Boreal stream
Mats Arstro¨m
Åbo Akademi University, Department of Geology and Mineralogy, Domkyrkotorget 1, 20500 Åbo, Finland (e-mail: mats.astrom@abo.fi)
ABSTRACT:
The concentrations and shale-normalized patterns of rare earth elements (REEs) were determined along the main stem of a medium-sized Boreal stream (catchment area, 107 km
2), to which areas covered with acid sulphate soils, developed in sulphide-bearing marine sediments, drain. In the upper reaches (headwater), where there are no acid sulphate soils, the REE concentrations were low at all sampling events (e.g. Nd=1.0 to 1.3 µg l
1) and the shale-normalized profiles were characterized by peaked middle-REE enrichment (centred on gadolin- ium) with a secondary maximum at ytterbium (Yb)/lutetium (Lu). Downstream there was an increase in the REE concentrations caused by leaching of acid sulphate soils which increase in abundance towards the basin outlet. The strongest down- stream increase occurred at high-water flow in autumn (Nd concentrations up to 53 µg l
1) and the weakest during baseflow conditions (Nd=2–7 µg l
1). Down- stream, at elevated REE concentrations, there was a depletion of the heavy REEs caused by: preferential scavenging (and thus removal) of the heavy REEs by oxy-hydroxides contained in the source soil; preferential complexation of the light and middle REEs by the dissolved SO
24; and/or the existence of a source mineral (in the acid sulphate soils) depleted in the heavy REEs. In two tributaries and a first-order stream sampled on a single occasion, the REE features were consistent with those along the main stem. In a subsurface drainpipe, also sampled once, there was, however, a convex REE-fractionation pattern characterized by a stronger depletion from gadolinium (Gd) to lanthanum (La) than from Gd to Lu. The origin of this pattern, which seems to be of minor importance in the REE-enriched waters in the catchment, remains unexplained.
KEYWORDS: rare earth elements, REE, acid soil, stream, hydrology, acidification, sulphate
INTRODUCTION
The behaviour of rare earth elements (REEs) in major rivers, which commonly have a near neutral pH, has been studied quite extensively in the last two decades (e.g. Keasler &
Loveland 1982; Goldstein & Jacobsen 1988; Elderfield et al.
1990; Sholkovitz 1992; Zhang et al. 1998). In general, whilst there commonly is an inverse correlation between the REE concentrations and water pH (Keasler & Loveland 1982;
Goldstein & Jacobsen 1988), a more important control of REE abundance in many rivers is the amount of particulate and colloidal-sized material commonly carrying the major load of the REEs (Sholkovitz 1992; Zhang et al. 1998; Ingri et al. 2000).
These colloidal and particulate fractions tend to be enriched in the light REEs (LREEs) and/or the middle REEs (MREEs) (Hoyle et al. 1984; Sholkovitz 1992, 1995; Zhang et al. 1998;
Land et al. 1999; Ingri et al. 2000). In contrast, the solution dissolved phase in river water, as in sea water, is commonly characterized by shale-normalized LREE depletion and heavy REE (HREE) enrichment (Hoyle et al. 1984; Elderfield et al.
1990; Sholkovitz 1992, 1995; Sholkovitz et al. 1994; Ingri et al.
2000) due to the increase in solution–complexation constants with important ligands (F, CO23 , OH, HPO24 ) across the lanthanide (Ln) series (Cantrell & Byrne 1987; Wood 1990;
Millero 1992; Johannesson et al. 1997).
The behaviour of REEs in acidic terrestrial waters has been investigated to some extent. In general, such waters are characterized by higher REE concentrations than those found in near-neutral to alkaline rivers, and by a high proportion of the free Ln3+and the LnSO+4 complex in the solution phase (Smedley 1991; Gosselin et al. 1992; Johannesson & Lyons 1995; Johannesson et al. 1996; Banks et al. 1999; Johannesson &
Zhou 1999). A characteristic feature of many of these acidic waters is a MREE-enriched pattern, the origin of which is as yet not fully understood as indicated by the many suggested controls (Gosselin et al. 1992; Johannesson & Lyons 1995;
Johannesson et al. 1996; Johannesson & Zhou 1999).
This study focuses on the REE hydrochemistry in a medium- sized Finnish stream (Munsala stream), whose catchment is partly underlain with sulphur-rich acidic (acid sulphate) soils from which, periodically, large amounts of acids and metals are leached into the stream (Arstro¨m & Bjo¨rklund 1996; Arstro¨m
Geochemistry: Exploration, Environment, Analysis, Vol. 1 2001, pp. 101–108 1467-7873/01/$15.00 2001 AEG/Geological Society, London
2001a). The main aims were to determine the concentration levels and fractionation patterns of REEs in the water in this occasionally acidified (pH 4) stream (Arstro¨m & Bjo¨rklund 1996) and, based on the identified hydrochemical features, discuss the aquatic REE controls in this ‘acidic’ catchment.
STUDY AREA
The Proterozoic (1.8 Ga) granitoidic bedrock in the 107-km2 Munsala stream catchment is overlain to a large extent with till, peat and glaciofluvial deposits (Fig. 1); this area is used for forestry. A major part of these forested areas is artificially drained by a network of c. 1 m deep open surface ditches reaching the organic (peat) and mineral soil layers underneath the forest topsoil. These ditching activities, carried out during the last few decades, have been undertaken in order to release excess surface water and thereby improve growth conditions for the tree stand.
Acid sulphate soils, classified as Sulfic Cryaquept or Typic Sulfaquept (Yli-Halla 1997; Yli-Halla et al. 1999) and developed on marine/lacustrine sulphide-bearing (S concentrations of c. 0.5%) fine-grained sediments, are located mainly along the main stem but to some extent also in peripheral parts of the catchment (Fig. 1). These cultivated (corn and grass) soils, which are 1 to 1.5 m deep and characterized by low pH (Fig. 2), are drained either by open surface ditches or efficient subsur- face pipes at depths of 100–110 cm and about 20 m apart.
Hence, both the forest and agricultural land within the catch-
ment have been artificially drained, which has changed the natural pattern of drainage channels in the catchment.
The climatic and hydrological conditions in the region are as follows: Between June and August, there is continuous daylight, a mean temperature of about 15C, high biological activity and commonly baseflow conditions. During the autumn months (September–November), there is a gradual decrease in tempera- ture from about 15C to c. 0C, with corresponding decreases in biological activity and evapotranspiration, and high-flow events of variable duration. The winter (December–April) is charac- terized by freezing temperatures and a snow cover ranging in maximum thickness from c. 20 to 80 cm. The melting of the snow cover starts in April, resulting in high spring flows, and a few weeks later the ground is completely defrosted. The mean annual precipitation in the area is c. 500 mm, part of which falls as snow.
Within the catchment, the population is in the order of 1000 persons (distributed in a few small villages), industrial activity is marginal and the traffic load is small. Hence, anthropogenic sources contribute only to a limited extent to the total load of chemical elements in the streams the catchment.
METHODS
A total of six sites (M1–M6) along the main stem of Munsala stream were included in the sampling programme (Fig. 1). Site M1, which drains the upper 13 km2 of the catchment covered entirely with till, peat and glaciofluvial deposits overlain with forest, is located at the point where the main stem reaches the acid sulphate soils (Fig. 1). The other five sampling sites are located downstream at appropriate and easily reachable lo- cations (Fig. 1). Calculations shows that there is a gradual increase in the relative proportion of the acid sulphate soils from 0% in the area that drains into site M1 to 31% at the basin outlet designated ‘M6’ (Fig. 1).
Water samples at these sites were collected twice during autumn high-water flow, twice during summer low-water flow, and once each during autumn low-water and summer high- water flow (Table 1). The reason for sampling the latter two events only once is simply that they are less common than the former. Discharge was not measured, but it is in the order of 100 l s1during low flows and two orders of magnitude higher at high flows. The sampling was carried out over four years (Table 1). However, this is not of major concern because it has been shown previously that, in the Munsala stream, there are Fig. 1. Sites of water sampling in Munsala stream (M1–M6) and the
occurrence of acid sulphate soils in the catchment. The till, peat and glaciofluvial deposits are not separated, and some minor bedrock exposures are omitted. The indicated acid sulphate soil occurrences can be considered approximate only, since they are not based on detailed field mapping but on the distribution of fine-grained sediments in the catchment (Arstro¨m & Bjo¨rklund 1996) and on general field observations by the authors.
Cultivated layer (frequent liming) Permanently oxidizing conditions
Acid sulphate soil
During high flows waterlogged During low-flow periods the ground- water table is located at the boundary with the parent sediment underneath and the conditions are oxidizing
Sulphide-bearing parent sediment Permanently waterlogged and re- ducing conditions
3 4 5 6 7 8
0.0
0.5
1.0
1.5
2.0 Depth
(m)
pH
Fig. 2. pH, redox and hydrological characteristics of a typical acid sulphate soil profile in mid-western Finland.
considerably stronger hydrochemical contrasts between various hydrological events than between various years (Arstro¨m &
Bjo¨rklund 1996). No sampling was undertaken in spring and winter as snow-melt water and frost in the ground are import- ant controls of stream-water chemistry in these periods (Arstro¨m & Bjo¨rklund 1996). On one of the autumn high-flow events (AHF-I; see Table 2), water samples were also collected in two major tributaries (Harjuxba¨cken and Eljasasstordiket), one first-order stream and in one subsurface drainpipe within the catchment.
Initially there was no intention to include REEs in the survey, therefore the stream water samples collected were (unfortunately) unfiltered. Only five selected water samples were filtered, in the field, through 0.45-µm polycarbonate screen filters (Nuclepore ). The water samples were acidified to a pH<2 with concentrated HNO3 (Suprapur quality, Merck ) and frozen. After the sampling was completed, each sample was thawed and analysed in random order by inductively coupled plasma mass spectrometry (ICP-MS) (Perkin Elmer-Sciex ELAN 6000). The results of the REEs, Fe, Al and Mn are presented in this paper; the results of the latter three for the main stem were reported by Arstro¨m (2001a).
Standards were run repeatedly to check the analytical accuracy.
Analysis of duplicates showed that for each of the REEs, Al, Fe and Mn, the combined sampling and analytical error was significantly (=0.01) smaller than the overall concentration variability. Hence, any patterns or trends of these metals in the data set are due to natural variation.
Total organic carbon (TOC) concentrations were determined using a TOC analyser; on samples collected in glass bottles and stored at 4C. Conductivity and pH were measured on the day of sampling. SO24 was determined gravimetrically by the precipitation, drying and weighing of BaSO4. The results for these variables for the two tributaries, the first-order stream and the drainpipe are presented in this paper, while their variations along the main stem are discussed in detail elsewhere.
RESULTS
The REE concentrations in the selected (n=5), filtered (0.45- µm) water samples were <10% lower than in the corresponding unfiltered ones. Hence, the variations that existed between the treated and untreated samples were small. However, as the vast majority of the samples were not filtered, the concentrations presented below (for unfiltered samples) should be considered as the sum of dissolved species and those fractions released from particulates in the acidified (pH<2) samples during storage.
At site M1 (the headwater) in Munsala stream, the TOC concentrations were high (34–50 mg l1), pH varied from weakly acidic to near neutral (4.6–6.7), and the concentrations of REEs (Neodymium=1.0–1.3 µg l1), Mn (51–92 µg l1), Al (0.29–0.92 mg l1), SO24 (5–14 mg l1) and Fe (2.2–6.0 mg l1) were low (Fig. 3) and similar to the background concen- trations in forested areas in this part of the country (Lahermo et al.1996). The ratios of LaSNto LuSN, where SN refers to the
shale-normalized value calculated using the NASC concen- trations given by Haskin et al. (1968) and Gromet et al. (1984), ranged from 0.69 to 0.98 (Fig. 4). In detail, there was an increase in the relative abundance from lanthanum (La) to gadolinium (Gd) (GdSN/LaSN=1.54–1.78), a decrease in the abundance from Gd to Tm (TmSN/GdSN=0.52–0.68) and then an increase in relative concentrations for the heaviest metals ytterbium (Yb) and lutetium (Lu) (Fig. 5). This fractionation pattern can be described as a peaked MREE enrichment (centred on Gd) with a secondary maximum on Yb and Lu (Fig. 5).
In all sampling events there was an increase in the REE concentrations from site M1 (headwater) to M6 (stream termi- nus) (Fig. 3), related to the increase in the acid sulphate soil cover from 0% at the former to 31% at the latter site. This downstream increase was strongest in the high-flow events in autumn, lower in summer and relatively weak during the low-flow periods (Fig. 3). The downstream variations in the REE concentrations closely follow those of Mn, Al and SO24 , and are to some extent inversely similar to the variations in pH, but do not correlate with the patterns for Fe and TOC concentrations (Fig. 3). Between sites M2 and M6, where the acid sulphate soil cover ranges from 11% to 31%, elevated REE concentrations (Nd=1.5–53 µg l1; Fig. 3), compared to those in the headwater (Nd=1.0–1.3 µg l1), are accompanied by ratios of LaSN to LuSN of 1.01 to 1.68 (Fig. 4). Typically, at these sites the shale-normalized profile was characterized by a flat pattern from La to samarium (Sm), a decrease in the relative abundance from Gd to thulium (Tm), and weak relative enrichments of Yb and Lu (Fig. 5).
The waters sampled in a single event (AHF-I) in the two tributaries (Harjuxba¨cken and Eljasasstordiket), the first-order stream and the subsurface drainpipe, whose catchments are dominated (or entirely covered) by acid sulphate soils, had low pH (2.9–3.6), low TOC concentrations (8–10 mg l1) and variable but overall high concentrations of REEs, SO24 , Al, Mn and Fe (Table 2). The shale-normalized REE patterns for the tributaries and the first-order stream are characterized by LaSN:LuSN ratios of 1.8–1.9 (Table 2) and an overall HREE depletion (Fig. 6), similar to the patterns for those sites along the main stem (M2–M6) affected by acid sulphate soils (Fig. 5).
Conversely, the water from the subsurface drainpipe had a convex pattern characterized by MREE-enrichment with a stronger depletion from Gd to La (GdSN/LaSN=3.6) than from Gd to Lu (GdSN/LuSN=1.7) (Fig. 6).
DISCUSSION Solubility of REE salts
To test whether the solubilities of REE salts have the potential to control REE concentrations in the catchment, the ‘activity’
products for REE sulphates for the first-order stream, which had the highest concentrations of both REEs (La=900 µg l1) and SO24 (3020 mg l1), were calculated and found to be as follows:
and
These values are orders of magnitude lower than the solubility product for Lu2(SO24 )3· 8H2O (109.64) reported by Rard (1990). This shows that the first-order stream, plus all the other
Table 1. Sampling periods
1995 1996 1997 1998
Summer low flow SLF-I SLF-II
Summer high flow SHF
Autumn low flow ALF
Autumn high flow AHF-I AHF-II
water samples collected, were far from saturated in REE sulphates. The REE salts of PO34 and CO23 are considerably less soluble than the sulphate salts (Johannesson et al. 1995).
However, these anions will also not control REE concen- trations because they are expected to occur in extremely small concentrations in the acidic (and REE-rich) waters, while in other less acidic waters, where these anions are likely to exist, the REE concentrations are too low to be precipitated as phosphate or carbonate salts. Hence, inorganic REE salts (sulphate, carbonate and phosphate) do not control aquatic REE concentrations in the catchment.
Behaviour of REEs in the headwater
The small range in REE concentrations (Nd=1.0–1.3 µg l1) and the consistent shale-normalized REE patterns at site M1
(headwater) throughout the sampling period (Fig. 5) show that the REE hydrochemistry in the upper catchment sections, where there are no acid sulphate soils, is not affected by processes which change substantially from low to high flows and from summer to autumn (i.e. climatic factors, biological activity, water pathways in the catchment soils, stream water pH). Other processes, consequently, control the REE hydrochemistry in the headwater, as discussed below.
Till, peat and glaciofluvial deposits cover, in unquantified proportions, the unpopulated forested area (13 km2) that drains into site M1 (Fig. 1). Of these sediment types, the former two are undoubtedly the major sources of the aquatic REE load in the headwater: the glaciofluvial deposits are composed of poorly weathered course-grained minerals (quartz, feldspars, mica) and bedrock fragments from which REEs are unlikely to leach in large amounts; the till has a substantial fine-fraction 0
6 12 18
Al (mg l )
0.0 0.5 1.0 1.5
Mn (mgl )
0 2 4 6 8 10
Fe (mgl )
Summer low-water flow (SLF-I, August 1995) Summer low-water flow (SLF-II, July 1998) Autumn low-water flow (ALF, September 1997) Autumn high-water flow (AHF-I, November 1996) Autumn high-water flow (AHF-II, November 1997) Summer high-water flow (SHF, June 1998)
0 100 200 300
SO (mg l)
0 20 40 60
TOC (mgl )
3 4 5 6 7
pH
0 20 40 60
Nd (µg l )
M1 M2 M3 M4 M5 M6 13 23 27 55 83 107 0 11 18 27 30 31
M1 M2 M3 M4 M5 M6 13 23 27 55 83 107 0 11 18 27 30 31 Site
km2 ASS %
M1 M2 M3 M4 M5 M6 13 23 27 55 83 107 0 11 18 27 30 31
-1
-1 -1-1-1-142-
Fig. 3. Variations in pH and in the concentrations of metals, SO24 and total organic carbon (TOC) from the headwater (site M1) to the terminus (site M6) of Munsala stream during six sampling events. For each sampling site (M1–M6), the catchment size (km2) and the proportion (%) of the catchment cover of acid sulphate soils (ASS) are indicated. The profiles for SO24 , pH, TOC, Al, Fe and Mn are from Arstro¨m (2001a).
component containing several potential REE-bearing minerals such as amphibole, epidote and apatite (O} hlander et al. 1996;
Arstro¨m & Bjo¨rklund 1997; Deng et al. 1998); and the peat deposits may contain appreciable amounts of REEs due to the strong affinity of these metals for organic ligands. It has, in fact, been documented that REEs are extensively leached from both the E- and B-horizons of soils developed on tills similar to that in the studied catchment (Land et al. 1999).
The absence of detailed source-material (till, peat) and stream water (dissolved and particulate fractions) REE data allow only speculation on the mechanisms of formation of the REE- fractionation pattern (Fig. 5) at site M1 in the stream. The following mechanisms, which may contribute to various extents, are suggested:
(1) The total REE content in the till is expected to have a flat NASC-normalized pattern (O} hlander et al. 1996; Land et al.
1999). However, selective mobilization and leaching of second- ary phases (contained within the till) such as amorphous oxides, commonly enriched in the middle REEs (Zhang et al.1998; Johannesson & Zhou 1999; Land et al. 1999), may explain the distinct MREE enrichment in the headwater (Fig. 5).
(2) Whereas the stability constants of the solution complexes of REEs and multidentate organic molecules are high and increase relatively smoothly from La to Lu (Wheelwright et al.
1953; Cantrell & Byrne 1987; Byrne & Li 1995; Li & Byrne 1997), reliable constants are unavailable for the diverse humic substances. However, because of the high abundance of the latter in the headwater (TOC=34–50 mg l1, of which a major part exists as humic substances) and the strong complexes formed between REEs and multidentate organic molecules, organic-REE complexes could, to a large extent, control both the abundance and fractionation of the REEs in the headwater.
(3) Solution complexation is an important control of REE- fractionation patterns in the solution phase in rivers, streams and lakes. Typically, this phase is characterized by LREE depletion and HREE enrichment due to preferential com- plexation of the latter by the ligands CO23 , F, OH and HPO24 (Cantrell & Byrne 1987; Wood 1990; Millero 1992;
Johannesson et al. 1997). Hence, the overall enrichment of the HREEs in the headwater (Figs 4 and 5) could, at least partly, be explained by complexation of the REEs by one or several inorganic ligands.
Origin of elevated REE concentrations
The REE concentrations were strongly elevated in the first- order stream, the subsurface drainpipe and the two tributaries (sampled in a single event), whose catchments are dominated by acid sulphate soils (Table 2). There was a downstream increase (along the main stem) in REE concentrations on all sampling events closely related to a corresponding downstream increase in the proportion of the catchment cover of acid sulphate soil (Fig. 3). This shows that the REEs are abundantly leached from the acid sulphate soils, in agreement with the large losses of La detected in a vertical profile of such a soil studied by Arstro¨m (2001b). Large variations, however, existed in the magnitude of the downstream increase in the REE concentrations among the various sampling events (Fig. 3).
Between June and August, when the soil temperatures are elevated and the groundwater table is low, there is extensive oxidation of sulphides to sulphuric acid (Purokoski 1958;
Lowson 1982; Palko 1994), production of acidity (Wiklander &
Hallgren 1949; Wiklander et al. 1950; Hartikainen & Yli-Halla 1986) and alteration of aluminosilicates (O} born 1991) in the acid sulphate soils. Since, in this dry period, the acid sulphate soils are moist but not wet enough to release large amounts of water (Fig. 2), the soluble weathering products, including REEs, are enriched in the stationary soil solution. Concomi- tantly, the runoff from other soils and sediments (till, glacio- fluvial deposits, peat), releasing only small quantities of REEs (see headwater), is relatively high resulting in extensive dilution of the limited amount of water, rich in REEs, discharged from the acid sulphate soils. The net result is relatively low REE concentrations in water during low-flow periods. During the heavy rains in autumn (November), precipitation soaks into the soil profile, mixes with the acid and REE-rich soil solution and ultimately discharges into the drainage. During high flows in autumn, therefore, REEs are abundantly leached from the acid sulphate soils resulting in strongly increased REE concen- trations in the affected streams, such as the sampled tributaries and low-order streams (Table 2) and the lower reaches of Munsala stream (Fig. 3). During the summer high-water flow (SHF), which is a rare event occurring only about once in ten years, soluble weathering products are not abundant in the acid sulphate soils due to extensive leaching during the preceding heavy rains in autumn and snow melt in spring (April).
Consequently, during this event, the REE concentrations along the main stem were considerably lower than during the corre- sponding high flows in autumn (Fig. 3), but higher than during the low-flow periods.
The contrasting downstream patterns of the REEs and those of Fe and TOC (Fig. 3) show that the increase in REE concentrations downstream is not explained by an increase in particulate/colloidal Fe oxy-hydroxides or particulate/dissolved organic molecules, for which the REEs have a strong affinity.
The main reason for these different patterns is that Fe oxhydroxides and humic substances, in contrast to the REEs, are in general immobile in the acid sulphate soils (Arstro¨m &
Bjo¨rklund 1995; Arstro¨m 1998). The downstream variation in the REE concentrations was, however, similar to those of Al, Mn and SO24 (Fig. 3). This is explained largely by a high mobility of the ions Ln3+, Al3+, Mn2+and SO24 in the acid sulphate soils, and thus leaching of these in large amounts when the soils are flushed during wet periods.
0.6 0.8 1.0 1.2 1.4 1.6 1.8
M1 M2 M3 M4 M5 M6
LaSN/LuSN
Fig. 4. Ratios of LaSNto LuSNfor water samples collected at six sites (M1–M6) along the Munsala stream during six events. At each site, the unfilled bars represent low-water flows (SLF-I, SLF-II and ALF, respectively) and the black bars high-water flows (AHF-I, AHF-II and SHF, respectively).
REE-fractionation patterns at elevated REE concentrations
The elevated REE concentrations in the middle and lower reaches of the main stem (Fig. 3), and in the two tributaries and the first-order stream (Table 2), caused by discharge of REE- rich water from the acid sulphate soils, are accompanied by a depletion of the HREEs (Figs 4–6). Because this REE pattern
occurred on all sampling events, it is undoubtedly the result of an overall smaller relative leaching (under any conditions) of the heavy REEs from the acid sulphate soils. There are at least three mechanisms, within the acid sulphate soils, which may result in such a REE pattern in the discharged soil water.
It has been shown (Arstro¨m 2001b) that, whilst a major part of the La mobilized in the acid sulphate soil is leached into the drainage (in agreement with the results of this study), a
-4.6 -4.5 -4.4 -4.3 -4.2 -4.1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Logsample/shale
site M1
-4.4 -4.2 -4.0 -3.8 -3.6 -3.4 -3.2
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Logsample/shale
site M3
-4.4 -3.9 -3.4 -2.9 -2.4
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Logsample/shale
site M6 Summer low-water flow (SLF-I, August 1995)
Summer low-water flow (SLF-II, July 1998) Autumn low-water flow (ALF, September 1997) Autumn high-water flow (AHF-I, November 1996) Autumn high-water flow (AHF-II, November 1997) Summer high-water flow (SHF, June 1998)
Fig. 5. Shale-normalized REE profiles at sites M1, M3 and M6 in Munsala stream during six sampling events. The patterns at the other sites (M2, M4, M5) are similar to those at sites M3 and M6 (Fig. 4) and are therefore not presented.
Table 2. Size of the catchment areas and the proportion of the catchment cover of acid sulphate soil (ASS) for a first-order stream, a subsurface drainpipe and two tributaries within the Munsala stream catchment. The chemical data are for waters collected on a single occasion during high-water flow in autumn (AHF-I). EC, electrical conductivity; TOC, total organic carbon;
subscript ‘SN’, shale-normalized
Stream Catchment km2 ASS % pH
EC µS cm1
Nd µg l1
Fe mg l1
Mn mg l1
Al mg l1
SO24
mg l1 TOC
mg l1 LaSN/LuSN
First-order stream 0.01 100 2.9 3770 771 90 13 276 3020 8 1.9
Subsurface drainpipe 0.002 100 3.4 2230 280 29 3.3 55 1017 9 0.46
Harjuxba¨cken 12 52 3.6 1300 168 6 2.9 44 684 10 1.8
Eljasasstordiket 2.1 85 3.4 1360 152 11 3.2 52 737 10 1.9
significant fraction of the mobilized La is not leached but rather is transported down the profile and subsequently immobilized and enriched in the lower soil horizons immediately above the sulphidic parent sediment (Fig. 2). In this ‘enrichment zone’, it is likely that La (and other REEs) are scavenged by Fe and Mn oxy-hydroxides, in a manner similar to other trace metals such as Co, Ni and Zn (Arstro¨m 1998). In this zone, the HREEs may be preferentially scavenged by the oxy-hydroxides because: (1) in the soil solution the ligands F, CO23 , OH, HPO24 and dissolved organic molecules, which preferentially complex the HREEs (Cantrell & Byrne 1987; Wood 1990; Millero 1992), are not quantitatively important, thus excluding the possibility of extensive REE binding and fractionation by such ligands; and (2) as the HREEs are not preferentially kept in solution by such ligands, they are instead likely to be preferentially scavenged by soil oxy-hydroxides as indicated by experimental data (Bau 1999) and the increase in the first hydroxide binding constant across the lanthanide series (Millero 1992; Bau 1999). Conse- quently, as the HREEs are preferentially removed by sorption, the light and middle REE become enriched in the soil solution, which is ultimately discharged into the drainage.
There is also the possibility that SO24 , which is by far the most abundant anion in the acid sulphate soils (Weppling 1993) and which has the potential to bind a substantial proportion of the aquatic REEs through the complex LnSO+4 (Johannesson
& Lyons 1995), is involved in the REE fractionation in the acid sulphate soils. According to Wood (1990), the SO24 - complexation constants do not vary significantly across the lanthanide series. However, the results of recent speciation and stability constant calculations (Johannesson & Lyons 1995;
Johannesson et al. 1996; Johannesson & Zhou 1999) show that the proportion of the REE-sulphate complex of the total dissolved REEs is equally high for the metals La to Eu/Gd, but decreases in importance from the latter to the heaviest REEs, in a pattern very similar to that in the studied acid sulphate soil affected waters (Figs 5 and 6). Thus, SO24 complexation has the potential to preferentially enrich the light and middle REEs in the soil solution.
A third possible mechanism is that the mobile REE pool within the acid sulphate soils in itself is depleted in the HREEs,
and that thus no further in-soil fractionation processes are needed to cause the observed REE pattern. A detailed REE analyses of the various soil minerals (phases) is needed to verify the existence of such HREE-depleted phases.
Origin of the convex REE pattern in the subsurface drainpipe
The shale-normalized REE pattern identified in the REE-rich subsurface drainpipe (Nd=280 µg l1), showing MREE enrich- ment characterized by a stronger depletion of the light than the heavy REEs (Fig. 6), is different from that of the other REE-rich waters collected in this study. It resembles, to some extent, the REE pattern of the (REE-poor) headwater (MREE enrichment), but is quite similar to REE-fractionation pat- terns frequently found elsewhere in rivers (Elderfield et al.
1990; Sholkovitz 1995), groundwater (Gosselin et al. 1992;
Johannesson et al. 1996) and lake waters (Johannesson & Zhou 1999). The origin of this type of convex REE-fractionation pattern has been explained by several mechanisms including:
fluid interaction with Fe–Mn coatings on carbonate minerals and/or secondary minerals in fractures (Gosselin et al. 1992);
solid–liquid exchange reactions or dissolution of surface coat- ings, suspended particulates and/or secondary phases as well as SO24 complexation (Johannesson et al. 1996); acid leaching or dissolution of MREE-enriched Fe–Mn oxy-hydroxides con- tained within the catchment rocks (Johannesson & Zhou 1999);
REE-phosphorus geochemistry (Sholkovitz 1995); and non- linear REE fractionation (Elderfield et al. 1990). It is intriguing that of the studied REE-rich waters, a convex pattern was found only in the subsurface drainpipe. However, since only one drainpipe was sampled and as no open surface ditch located close to the drainpipe was included in the study, it is not possible to assess whether the MREE enrichment is due to spatial variations in the acid sulphate soil geochemistry or to processes occurring as a result of drainage though the pipe.
Furthermore, because the water samples were not filtered, there is the possibility that the drainpipe carried a high particulate load (e.g. oxy-hydroxides) to which the middle REEs were preferentially scavenged.
CONCLUSIONS
Despite the fact that the water samples were analysed unfiltered and the REEs were not determined in the source soils and minerals, the overall REE hydrochemistry and its controls in the catchment were identified. In the headwater, to which areas of till, peat and glaciofluvial material drain, the REE concen- trations were low and characterized by a peaked MREE enrichment (centred on Gd) with a secondary maximum of Yb/Lu. Conversely, the REE concentrations strongly increased downstream, related to a downstream increase in the acid sulphate soil cover, and also in two tributaries, a first-order stream and a subsurface drainpipe draining such soils. This shows that REEs are leached much more extensively from these acidic soils than from the till and peat deposits. Except for the drainpipe, in which there was an enrichment in the MREEs, all other acid sulphate soil affected waters were depleted in the HREEs. This pattern is explained by preferen- tial scavenging (and thus removal) of the HREEs by oxy- hydroxides contained in the lower soil horizons, preferential solution complexation of the light and middle REEs by the SO24 dissolved in the soil solution and/or the existence of a source mineral depleted in the HREEs.
0.001 0.01 0.1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Sample/shale
FS
SD HB ES
Fig. 6. Shale-normalized REE profiles in two tributaries (HB, Harjuxba¨cken; ES, Eljasasstordiket), a first-order stream (FS) and in a subsurface drainpipe (SD) during high-water flow in autumn (for more details on these streams, see Table 2).
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