Aqueous geochemistry in the Udden pit lake,northern Sweden

Madeleine Ramstedta,*, Erik Carlssonb, Lars Lovgrena

aInorganic Chemistry, Department of Chemistry, Umea University, SE-901 87 Umea, SwedenbDivision of Applied Geology, Lulea University of Technology, SE-971 87 Lulea, Sweden

Received 3 April 2001; accepted 17 January 2002

Editorial handling by B.A. Kimball

Abstract

The Udden pit lake in northern Sweden was studied from June 1998 to February 1999 in order to increase knowl-edge of the geochemistry in lakes created as a result of decommissioning open pit mines. The vertical water prole inthe lake was sampled on 4 dierent occasions, in June, August, September and February. Water samples were analysedfor total concentrations of Fe, As, Cu, Cd, Zn, Pb, Al, Ca, K, Mg, Na, Mn, S, Cl, N and P. Temperature, concentra-

tion of dissolved O2, conductivity, pH, and redox potential were measured in situ at dierent depths. Four layers couldbe observed in the lake during summer, and 3 layers during winter. A thermocline was observed during summer at adepth of 5 m and on all 4 occasions a halocline was observed at a depth of 20 m, and a redoxcline at 35 m. Oxygenconcentration decreased dramatically at a depth of 20 m. pH increased downwards in the lake from 4.8 at the surfaceto 6.4 at the bottom of the lake. Geochemical processes occurring in the lake, the origins of the layers, the metal con-centrations and the anion concentrations are discussed in this article.# 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

Decommissioning of open pit mines in order to pre-vent acid mine drainage often involves ooding the openpit and thereby creating an articial lake. In many cases

waste rock is deposited in the pit prior to ooding. Byooding it is expected that O2 will be prevented fromdiusing into mine wastes and rock walls, leading to a

minimised oxidation of sulphide minerals and a subse-quently minimised release of heavy metals and acidity(MEND, 1989). However, oxidation can still occur in

the absence of O2 but with Fe(III) as the oxidising agent(Herbert, 1999).These articial lakes are called pit lakes and usually

have a small diameter to depth ratio compared to nat-ural lakes. This, together with the saline waters, often

results in a pronounced stratication and with time, the

development of an anoxic bottom layer (Miller et al.,1996; Tempel et al., 2000). Doyle and Runnells (1997)stated that one of the most important variables con-trolling water circulation in lakes is the relative depth

(the relation between the depth and the diameter of thelake). Other important variables are the length of free windaction and the wave fetch (Klapper and Schultze, 1997).

The distribution and speciation of trace metals instratied lakes are inuenced by 3 major geochemicalprocesses; cycling across redox boundaries, sorption and

coprecipitation on/with particles and formation of metalcomplexes in aqueous solution with inorganic ligands(e.g. OH, HS) and organic matter (Balistrieri et al.,1994). Iron and Mn are important metals for controllingthe dissolved concentration of trace metals in oxicwaters since particles of Fe (oxy)hydroxides and Mn(oxy)hydroxides act as scavengers for trace metals.

Adsorption of metal ions onto Fe (oxy)hydroxidesincreases with pH (Dzombak and Morel, 1990), whileelements like As, forming oxo-anions, are preferably

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* Corresponding author. Fax: +46-90-786-9195.

E-mail address: madeleine.ramstedt@chem.umu.se

(M. Ramstedt).

adsorbed at lower pH (Miller et al., 1996; Lutzenkirchenand Lovgren, 1998). The objective of the present workwas not only to study and describe the aqueous geo-chemistry in a pit lake by eld sampling and in situ

measurements but also to discuss the obtained results interms of geochemical processes such as weathering ofrock walls, adsorption and coprecipitation of heavy

metals on Fe and Mn (oxy)hydroxides, sulphide pre-cipitation and oxidation of organic material.

2. Site description

The Udden open pit is situated approximately 70 kmwest of Skelleftea in northern Sweden (Fig. 1). TheUdden sulphide ore deposit was part of the Skelleftedistrict volcanic series. The bedrock in the area consists

mainly of tute, some amphibolite and quartz-sericiterocks, containing quartz and sericite with subordinateamounts of chlorite. The Udden ore deposit was part of

a greater vein, which is called Kedtraskfaltet. This veinalso includes 3 other ore deposits: Kedtrask, Asen andSvansel (Boliden, 1955, 1971, 1980). The principle sul-

phide minerals at Udden were pyrite (FeS2), pyrrhotite(Fe1-xS), sphalerite (ZnS), chalcopyrite (CuFeS2), galena(PbS) and arsenopyrite (FeAsS). The average grade ofthe ore is shown in Table 1. The mining in Udden star-

ted in 1971 with an open pit. After 3a of mining thesurface deposit was depleted and the mining operationscontinued underground. The mining ended in 1991. A

total of 690 000 ton ore and 1000 000 ton of waste rockwas excavated from the open pit (Boliden, 1980).The major waste rock deposits are situated south of

the pit lake and cover an area of 12 ha (Fig. 1). Thewaste rock consists of approximately 1 wt.% S and 25wt.% rocks with a buering ability. The calculated

concentration of calcite is less than 0.5 wt.% of thebuering rock. The waste rock deposit has been coveredwith till (Boliden, 1991). The decommissioning pro-gram, based on ooding the open pit, started in 1991.

All buildings, including the shaft tower, were demol-ished and the ground at the northern side of the open pitwas levelled out. The excavated material and the waste

material from the buildings were deposited into the pit(Boliden, 1990) together with waste rock from theclosely located Kedtrask mine. The waste rock from

Fig. 1. The location of the Udden pit lake in Sweden. Sampling localities marked with A, B and C.

98 M. Ramstedt et al. / Applied Geochemistry 18 (2003) 97108

Kedtrask had previously been stored in open air andwas thus signicantly oxidised (T. Goransson, Boliden,pers. comm.). The average grade of the ore in Kedtraskis also shown in Table 1. The surrounding rock has a

SiO2 content higher than 52 wt.% (Boliden, 1982). Thecomposition of the waste rock from Kedtrask wasunknown but, considering the composition of the ore,

the waste rock most likely had a high sulphide content.The shafts leading down to the underground mine inUdden were covered with blocks of concrete prior to the

remediation. Natural sand and tailings were disposed ofin the western part of the open pit. All surroundingareas were sown with grass. Due to a high inow of

groundwater the pit was lled within 2a (Boliden, 1990).Today, the pit lake is approximately 390 m long and150 m across (Fig. 1). The maximum depth of thelake was found to be 50 m using a sonic altimeter.

Because of the large relative depth (18%) (relativedepth 50zm

p=

A0

pwhere zm is maximum depth and

A0 is surface area. Doyle and Runnells, 1997) the lake is

possibly meromictic.The only inows to the lake are rainwater, surface

runo and groundwater. There is no outow of surface

water except at the northern side, where the lake over-ows into a small creek during periods when the watersurface is elevated. A site-monitoring program collects

samples for analysis of the water quality when surfacewater ows out from the lake. This happened in 1993,1994, 1995 and 1998.

3. Methodology

The water quality at dierent depths in the Udden pitlake was examined by collecting water samples and per-forming in situ measurements with a probe. Measure-

ments were carried out on 4 dierent occasions during1998 and 1999. Water samples were collected in June,August and September for analysis of metals and majoranions. The in situ measurements were performed on

the same occasions with additional measurements inFebruary. The transparency of the water was estimatedby measuring the Secchi-depth with a Secchi disk and

water binoculars.

3.1. In situ measurements

Conductivity, temperature, concentration of dissolvedO2 (DO), pH and redox potential were measured in situ

with a probe (Hydrolab MiniSonde) (The MiniSondemeasures redox as a potential between a Pt electrodeand a AgCl reference electrode immersed in a KCl

solution. Conductivity is measured as a current between2 electrodes held at a xed potential. Dissolved O2 ismeasured as a current resulting from the electrochemical

reduction of O2 at a Au membrane. For measuring pHthe probe has a glass electrode.). The probe was cali-brated in the laboratory prior to the measurements and

the measurements were performed at 3 locations in theUdden pit lake, points A, B and C (Fig. 1). In Junemeasurements were done only at point A, in August andSeptember, points A and B and in February, point C.

The DO data were recalculated into O2 saturation (%)according to the equation given by Weiss (1970).

3.2. Water samples

Water samples were collected using a metal free

Ruttner sampler at dierent depths at two locations inthe Udden pit lake, points A and B. Point A corre-sponds to the deepest part of the lake. Some duplicate

samples were collected at point B to study possible var-iations in metal concentration towards the shore. Sam-ples were collected at depths from 0.5 to 45 m.Replicates were taken at a depth of 10 m at point A.

Equipment blanks and travel blanks were collected. Therecovery of Cu, Cd, Pb and As in spiked samples wasbetween 89 and 96%. Water samples were collected on 3

dierent occasions during the period JuneSeptember1998; 2226 June, 310 August and 714 September. Toestimate natural background concentrations in the area,

samples were also collected in a nearby natural lake,Lake Ljustrasket.Water samples were collected in 50 ml acid washed

polypropylene tubes. Two samples were taken at each

depth, one ltered immediately through a 0.22 mmMILLEX1-GS MILLIPORE lter, to remove the par-ticulate phase, and one which was not ltered. Both

samples were acidied with concentrated suprapureHNO3 to obtain pH

using 200 ml 1.2 M HCl and stored in the dark at 4 C.None of the samples had any smell and no particulatephase or colour could be observed during the sampling,except for a few white particles below a depth of 30 m in

September.

3.3. Chemical analysis

The dierent samples collected were analysed accord-ing to Table 2. Samples analysed using inductively cou-

pled plasma mass spectrometry (ICPMS) were dilutedwith Millipore water (Super Q plus) and acidied. Theconcentration of As was corrected for interference with

ArCl. The total concentration of Fe was determinedafter reduction of Fe3+ in the samples. For reduction,0.3 ml of 6 M HCl / 1.2 M NH2OHHCl was added to15 ml of sample and the solution was kept at 60 Cfor 45 min and 5 ml of the reduced sample were mixedwith 5 ml phenanthroline in 40 ml of acetate buer. Theabsorbance was measured at 510 nm using UV/vis

spectrometry.The August samples were also analysed with ICP

AES by SGAB Analytica, Lulea, Sweden for validation

and to obtain data for additional elements. (All data arenot shown in this article, for a complete dataset seeRamstedt et al., 2002).

4. Results and discussion

4.1. In situ measurements of physical parameters

All physical parameters measured in situ, except for

temperature and conductivity, appeared to be ratherconstant during the period JuneFebruary (Fig. 2). Thetemperature was constant at 5 C below 20 m butshowed some variation in the upper 20 m of the watercolumn. During summer the surface water was heated toa temperature of 16 C and was cooled during autumnand winter. In the water column between 5 and 20 m,

warming was observed from June to September and

cooling from September to February. In February, theuppermost part of the water column showed lower tem-peratures than the rest of the lake.During summer a thermocline and a halocline were

developed at 5 m. The uppermost part of the water col-umn had a higher degree of O2 saturation during sum-mer due to contact with the atmosphere and mixing by

wind and waves. The conductivity increased stepwisedownwards in the lake from 1 to 2 mS/cm. BetweenJune and September, a halocline was found at the ther-

mocline and another one was found at a depth of 20 m.In February there was only one halocline at a depth of20 m. According to Levy et al. (1997), the distribution

of O2 and other constituents in a lake may be aectedby the circulation patterns. In Udden, dissolved O2 wasfound down to 20 m, thus indicating a possible seasonalturnover of the top 20 m in the lake. The February

measurements, showing a rather homogenous top 20 m,support this assumption.pH showed the lowest value, 4.8, at the surface of

the lake. Below the halocline (at 20 m) pH increased to5.5 and remained rather constant down to 35 m,where it increased to 6.4. Pit lakes created from mineswhere sulphide ore has been extracted are often acidic.Two examples of this are the Spenceville Cu pit inNevada USA, with a pH around 2.5 (Levy et al., 1997),

and the Berkeley pit in Montana USA with a pHaround 3 (Davis and Ashenberg, 1989). The reason forthe relatively high pH in Udden is thought to be therather strong buering ability of the surrounding rock,

the shaft tower and the blocks of concrete covering theunderground mine. Another important factor is thatcomparatively small areas of sulphurous rock are

exposed at Udden, thus making the continuous produc-tion of acid rather limited (Nils Eriksson, Boliden, pers.comm.). The relatively low pH at the surface of the lake

is likely caused by oxidative weathering of mainly pyrite.From the in situ measurements, 4 distinct layers could

be dened in the pit lake during summer and autumn, asa result of dierences in temperature and salinity (Doyle

and Runnells, 1997). The layers are indicated in Fig. 3

Table 2

Techniques used for analysis of dierent elements on dierent sampling occasions during 1998 in Udden

Analysis Elements Sampling period

ICPMS As, Cu, Cd, Zn, Pb June and August

UV/vis Fe June, August and September

ICPAESa Li, Na, K, Ca, Mg, Mn, Al, Si, Ba, Sr, Co, Cr,

Mo, Ni, S, V, B, Zn, Cu, Cd, As, Pb, Fe

August

Ion chromatographyb S, Cl June and August

Continuous ow analysisUV/visc N, P June, August and September

Pt- catalysed high temperature combustionc C (TOC) September

a The analyses were performed at SGAB, Lulea, Sweden.b The analysis was performed at SLU, Umea, Sweden.c The analysis was performed at UMF, Norrbyn, Sweden.

100 M. Ramstedt et al. / Applied Geochemistry 18 (2003) 97108

and will be referred to as layer I, II, III, IV in the fol-lowing text. In Udden, layer I reaches from the surfacedown to the thermocline situated at a depth of approxi-mately 5 m. Layer II reaches between 5 m and thehalocline at 20 m. Layer III is dened as the section ofwater between 20 and 35 m. In this layer the O2saturation decreases dramatically to reach a value of 0.3

mg/l at 35 m depth. Layer IV stretches from 35 m anddownwards. It is anoxic and the redox potential in thislayer is lower than in the other 3. In February the

uppermost layering, i.e. the thermocline, had dis-appeared. The dierent layers found in the Udden pitlake can be compared with redox layers found in natural

meromictic lakes reported in the literature, e.g. HallLake in Washington, USA. Hall Lake is 15 m deep with3 redox layers (oxic, suboxic and anoxic layer) which areeach approximately 5 m deep (Balistrieri et al., 1994).

The redox potential (Eh) measured was rather stableat 500 mV down to a depth of approximately 30 m. Inlayer IV Eh decreased with depth to a value of 300 mVclose to the bottom of the lake. The redox readings inJune were unstable and are thus excluded. The degra-

dation of organic material requires an oxidising agentand, since there is no mixing vertically, O2 and otheroxidising agents will be depleted. The redox potential isthus expected to decrease towards the bottom of the

lake in a similar manner as was observed in Udden. Theredox level in the 4 dierent layers is thought to begoverned mainly by dissolved O2 in layer I and II and

by Fe in layer IV. Which species govern the redox levelin layer III is yet an unresolved question as is the smallbut consistent increase in Eh at 20 m. One should bear in

mind that redox potentials measured in natural systemsmay be dicult to interpret due to the fact that naturalwaters seldom are in internal equilibrium, and the redox

potential measured should rather be viewed as an indi-cation of the overall redox state of the water. Apparentredox levels can be calculated based on equilibriabetween dierent redox indicator species e.g. dis-

solved O2, Fe(II)/Fe(III), HS-/SO4

2- etc. (Lindberg andRunnells, 1984). From these apparent levels, saturationindices can be calculated for the redox reactions in

question. This approach will result in a constant inwhich all uncertainties are accumulated such as errors in

Fig. 2. Physical parameters measured in situ during the period June 1998February 1999.

M. Ramstedt et al. / Applied Geochemistry 18 (2003) 97108 101

redox measurements and non equilibrium status ofthe water. Such calculations, with respect to Fe2+/FeOOH(s) equilibria, will be reported in a later sectionof this paper.

4.2. Nutrients and organic carbon

The total concentration of N increased with depththrough the upper 3 layers of the lake, but appeared todecrease downwards in layer IV (Fig. 4). The con-

centration of N in Udden was much higher compared toL. Ljustrasket; the concentration near the surface wastwice as high, and at a depth of 30 m almost 5 times

higher than the surface water in L. Ljustrasket (surfaceconcentrations: 0.5 mg/l in Udden and 0.2 mg/l in L.Ljustrasket). Although none of the values can be regar-ded as high for surface waters one should bear in mind

that the Udden pit lake has mainly been lled by a largeinow of groundwater, which in this area generallyexhibits very low N concentrations. A possible source of

the high total concentration of N is diusion of remnantundetonated explosives from the rock walls and thedeposited waste rock. The concentration of N decreases

below 30 m, which can possibly be explained by deni-trication, i.e. reduction of NO3 to elemental N2, whichin turn can escape the lake in the gaseous phase.

The total concentration of P was below 5 mg/l on all 3sampling occasions. Doyle and Runnells (1997) statedthat meromictic lakes generally are oligothrophic andhave low biological productivity, which also was

observed in Udden. Simultaneously with this study ofthe geochemistry of the Udden pit lake, an investigationof the phytoplankton community in the two lakes was

made (not reported here). The amount of algae wasmuch lower (1/10) than in L. Ljustrasket and some ofthe plankton species found in Udden are species that are

known to be able to survive in waters with high metalcontent (Soe Abser, pers. comm.).The concentration of total organic C was measured in

the September samples from each layer of the lake. Inlayer I, at 3 m, the concentration was 0.6 mg/l. At 10 mit was slightly less, 0.4 mg/l. Further down in the lakethe content of organic C increased and at 30 and 45 m it

was 1.0 and 2.7 mg/l, respectively. A comparison ofthese concentrations with the concentration in LakeLjustrasket shows that the content of organic C in

Udden was low, 0.6 mg/l compared with 5.8 mg/l at 3 m.The low content of particles and total organic C was alsoreected in the water transparency. The Secchi depth

was 9 m in June and 12 m in September and August.The lower transparency of the water in June is thoughtto be due to an increased amount of algae at that time.

Fig. 3. Visualisation of the observed layers in the lake. The

layers are numbered from the surface and downwards. Data

from measurements in August 1998 (O2 sat=saturation in %,

cond=conductivity in mS/cm, Eh=redox potential in mV). Fig. 4. Concentration of N at dierent depths in Udden.

102 M. Ramstedt et al. / Applied Geochemistry 18 (2003) 97108

4.3. Main elements

For most water samples no signicant dierence inconcentration could be found comparing the ltered

(0.22 mm) and the non ltered samples from the samedepth. Nor could any signicant dierence be foundbetween sampling points A and B. These two observa-

tions indicate that the water is well mixed horizontallyand that the metals present in the water either occur assoluble species or the colloidal phase passes through the

lter. Previous studies by e.g. Kimball et al. (1992) haveshown that particulate material can pass through thelters and, thus, we have chosen to include discussions

considering possible sorption onto a colloidal particu-late phase although we do not have any data supportingthis.The proles for SO4

2 and Cl concentrationsstrongly resemble those for the conductivity measure-ments (Figs. 2 and 5). The concentration of Cl

increased downwards in Udden but appeared to be

rather stable within each layer. The origin of this Cl isprobably weathering of Cl-containing silicate minerals.The largest dierence in concentrations was found at the

border between layers I and II and between layers II andIII. The concentration of Cl was 5 times as high as inL. Ljustrasket at the surface, and 9 times as high in the

bottom water. The SO42 concentration prole showed

exactly the same pattern but with a higher concentrationdierence between the pit lake and Ljustrasket (Udden140 400 mg/l; L. Ljustrasket 0.7 mg/l). The origin of

SO42 is obviously oxidation of sulphide containing

minerals in the rock walls and waste rock.The authors attempted to distinguish between SO4

2

and reduced S species in the water samples. However,samples collected in June and August for analysis of Cl

and SO42, although stored in a freezer until analysis,

showed signs of oxidation in the sense that an Fe oxideprecipitate had formed. This might indicate that theanalysed concentration of SO4

2 possibly represents thetotal concentration of S.

The concentrations of Na, K, Mg and Ca increaseddownwards in the lake and the depth prole resemblesthat of the conductivity. Calcium was the dominating

cation with concentrations of 140 mg/l at a depth of 3 mand increased levels downwards in the water column toa concentration of 440 mg/l at 45 m depth. The second

most common cation in the lake was Mg with con-centrations between 24 and 45 mg/l. The positivecharge, formed by those major cations, is mainly

balanced by SO4 ions. The high concentrations of Caand SO4

2 implies that formation of gypsum is possible.Thermodynamic calculations (using equilibrium con-stants from Ball and Nordstrom (1991) adjusted with

respect to temperature and ionic strength) result insaturation indices close to zero in the fourth layer of thelake, which indicates that gypsum is likely to be formed

at that depth and presumably the white particlesobserved in September were small particles of gypsum.

4.4. Iron, manganese and trace metals

The concentrations of Fe in mine waters are oftenhigh due to the slow oxidation of Fe(II) in acidic water

(Nordstrom and Alpers, 1999) and the origin is oftenconsidered to be weathering of the rock walls (e.g. Davisand Eary, 1997; Miller et al., 1996; Levy et al., 1997).

Bacteria increase the oxidation rate of sulphide miner-als, especially at low pH (MEND, 1995), and since bac-teria are ubiquitous, the oxidation of rock walls in the

Udden pit lake can be considered as micro-organismmediated.The concentration prole for Fe in Udden corre-

sponds well with what could be expected from the redox

prole, with a rather constant concentration in theupper 20 m and a sharp increase in Fe below 35 m(Fig. 6). It can be expected that Fe(III) dominates under

oxidised conditions, while Fe(II) is stable at greaterdepths with anoxic conditions. However, the high Feconcentrations found in the ltered samples from the

upper 20 m, 3 mg/l, are much higher than thethermodynamic solubility of Fe (oxy)hydroxide in the

Fig. 5. Concentrations of Cl and SO24 at dierent depths inthe pit lake.

M. Ramstedt et al. / Applied Geochemistry 18 (2003) 97108 103

H GallardTexte surlign

current pH-range. This can be caused either by a pro-nounced disequilibrium explained by slow oxidationkinetics or by the presence of colloidal Fe(III) (oxy)-hydroxides which have passed the lter (

Mn(IV) (oxy)hydroxides dissolve at lower pH. Besidesbeing more thermodynamically stable than Fe(II), theoxidation of Mn2+ is known to be a considerably slowerprocess than Fe2+ oxidation (Stumm andMorgan, 1996).

The concentration prole for As in Udden showssimilarities to the prole for Fe (Fig. 6), i.e. low con-centrations down to a depth of 35 m and increasing

concentrations with depth in layer IV. This observationmay be explained by a release of adsorbed As from sur-faces of colloidal Fe (oxy)hydroxide or could be a con-

sequence of reductive dissolution of FeAsO4 and/oroxidative dissolution of FeAsS by Fe(III) originatingfrom secondary Fe minerals in the deposited waste.

Arsenic can exist either as As(V) or as As(III), eitherway it is present as oxyanions, HnAsO3

n3 orHmAsO4

m3 (n, m=0,1,2,3).Both forms can be adsorbed onto surfaces of Fe

(oxy)hydroxides, a process which is to some extent pHdependent. Arsenic has been shown to adsorb onto Fe(oxy)hydroxide within a wide pH range (pH 310),

although the adsorption is strongest at lower pH (Lut-zenkirchen and Lovgren, 1998). The pH of the Uddenpit lake (pH 4.56.5) is within the range of pronounced

As adsorption and, thus, if colloidal Fe (oxy)hydroxidesare present, the origin of As is likely to be the reductivedissolution of these.

The concentration of Pb was rather constant around1 mg/L through the rst two layers and decreased in theupper part of layer III, where the O2 concentration alsodecreased (Figs. 2 and 7). The concentration remains

low through layers III and IV. Comparing these con-centrations with guideline concentrations for surfacewater provided by the Swedish Environmental Protec-tion Agency (Naturvardsverket), shows that the Pb

concentrations would be classied as low concentrations(Naturvardsverket, 1999).The Cu concentration increased from 80 to 140

mg/l at the interface between layers I and II (Fig. 7). Theconcentration is rather stable within layer II butdecreases dramatically in the upper parts of the suboxic

layer III and even more in layer IV.The concentrations of Cd and Zn are higher in layer

II compared to layer I (Fig. 7). The concentrations

increase downwards through the second layer, dropslightly just at top of the suboxic layer III and decreasedownwards in the same layer and nally, in layer IV, theconcentrations decrease to a level equal to or below the

concentration at the surface. The dip in the concentra-tion curves for Zn and Cd at approximately 20 mappears at the same depth as the temporary rise in the

redox proles. Nickel concentrations are highest in layerII (450 mg/l) and decrease towards the bottom of thelake similarly to Cd, Zn, Pb and Cu (Fig. 7).

Metals expected to precipitate as sulphides underanoxic conditions are e.g. Pb, Cu, Zn, Cd and Ni. Thus,the aqueous concentrations of these metals could be

expected to decrease across an oxicanoxic interface.Since the solubility of metal sulphides is increasing inthe order CuS

form before CdS and ZnS. At high concentrations ofsulphide, aqueous sulphide and bisulphide complexesmay be formed, which would lead to increased metalsolubility (Balistrieri et al., 1994). However, the

observed redox potentials in Udden, even at the greatestdepths, are all in a range unfavourable for SO4

2 reduc-tion. Thus, the concentration of sulphides can be

expected to be low and the precipitation of metals asmetal sulphides should therefore be limited and theformation of bisulphide complexes nonexistent. On the

other hand, the redox potential measured is an averageredox level and will mainly be inuenced by the highamounts of Fe in solution. Thus, sulphide species might

have been formed in the bottom water without aectingthe measured redox potential. Furthermore, sulphideminerals, such as pyrite (FeS2), may dissolve to a certainextent in contact with water and form free metal ions

and sulphide ions, as well as other reduced S anions.Slow oxidation kinetics would allow this sulphide toreact with heavy metals in the water and form solid

metal sulphides. It seems likely, from the data obtained,that the metals in the suboxic and anoxic region are to alarge extent associated with solids which are removed

from the water column by settling. However, theauthors do not have any data supporting precipitationof metal sulphides and pH is too low for formation of

metal oxides. The mechanism behind trace metalremoval from the water column in Udden at largedepths is yet to be resolved.Correlation coecients between the dierent elements

and the dierent physical parameters were calculated.High correlation coecients were obtained betweenconductivity, Ca, K, Na, Mg, S, Si, Mn, and Sr con-

centrations. The redox potential strongly correlates withCd and Fe concentrations, and Al, Ba and Cu con-

centrations are correlated with pH. The strong correla-tion between conductivity and the dierent metalsmentioned is not surprising since the conductivity is ameasure of charged ions and particles.

4.5. Temporal trends

The Udden pit lake has been sampled by Boliden ABon several occasions since 1993 as part of a monitoringprogramme. A decrease in metal concentrations from

1993 to 1998 can be noticed (Table 3). This may be theresult of a number of processes. Some of the metal ionsthat were leached from the deposited waste material and

the rock walls after ooding may have adsorbed ontosettling mineral particles or precipitated as secondaryminerals. Furthermore, the weathering of the pit wallsand waste rock can be expected to be substantially

reduced. The observed trend is an indication of a suc-cessful decommissioning of the pit mine. However, itshould be noted that the exchange of water between the

pit and its surrounding soil and bedrock has not beenquantied. There is no inow of surface water into thelake besides the annual precipitation, which in this area

is fairly low, 518 mm in 1999 (P. Lindstrom, Boliden,pers. comm.). The only other sources of water aregroundwater and the increased amount of surface runo

during the spring when the snow melts. The rapid llingof the abandoned pit implies a large ux of ground-water, however, the topography of the area does notindicate the presence of a large hydraulic gradient after

the pit was lled. The extent to which decreased metalconcentrations can be explained by an outow of waterthrough the ground is an open question. However, the

stability in e.g. temperature, redox potential, SO42 and

Cl concentrations suggests a rather low ux of water.

Table 3

Water quality data for surface water collected in Udden pit lake previously and during 1998

Date Cu (mg/l) Zn (mg/l) Pb (mg/l) Cd (mg/l) Cond (mS/m) SO42- (mg/l) pH (units)

Surfacea 931116 0.17 51.6 62 0.08 160 976 4.7

Surfacea 940803 0.17 81.1 10 0.13 184 1270 5.8

Surfacea 950529 0.04 14.8 18 0.024 66 303 5

Surfacea 950609 0.11 2.58 6 29.5 932 2.7

Surfacea 950816 0.12 67.3 8 0.11 174 1081

Surfacea 980519 0.03 2.15 18 34.5 172 4.3

Surfacea 980812 0.10 35.0 26 98.0 746 5.9

0.5 m 980625 0.076 33 0.66 0.033 93 159 5.14

980806 0.086 34 1.2 0.036 95 148 4.93

980914 107 4.87

Surface=surface water.

Cond=conductivity.a =Data collected by the monitoring programme at Boliden (unpublished data).

106 M. Ramstedt et al. / Applied Geochemistry 18 (2003) 97108

4.6. Comparison with other pit lakes

In comparison with other pit lakes described in theliterature, the water quality of the Udden pit lake is

relatively good in terms of pH and heavy metal content.However, the concentrations for most metals are muchhigher than background concentrations and guideline

concentrations given for lake and surface water byNaturvardsverket (1999). The extent to which the higherpH can be explained by the acid neutralising eect of

the concrete, in e.g. the shaft tower, or a high bueringability of the surrounding rock cannot be deduced fromthe present data. The content of Fe, Cu, Zn, Cd and

SO42 is much higher than in natural lakes due to dis-

solution of old weathering products and weathering ofsulphide minerals. Comparing the concentrations foundin Udden to those in Berkeley pit and in the Spenceville

Cu pit, the metal concentrations in the Udden pit lakegenerally appear to be lower. Comparing the dierentpit lakes mentioned in Miller et al. (1996), it can be

observed that pit lakes with higher pH tend to havelower aqueous metal concentrations. This can be due toseveral mechanisms: decreased solubility of both pri-

mary and secondary mineral phases and increasedadsorption to particle surfaces, mainly secondary Fe(oxy)hydroxides. The oxidation rate of sulphides is also

higher in acidic waters. Comparison of the results fromUdden with measurements made in Hall Lake (Balis-trieri et al., 1994) shows a similar behaviour of 3 ele-ments measured in both lakes (Cu, As, Zn). Hall lake is

a natural lake and most metals in this lake precipitatebelow the anoxic border where sulphide is generated.

5. Conclusions

Four distinct layers could be observed in the Uddenpit lake from the measurements performed during sum-mer and autumn 1998. The layers seem to be establisheddue to temperature gradients and dierences in salinity

in the water column. A thermocline and a markedlyincreased salinity were found at a depth of 5 m duringsummer, separating layer I from layer II. Layers II and

III are separated due to a change in salinity at a depthof 20 m. A redoxcline seems to have been formed at adepth of 35 m as a result of this layering. The strati-cation of the lake leads to depletion of oxidising agentsand the oxidation of organic material may occur dier-ently in the 4 layers. The metal concentrations were

rather stable throughout the entire period, but dier-ences in concentration could be observed at dierentdepths. Iron and As showed a pronounced increasebelow 35 m where Fe(II) is thermodynamically stable.Lead concentrations decreased downwards in the lake.The concentrations of Cu increased signicantly at adepth of 5 m and remained at that level down to a

depth of 20 m, below which they decreased. The Cdand Zn concentration proles showed similar patternsto Cu but with a less striking increase at 5 m. Thedecrease of Cu, Cd, Pb and Zn may be explained by

precipitation of the metals as sulphides, although itshould be noted that the measured redox potential atthis level is too high to support SO4 reduction. The

dominating anion in the lake is SO42 and the concentra-

tion of SO42 increases downwards in the water column

but appears to be rather stable within each layer

observed. Close to the bottom of the lake the con-centration of SO4

2 is at a level corresponding to thesolubility of gypsum.

This investigation is a description of the prevailinggeochemical conditions in the Udden pit lake. For animproved and more complete understanding it would benecessary to also include a detailed hydrological study

to form the basis for mass balance calculations. Withmore knowledge of sediment composition, sedimentdepth and composition of the colloid phase together

with a detailed hydrological study, it would be possibleto gain a better understanding of dierent processes inthe lake.

Acknowledgements

The authors would like to thank Nils Eriksson, JohanLjungberg, Staan Sjoberg, Kristina Axe, Julia Shealsand two anonymous referees for valuable comments on

the manuscript and Milan Vnuk for help with the site-map. The eld study was nanced by Boliden MineralAB. This work was also supported by funds from the

Foundation for Environmental Strategic Research(MISTRA) provided to the senior author (L.L.) throughthe research programme MiMi (Mitigation of the

Environmental Impact of Mining waste).

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atl1IntroductionSite descriptionMethodologyIn situ measurementsWater samplesChemical analysis

Results and discussionIn situ measurements of physical parametersNutrients and organic carbonMain elementsIron, manganese and trace metalsTemporal trendsComparison with other pit lakes

ConclusionsAcknowledgementsReferences