (5) sulfide minerals' fraction ap-plying combination of KClO3 and HCl, followed by 4 M HNO3 boiling;and (6) residual fraction using HF-HClO4 extraction. The extraction pro-cedure and the subsequent chemical analysis by ICP-MS and inductivelycoupled optical emission spectroscopy (ICP-OES; Thermo iCAP jaThermo IRIS) were performed at the FINAS accredited (T025) chemicallaboratory LabtiumOy. Analytical quality controlwas based on the prac-tices and standards of the laboratories. The samples collected in 2011were also analyzed by bulk powder X-ray diffraction (XRD) usingPanalytical CubiX3 Minerals at the Geological Survey of Finland, andthe parameters were set to 45 kV, 35 mA, and a scan range of 5–70°2θ with 0.02° 2θ step size and 0.15 s counting time per step.

Results

Chemical characteristics of glacial till

The results of till samples for Cu, Ni, and Zn are presented in Table 1and Fig. 3, where the samples are divided into five groups based on thegeological context as the sampling lines crosscut from SW to NE gneissgranite, quartzite, Cu–Ni–Zn mineralization in black schist, black schist,and mica schist areas (Fig. 1). The till samples exhibited geochemicalvariation according to the bedrock context. The glacial till containingabundant material from weathered bedrock reflects the compositionof the bedrock as the transportation distance (from NW) of the glacialsediments was relatively short. The till samples in black schistareas showed highest concentrations of Cu, Ni, and Zn with up to2900 mg/kg (median value 70), 3100 mg/kg (56), and 17,900 mg/kg(150), respectively, whereas the till samples collected from non-mineralized areas exhibited lower values (up to 270 mg/kg, 400 mg/kg,and 380 mg/kg, respectively). The Cu (median values ranging 30–31 mg/kg) and Ni (18–22 mg/kg) concentrations were similar in all ofthe non-mineralized bedrock areas, but Zn concentrations were higherin the mica schist area (44 mg/kg) than in the quartzite (28 mg/kg)and gneiss granite areas (29 mg/kg). However, the median till samplevalues for Cu, Ni, and Zn in any bedrock context did not surpass thelower guideline values for contaminated soils in a Decree set by theFinnish Council of State (150, 100, and 250 mg/kg, respectively, Finlex214/2007). Only random samples exceeded the lower guideline values.The median values in weathered bedrock and fresh bedrock sampleswere distinctively higher at mineralized areas than at non-mineralizedareas. Copper exhibited values up to 600 mg/kg in weathered bedrocksamples and 875 mg/kg in bedrock samples, Ni 500 and 2450 mg/kg, re-spectively, and Zn 1550 and 4300 mg/kg, respectively. The results showthat till concentrations reflect the underlying bedrock composition al-though the concentrations are not uniformly distributed in the till sam-ples, and they also agree with the conclusion by Murtoniemi (1984)that the transportation distance of the till fine fraction was 200 m atmost in the study area.

General characterization of peat samples

The characteristics of each of the peat sampling sites are listed inTable 2. Generally, Sphagnum (moss) peat dominated in the surface



Table 1Minimum, maximum, and median values of the Ni, Cu, and Zn concentrations (mg/kg) in till, weathered bedrock, and bedrock samples. The samples are sorted by the type ofunderlying bedrock (Fig. 1), and bold faces correspond to samples from mineralized areas. Guideline values for contaminated soil are based on the Decree of the Finnish Council ofState (Finlex, 214/2007). N = number of samples.

Bedrock context Till Weathered bedrock Bedrock

N Min. Max. Med. N Min. Max. Med. N Min. Max. Med.

Cu (lower guideline value 150 mg/kg)Granite gneiss 110 8.0 180 30 17 4.0 380 38 18 3.0 530 9.0Quartzite 113 1.0 270 31 15 3.0 530 36 15 2.0 220 12Mica schist 223 7.0 270 30 19 13 168 26 34 7.0 179 24Black schists 586 8.0 2900 70 222 8.0 5200 200 29 4.0 1270 154Cu–Ni–Zn ore 131 8.0 1310 70 32 70 1500 600 10 90 3220 875

Ni (lower guideline value 100 mg/kg)Granite gneiss 110 4.0 290 20 17 3.0 540 340 18 4.0 390 14Quartzite 113 2.0 90 18 15 6.0 115 26 15 11 120 22Mica schist 223 7.0 400 22 19 19 131 52 34 13 137 47Black schists 586 6.0 3100 56 222 6.0 4400 220 29 11 2160 210Cu–Ni–Zn ore 131 9.0 800 45 32 22 1900 500 10 540 5000 2450

Zn (lower guideline value 250 mg/kg)Granite gneiss 110 5.0 110 29 17 11 360 120 18 5.0 159 30Quartzite 113 4.0 90 28 15 7.0 240 78 15 1.0 1600 21Mica schist 223 16 380 44 19 31 190 90 34 7.0 190 78Black schists 586 14 17,900 150 222 18 41,000 472 29 14 6000 360Cu–Ni–Zn ore 131 33 1700 110 32 90 7800 1550 10 1700 13,000 4300


layers, whereas Carex (sedge) peat became dominant in the bottomlayers. The subsoil beneath all sampled peat profiles was composed ofsandy till. The samples at site M1, corresponding to non-mineralizedzone of mica schist, consisted of woody Sphagnum peat with Carexpeat at the bottom representing a transformed pine bog. A 10-cmlayer of sandy peat was encountered between the peat layer and themineral soil. Well-decomposed woody Carex peat was the main peattype at the mineralized site M2 representing a transformed sprucepeatland. The site M3 comprised moderately decomposed woodySphagnum and Carex peat representing transformed spruce peatlandtype. The median humification degrees of the peat at sites M1, M2,and M3 were 5.0; 7.0; and 5.0, respectively, whereas median water(%) and ash contents (%) were 87.3 and 2.1; 85.2 and 35.2; and 86.6and 5.5, respectively. The median dry bulk densities (kg/m3) were122.4; 136.5; and 129.6, respectively (Table 2). The surface layer ofthe site M2* was compacted as a consequence of deforesting in thebeginning of themining. The composite samples ofM2* had similarme-dian water content (85%) to samples of M2. However, the field

Fig. 3. Cu, Ni, and Zn concentrations in glacial till (mg/kg). The lithological areas are ex


observations in 2011 showed that the top layer had experienced drying.The ash content ofM2* increasedwith depth, but showed lowermedianvalue (17%) than the respective samples in 2005.

The pH values showed a downward increasing trend at sites M1 andM3 ranging from 3.1 to 3.9 and from 3.4 to 4.6, respectively (Fig. 4). Atsite M2, pH values decreased abruptly from 4.4 to 3.1 below 50 cm,and the lowest recorded value was 2.8 in the bottom sample. The elec-tric conductivity of the peat samples decreased with progressing depthat the site M1, whereas it increased at the sites M2 and M3 (Table 2,Fig. 4), where the conductivity values were well-correlated with S andFe concentrations. The median electric conductivity was 5.1; 10.1; and8.1 mS/m, respectively. At site M2, where the highest metal and ashconcentrations were recorded, conductivity was also correlated withmetals (Fe with R2= 0.81, Ni 0.77, Zn 0.74), S (R2= 0.91), ash content,and dry bulk density.

After the catastrophic discharge of the Sotkamo Ice Lake (10500 years cal BP; Saarelainen and Vanne, 1997), paludification of thedepressions commenced. Paludification at site M1 began when sandy

plained in Fig. 1, and the pink area defines the Kuusilampi ore body at Talvivaara.



Table 2Sampling depth (cm), humification, peat type, pH, conductivity (mS/m corrected to 18°), carbon (% of dry weight), water (% of wet weight), and ash content (% of dry weight), as well asdry density (kg/m3) for peat samples at sitesM1,M2,M2*, andM3. Thepeat type is described using the following symbols: C Carex (sedge), S Sphagnum (moss), B Bryales (brownmoss), EREriophorum (cotton grass), L wood residues, EQ Equisetum (horsetail), N Nanolignid. H degree of humification 1–10 in von Post's scale. Sphagnum-predominant peat is marked with italicletters and Carex-predominant peat with normal letters.

Sampling site Depth Humification Peat type pH Conductivity C% Water% Ash% Dry density

M1 10 2 LS 3.4 12.8 43.6 87.2 3.5 100.620 7 LS 3.2 12.8 51.2 88.3 3.1 114.230 7 LS 3.1 12.8 48.3 86.2 2.1 135.440 5 ERS 3.1 8.0 48.7 85.5 1.7 142.550 5 ERS 3.2 8.0 49.8 84.4 1.4 153.660 5 ERLS 3.3 8.0 51.1 84.4 1.9 153.570 5 ERLS 3.3 6.7 51.0 84.6 1.8 151.580 5 ERLS 3.4 6.7 50.9 86.3 1.7 134.590 5 ERLS 3.6 4.3 51.5 88.2 1.8 115.5100 5 CS 3.6 4.3 51.6 90.2 1.7 95.5110 7 SC 3.5 4.3 51.0 89.0 1.9 107.3120 7 SC 3.6 5.1 51.2 88.7 2.5 110.2130 7 SC 3.6 5.1 51.8 87.3 3.2 123.8140 6 SC 3.7 4.5 52.7 84.6 3.0 150.6150 6 SC 3.7 4.5 51.1 84.3 3.2 153.5160 5 SC 3.7 4.5 51.5 87.6 2.7 121.0170 5 SC 3.9 3.7 51.6 89.9 2.2 98.4180 5 SC 3.9 3.7 49.8 90.4 3.4 93.2190 Sandy peat 4.1 3.3 35.9 32.3200 Peaty till 4.4 3.3 11.5 81.9Median 5 3.6 5.1 51.1 87.3 2.1 122.4

M2 10 2 NS 3.5 3.5 43.8 85.3 6.6 100.620 5 LCS 3.6 3.5 48.2 84.1 6.4 155.530 6 LCS 3.7 3.5 46.9 85.1 10.1 144.840 7 SC 4.2 4.5 40.3 85.7 20.8 135.550 7 SC 4.4 4.5 29.7 86.3 41.7 124.760 8 SC 4.3 4.5 34.9 87.1 29.8 119.670 7 LSC 3.1 8.5 30.5 88.0 31.7 110.280 6 SC 3.2 8.5 35.1 89.0 21.9 102.790 7 SC 3.3 10.1 30.7 87.7 30.3 113.5100 7 SC 3.3 10.1 26.1 87.5 36.7 114.0110 8 LSC 3.3 10.1 28.0 87.8 32.5 112.0120 8 LSC 3.3 9.5 26.5 87.7 34.1 112.7130 7 LSC 3.2 9.5 25.0 87.2 35.2 117.3140 7 LSC 3.4 11.7 23.8 86.1 36.4 127.9150 7 SC 3.3 11.7 23.2 83.8 36.9 150.6160 7 SC 3.7 11.7 23.5 84.9 37.3 139.6170 8 SC 3.6 12.5 23.5 84.6 35.6 142.9180 7 SC 3.8 12.5 23.2 84.6 34.9 143.1190 6 BC 3.2 16.5 21.1 85.2 37.6 136.5200 6 BC 3.1 16.5 20.6 84.4 35.7 144.9210 6 BC 3.7 16.5 22.2 85.1 35.3 138.1220 6 SC 3.5 21.5 18.8 83.5 36.9 153.5230 7 LSC 3.6 21.5 14.9 80.3 43.3 183.7240 7 LSC 3.2 27.8 16.1 79.5 40.9 192.1250 7 EQLSC 2.8 27.8 24.8 82.4 33.7 165.1260 Peaty till 36.7 9.7 68.3Median 7 3.6 8.5 29.7 86.3 32.5 122.4

M2* 0 3.7 55.2 83.8 7.650 3.6 44.9 86.2 16.8100 3.7 43.1 87.8 17.2150 3.2 27.9 79.9 30.1Median 3.6 44.0 85.0 17.0

M3 10 2 NS 3.4 7.3 45.1 83.3 6.4 100.620 7 LS 3.5 7.3 47.9 83.5 4.7 161.930 7 LS 3.6 7.3 48.2 85.2 5.2 144.840 6 LCS 3.7 5.4 48.6 85.3 5.3 143.850 6 LCS 3.8 5.4 47.9 84.8 5.9 148.760 4 LCS 4.2 5.4 47.8 88.2 5.1 114.570 5 LSC 4.1 8.9 47.5 86.7 5.6 129.280 5 LSC 4.2 10.4 48.0 87.5 6.1 121.290 5 LSC 4.3 13.6 47.5 86.6 6.6 130.0100 5 LSC 4.3 13.6 48.3 86.9 5.2 127.3110 5 LSC 4.6 13.6 49.5 87.8 4.6 118.6120 5 LSC 4.5 15.1 48.8 85.2 6.5 143.8Median 5 4.2 8.1 48.0 86.0 5.5 129.6


till emerged in the depressions between the moraine ridges. Peatlandformation began 7980 cal BP years ago (Poz-43623) on mineralsoils at the sloping site M2. The rate of peat formation averaged


0.24 mm per year. The peat at site M3 was formed by progressivepaludification of forest; wood residues are found at the bottom of thepeat formation.



Fig. 4. Graphs showing pH and electric conductivity (mS/m corrected to 18 °C) of peat samples as a function of depth (cm) at sites M1, M2, M2*, andM3 (Fig. 1). The red, dashed line forM2* simulates the compaction of the surface layer of the peat correcting the sample depth in comparison to M2. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)


Chemical and mineralogical characteristics of peat

The elemental concentrations in the non-mineralized site M1 werelower in comparison to the concentrations of the sitesM2 andM3 locat-ed in mineralized areas (Fig. 5). Only Pb and U presented similar con-centrations at all sampling sites with relatively low values rangingfrom 0.21 to 23.9 mg/kg (median values of 1.41 mg/kg for Pb and1.61 mg/kg for U). Lead was enriched in the surface samples of eachsite with average 20 mg/kg. Independently of the geological location,the concentrations ofMn and Pb, aswell as, Fe and Zn at siteM1, peakedimmediately below the surface layer of the peat. At sites M2 and M3,manganese experienced a decrease of one order of magnitude 20 cmbelow surface and peaked again at the depth of 50–100 cm. In mineral-ized areas, the general increasing trend of metals with increasing depthwas evident from Fig. 5, which highlights the elevated Fe (max.12.0 wt.%), Ni (758 mg/kg), Zn (5270 mg/kg), and S (13.6 wt.%) con-centrations, as well as Co (28.1 mg/kg) and Cu (324 mg/kg) to lesserextent, in the basal Carex peat layers. The downward increasing Feconcentrations at sites M2 and M3 were well-correlated with S content(R2 = 0.96) and conductivity (R2 = 0.88), and additionally, with sometrace elements (especially, Ni with R2= 0.74 and Cowith R2= 0.64) atsite M2. Merely, the concentrations of Cu, Ni, and S increased as a func-tion of depth at site M1.

The depth-corrected results for site M2* in Fig. 5, show similar ele-mental trends as for M2. The concentrations of Co, Ni, and Zn exhibitednearly same values, whereas Cu, Pb, and U concentrations were higherin the samples from M2*. Iron and S, on the other hand, exhibited sim-ilar patternswith a z-shaped curve. The concentrationsmeasured for Feand S at 100 and 150 cmdepth inM2* were lower than the correspond-ing samples fromM2.

The bulk XRD analysis of the M2* samples showed that at all of thesampled depths clay minerals, such as montmorillonite and illite, andgoethite [α-FeOOH] were present. In the samples below 0.5 m, pyritewas detected and the amount increased in the deeper samples. Chloritewas also detected in the samples below 1 m. The presence of quartzcould not be verified because of peak overlapping.

Sequential extractions

The sequential extractions unraveled the elemental distribution inthe different mineral fractions elucidating the possible elemental be-havior and tendencies in the peat (Fig. 6). The Ba (median value of thesum of all fractions 14.05 mg/kg), Cd (7.75 mg/kg), Cr (5.93 mg/kg),Mo (2.59 mg/kg), Pb (1.09 mg/kg), Sb (0.20 mg/kg), Se (8.30 mg/kg),Sr (8.72 mg/kg) and U (3.47 mg/kg) concentrations were generallyvery low and are not shown in Fig. 6. The concentrations of major ele-ments, such as Ca, Mg, and Mn, exhibited similar behavior with majorconcentrations in the water-soluble fraction increasing with depth(41–94% of the total content). In the superficial sample, these elementsappeared also in the exchangeable, Fe(III)hydroxides, organic, and sul-fide fractions, while the proportion of these fractions diminished in


the deeper samples to less than 10%. On the other hand, Al and Fe exhib-ited similar trends with exchangeable (Fe as monosulfides) and Fe(III)hydroxides as major fractions in the three top most samples. TheFe(III)hydroxide fraction was especially important covering from 57 to81% of the total concentrations. Goethite was detected in the XRD anal-ysis, but the S concentrations (probably as SO4) in this fraction imply thepresence of Fe(III)oxyhydroxysulphate phases such as schwertmannite[Fe16O16(OH)12(SO4)2]. The acidic pH of theM2* favors the precipitationof these phases. In the bottom sample, almost half of the Al content waspresent as water-soluble phase, whereas roughly 50% of Fe was boundto sulfide fraction. The Fe and S in the sulfide fraction probably representpyrite which is in accordance with the XRD analysis confirming thepresence of pyrite in the samples below 0.5 m (especially abundant inthe bottom sample).

Most of the As was adsorbed and co-precipitated in the Fe(III)hy-droxide phases throughout the profile, though it presented low totalconcentrations (b15.8 mg/kg). For other trace elements, including Co,Ni, and Zn, the exchangeable and Fe(III)hydroxide fractions were im-portant in the surface samples, but in deeper samples water-solublefraction (with 60 to80% of the total concentrations) gainedmore impor-tance. 20 to 25% of Cu was present in the organic matter fraction in thethree top samples, but the other trace elements showed much smallerproportions in this fraction. In the bottom samples, the proportion ofAs, Co, and Ni in sulfide fraction increased to about 25%,whereas almost70% of Cu was bound to sulfides. This implies the presence of smallamounts of chalcopyrite or other Cu-sulfide in the peat, even though itwas not confirmed by XRD. There were no detectable concentrationsof the analyzed elements in the residual fraction.

Discussion

The chemical composition of peat is generally affected by numerousfactors including plant physiology, geochemical and microbiologicalprocesses, and capillary flow of water through underlying soil, i.e.groundwater composition (e.g. Rose et al., 1979). The concentrationand chemical form of metals precipitated in peatlands depend on thepH and redox conditions, as well as, on the ion exchange capacity andthe number of complex compounds (Rose et al., 1979). The geochemicalcharacteristics of the studied till and peat samples varied according tothe geology of the bedrock, and abundant accumulation of metals andsulfur was detected in mineralized black schist areas. The till subsoillayer between the bedrock and peat layer exhibited markedly highermetal concentrations in the black schist areas than in mica schist dom-inated area. According to Loukola-Ruskeeniemi et al. (1998), groundwa-ter samples from dug wells in the Talvivaara area show higherconcentrations of Cu, Fe, Mn, Ni, and Zn in areas overlying black schiststhan non-mineralized areas. This implies that the composition of thebedrock and of the overlying subsoil reflects in the groundwater chem-istry, and consequently, the chemistry of the overlying peat layers.

At the site M2, the quality of both surface water and groundwaterwas influenced by sulfide-rich bedrock and till. The elevated metaland S concentrations, and subsequent high electric conductivity, at



Fig. 5.Graphs showing geochemical composition (mg/kg in logarithmic scale, except for Fe and S inwt.%) of peat samples as a function of depth (cm) at sitesM1,M2,M2* andM3. The red,dashed line for M2* simulates the compaction of the surface layer of the peat correcting the sample depth in comparison to M2. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)


Please cite this article as: Parviainen, A., et al., Pre-mining acid rock drainage in the TalvivaaraNi–Cu–Zn–Co deposit (Finland): Natural peat layersas a natural analog to construc..., J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.03.024


Fig. 6. The results of the six-step sequential extraction procedure for the elements of interest (total concentration in mg/kg) in samples from site M2*.


site M2 were influenced by the ARD derived from fractured black schistoutcrop (Fig. 2) rather than solely by the groundwater composition.ARD has been described as a natural analog to AMD in areas where sul-fide rich ore deposits are exposed to atmospheric conditions andweathering (Nordstrom, 2011; Plumlee et al., 1999). Generally, theblack schist deposits in Finland are found in topographical low areascovered by peatlands or lakes as a consequence of heavy erosion ofthis easily weathered rock type during the glaciations. At Talvivaara,the black schist deposit was so large before the last glaciationsthat even intensive glacial abrasion could not entirely level the land-scape. The sulfide-rich black schists weather more easily than othersurrounding rock types causing ARD and, consequently, acidificationof surface waters (Gustavsson et al., 2012; Kwong et al., 2009;Loukola-Ruskeeniemi et al., 1998). When the water level in TheSotkamo Ice Lake suddenly dropped and the black schist outcropswere exposed to weathering, ARD was intense in the area as evidencedfrom the lake sediment strata (Loukola-Ruskeeniemi et al., 1998).

The elevatedmetal and S concentrations at siteM3,while presentinglower values than at site M2, were mainly influenced by elevated con-centrations in groundwater derived from underlying black schist(Loukola-Ruskeeniemi et al., 1998). Here the effect of capillary actionbecomes more important as it is efficient in sandy till allowing the mo-bilization of nutrients and metals several meters above the bedrock in-terface. The capillary actionmost probably occurs at siteM1 aswell, butthe lowermetal concentrations of the underlying non-mineralized bed-rock and till subsoil were reflected as lower concentrations in the peat.

Carex peat which is dominant in the deeper parts of the sampledprofiles tends to be more dependent on the nutrients than Sphagnumpeat dominated at the surface layers. Carex peat has a greater potentialto bind metals than Sphagnum peat, especially Cu and Zn, from waste-water, such as, sulfide mine leachate (Ringqvist and Öborn, 2002;Ringqvist et al., 2002). In the samples from profile M2*, Cu was found


in the organic matter fraction of the sequential extractions, but othermetals exhibited very low concentrations in this fraction.

Electrical conductivity of peat correlates with the nutrient content,temperature and acidity of peatland waters, and provides a reliable in-dication of anomalies in peat (Puranen et al., 1999). In this study, highconductivity anomalieswere detected in Carex peat in black schist dom-inated areas. Moreover, higher acidity and increasing ash contentenhance the conductivity of peat at site M2. Similarly, Lerssi andVirtanen (2006) describe high conductivity in Carex-dominated bottomand middle layers of peatlands in close relation with black schist. Theash content in the samples at site M1was in the range of average valuesin the Finnish peat (2.6% in Sphagnum peat and 4.2% in Carex peat;Mäkilä, 1994), whereas at site M3 these values were slightly exceeded.However, the values recorded at M2 were anomalously high rangingfrom 6.6 to 43.3% (median 35.2%, Table 2). The peat type and the degreeof humification affect the ash content of peat, but at site M2 the metal-rich waters of ARD transport dissolved elements, which precipitate inthe peat layers increasing the mineral matter component and conduc-tivity, especially in the deeper layers. This is in agreement with the ele-vated metal and S concentrations in samples fromM2 and M2* (Fig. 5).The results of XRD and sequential extractions suggested that sulfideswere very scarce or absent in the surface sample, but their amountincreased in the bottom sample, implying that sulfides were notprincipally derived from surface runoff as detrital grains but as a re-sult of in situ precipitation. Metal and S-rich waters were derivedfrom ARD which flows on the interface of till and peat layers, aswell as, from capillary waters. The results highlighted also that pre-cipitation of Fe(III) hydroxides, such as goethite, was abundantthroughout the profile contributing to the high ash content. TheFe(III) hydroxide fraction, as well as adsorbed fraction in the sequen-tial extractions proved that these phases are an important trap formetals. Sobolewski (1999) also suggest that metal removal in peat




by hydrolysis of Fe(III) is efficient at pH above 3.5, which is in accor-dance with the sequential extraction results.

Wetland drainage, excavation or other types ofmanipulation ofwet-lands lowering thewater table have been shown to lower the pH and tocause mobilization of elements such as Al, As, Fe, and Sb (Appleyardet al., 2004; Ramchunder et al., 2009; Rothwell et al., 2010). In theTalvivaara area, the human exposure to Hg is attributed to the drainingactivities of peat, which increasedmobilization of Hg from soil especial-ly during snowmelt (Loukola-Ruskeeniemi et al., 2003). Studies on theimpact of drainage in a peatland in Central Finland corroborate thatlowering the water table causes subsidence of the surface layer alongwith lowering the pH and changes in the vegetation (Minkkinen et al.,1999).

The low pH at site M2 is attributed to the ARD, but was also influ-enced by the extensive wetland draining executed in the area roughly50 years ago. The pH dropped permanently, and subsequent changesin redox-conditions and microbiological activity in peat probably oc-curred. The underlying till layer and, at places, the Ni-rich bedrockwere exposed enhancing the acidification and mobilization of metalsand S. As both the bedrock and till at Talvivaara contain abundant sul-fides, the surface waters flowing in peatland drainage channels reactedwith sulfide-richmaterial resulting in acid runoff in the nearby streams.A dense network of drainage channels also promotes vegetationgrowth, which in turn enhances the water movement and runoff inpeatlands because the permeability of peat is dependent primarily onthe structure of the peat and the presence of tree roots and debris.Hence,wetland drainage directly lowers the residence time ofwater de-teriorating the possibilities of natural attenuation processes. By the timeof the first peat sampling campaign, the water table had recovered al-most to the same level as before drainages activities.

The results from the second sampling at site M2, corroborated thatthe deforesting of the area after the onset of Talvivaara mine in 2008provoked the compression of the surface layers of the peat, but drasticmetal mobilization was not observed. Merely, Cu, Pb, and U exhibitedslightly higher values in profile M2* than in M2 six years earlier, whileother elements showed similar concentrations.

Sequential extractions

Sequential extractions have been criticized for not being always per-fectly selective (Hall et al., 1996;McCarty et al., 1998), and this handicapmust be kept in mind. However, when combined with good knowledgeabout themineralogical composition of the studied material it has beenrecognized as a powerful tool for characterizing the elemental distribu-tion in different mineral fractions (Dold, 2003; Sobolewski, 1999).

The proportion of water-soluble fraction increased and provedimportant in the samples below 0.5 m for Co, Ni, Zn, Ca, Mg, and Mnranging from 61 to 100% of total concentrations. The proportion of S inthis fraction ranged also from 28 to 41% corresponding to up to29,300 mg/kg. The water-soluble fraction probably reflected the porewater chemistry as the metal concentrations in solution are forced toprecipitate as efflorescent salts, principally soluble sulfates, upon dryingthe peat sample. This implies high dissolvedmetal concentrations in thedeeper sections of the peat profile.

The concentrations of Al, Ca, Fe, K, and Mg in the second extractionstep (at pH 4.5) may derive from the clay minerals present in the peatdue to their exchangeable character. Choi et al. (2005), Köhler et al.(2003), and Rozalén et al. (2008) also described that the dissolution ki-netics of montmorillonite and illite increase at acidic conditions belowpH 5, though over much longer dissolution exposure time thanemployed here. The Al and Fe concentrations in the Fe(III)hydroxidefraction derive principally from secondary precipitates. The adsorbedand Fe(III)hydroxide fractions proved also important for the trace ele-ments especially in the surface samples trapping from 40 to 72% ofthese elements. Additionally, 20 to 25% of Cu was also retained by or-ganic matter in the surface sections.


The concentrations in the residual fraction were below detectionlimits implying there were no resistant silicates in the peat samples.Quartz, if present, could not be measured in the residual fraction be-cause the analysis of Si is not possible in the hydrofluoric extraction.

Analog to constructed wetlands

Peatlands are a natural analog to constructed wetlands, which areused as passive remediation method in areas receiving AMD or othertype of contaminated waters. The use of natural or constructed wet-lands in the remediation is based on their ability to filter particulatematter reducing suspended solids, their ability to remove and store nu-trients and heavy metals, and their natural buffering capacity (Bloweset al., 2003, and references therein). At near-neutral pH, constructedwetlands have been reported to retainmetals more efficiently than nat-ural wetlands (Mays and Edwards, 2001). However, the performancecapacity of constructed wetlands seems to be influenced by pH. Accord-ing to Woulds and Ngwenya (2004), the low efficiency in contaminantremoval (average 25%) was attributed to low pH (2.6), whereas higherefficiency has been recorded at pH N6 (Jarvis and Younger, 1999; Maysand Edwards, 2001). Extremely low pH may increase the solubility ofFe(III) oxyhydroxides and also suppress sulfate reduction by sulfate re-ducing bacteria (White and Gadd, 1996). Liming is used to buffer con-structed wetlands in the reducing and alkalinity producing system(RAPS; Younger et al., 2003) and successive alkalinity producing system(SAPS; Kepler and McCleary, 1995), which are designed for net-acidicdrainage. In natural wetlands, the buffering capacity may be drasticallyand permanently affected by AMD or natural ARD. Highly acidic andmetal-rich drainage may also deteriorate the vegetation, which maynot support the hostile conditions, consequently limiting plant uptake.This may be the case at M2, where throughout the peat formation theARD and half a century ago drainage activities have influenced the pHevolution of the site. The pH presented lowest values in the bottom sec-tion of the site, where the metal concentrations, including Cu, were thelowest in the organic matter fraction (Fig. 6).

However, the results for site M2* show that natural peatlands havecapacity to retain trace metals at net-acidic conditions attenuating theeffect of the low quality runoff. The Fe(III) hydroxides and sulfides con-tributed to the metal retention at different sections of the peat profile.Major proportion of Fe (and Al) was present as hydroxides, such as goe-thite, and probably as hydroxylsulfates which have great capacity totrap trace elements (Acero et al., 2006; Asta et al., 2010). Additionally,precipitation of Fe and S as pyrite contributed to the retention of theseelements in the bottom sections of the peat. Nevertheless, as discussedearlier local ARD and past drainage activities probably permanentlylowered the pH conditions which prevented more efficient metal andSO4 retention at this site. The real retention capacity cannot be evaluat-ed due to lack of information about the quality of the inflowing andoutflowing water.

Conclusions

This study reports on the variation in chemical characteristics of peatin different lithological areas. The higher heavy metal concentrations ofmineralized than in non-mineralized bedrock types were reflected aselevated concentrations both in the overlying glacial till and peat layers.Previously, groundwater quality has been described to be influenced ofthe high sulfide contents of the black schist aswell. At the study sitesM2andM3 located inmineralized black schist areas, the elemental concen-trations (especially Ni, Fe, S, and Zn) in peat showed an increasing trendwith depth. At the non-mineralized study siteM1, the concentrations ofCu and S increased as a function of depth, yet more moderately than atmineralized areas. The concentrations ofMn and Pbwere greatest at thepeatland surface, immediately below the surface layer of peat. Varia-tions in concentrations were caused by fluctuations in groundwater




level as well as changes in the mobilization and accumulation of theelements.

The highest metal and ash concentrations in the Talvivaara studyarea were recorded at the black schist study site M2. These results,thus, highlight that inmineralized areas the heavymetal concentrationsin peat can reachmuchhigher than average values in natural conditions.This ought to be taken in consideration in evaluating the environmentalimpact of the current mining operations, because the capacity ofpeatlands to capture heavymetals may be lower than in areaswith nat-urally low sulfide concentrations in the underlying bedrock.

Instead of attributing the elevatedmetal concentrations solely to theinfluence of metal-rich groundwater, the peat chemistry at this site wasmore likely governed by the ARD derived from the weathering ofsulfide-rich black schist outcrop on the adjacent hillside throughoutthe entire history of peat formation. In addition, peatland drainage ac-tivities in the 1960s resulted in acidic runoff wherever excavations pen-etrated to the underlyingNi-rich bedrock or glacial till. The precipitationand accumulation of metals in the peat at site M2 act as a natural atten-uation mechanism, therefore, representing a natural analog to con-structed wetlands which are used as a remediation method at miningareas impacted by low quality drainage. However, the influence ofARDanddrainage activities lowering the pH conditions probably dimin-ished the natural buffering capacity and the potential to retain metals,which has been proven more efficient in constructed wetlands withcircumneutral pH values. Therefore, it seems that the capacity of thepeatland at site M2 to capture metals from surface and groundwaterwas decreased already under natural conditions before commencementof the mining activities.

Acknowledgments

We thank Markku Tenhola, Pekka Lestinen, and Alpo Eronen, all re-tired from the Geological Survey of Finland, for the till geochemistrydata. Additionally, Heikki Säävuori and Harri Kutvonen are acknowl-edged for their contributions to this work.

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Documents

Pre-mining acid rock drainage in the Talvivaara Ni–Cu–Zn–Co deposit (Finland): Natural peat layers as a natural analog to constructed wetlands