Environmental Geochemistry and Health (1997), 19, 155±164

Geochemistry and acidity of sulphide-bearing

postglacial sediments of western Finland

Mats AÊ stroÈm* and Alf BjoÈ rklund

AÊ bo Akademi University, Department of Geology and Mineralogy, FIN-20500 AÊ bo, Finland

The geochemistry, mineralogy, acidity and electric conductivity of sulphide-bearing postglacial sediments located

on the coastal plains of western Finland were studied. Grain size andmineralogical analysis (eight samples studied)

indicated that the sediments are silts dominated by primary minerals (quartz, feldspars), and that the clay fraction

(<2 �m), which made up less than one-third of the bulk samples, is enriched in phyllosilicates but depleted in quartz

and feldspars, relative to bulk samples. As compared to the metal contents of the fine-fraction of glacial till (data of

the Geological Survey of Finland), the fine sediments appear to be enriched in sulphur (median = 0.54%) and trace

metals such as Ni (31 �g gÿ1), Zn (90 �g gÿ1), Co (13 �g gÿ1), Cr (48 �g gÿ1) and V (50 �g gÿ1). Statistical analysis

indicated that aluminosilicates are the principal carrier phases of trace metals, and that sulphides and organic

matter only partially control the distribution of elements other than S and C. A 3-g portion of 232 samples was

oxidised in the laboratory by atmospheric oxygen for a period of 1 year. After this period of oxidation, 15 ml of water

was added, after which pH and electric conductivity of the suspensions (sediment:water, 1:5) were measured. The

quantities of elements mobilised by the oxidation was calculated from the dissolved concentrations found in 3 g of

oxidised sediment suspended in 30ml of deionised water (eight samples studied). The results of the experiment

showed that the impact of sulphide oxidation on the pH and conductivity of the sediments is large, and that a high

proportion (>13%) of the aqua-regia extractable contents of several elements (Co, Mn, Ca, Ni, Zn, Sr, Na and B) are

mobilised at oxidation. Because of the high quantities of acidity and metals liberated at oxidation, it is argued that

stream-water quality, which is poor in the study area, will not improve unless artificial draining is restricted in areas

of sulphide sediments.

Keywords: Geochemistry, metals, sulphide sediment, oxidation, acidification

Introduction

Large areas along the coast of Finland are coveredwith fine-grained sulphide-bearing sediments. Whenthese are brought into the oxidation zone, either bynatural processes such as postglacial isostatic landuplift or by ditching, dredging and other land use,sulphuric acid is formed and metals are released intothe soils (Palko and Yli-Halla, 1988, 1990). This hasresulted in environmental problems such as soilacidification (e.g. Palko, 1994), increased contentsof Ni, Co and Cr in timothy grass (Palko, 1986) andof Fe and Co in oat grains (Yli-Halla and Palko,1987), elevated concentrations of toxic metals (Cd,Tl, Al, Mn) and acid compounds in stream waters(EdeÂn and BjoÈrklund, 1993; Palko and Weppling,1994, Lahermo et al., 1995, AÊ stroÈm and BjoÈrklund,1995, 1996) and the sudden dying of fish in streams,rivers, estuaries and in coastal waters (Hudd et al.,1984). Similar environmental problems occur inother regions covered with corresponding sediments(e.g. eastern Australia; Willett et al., 1993) and inareas affected by active and abandoned sulphide

mines (Fuge et al., 1991; Axtmann and Luoma,1991; King, 1995) and by oxidising sulphide-con-taining bedrock (Hindar and Lydersen, 1994).

The soils developing on sulphide-bearing sedimentsare characterised by a low pH (3±4) and highsulphate contents, and are therefore classified as acidsulphate soils (Brinkman and Pons, 1973). The dis-tribution of these soils in Finland has been studied onseveral occasions. Based on pH and S determinationsand on field observations, Purokoski (1958) esti-mated that acid sulphate soils cover an area of 380km2 in coastal regions. Later studies, however,showed that his estimate is too low (ErvioÈ , 1975).Based on pH measurements of vertical profiles at740 randomly selected field sites, Palko (1994)estimated the area covered by this soil type to beas large as 3360 km2.

Despite the environmental problems related to theoxidation and weathering of the sulphide-bearingsediments, the geochemistry of such sediments hasbeen studied to only a limited extent. Hartikainenand Yli-Halla (1986) showed in a laboratory experi-ment that Al, Fe and alkali and alkaline earth metalsare liberated at oxidation due to cation exchange andto dissolution of hydrous oxides. Palko and Yli-Halla

0269-4042 # 1997 Chapman & Hall

*To whom correspondence should be addressed.

(1988, 1990) showed that the mobility of metals (Co,Ni, Zn, Al) are higher in acid sulphate soils than inother soil types. In corresponding sediments studiedat three sites in Sweden (OÈ born, 1991), the contentsof Mn, Ca and Mg are considerably lower in the acidsulphate soil than in the parent material, indicatingextensive leaching of the metals during soil forma-tion.

In the present investigation, the contents of metalsand S, C and P were determined in 317 samples ofunoxidised sulphide-bearing sediments collected inwestern Finland. The acidity, electric conductivityand easily mobilisable element contents were deter-mined in selected samples subjected to experimentaloxidation. The main objectives were to study therelation between the geochemistry and potential

acidity of the sulphide sediments, and to estimatethe extent of metal mobility caused by the oxidationof this type of overburden.

Physiography and Geology

In western Finland, the geographical relief is verylow owing to an old and deeply eroded bedrocksurface and to thick post-glacial sediments fillingthe depressions. The bedrock is dominated by Sve-cofennian granites, gneisses and schists. Carbonateand sedimentary rocks are scarce (Simonen, 1980).The area is covered mainly by Quaternary deposits,including till and glaciofluvial material, marine andlacustrine clay and silts, littoral deposits and peat(Kujansuu and NiemelaÈ, 1984). The clay and siltsediments were deposited mainly during the stages of

Figure 1 Sites of sediment sampling in the coastal area of western Finland.

156 M. AÊstroÈm and A. BjoÈrklund

the development of the Baltic Sea, dating to the ±Yoldia (10 000±9300 BP), Ancylus (9300±8000 BP)and Litorina (after 8000 BP) periods. During theLitorina period, large quantities of decaying organicmatter on the sea bottoms and brackish water condi-tions resulted in extensive reduction of sea-watersulphates and precipitation of metal sulphides inthe sediments (Palko, 1994).

Precipitation, which normally is in the form of snowfrom December to April, is around 500 mm yearÿ1.

Sampling and Analysis

Samples of fine-grained (clay/silt) sediments werecollected at 317 sites (Figure 1) from the ground-water zone at depths of 100 to 350 cm. A 0.5 gportion was analysed for (Ag), Al, As, (Au), B, Ba,(Bi), Ca, (Cd), Co, Cr, Cu, Fe, K, La, Mg, Mn, (Mo),Na, Ni, P, Pb, (Sb), Sr, Th, Ti, (U), V, (W) and Znwith ICP-AES after digestion in 3ml 3:1:2HCl:HNO3:H2O (aqua regia) for 1 h at 958C anddilution to 10ml with water. Fifty-nine samples wereanalysed in duplicate. The concentrations of theelements in brackets were below the detection limitin most samples. The aqua regia leach dissolvesorganic matter, metal sulphides and phyllosilicatessuch as clay minerals, mica and chlorite, whilequartz, feldspars and amphiboles are almost unal-tered (RaÈisaÈnen et al. 1992; Koljonen and Malisa,1991; Chao, 1984) Total C and total S contents weredetermined with the Leco method at 8508C.

Subsamples from eight sites (A±H; Figure 1) weresent to the Macaulay Land Use Research Institute,Aberdeen, Scotland, for semi-quantitative X-ray dif-fraction analysis, and to the laboratory of the Geo-logical Survey of Finland for grain-size determina-tion by the laser method. Samples from these siteswere analysed also for carbonate (CO2 released inacid) and organic C contents (difference betweentotal and carbonate C).

At 232 sites (73%), a 9 ml portion of the sedimentwas placed in a plastic tube and was mixed with 15ml of deionised water for 1 min. Three days later, thepH and the conductivity of the suspensions weremeasured (First measurement), whereafter the sam-ples (sediment+water) were dried at 508C for tendays (Figure 2). After disaggregation, a 3 g portion ofeach sample was kept in room temperature for oneyear while wetted with 3 ml of deionised water everymonth. After this period of oxidation 15ml of waterwas added (sediment:water 1:5), the suspensionswere shaken for 1 min and two weeks later thesupernatants were measured for pH and conductivity(Second measurement). Another 15 ml of deionisedwater was then added to the samples from sites A±H(sediment:water 1:10; Figures 1 and 2). The super-natants of these samples were filtered (0.45 �m) fivedays later and analysed for (Ag), Al, As, B, Ba, Be,(Bi), Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, (Mo),Na, Ni, Pb, Rb, Sb, Se, Si, Sr, Th, Tl, U, V and Znwith ICP (MS and AES).

Figure 2 Experimental oxidation of fine-grained sediments.

Sulphide-bearing postglacial sediments of western Finland 157

Analytical precision

The analytical precision at the 95% level of con-fidence calculated from 59 duplicate sediment sam-ples (only 29 pairs for S and C) by the methodproposed by Garret (1969, 1973) was 12%, 13%and 18% for Cu, Th and Pb respectively, and betterthan 10% for Zn, Ni, Co, Mn, Fe, Sr, V, Ca, P, La,Cr, Mg, Ba, Ti, Al, Na, K, S and C. The precision forAs (50%) and B (25%) were poor and the results ofthese elements are, therefore, not presented. Thecorresponding figures were 4% for pH (47 replicatemeasurements) and 1% for conductivity (26 replicatemeasurements). The precision of the ICP analysis ofthe leachates was not determined because of the lownumber of the samples (n=8).

Results and Discussion

Sediments

Grain size and mineralogy. The grain-size distribu-tion of the sediment samples collected at sites A±Hare given in Table 1. The samples are dominated bysilt (68±92% by volume), while the clay (7±32%)and the fine sand fractions (� 10%) are less impor-tant.

The most abundant minerals are quartz (28±39%)and plagioclase (28±36%; Table 2). The estimatedcontents of alkali feldspar, mica and chlorite+kaoli-nite are 14±18%, 7±18% and 3±6%, respectively(Table 2). In the clay fractions (< 2 �m) separatedby centrifugation mica is the dominating mineral(>45%) followed by chlorite (8±23%). This fractionis depleted in quartz and feldspars relative to the bulksamples (Table 2). Other detected phases in thesamples are amphibole, pyrite and a 14AÊ swellingclay mineral (vermiculite and/or smectite; Table 2).Similar mineral sequences are reported for sulphide-bearing fine sediments from Sweden (OÈ born, 1991)and for the fine fraction of till in Finland (RaÈisaÈnenet al., 1992; Lintinen, 1995).

Carbon and sulphur. The median and range of the Ccontent for the 317 sediment samples are 1.37% and0.34±3.95%, respectively (Table 3). Only a smallfraction (1±5%) of the C in samples A±H was releasedin acid, indicating that organic C is the dominatingform and that the content of carbonate material is lowin the samples. This was expected, as carbonate rocksare scarce in the study area.

Sulphur contents between 0.2% and 2% are fre-quently reported for fine-grained sediments locatedon coastal areas of Scandinavia (Purokoski, 1958;Palko, 1994; OÈ born, 1989; Wiklander and Hallgren,1949; Hartikainen and Yli-Halla, 1986; Georgala,1980). The median (0.54%) and maximum contents(1.78%) of the samples of this study (n=317) arewithin this range. In corresponding sediments oftropical areas, the S contents are normally higher(1±5%; van Breemen, 1973; Dent, 1980). The med-ian content in glacial till (<60 �m) in the wholecountry is 140 �g gÿ1 (Koljonen, 1992). However,part of these samples were oxidised which may havecaused the loss of some S.

The Spearman correlation coefficient between thecontents of S and C is 0.65. This positive correlationis mainly a result of the increased sulphate reductionand thus increased sulphur accumulation with high

Table 1 Particle-size distribution (V%) of eight fine-grained

sediments (A±H) from western Finland.

Clay (<2 �m) Silt (2±60 �m) Fine sand(>60 �m)

A 8 83 9B 19 80 1C 10 80 10D 13 79 8E 10 82 8F 32 68 0G 7 92 1H 17 78 5

Table 2 Estimated mineralogical composition of bulk samples and the separated clay fraction (<2 �m) of eight fine-grainedsediments (A±H) from western Finland.

Bulk sample (%) Clay fraction (%)

Q Pf Af Mi Ch+K Am Py SM Q Pf Af Mi Ch K Am Py SM

A 35 32 18 11 3 T ND ND ND 19 12 49 11 9 T ND NDB 32 31 16 13 6 T T ND 8 11 9 51 8 6 T ND 7C 39 28 15 13 4 T T ND ND 5 5 71 14 ND T ND 4D 36 34 16 9 5 T ND ND 6 9 8 53 10 7 T ND 7E 34 32 15 12 4 T T ND 4 6 5 61 9 6 T ND 9F 28 30 18 18 6 T ND ND 3 7 7 49 23 ND T ND 8G 37 36 15 7 3 T ND ND ND 16 14 47 22 ND T ND NDH 31 29 14 18 5 T T ND 3 5 5 60 8 6 T ND 12

Q=quartz, Pf-plagioclase feldspar, Af=alkali feldspar, Mi=mica, Ch=chlorite, K=kaolinite, Am=amphibole, Py=pyrite, SM=swelling clay

mineral (14AÊ ), T=trace, ND=not detected.

158 M. AÊstroÈm and A. BjoÈrklund

contents of degrading organic matter in the sedi-ments. The fact that organic substances containvarious amounts of sulphur (Pettersson, 1992), willalso to some extent contribute to this correlation.

Metals. The aqua regia extractable contents ofmetals and P in the samples (n=317) are given inTable 3. Iron is most abundant (median = 3.8%)followed by Al (2.02%), Mg (1.04%) and K (0.60%).The most abundant trace metals are Zn (90 �g gÿ1),Ba (87 �g gÿ1), V (50 �g gÿ1) and Cr (48 �g gÿ1). Ascompared to the median contents of the fine-fraction(<0.06 mm) of glacial till samples from the same area(data of the Geological Survey of Finland; Table 3),the levels of Fe, Al, Mg, K, Mn, Zn, Ba, Cr, Sr, Ni andCo are more than twice as high and those of Ca, Ti, V,La, Cu, Pb and Th between 1.5 and 2 times higher inthe fine sediments (Table 3). This may be due tohigher contents of clay-sized material andphyllosilicates such as mica and chlorite in the latteroverburden type. However, as the two sample mediawere analysed in different laboratories, possibledifferences in analytical levels between thelaboratories could explain at least part of thedifferences.

Copper, Pb, Zn, Ni, Co and Fe, which are commonlyassociated with sulphides (Moore et al., 1988; Morseand Arakaki, 1993) and organic material (Rashid,1974; Nissenbaum and Swaine, 1976; Tipping andHurley, 1992), do not correlate significantly with the

S and C contents of the sediments (Table 4). How-ever, these metals and also a number of other tracemetals (Th, V, La, Cr, Ba, Ti) correlate with majorelements such as Al, Mg and K (rs > 0.70; Table 4).It is, therefore, argued that metals are associatedlargely with silicates, mainly such as clay minerals,mica and chlorite, and that sulphides and organicmatter control only to a limited extent the distribu-tion of elements other than S and C. Hydrous oxidesof Fe and Mn, which in many environments formpowerful traps for a range of heavy metals (e.g.Jenne, 1968; FoÈrstner and Wittmann, 1979), areunstable in reduced sediments.

The geochemistry of the leachates

pH. The median and range of pH of the sedimentsamples in reduced states (First measurement;Figure 2) were 6.7 and 5.0±7.8, respectively. Thehigh pH shows that the majority of the samples weretaken from the unoxidised zone, where pH is normallynear neutral. Some of the weakly acid samples (pH 5±6) may have been collected from layers thatoccasionally have been exposed to atmosphericoxygen.

When the sediment samples had been exposed to theatmosphere for one year and the added waters hadleached the samples for two weeks (Figure 2), the pHof the supernatants was much lower (median = 3.0,

Table 3 Element contents (minimum, maximum, median and 10th and 90th percentiles) of fine-grained sediments and of thefine faction (<0.06 mm) of glacial till from western Finland.

Fine sediments (n=317) Glacial till (n=1826)*

Min 10% Median 90% Max Min 10% Median 90% Max

Fe (%) 0.78 2.29 3.80 4.81 5.70 0.13 0.91 1.49 2.69 5.80Al (%) 0.35 0.96 2.02 2.95 3.58 0.05 0.41 0.78 1.67 2.78C (%) 0.34 0.56 1.37 2.40 3.95 ± ± ± ± ±Mg (%) 0.12 0.44 1.04 1.43 1.83 0.03 0.20 0.39 0.86 1.71K (%) 0.07 0.28 0.60 0.94 1.40 BD 0.10 0.25 0.58 1.45S (%) BD 0.04 0.54 1.01 1.78 ± ± ± ± ±Ca (%) 0.25 0.37 0.50 0.58 0.75 0.02 0.20 0.29 0.39 0.61Ti (%) 0.07 0.13 0.22 0.27 0.30 0.01 0.08 0.13 0.22 0.42Na (%) BD 0.05 0.10 0.19 0.31 ± ± ± ± ±P (�g gÿ1) 40 530 640 840 1810 117 606 756 858 1400Mn (�g gÿ1) 90 223 448 713 1147 14 92 160 288 1160Zn (�g gÿ1) 18 43 90 114 142 2.2 18 34 75 194Ba (�g gÿ1) 14 45 87 168 229 2.3 18 39 91 267V (�g gÿ1) 12 28 50 64 76 4.5 17 29 61 155Cr (�g gÿ1) 11 28 48 62 76 2.6 12 23 50 116La (�g gÿ1) 13 25 38 50 62 3.5 15 22 36 80Sr (�g gÿ1) 10 22 36 48 60 1.7 6.7 10 17 38Ni (�g gÿ1) 6.0 16 31 42 58 1.5 6.0 12 30 143Cu (�g gÿ1) 2.0 14 27 46 65 0.45 7.6 15 37 137Co (�g gÿ1) 2.0 7.0 13 18 31 BD 2.2 5.0 13 32Pb (�g gÿ1) BD 6.0 12 17 25 BD 0.32 7.9 16 141Th (�g gÿ1) BD 4.6 10 16 22 BD 2.0 6.2 11 37

*Data of the Geological Survey of Finland.

Carbon and S were analysed with Leco. All other elements were extracted with aqua regia. BD=below detection limit.

Sulphide-bearing postglacial sediments of western Finland 159

Table 4 Matrix of Spearman correlation coefficients for element contents of fine-grained sediments: coefficients not significant at the 99.9% level of confidence are omitted for clarity (n=317).

S C Al Mg K Fe Ca Ti Na P Mn Zn Ba V Cr La Sr Ni Cu Co Pb

SC 0.65Al ± ±Mg ± ± 0.97K ± ± 0.91 0.90Fe ± 0.20 085 0.82 0.75Ca ± ± 0.66 0.69 0.53 0.61Ti ÿ0.28 ÿ0.23 0.90 0.92 0.76 0.72 0.77Na 0.20 0.33 0.40 0.39 0.43 0.39 0.34 0.26P ± 0.44 0.23 ± 0.26 0.38 0.22 ± 0.53Mn ± ± 0.70 0.72 0.64 0.73 0.62 0.69 0.30 0.30Zn ± ± 0.90 0.87 0.82 0.85 0.63 0.79 0.35 0.27 0.66Ba ÿ0.23 ÿ0.23 0.90 0.90 0.82 0.68 0.62 0.91 0.28 ± 0.62 0.74V ± ± 0.96 0.95 0.88 0.84 0.66 0.89 0.44 0.30 0.70 0.86 0.88Cr ± ± 0.88 0.90 0.73 0.76 0.66 0.87 0.32 ± 0.67 0.81 0.83 0.86La ÿ0.21 ± 0.95 0.92 0.85 0.81 0.64 0.87 0.44 0.27 0.62 0.88 0.86 0.95 0.86Sr ± 0.46 0.62 0.60 0.62 0.70 0.63 0.46 0.67 0.55 0.50 0.63 0.41 0.63 0.52 0.60Ni ± ± 0.92 0.90 0.91 0.79 0.54 0.79 0.38 0.23 0.64 0.90 0.83 0.91 0.80 0.90 0.56Cu ± ± 0.89 0.89 0.87 0.74 0.57 0.81 0.38 ± 0.54 0.82 0.87 0.91 0.81 0.91 0.57 0.90Co ± ± 0.84 0.83 0.76 0.79 0.64 0.78 0.23 0.20 0.69 0.91 0.71 0.82 0.78 0.80 0.52 0.87 0.74Pb ± ± 0.76 0.75 0.80 0.64 0.42 0.64 0.35 0.22 0.52 0.74 0.71 0.76 0.65 0.75 0.50 0.80 0.78 0.69Th ÿ0.24 ± 0.88 0.88 0.79 0.68 0.58 0.85 0.38 ± 0.58 0.75 0.88 0.89 0.81 0.88 0.50 0.82 0.85 0.69 0.72

160

M.A ÊstroÈ m

andA.BjoÈ rklu

nd

range = 2.4±5.8) than in the first measurement. In82% of the samples, pH was <4.0 and in 73% <3.5,which shows that some three-quarters of the finesediments in the study area have a high potentialacidity. In most S-rich samples (>0.2%) high acidityhad developed at oxidation (pH< 3.7; Figure 3),whereas those containing < 0.1% S were less acidic(pH 3.8±5.8; Figure 3). The Spearman correlationcoefficient between the pH and S is ÿ0.73 (Table 5).Thus, the metal-sulphide oxidation had a strongimpact on the pH. The mechanisms of the oxidationreactions of reduced compounds of S and Fe hasbeen described by Wiklander et al. (1950), Lowson(1982), Morse et al. (1987) and others.

The large pH drop also at relatively low sulphidecontents (0.2% S; Figure 3) shows that the bufferingcapacity of the sediments is low. This is probably

due to low contents of carbonate material and miner-als with high ion-exchange capacity such as smectiteand vermiculite in the sediments. The pH of theoxidised samples correlated positively and signifi-cantly (�=0.1%) with the aqua regia extractablecontents of Al, Ba, Ca, Co, Cr, Cu, Fe, La, Mg, Ni,Th, Ti, V, Zn (Table 5), which indicates a slightlyincreased hydrogen-ion buffering capacity at ele-vated metal contents.

Conductivity. When the sediment samples were inreduced states (First measurement; Figure 2), themedian and range of the conductivity of thesupernatants were 64 mS mÿ1 and 12±323 mS mÿ1,respectively. In their natural state, marine sedimentscontain relict sea salts trapped in the pores. As aconsequence, the conductivity correlates with the Nacontents (rs = 0.59; Table 5).

Figure 3 Relation between S contents and the pH and electric conductivity of fine-grained sediments.& = reduced state;

. = oxidised state.

Electricconductivity(m

Smÿ1)

Sulphide-bearing postglacial sediments of western Finland 161

After the oxidation period, the conductivity of thesupernatants was much higher (median = 325 mSmÿ1, range = 17±628 mS mÿ1) than in the firstmeasurement. The correlation between the conduc-tivity and the S contents (rs = 0.81) and pH (ÿ0.73)is high and the former is approximately linear(Figure 3), whereas the correlation is weak betweenthe conductivity and the metal contents (Table 5).Considerable quantities of soluble components (sul-

phate, cations) were, thus, liberated in the sedimentsas a result of sulphide oxidation and related acidifi-cation.

Chemical elements. The contents of the easilymobilisable elements in the sediment samples fromsites A±H (Figure 1), calculated from theconcentrations found in the leachates after theoxidation period (Figure 2), are given in Table 6.The highest contents are recorded for Ca (median =1015 �g gÿ1), Mg (842 �g gÿ1) and Fe (363 �g gÿ1).The most abundant trace elements are Zn (13 �g gÿ1),Sr (10 �g gÿ1), Ni (4.6 �g gÿ1) and B (3.4 �g gÿ1).There are large variations in the leachability of theelements in the sediments. Between 20% and 31% ofthe contents of Na, Sr, Co, Mn, B, Ca and Ni (aquaregia extractable) were liberated at oxidation, whilethe corresponding figures are 13, 7.5, 4 and 1.1 forZn, Mg, Cu and Al, respectively, and <1 % for V, Th,Sb, Pb, Cr, Ba and Fe (Table 6). Water-soluble,exchangeable and sulphide-bound fractions wereprobably largely liberated during the oxidationperiod, while elements may not have been releasedin large quantities from structural sites of alumino-silicates.

A comparison with the hydrogeochemical data forwestern Finland presented by AÊ stroÈm and BjoÈrklund(1995) shows that trace elements that were mobilisedin large quantities by the oxidation of the sediments(Zn, Mn, B, Co, Ni, Sr) are also abundant in thewater of streams draining this overburden type, andthat the concentrations of elements that are relatively`immobile' in the sediments (Cr, Sb, Ba, V, Pb) arenot, in general, elevated in the water of such streams.There is, thus, good agreement between existinghydrogeochemical data and the experimental resultspresented in this paper.

Table 5 Spearman correlation coefficients between element

contents and the pH and electric conductivity of fine-

grained sediments in reduced and oxidised states: coeffi-

cients not significant at the 99.9% level of confidence are

omitted for clarity (n=232).

pH Conductivity

(red) (ox) (red) (ox)

S 0.30 ÿ0.73 0.31 0.81C ± ÿ0.46 0.41 0.64Al ± 0.33 ± ±Ba ± 0.41 ÿ0.36 ÿ0.29Ca ± 0.30 ± ±Co ± 0.28 ± ±Cr ± 0.30 ± ±Cu ± 0.38 ÿ0.25 ±Fe ± 0.26 ± ±K ± ± ± ±La ± 0.42 ± ±Mg ± 0.33 ± ±Mn ± ± ± ±Na 0.43 ± 0.59 0.32Ni ± 028 ± ±P ± ± 0.30 ±Pb ± ± ± ±Sr ± ± 0.26 ±Th ± 0.40 ÿ0.23 ÿ0.28Ti ± 0.43 ÿ0.31 ÿ0.31V ± 0.36 ÿ0.25 ±Zn ± 0.28 ± ±

Table 6 Easily mobilisable element contents (median, minimum, maximum) of eight fine-grained sediments and the relative

amount (%) of easily mobilised elements as compared to the contents extracted by aqua regia.

Element(�g gÿ1)

Median Min Max (%) Element(ng gÿ1)

Med-ian

Min Max (%)

Ca 1015 238 1410 22 Cu 896 64 3380 4.0Mg 842 112 1240 7.5 Ba 308 9.5 596 0.32Fe 363 0.70 1300

0.87Rb 176 93 306 ±

Si 202 89 261 ± Be 123 8 230 ±Na 201 25 1210 31 Cr 92 9.3 342 0.20Al 146 2.3 737 1.1 U 74 4.1 295 ±K 118 8.7 265 3.3 Se 62 19 122 ±Mn 118 25 331 29 Cd 58 17 74 ±Zn 13 0.80 16 13 As 9.0 2.6 55 ±Sr 10 2.2 20 30 Pb 8.4 1.7 25 0.08Ni 4.6 0.89 9.1 20 V 5.4 1.0 271 0.02B 3.4 0.42 7.5 29 Th 3.4 0.40 72 0.06Co 2.6 0.57 4.3 29 Sb 2.3 0.70 4.4 0.12Li 1.5 0.38 2.9 ± Tl 1.4 0.80 2.5 ±

162 M. AÊstroÈm and A. BjoÈrklund

Conclusions

The fine-grained (mainly silt) sediments located onthe coastal plains of western Finland were shown tobe highly sensitive to oxidation. In the reduced state,the pH of the sediments is near neutral and theconductivity is low (median = 64 mS mÿ1), but aftera one-year period in contact with the atmospherestrong acidity (median pH = 3.0) and high conduc-tivity (median = 325 mS mÿ1) developed as a resultof oxidation of sulphides. The results also showedthat several trace elements (Ni, Zn, Co, Mn, B, Sr)are liberated in large quantities at oxidation, whereasothers are mobilised only to a limited extent (Cr, Sb,Ba, V, Pb).

Because of the high amounts of acids and metalsliberated by the oxidation of the sediments, activitiesaiming at lowering the groundwater table (ditching,dredging) will increase the load of chemical com-pounds on the drainage waters. Therefore, the qualityof surface waters of western Finland will not im-prove, unless artificial draining is restricted on alocal and regional scale, and prohibited in areascovered with high-S sediments.

Acknowledgements

This work was financed by the Academy of Finlandand the authors are grateful for this support. Theauthors also wish to thank Dr Jeff Wilson and MrDonald Duthie at the Macaulay Land Use ResearchInstitute for the mineralogical analysis, and Dr AlanNewman for many constructive discussions and va-luable help concerning clay-mineralogical problems.

References

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[Manuscript No. 424: submitted August 21, 1995 andaccepted after revision February 5, 1996.]

164 M. AÊstroÈm and A. BjoÈrklund