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Geochemistry of manganese in the Kalix River, northern Sweden
0016-7037/92/$5.00 + .OO
Geochimica et Cosmoehimica Aaa Vol. 56. pp. 1485-1494 Copyright 0 1992 Pergamon Press Ltd. Printed in U.S.A.
Geochemistry of manganese in the Kalix River, northern Sweden CHRISTERPONT~~R, * JOHAN INGRI,+ and KURT BOSTROM* Department of Economic Geology, LuleH University of Technology, S-95 1 87 LuleB,Sweden (Received November 28, 1990; accepted in revised form January 22, 1992 )
Abstract-Dissolved and suspended Mn in the Kalix River, northern Sweden, were measured weekly over a period of eighteen months. During the same period four lakes in the Kalix catchment were sampled at their outlets and in vertical profiles within the lakes, together with a stream draining a series of mires with shallow lakes. Snow melting in mid-May increased the dissolved Mn concentration in the river tenfold, compared with a concentration of 5 pg L-’ during the winter discharge (January to April). We suggest that the increase was caused by Mn-rich mire water mixing with melting snow and being transported to the river. Large concentrations of dissolved Mn built up in the hypolimnion of the lakes studied during the icecovered period. Break-up of the ice and spring-overturn in June increased the dissolved Mn concentration tenfold in lake discharge and a concomitant peak in the dissolved Mn concentration was observed in the river. Lake-derived Mn was the dominant source for Mn in the river during this time. Suspended Mn in the river was hosted mainly in detrital particles during flood in May. In mid-June, non-detrital suspended Mn started to accumulate and reached a maximum in late July and early August. The Mn / Al ratio was 25 times higher during this period than during flood in May, suggesting the precipitation of an Mn-oxyhydroxide phase. The precipitation of the non-detrital Mn-rich phase was correlated in time with increased temperature, increased pH, and increased concentration of suspended biogenic particles. The precipitation of dissolved Mn was biologically mediated. Sedimentation and mineralisation of the non-detrital Mn phase in river and lake sediments resulted in a steady increase of the dissolved Mn concentration in the river water during autumn. INTRODUCTION
dissolved and suspended Mn was exported from a small lake in central Sweden during different seasons. Temporal variations in dissolved and suspended Mn in rivers and streams have been observed in a number of studies ( SILKER, 1964; SLACKand FELTZ, 1968; GUSTINGand RAN-
REDUCTION/OXIDATION(redox) cycles are important components of the biogeochemistry of many lake waters and sediments. The redox cycles of Fe and Mn in lakes are well documented and are two of the major biogeochemical processes in seasonally anoxic lakes ( WETZEL, 1983). Depletion of dissolved oxygen during summer stratification and during ice formation decreases the redox potential in the surface sediment and hypolimnion. This process often begins in the surface sediment which becomes a reducing environment earlier than the overlying water column. Large concentrations of dissolved Mn and Fe appear in the pore waters ( VERDOUW and DEKKERS, 1980; HAMILTON-TAYLOR and MORRIS, 1985 ) . Both these elements migrate up into the bottom water. SHOLKOVITZand COPLAND( 1982) demonstrated that dissolved and suspended Mn increased in the surface water of Esthwaite Water due to advective and diffusive transport from bottom water. In ice-covered lakes in Alaska elevated Mn concentrations have been observed in surface water during late winter ( BARSDATEand MATSON, 1967). DAVISONet al. ( 1982 ) concluded that Mn released to the oxic epilimnion was partially transported out of the lake. LIDBN ( 1983) also suggested that lake Liiviisundet, in northern Sweden, exported Mn seasonally. ANDERSSON( 199 1) has shown in detail how
CITELLI, 1972; AHL, 1977; EISENREICH et al., 1980; BURMAN, 1983; SUGAIand BURELL, 1984; PONT~R et al., 1990; COSSA et al., 1990). Suspended matter transported by the Kalix River, which discharges into the Gulf of Bothnia in the northern Baltic Sea, is strongly enriched in Mn compared to world rivers and average crust and a pronounced seasonal variation in dissolved and suspended Mn has been observed
(AHL, 1977; BURMAN,1983; PONTER~~al., 1990). The origin of particles rich in Mn in the Kalix River is of special interest in view of the vast occurrence of Mn-rich concretions and sediments in the Gulf of Bothnia ( WINTERHALTER, 1966; BOSTR~M et al., 1982; INGRI and PONTI?R, 1986, 1987). In an earlier study of the Kalix River, we suggested that lake and/or river sediments might add Mn to the river seasonally ( PONTBRet al., 1990). Only a small number of direct measurements of Mn in lake outlets have been made. To the authors’ knowledge no attempt has previously been made to correlate seasonal variation of Mn concentrations in lakes with variations in the Mn concentration in streams and rivers. SAMPLE AREA
* Present address: Swedish Geological AB, SGAB ANALYS, Box 80 1, S-95 1 28 Luld, Sweden. t Author to whom correspondence should be addressed. * Present address: Department of Geochemistry and Petrology, University of Stockholm, S- 106 9 1 Stockholm, Sweden.
The Kalix and Torne rivers, connected by the T&end6 River bifurcation, are the last major pristine rivers in Scandinavia (Fig. 1). The catchment of the Kalix River is 23600 km2 Half of the discharged water from the Tome River above TiirendS river drains via the Tlrendij River into the Kalix River. The drainage area is covered mainly 1485
C. PontCr, J. Ingri, and K. Bostriim
1486
3 I i I
69O [ 1
ba f>
KARESUANDO”-
F
-2-
100km
-
--
FIG. 1. The Kalix River drainage basin. A, B, C, and D denote the sampled lakes VettasjBrvi, Mettajtirvi, KZintiijWi, and Lapptrssket. E represents the sampled SkSrdtjiirnsbicken stream draining a peatland area. by coniferous forest (565%) and peatland ( 17-205’0).Four percent of the area is covered by lakes and less than 1% is farmland ( HJORT, 197 1) Quatemary deposits in the area consist primarily of till, showing well-developed podzol profiles and mixed mires ( FROMM,1965).
.
The bedrock of the river basin consists of two main units, the Precambrian basement and the Caledonian mountain range. The Kalix River has its source in a region around the highest mountain of Sweden, Kebnekaise (2120 m), in the Caledonian mountains.
Seasonal effects on the geochemistry of Mn Dominant rock types are mica schist, quartzite, and deformed amphibolite. Approximately 5% of the drainage area is situated within this lithological region. In the Precambrian basement, above the inflow of the Kaitum River (Fig. I), acid, intermediate, and basic volcanic rocks alternate. Below this tributary granites predominate (GAAL ~~~G~RBA~CHEV, 1987). Water samples in the river were taken 2 km north of the village of Kahx (Fig. 1) in the last rapid before the river reaches the Bothnian Bay archipelago. Outlets from four different lakes together with a stream (E in Fig. I ) which drain a series of shallow lakes and mires were sampled (Fig. 1). Lake Lapptrasket (D) is situated 15 km up stream of the mouth of the Kahx river and has a surface area of 4. I km2 and a maximum depth of approximately 12 m. Lake KZntiijarvi (C) and Metmjarvi (B) have surface areas of 3.0 km* and 6.6 km2, respectively, with maximum depths of around 10m. Lake Mettajarvi drains into lake Ka&tijjlrvi via the stream Vahjoki. Lake Vettasjlrvi (A) has a surface area of 8.7 km2 with a maximum depth of approximately 20 m. Lake Vettasj&rvi drains into the Kalix River via the Angela River. METHODS The river water was sampled two times a week during the period April 29 to September 4, 1985, and once a week from September 11, 1985, to September 30, 1986. Outlet water from the lakes was measured once a week from April 1985 to the end of August 1985. Samples were taken close to lake outlet except in Lake Ktintojijlrvi andLakeMettajlrvi, whichweresampled500and300mdownstream, respectively. Water was filtered in the field, in situ Silicone tubing was lowered below the water surface. The water was then pumped by using a portable Masterflex@ peristaltic pump and filtered “on line” through Milliporee filters. The filters had a diameter of 142 mm and a pore size of 0.45 pm. They were mounted in a Geoteche polycarbonate filter holder. In the streams, the samples were taken in the middle of the stream and in the middle of the water column shortly downstream from a rapid, so that a fairly well-mixed suspended sample would be obtained. In the river, samples were taken approximately 3 m from the shore and approximately 30 cm below the water surface, in a rapid. In vertical lake profiles the tubing was lowered from the surface to the lake bottom. Water discharge was measured in small concrete culverts made for crossing streams. At two locations the discharge measurements were performed below small bridges across the stream. A plastic bottle, partly filled with sand to obtain a density close to water, was used as a marker for the determination of water velocity in these streams. Discharge values in the Kalix River were not measured by the authors.
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They were obtained from continuous discharge measurements performed by the Swedish Meterological and Hydrological Institute (SMHI). pH values were measured in the field with a portable Metrohme 602 pH meter equipped with a standard glass combination electrode. The pH meter was calibrated with standard buffer solutions in the field before each determination. Half a litre of unfiltered water was allowed to equilibrate with the electrode for 30 min before measurement. The filtered water volume, which varied between 3 and 30 L depending on the suspended load, was collected in an acid-leached plastic container. Subsamples for analysis were taken from this bulk sample. This was done to obtain comparable “dissolved” element v-dues. The quantity of Mn passing the filter varies during the filtration as accumulation of suspended material continously decreases the nominal pore size (KENNEDYet al., 1974; LAXENand CHANDLER, 1983). From 2 L of filtered water manganese was coprecipitated with magnesium hydroxide at pH 10 after addition of magnesium sulfate and sodium hydroxide. This method has been described by ANDERSSONand INGRI ( 199 1). The precipitate was siphoned off and centrifuged, dissolved with 5 mL of concentrated nitric acid, and diluted to 10 mL with distilled water before measurement. Two filters with suspended material were wet-ashed in concentrated nitric acid in a platinum crucible at 70°C and then dry ashed at 450°C. The ashed inorganic matter was weighed to an accuracy of within flO%, fused with lithium metaborate at lOOO”C, and the bead formed was dissolved in 1.5mol L-’ nitric acid ( BURMANet al., 1978). Suspended and dissolved Mn was determined with a precision of approximately + 1.5%.Manganese analyses of the dissolved and suspended phase were performed by atomic emission spectroscopy, with an inductively coupled plasma as excitation source (ARL 3580) at SGAB ANALYS, Lulea, Sweden.
RESULTS AND DISCUSSION Melting snow in the Kalix River catchment usually results in a strong discharge maximum in the middle of May, as seen in 1986 (Fig. 2). In 1985, however, the spring flood was divided into three discharge peaks because of a longer melting period. A late rainy summer increased the water discharge in late August and early September to values comparable with the melt-water peaks. The total suspended sediment concentration (ashed) was almost constant at around 2 mg L-’ throughout the two sampling years (except during the flood in May, when the
suspended
FIG. 2. Water discharge and ashed suspended sediment concentration in the Kalix River during the period April 29, 1985, (day 119) to September 30, 1986 (day 638).
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concentration increased by a factor of approximately ten). The transient peak on the 16th of October (day 289 in Fig. 2) was related to a violent storm that swept over the drainage basin. Maximum suspended Mn concentrations in the river were observed during maximum melt-water discharge in midMay (Fig. 3a). The Mn/Al ratio in the suspended phase indicated that suspended Mn during the spring flood mainly was hosted in detrital particles. The Mn/Al ratio during maximum melt-water discharge was 0.02 (Fig. 3b). This is the same ratio as in average “world river” and average crust (MARTIN and WHITFELD, 1983). In mid-June suspended Mn increased and reached a local maximum in late July and early August. In late July the Mn/AI ratio was up to 2.5times higher than its value during maximum melt-water discharge. This indicated a significant increase of a non-detrital Mn phase in the river during this period (Fig. 3b). The dissolved Mn concentration increased rapidly during flood in May and showed the highest concentration shortly
before maximum melt-water discharge. A smaller dissolved concentration peak was observed in early June and increased dissolved Mn concentrations were observed also in November and December (Fig. 3a). When non-detrital suspended Mn was high during July, August, and September, the dissolved Mn concentration decreased to 1.8 pg L-’ (mid-July). This was half the dissolved concentration during low water discharge in winter (January to April; Fig. 3a). When non-detrital suspended Mn declined in mid-October, the dissolved concentration started to increase and a local maximum was reached on Nov 7 ( 15.9KgL-l). During winter the variations in the dissolved and suspended concentrations were small, and an almost constant total Mn concentration of 7 rg L-’ was measured. It seems reasonable to assume that the concentration of Mn in the river during winter, when the whole area is covered by snow, represents the average groundwater concentration in the drainage basin. This concentration doubled during summer and there was a tenfold increase during
a
80-
60-;
502 ; 40I 30-
100
150
200
250
300
350
400
450
500
550
600
650
IMIJIJIAIsIoINIDIJIF~M~A~M~J~J~A~s~
--NV. Temperature
FIG. 3. (a) Dissolved and suspended Mn concentrations in the Kalix River from April 29, 1985, to September 30, 1986. (b) The suspended Mn/AI ratio, temperature, and pH value in the Kalix River during the period of investigation.
1489
Seasonal effects on the geochemistry of Mn
l-
,
j_:;:l 1 0
thl di66Obd
Mn suspend6d (pg I”)
(M r’)
40
Mn
I
(M r’)
Mn suspended Qls I-‘)
Mn
160
120
I
I
0
ratio
I
0.4
)
Mn diisoiwd
60
/ Al 6U6p6nded
2
4
6
02dissobd
6
12
10
6.5
(mg I“)
7.0
,
I
0.6
/ Al suspended
6.2
4
ratio
I
7.5
P’-’
02di660hred (mg I.‘)
I
6.6 PH
FIG. 4. Dissolvedand suspended Mn concentrations, the suspended Mn/Al ratio, dissolved oxygen concentrations, and pH value on May 18, 1985, (unfilled dots) and August 9, 1985, (filled dots) in Lake Vettasjarvi (a) and on May 14 and August 8 in Lake Lapptdsket (b).
70
0
50
al
-I
b
-, 40: ;
30-
s % 20-
:
i 4 i ':.
$j 5
1010120 130 140 150 180 170 180 190 200 210 220 230 240 1
May
1
June
1
July
1
August
\ ?.
lo-
......c--0. ..& b..,.... “~O-+.._.~” A..< ow;.";o:'"'I 1”’ z ’ * ’ 8 ’ 8 “1 b’ I ’ * ’ 81 110 120 130 140 150 180 170 180 190 200 210 220 230 24(
1
May
July
June
I
August
I
!:;..;?-i,y+_%yhQo :lO 120 130 140 150 180 170 180 190 200 210 220 230 241
I
May
I
June
I
July
I
August
I
I
May
I
June
I
July
I
FIG. 5. Dissolved Mn concentrations in lake outlet waters during the period April 22,1985, to August 30, 1985. (a) Vettasjiirvi (b) Ktintiijlrvi (c) Mettajarvi (d) Lapptrasket. The arrow denotes ice break-up and spring overturn.
August
I
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hood in May. Hence, some other source(s) for Mn-rich water must exist in the drainage basin. Large concentrations of dissolved Mn accumulated in the hypolimnion of the lakes studied during the period of ice cover. Lake Vettasjlrvi was anoxic in the bottom water during late winter and early spring 1985. On May 18, 12 100 pg L -’ of dissolved Mn was measured 60 cm above the lake bottom (Fig. 4a). In Lake Lapptmsket anoxic conditions in the hypolimnion were not attained during the winter stratification of 1985. On May 14, the oxygen saturation was close to 40%, 50 cm above bottom, and the dissolved Mn concentration was 213 pg L-' (Fig. 4b). The thawing period in 1985 started in early May and maximum melt-water discharge was observed around the 20th of May in the lake outlets. Slightly elevated dissolved Mn concentrations were observed in the outlets of the lakes during this period (Fig. 5). Ice break-up occurred approximately two weeks after most snow had melted. Within a week of ice break-up and spring overturn dissolved Mn concentrations in the outlet waters increased by more than a factor of ten (Fig. 5 ). The peak of dissolved Mn in early June in the river (Fig. 3a) was correlated in time with peaks of dissolved Mn in the outlets of the lakes studied (Fig. 5). In the outlet from Lake Vettasjarvi the increased dissolved Mn concentration was accompanied by an increased suspended Mn concentration (Fig. 6a). Dissolved Mn declined rapidly to values below the concentration observed in early May. Suspended Mn decreased after the initial increase, but a relatively high concentration was maintained during July and increased steadily during August. Suspended matter with approximately 3% Mn (on an ashed weight basis) was observed in the outflowing water from June 18 to the end of August. In the stream from Lake Ktintiijlrvi, maximum suspended Mn concentrations were observed three weeks after the dissolved Mn peak (Fig. 6b). From July 5 to the end of August approximately 3% (ashed weight) of the discharged suspended phase was Mn, corresponding to high Mn/Al suspended ratios (Fig. 6~). Discharge of Mn-rich particles from lakes correlated in time with the increase of non-detrital suspended Mn in the river. It has been shown that precipitation ofdissolved Mn in freshwater results in a combination of Mn-oxyhydroxides, most commonly resembling vemadite, 6-Mn02 (TIPPING et al., 1984). It therefore seems reasonable to assume that the increased Mn/Al ratios observed in this study arose from precipitated Mn-oxyhydroxides. Daily discharges of suspended and dissolved Mn from Lake Vettasjlrvi, Lake KGntijjarvi-Mettajarvi (Lake Mettajarvi drains into Lake KGntiijarvi) and the Kalix River are shown in Fig. 7. The total input of Mn from the lakes into the river has been calculated from these data, assuming that the three lakes represent an average value for lake discharge of Mn. It should be remembered that these lakes represent only 2% of the total lake area in the watershed and that the absolute values for lake discharge of Mn could change significantly if our assumption is wrong. However, the relative changes in Mn discharge from lakes between different months in relation to the river discharge is probably valid for most lakes in the area. Table 1 indicates that lakes were a major source for Mn throughout the summer. Lake discharge of Mn was of the same order as the total Mn load in the river during June.
110 120 130 140 150 160 170 180 190 200 210 220 230 240
1
May
1 June
50
-
1
July
1 August
P
b
/\
l Mnsuspsnded
40- o Mndissolved
t
i\ ; :
110 120 130 140 150 160 170 180 190 200 210 220 230 24(
I
May
I
June
I
July
I
I
August
A stream Vettasjoki n stream Kaantyjoki
110 120 130 140 150 180 170 180 190 200 210 220230
I
May
I
June
I
July
I
August
240 I
FIG. 6. Dissolvedand suspended Mn concentrations in outlet water from Lake Vettasjarvi during the period April 22, 1985, to August 30, 1985, (a) and Lake KSlntiijlrvi (b). (c) The suspended Mn/AI
ratio in outlet water from Vettasjtirvi and Wnt6jlrvi.
This supports the conclusion that the peak of dissolved Mn in June in the river was the result of spring overturn in the lakes. However, data clearly showed that large and relatively deep lakes of the type studied did not contribute significantly to the high Mn concentration in the river during flood in May. Hence, there must be some other source(s) for Mn during spring flood. Major elements, Ca, Si, Na, S, and Mg were diluted by the spring flood in the river. This indicates that melt-water or surface run-off contributed to the discharge peaks during flood in May. Mixed mires of the type found in the Kalix River drainage basin are effluent areas for groundwater where little infiltration can occur. It is therefore reasonable to assume that much of the melt-water run-off observed during May in the Kalix River originated from peatland.
Seasonal effectson the geochemistry of Mn
k -
0 100
....* ....
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suspsndsd disbud
“,““‘,““‘,I”“’
150 200 250 300 350 400 450 IMI JIJ~AIS~O~NIDIJIFIMIA~M~
500
550
600
RG.7.Dailyloadof dissolved and suspended Mn in lake outlet waters from (a) Lake Vettasjti and in the Kalix River (c ) .
An average Mn concentration of 0.7 pg L-’ can be estimated for snow in the drainage area (ROSS and GRANAT, 1986). The total precipitation was 189 mm during the period from October 1984 to April 1985. From these values (and
Table 1.
dissolved SlJSpCNld8d
0
(b) Lake Wntiijiiwi,
with a total drainage area of 23600 km2), the total amount of Mn in snow can be calculated
to be 3 122 kg. Twenty
percent, or 624 kg, of this was deposited on peatland. This is 0.9% of the total amount of Mn transported in the river
Measured Mn load in the Kalix River during late spring and summer 1965 compared with the calculated quantity of Mn discharged from lakes within the drainage basin during the same period.
May Mn (kg)
f
JI JIAISI
June
July
river
lakes
river
lakes
river
lakes
45800 29500
4800 1700
14600 10900
18000 8500
3200 15600
3800 7000
August river lakes
4800 23800
3100 6700
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25
250 l
yr: 200
Mn diss kg(24h).
& 2
9 _
150
15 z
100
10
50
5z
1" g +j 5
$ g3 r' 0 110 120 130 140 150 160 170 160190
1
May
1
June
1
200 210 220 230 240'
July
1 August
1
FIG. 8. Dissolvedconcentrations and daily dissolvedload of Mn in the Skordtjlmsbacken stream draining a peatland
area. during May 1985. Hence, the Mn load in snow cannot explain the high Mn load during May in the river. Analyses of peat cores from poor mires and from ombrotrophic Scandinavian bogs have shown an enrichment of Mn in the uppermost living part and a pronounced loss in the peat below the low water level ( SONESSON, 1970; DAMMAN, 1978). Accumulation of dissolved Mn in mire waters has been known for a long time, but the mechanism causing this enrichment relative to groundwater is incompletely understood ( SHOTYK, 1988). The Skordtjlmsbacken stream drains a peatland area with small and shallow lakes (Fig. 1, E). Measurements showed high dissolved Mn concentrations throughout the sampling period, especially in late April and early May (Fig. 8). Shortly before maximum melt water discharge the dissolved Mn concentration was 238 pg L-‘. Mn was diluted by melt-water discharge in mid-May and on June 6 the dissolved concentration was 20.9 fig L-’ . This increased to approximately 35 pg L-’ between June 13 and June 25. A new local maximum was observed in early August. The average discharge of dissolved Mn was 225 kg/day during the maximum melt-water discharge and decreased rapidly down to 50 kg/day in late May. It seems reasonable to assume that the enrichment of dissolved Mn in runoff during meltwater discharge in the Skiirdtjlmsbacken stream was caused by melt-water mixed with Mn-rich mire water. The high concentration of dissolved Mn shortly before melt-water maximum in May in the Kalix River could therefore be explained by an increased supply of Mn-rich mire water. COSSAet al. ( 1990) observed a transient peak of dissolved Mn in the St. Lawrence River during snowmelt. This was very similar to the peak observed in the Kalix River, but they did not explain the origin of the peak. The increase in outflowing Mn in early August from Skiirdtjlmsblcken was probably related to heavy rains (the rainfall in this area was 116 mm during August in the area compared with 54 mm during July). Rain water might displace Mn-rich water from mires and shallow lakes in the same way that it is displaced by melting snow during flood in spring. This source could supply Mn to the in periods of heavy rainfall. However, we do not believe that this would explain the almost constant increase in concentration of dissolved Mn from September to November in the Kalix River (Fig. 3a).
The Si/Al ratio measured in lakes has been shown to reflect the growth of diatoms ( STABEL,1985 ). In the Kalix River a decrease in dissolved Si and a simultaneous increase in the suspended B/Al ratio was measured during the growth season (Fig. 9). This probably reflects the uptake of dissolved Si into the suspended biomass. The demand for Mg in metabolism is small compared with that generally available in fresh water. Because of the solubility characteristics of Mg and its minor biotic demand, concentrations of Mg are maintained at a relatively uniform level ( WETZEL, 1983) and Mg has been used to calculate groundwater influxes into a lake ( WETZEL and OTSUKI, 1974). We have used Mg-normal&d data for dissolved Si and Mn in the Kalix River to display temporal variations unrelated to changes in discharge. The constant slow increase of normal&d dissolved Si during autumn most likely reflects mineralisation of diatoms in river and lake sediments (Fig. 10). The correlation between normalised dissolved Mn and Si concentrations is striking (Fig. 10). The slow and constant increase of dissolved Mn during autumn can be explained analogously to the increase of dissolved Si. Settling Mn-rich particles were dissolved during early diagenesis in river and lake sediments, and dissolved Mn was
5.5
5.5
--c suspended ......o...... dissolved 150
200
250
300
350
I JIJIAIS~OINIDI
FIG. 9. The dissolved Si concentration and suspended Si/~l ratio duringsummer and autumn 1985 in the Kalix River.
1493
Seasonal effectson the geochemistry of Mn
100
150
200
250
300
350
400
450
500
550
600
65
FIG. 10. The Mg-nonnalised dissolved Si and Mn concentrations in the Kalix River from May 1985 to September 1986.
added continuously to the river water in September, October, November, and December from this source. There are several possible explanations to the observed phase change of Mn in the Kalix River during the summer. Laboratory studies suggest that for the pH values and temperatures found in lake water the oxidation rate of dissolved Mn should be very slow. However, in real lake waters Mn can precipitate within a few days, at pH values down to 6.5. The rapid oxidation of dissolved Mn in the lake waters depends on the presence of natural particulate matter, implying either that the reaction is biologically mediated or that it is catalysed by particles ( CHAPNICKet al., 1982; TIPPING et al., 1984). Biological oxidation of Mn generally occurs between pH values of 6.5 to 7.5 (BROMRELDand DAVID, 1976; UREN and LEEPER, 1978). Several workers have emphasized the importance of microbially mediated Mn oxidation in fresh water ( CHAPNICKet al., 1982; DIEM and STUMM, 1984; TIPPING, 1984). GRAHAM et al. (1976) concluded that the speciation of Mn in rivers is a function primarily of pH-dependent oxidation processes. LAXENet al. ( 1984), on the other hand, did not find any coupling between pH and soluble (co.0 15 pm) Mn in their investigation of fifteen rivers and streams. The rapid decrease of dissolved Mn and increase of suspended Mn in June, in the Kalix River, was correlated in time with an increase in temperature and pH value (Fig. 3b). High suspended Mn/Al ratios were observed during the summer and early autumn months, when the water temperature and pH value were high and when large quantities of biogenic particles were present in the suspended load, as indicated by high Si/ Al ratios. Data in Fig. 6 showed that Mn-rich particles were formed in lakes and exported to the river. The oxidation rate of dissolved Mn may be increased by suspended Mn-Fe oxyhydroxide surfaces ( STUMM and MORGAN, 198 1). These par-
ticles may have catalysed the oxidation of dissolved Mn in the river. The export of these particles ceased when the lakes were stratified in the autumn, thus explaining the low sus-
pended/dissolved ratio in the river in late autumn and winter. However, comparable high suspended Fe/ Al and Mn/ Al ratios have been measured also during autumn and winter in the river. Approximately half of the suspended load during winter consisted of Fe in a non-detrital form, most likely Feoxyhydroxides. Scavenging of dissolved Mn onto Mn-Feoxyhydroxides thus appears to be of minor importance for the phase change of Mn during summer in the river. The rapid increase of the suspended Mn / Al ratio started at a pH of approximately 6.7 both sampling years. However, during the winter months (December-March ) a constant pH of ap proximately 6.7 was measured. During this period, low suspended Mn / Al ratios were observed and the total Mn transport was dominated by the dissolved phase. Furthermore, the pH was higher than 6.7 during autumn (September-November) but the suspended Mn/ Al ratio was low. The Mn/Al ratio started to increase when the water temperature in the river reached approximately 15°C. This was measured both sampling years. The Mn/AI ratio remained high so long as the temperature remained above 15°C and decreased when the temperature fell below 15“C. It has been shown that the oxidation rate ofdissolved Mn in English lake waters has a well-defined optimum temperature, varying from 15°C to 3O”C, in the pH range 7-8. This is a strong indication of biological mediation of the oxidation (TIPPING, 1984). It is plausible that increased activity of Mn-oxidising bacteria catalysed the precipitation of dissolved Mn in the Kalix River. This could explain the rapid decrease of dissolved Mn in late June, the high suspended Mn/Al ratios during summer, and the relatively low suspended Mn/ Al ratios during winter. Acknowledgments-The study was financed by grants from NFR. This support is greatfully acknowledged. We also wish to thank W. Davison, E. Tipping, and an anonymous reviewer for their constructive comments on the manuscript and P. Digby for his improvements of the English. Editorial handling: J. I. Drever
C. Ponter, J. Ingri, and K. Bostriim
1494 REFERENCES
T. ( 1977) River discharges of Fe, Mn, Cu, Zn and Pb into the Baltic Sea from Sweden. Ambio. Spec. Rep. 5, 2 19-228. ANDERSSON P. ( 199 1) Hydrogeochemistry of iron, manganese and sulphur and strontium isotopes in a coniferous catchment, central Sweden. Ph.D. dissertation, Univ. of Stockholm. ANDERSSON P. and INGRI J. ( I99 1)A rapid preconcentration method for multielement analysis of natural freshwaters. War. Res. 25, 6 17-620. BARSDATER. J. and MATSONW. R. ( 1967) Trace metals in arctic and sub-arctic lakes with reference to the organic complexes of metals. In Radioecological Concentration Processes. (ed. B. ABERG and F. P. HUNGATE)pp. 71 I-719. Oxford Pergamon Corp. BOSTR~MK., WIBORGL., and INGRI J. (1982) Geochemistry and origin of ferromanganese concretions in the Gulf of Bothnia. Mar. Geol. SO, 1-24. BROMFIELDS. M. and DAVIDD. J. ( 1976) Sorption and oxidation of manganous ions and reduction of manganese oxide by cell suspensions of a manganese-oxidizing bacterium. Soil Biol. Biochem. 8, 37-43. BURMANJ-O. ( 1983) Element transport in suspended and dissolved phases in the Kalix River. Ecol. Bull. 35, 99-I 13. BURMANJ-O., PONT~R C., and BOSTROMK. ( 1978) Metaborate digestion procedure for inductively coupled plasma-optical emission spectrometry. Anal. Chem. 50, 679-680. CHAPNICKS. D., MOOREW. S., and NEALSONK. H. ( 1982) Microbially mediated manganese oxidation in a freshwater lake. Limnol. Oceunogr. 27, 1004-1014. COSSA D., TREMBLAY G. H., and GOBEILC. ( 1990) Seasonality in iron and manganese concentrations ofthe St. Lawrence River. Sci. Total Environ. 97198, 185-190. CUSTINGC. E. and RANCITELLIL. A. ( 1972) Trace element analysis of Columbia river water and phytoplankton. N W Sci. 46, 11S121. DAMMANA. W. H. ( 1978) Distribution and movement of elements in ombrotrophic peat bogs. Oikos 30,480-495. DAVISONW., WOOF C., and RIGG E. ( 1982) The dynamics of iron and manganese in a seasonally anoxic lake; Direct measurement of fluxes using sediment traps. Limnol. Oceunogr. 27,987-1003. DIEM D. and STUMMW. ( 1984) Is dissolved Mn2+ being oxidized by O2 in absence of Mn-bacteria or surface catalysts? Geochim. Cosmochim. Acta 48, 157 I-1573. EISENREICHS. J., HOFFMANNM. R., RASTETTERD., YOSTE., and MAIERW. J. ( 1980) Metal transport phases in the upper Mississippi river. Adv. Chem. Ser. 189, 135-176. FROMM E. ( 1965) Beskrivning till jordartskartan over norrbottens Ian, nedanfor lappmarksgransen. SGU ser. Ca 41, 1-15 1. (in Swedish with English summary). GAAL G. and GORBATSCHEV R. ( 1987) An outline of the Precambrian evolution of the Baltic Shield. Precambrian Res. 35, 15-52. GRAHAMW. F., BENDERM. L., and KLINKHAMMERG. P. ( 1976) Manganese in Narragansett Bay. Limnol. Oceanog. 21,665-673. HAMILTON-TAYLOR J. and MORRISE. B. ( 1985) The dynamics of iron and manganese in the surface sediments of a seasonally anoxic lake. Arch. Hydrobiol. Suppl. 72, 135-165. HJORT S. ( 197 1) Tome och Kalix %lvar, de1 1 allmln beskrivning. University of Uppsala, UNGI Report 12. (in Swedish with English summary). INGRI J. and PONTBRC. ( 1986) Iron and manganese layering in recent sediments in the Gulf of Bothnia. Chem. Geol. 56, 105116. INCRI J. and PONTBRC. ( 1987) Rare earth abundance patterns in AHL
ferromanganese concretions from the Gulf of Bothnia and the Barents Sea. Geochim. Cosmochim. Acta 51, 155- 161. KENNEDY V. C., ZELLWEGER G. W., and JONESB. F. ( 1974) Filter pore-size effects on the analysis of Al, Fe, Mn and Ti in water. Water Resources Res. 10, 785-790. LAXEND. P. H. and CHANDLERI. M. ( 1983) Size distribution of iron and manganese species in freshwaters. Geochim. Cosmochim. Acta 47, 73 l-74 1. LAXEN D. P. H., DAVISONW., and WOOF C. (1984) Manganese chemistry in rivers and streams. Geochim. Cosmochim. Acta 48, 2107-2111. LIDBNJ. ( 1983) Equilibrium approaches to natural water systems. A study of anoxic-and ground waters based on in situ data acquisition. Ph.D. dissertation, Univ. of Umea. MARTINJ.-M. and WHITFIELDM. ( 1983) The significance of the river input of chemical elements to the ocean. In Truce Metals in Sea Water (ed. C. S. WONG et al.) pp. 265-269. Plenum. PONTBRC., INGRIJ., BURMANJ-O., and BOSTROMK. ( 1990) Temporal variation in dissolved and suspended iron and manganese in the Kalix River, northern Sweden. Chem. Geol. 81, 12 I- 13 I. Ross H. B. and GRANATL. ( 1986) Deposition of atmospheric trace metals in northern Sweden as measured in the snowpack. Tellus 38B, 27-43. SHOLKOVITZE. R. and COPLANDD. ( 1982) The chemistry of suspended matter in Esthwaite water, a biologically productive lake with seasonally anoxic hypohmnion. Geochim. Cosmochim. Acta 46,393-410. SHOTYKW. ( 1988) Review of the inorganic geochemistry of peats and peatland waters. Earth Sci. Rev. 25, 95- 176. SILKERW. B. ( 1964) Variations in elemental concentrations in the Columbia River. Limnol. Oceanogr. 9, 540-545. SLACKK. V. and I%LTZH. R. ( 1968) Tree leaf control on low flow water quality in a small Virginia stream. Environ. Sci. Tech. 2, 126-131. SONESSON M. ( 1970) Studies on mire vegetation in the Tometrask area, northern Sweden. IV. Some habitat conditions of the poor mires. Bot. Not. 123, 67-l 11. STABELH-H. ( 1985) Mechanisms controlling the sedimentation sequence of various elements in prealpine lakes. In Chemical Processes in Lakes (ed. W. STUMM), Chap 7, pp. 143-169. J. Wiley & Sons. STUMMW. and MORGANJ. J. ( 1981) Aquatic Chemistry. 2d ed. J. Wiley & Sons. SUGAIS. F. and BURELLD. C. ( 1984) Transport ofdissolved organic carbon, nutrients and trace metals from the Wilson and Blossom rivers to Smeaton Bay, southeast Alaska. Canadian J. Fish. Aquat. Sci. 41, 180-190. TIPPING E. ( 1984) Temperature dependence of Mn (II) oxidation in lakewaters: A test of biological involvement. Geochim. Cosmochim. Acta48, 1353-1356. TIPPINGE., THOMPSOND. W., and DAVISONW. (1984) Oxidation products of Mn (II) in lake waters. Chem. Geol. 44,359-383. URENN. C. and LEEPERG. W. ( 1978) Microbial oxidation of divalent manganese. Soil Biol. Biochem. 10, 85-87. VERDOUWH. and DEKKERSE. M. J. ( 1980) Iron and manganese in lake Vechten: Dynamics and role in the cycle of reducing power. Arch. Hydrobiol. 89, 509-532. WETZELR. G. ( 1983) Limnology. 2d ed. CBS College Publishing. WETZELR. G. and OTSUKIA. ( 1974) Allochthonousorganic carbon of a marl lake. Arch. Hvdrobiol. 73. 31-56. WINTERHALTERB. ( 1966 ) Pohjanlahden ja Suomenlahden rautamangaanisaostumista. Gotekn. Julk. 69, l-78. (in Finnish with English summary).
Geochimica et Cosmoehimica Aaa Vol. 56. pp. 1485-1494 Copyright 0 1992 Pergamon Press Ltd. Printed in U.S.A.
Geochemistry of manganese in the Kalix River, northern Sweden CHRISTERPONT~~R, * JOHAN INGRI,+ and KURT BOSTROM* Department of Economic Geology, LuleH University of Technology, S-95 1 87 LuleB,Sweden (Received November 28, 1990; accepted in revised form January 22, 1992 )
Abstract-Dissolved and suspended Mn in the Kalix River, northern Sweden, were measured weekly over a period of eighteen months. During the same period four lakes in the Kalix catchment were sampled at their outlets and in vertical profiles within the lakes, together with a stream draining a series of mires with shallow lakes. Snow melting in mid-May increased the dissolved Mn concentration in the river tenfold, compared with a concentration of 5 pg L-’ during the winter discharge (January to April). We suggest that the increase was caused by Mn-rich mire water mixing with melting snow and being transported to the river. Large concentrations of dissolved Mn built up in the hypolimnion of the lakes studied during the icecovered period. Break-up of the ice and spring-overturn in June increased the dissolved Mn concentration tenfold in lake discharge and a concomitant peak in the dissolved Mn concentration was observed in the river. Lake-derived Mn was the dominant source for Mn in the river during this time. Suspended Mn in the river was hosted mainly in detrital particles during flood in May. In mid-June, non-detrital suspended Mn started to accumulate and reached a maximum in late July and early August. The Mn / Al ratio was 25 times higher during this period than during flood in May, suggesting the precipitation of an Mn-oxyhydroxide phase. The precipitation of the non-detrital Mn-rich phase was correlated in time with increased temperature, increased pH, and increased concentration of suspended biogenic particles. The precipitation of dissolved Mn was biologically mediated. Sedimentation and mineralisation of the non-detrital Mn phase in river and lake sediments resulted in a steady increase of the dissolved Mn concentration in the river water during autumn. INTRODUCTION
dissolved and suspended Mn was exported from a small lake in central Sweden during different seasons. Temporal variations in dissolved and suspended Mn in rivers and streams have been observed in a number of studies ( SILKER, 1964; SLACKand FELTZ, 1968; GUSTINGand RAN-
REDUCTION/OXIDATION(redox) cycles are important components of the biogeochemistry of many lake waters and sediments. The redox cycles of Fe and Mn in lakes are well documented and are two of the major biogeochemical processes in seasonally anoxic lakes ( WETZEL, 1983). Depletion of dissolved oxygen during summer stratification and during ice formation decreases the redox potential in the surface sediment and hypolimnion. This process often begins in the surface sediment which becomes a reducing environment earlier than the overlying water column. Large concentrations of dissolved Mn and Fe appear in the pore waters ( VERDOUW and DEKKERS, 1980; HAMILTON-TAYLOR and MORRIS, 1985 ) . Both these elements migrate up into the bottom water. SHOLKOVITZand COPLAND( 1982) demonstrated that dissolved and suspended Mn increased in the surface water of Esthwaite Water due to advective and diffusive transport from bottom water. In ice-covered lakes in Alaska elevated Mn concentrations have been observed in surface water during late winter ( BARSDATEand MATSON, 1967). DAVISONet al. ( 1982 ) concluded that Mn released to the oxic epilimnion was partially transported out of the lake. LIDBN ( 1983) also suggested that lake Liiviisundet, in northern Sweden, exported Mn seasonally. ANDERSSON( 199 1) has shown in detail how
CITELLI, 1972; AHL, 1977; EISENREICH et al., 1980; BURMAN, 1983; SUGAIand BURELL, 1984; PONT~R et al., 1990; COSSA et al., 1990). Suspended matter transported by the Kalix River, which discharges into the Gulf of Bothnia in the northern Baltic Sea, is strongly enriched in Mn compared to world rivers and average crust and a pronounced seasonal variation in dissolved and suspended Mn has been observed
(AHL, 1977; BURMAN,1983; PONTER~~al., 1990). The origin of particles rich in Mn in the Kalix River is of special interest in view of the vast occurrence of Mn-rich concretions and sediments in the Gulf of Bothnia ( WINTERHALTER, 1966; BOSTR~M et al., 1982; INGRI and PONTI?R, 1986, 1987). In an earlier study of the Kalix River, we suggested that lake and/or river sediments might add Mn to the river seasonally ( PONTBRet al., 1990). Only a small number of direct measurements of Mn in lake outlets have been made. To the authors’ knowledge no attempt has previously been made to correlate seasonal variation of Mn concentrations in lakes with variations in the Mn concentration in streams and rivers. SAMPLE AREA
* Present address: Swedish Geological AB, SGAB ANALYS, Box 80 1, S-95 1 28 Luld, Sweden. t Author to whom correspondence should be addressed. * Present address: Department of Geochemistry and Petrology, University of Stockholm, S- 106 9 1 Stockholm, Sweden.
The Kalix and Torne rivers, connected by the T&end6 River bifurcation, are the last major pristine rivers in Scandinavia (Fig. 1). The catchment of the Kalix River is 23600 km2 Half of the discharged water from the Tome River above TiirendS river drains via the Tlrendij River into the Kalix River. The drainage area is covered mainly 1485
C. PontCr, J. Ingri, and K. Bostriim
1486
3 I i I
69O [ 1
ba f>
KARESUANDO”-
F
-2-
100km
-
--
FIG. 1. The Kalix River drainage basin. A, B, C, and D denote the sampled lakes VettasjBrvi, Mettajtirvi, KZintiijWi, and Lapptrssket. E represents the sampled SkSrdtjiirnsbicken stream draining a peatland area. by coniferous forest (565%) and peatland ( 17-205’0).Four percent of the area is covered by lakes and less than 1% is farmland ( HJORT, 197 1) Quatemary deposits in the area consist primarily of till, showing well-developed podzol profiles and mixed mires ( FROMM,1965).
.
The bedrock of the river basin consists of two main units, the Precambrian basement and the Caledonian mountain range. The Kalix River has its source in a region around the highest mountain of Sweden, Kebnekaise (2120 m), in the Caledonian mountains.
Seasonal effects on the geochemistry of Mn Dominant rock types are mica schist, quartzite, and deformed amphibolite. Approximately 5% of the drainage area is situated within this lithological region. In the Precambrian basement, above the inflow of the Kaitum River (Fig. I), acid, intermediate, and basic volcanic rocks alternate. Below this tributary granites predominate (GAAL ~~~G~RBA~CHEV, 1987). Water samples in the river were taken 2 km north of the village of Kahx (Fig. 1) in the last rapid before the river reaches the Bothnian Bay archipelago. Outlets from four different lakes together with a stream (E in Fig. I ) which drain a series of shallow lakes and mires were sampled (Fig. 1). Lake Lapptrasket (D) is situated 15 km up stream of the mouth of the Kahx river and has a surface area of 4. I km2 and a maximum depth of approximately 12 m. Lake KZntiijarvi (C) and Metmjarvi (B) have surface areas of 3.0 km* and 6.6 km2, respectively, with maximum depths of around 10m. Lake Mettajarvi drains into lake Ka&tijjlrvi via the stream Vahjoki. Lake Vettasjlrvi (A) has a surface area of 8.7 km2 with a maximum depth of approximately 20 m. Lake Vettasj&rvi drains into the Kalix River via the Angela River. METHODS The river water was sampled two times a week during the period April 29 to September 4, 1985, and once a week from September 11, 1985, to September 30, 1986. Outlet water from the lakes was measured once a week from April 1985 to the end of August 1985. Samples were taken close to lake outlet except in Lake Ktintojijlrvi andLakeMettajlrvi, whichweresampled500and300mdownstream, respectively. Water was filtered in the field, in situ Silicone tubing was lowered below the water surface. The water was then pumped by using a portable Masterflex@ peristaltic pump and filtered “on line” through Milliporee filters. The filters had a diameter of 142 mm and a pore size of 0.45 pm. They were mounted in a Geoteche polycarbonate filter holder. In the streams, the samples were taken in the middle of the stream and in the middle of the water column shortly downstream from a rapid, so that a fairly well-mixed suspended sample would be obtained. In the river, samples were taken approximately 3 m from the shore and approximately 30 cm below the water surface, in a rapid. In vertical lake profiles the tubing was lowered from the surface to the lake bottom. Water discharge was measured in small concrete culverts made for crossing streams. At two locations the discharge measurements were performed below small bridges across the stream. A plastic bottle, partly filled with sand to obtain a density close to water, was used as a marker for the determination of water velocity in these streams. Discharge values in the Kalix River were not measured by the authors.
16001
,
-o-
1487
They were obtained from continuous discharge measurements performed by the Swedish Meterological and Hydrological Institute (SMHI). pH values were measured in the field with a portable Metrohme 602 pH meter equipped with a standard glass combination electrode. The pH meter was calibrated with standard buffer solutions in the field before each determination. Half a litre of unfiltered water was allowed to equilibrate with the electrode for 30 min before measurement. The filtered water volume, which varied between 3 and 30 L depending on the suspended load, was collected in an acid-leached plastic container. Subsamples for analysis were taken from this bulk sample. This was done to obtain comparable “dissolved” element v-dues. The quantity of Mn passing the filter varies during the filtration as accumulation of suspended material continously decreases the nominal pore size (KENNEDYet al., 1974; LAXENand CHANDLER, 1983). From 2 L of filtered water manganese was coprecipitated with magnesium hydroxide at pH 10 after addition of magnesium sulfate and sodium hydroxide. This method has been described by ANDERSSONand INGRI ( 199 1). The precipitate was siphoned off and centrifuged, dissolved with 5 mL of concentrated nitric acid, and diluted to 10 mL with distilled water before measurement. Two filters with suspended material were wet-ashed in concentrated nitric acid in a platinum crucible at 70°C and then dry ashed at 450°C. The ashed inorganic matter was weighed to an accuracy of within flO%, fused with lithium metaborate at lOOO”C, and the bead formed was dissolved in 1.5mol L-’ nitric acid ( BURMANet al., 1978). Suspended and dissolved Mn was determined with a precision of approximately + 1.5%.Manganese analyses of the dissolved and suspended phase were performed by atomic emission spectroscopy, with an inductively coupled plasma as excitation source (ARL 3580) at SGAB ANALYS, Lulea, Sweden.
RESULTS AND DISCUSSION Melting snow in the Kalix River catchment usually results in a strong discharge maximum in the middle of May, as seen in 1986 (Fig. 2). In 1985, however, the spring flood was divided into three discharge peaks because of a longer melting period. A late rainy summer increased the water discharge in late August and early September to values comparable with the melt-water peaks. The total suspended sediment concentration (ashed) was almost constant at around 2 mg L-’ throughout the two sampling years (except during the flood in May, when the
suspended
FIG. 2. Water discharge and ashed suspended sediment concentration in the Kalix River during the period April 29, 1985, (day 119) to September 30, 1986 (day 638).
1488
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Ponttr, J. Ingri, and K. Bostrijm
concentration increased by a factor of approximately ten). The transient peak on the 16th of October (day 289 in Fig. 2) was related to a violent storm that swept over the drainage basin. Maximum suspended Mn concentrations in the river were observed during maximum melt-water discharge in midMay (Fig. 3a). The Mn/Al ratio in the suspended phase indicated that suspended Mn during the spring flood mainly was hosted in detrital particles. The Mn/Al ratio during maximum melt-water discharge was 0.02 (Fig. 3b). This is the same ratio as in average “world river” and average crust (MARTIN and WHITFELD, 1983). In mid-June suspended Mn increased and reached a local maximum in late July and early August. In late July the Mn/AI ratio was up to 2.5times higher than its value during maximum melt-water discharge. This indicated a significant increase of a non-detrital Mn phase in the river during this period (Fig. 3b). The dissolved Mn concentration increased rapidly during flood in May and showed the highest concentration shortly
before maximum melt-water discharge. A smaller dissolved concentration peak was observed in early June and increased dissolved Mn concentrations were observed also in November and December (Fig. 3a). When non-detrital suspended Mn was high during July, August, and September, the dissolved Mn concentration decreased to 1.8 pg L-’ (mid-July). This was half the dissolved concentration during low water discharge in winter (January to April; Fig. 3a). When non-detrital suspended Mn declined in mid-October, the dissolved concentration started to increase and a local maximum was reached on Nov 7 ( 15.9KgL-l). During winter the variations in the dissolved and suspended concentrations were small, and an almost constant total Mn concentration of 7 rg L-’ was measured. It seems reasonable to assume that the concentration of Mn in the river during winter, when the whole area is covered by snow, represents the average groundwater concentration in the drainage basin. This concentration doubled during summer and there was a tenfold increase during
a
80-
60-;
502 ; 40I 30-
100
150
200
250
300
350
400
450
500
550
600
650
IMIJIJIAIsIoINIDIJIF~M~A~M~J~J~A~s~
--NV. Temperature
FIG. 3. (a) Dissolved and suspended Mn concentrations in the Kalix River from April 29, 1985, to September 30, 1986. (b) The suspended Mn/AI ratio, temperature, and pH value in the Kalix River during the period of investigation.
1489
Seasonal effects on the geochemistry of Mn
l-
,
j_:;:l 1 0
thl di66Obd
Mn suspend6d (pg I”)
(M r’)
40
Mn
I
(M r’)
Mn suspended Qls I-‘)
Mn
160
120
I
I
0
ratio
I
0.4
)
Mn diisoiwd
60
/ Al 6U6p6nded
2
4
6
02dissobd
6
12
10
6.5
(mg I“)
7.0
,
I
0.6
/ Al suspended
6.2
4
ratio
I
7.5
P’-’
02di660hred (mg I.‘)
I
6.6 PH
FIG. 4. Dissolvedand suspended Mn concentrations, the suspended Mn/Al ratio, dissolved oxygen concentrations, and pH value on May 18, 1985, (unfilled dots) and August 9, 1985, (filled dots) in Lake Vettasjarvi (a) and on May 14 and August 8 in Lake Lapptdsket (b).
70
0
50
al
-I
b
-, 40: ;
30-
s % 20-
:
i 4 i ':.
$j 5
1010120 130 140 150 180 170 180 190 200 210 220 230 240 1
May
1
June
1
July
1
August
\ ?.
lo-
......c--0. ..& b..,.... “~O-+.._.~” A..< ow;.";o:'"'I 1”’ z ’ * ’ 8 ’ 8 “1 b’ I ’ * ’ 81 110 120 130 140 150 180 170 180 190 200 210 220 230 24(
1
May
July
June
I
August
I
!:;..;?-i,y+_%yhQo :lO 120 130 140 150 180 170 180 190 200 210 220 230 241
I
May
I
June
I
July
I
August
I
I
May
I
June
I
July
I
FIG. 5. Dissolved Mn concentrations in lake outlet waters during the period April 22,1985, to August 30, 1985. (a) Vettasjiirvi (b) Ktintiijlrvi (c) Mettajarvi (d) Lapptrasket. The arrow denotes ice break-up and spring overturn.
August
I
1490
C. Ponter, J. Ingri, and K. Bostriim
hood in May. Hence, some other source(s) for Mn-rich water must exist in the drainage basin. Large concentrations of dissolved Mn accumulated in the hypolimnion of the lakes studied during the period of ice cover. Lake Vettasjlrvi was anoxic in the bottom water during late winter and early spring 1985. On May 18, 12 100 pg L -’ of dissolved Mn was measured 60 cm above the lake bottom (Fig. 4a). In Lake Lapptmsket anoxic conditions in the hypolimnion were not attained during the winter stratification of 1985. On May 14, the oxygen saturation was close to 40%, 50 cm above bottom, and the dissolved Mn concentration was 213 pg L-' (Fig. 4b). The thawing period in 1985 started in early May and maximum melt-water discharge was observed around the 20th of May in the lake outlets. Slightly elevated dissolved Mn concentrations were observed in the outlets of the lakes during this period (Fig. 5). Ice break-up occurred approximately two weeks after most snow had melted. Within a week of ice break-up and spring overturn dissolved Mn concentrations in the outlet waters increased by more than a factor of ten (Fig. 5 ). The peak of dissolved Mn in early June in the river (Fig. 3a) was correlated in time with peaks of dissolved Mn in the outlets of the lakes studied (Fig. 5). In the outlet from Lake Vettasjarvi the increased dissolved Mn concentration was accompanied by an increased suspended Mn concentration (Fig. 6a). Dissolved Mn declined rapidly to values below the concentration observed in early May. Suspended Mn decreased after the initial increase, but a relatively high concentration was maintained during July and increased steadily during August. Suspended matter with approximately 3% Mn (on an ashed weight basis) was observed in the outflowing water from June 18 to the end of August. In the stream from Lake Ktintiijlrvi, maximum suspended Mn concentrations were observed three weeks after the dissolved Mn peak (Fig. 6b). From July 5 to the end of August approximately 3% (ashed weight) of the discharged suspended phase was Mn, corresponding to high Mn/Al suspended ratios (Fig. 6~). Discharge of Mn-rich particles from lakes correlated in time with the increase of non-detrital suspended Mn in the river. It has been shown that precipitation ofdissolved Mn in freshwater results in a combination of Mn-oxyhydroxides, most commonly resembling vemadite, 6-Mn02 (TIPPING et al., 1984). It therefore seems reasonable to assume that the increased Mn/Al ratios observed in this study arose from precipitated Mn-oxyhydroxides. Daily discharges of suspended and dissolved Mn from Lake Vettasjlrvi, Lake KGntijjarvi-Mettajarvi (Lake Mettajarvi drains into Lake KGntiijarvi) and the Kalix River are shown in Fig. 7. The total input of Mn from the lakes into the river has been calculated from these data, assuming that the three lakes represent an average value for lake discharge of Mn. It should be remembered that these lakes represent only 2% of the total lake area in the watershed and that the absolute values for lake discharge of Mn could change significantly if our assumption is wrong. However, the relative changes in Mn discharge from lakes between different months in relation to the river discharge is probably valid for most lakes in the area. Table 1 indicates that lakes were a major source for Mn throughout the summer. Lake discharge of Mn was of the same order as the total Mn load in the river during June.
110 120 130 140 150 160 170 180 190 200 210 220 230 240
1
May
1 June
50
-
1
July
1 August
P
b
/\
l Mnsuspsnded
40- o Mndissolved
t
i\ ; :
110 120 130 140 150 160 170 180 190 200 210 220 230 24(
I
May
I
June
I
July
I
I
August
A stream Vettasjoki n stream Kaantyjoki
110 120 130 140 150 180 170 180 190 200 210 220230
I
May
I
June
I
July
I
August
240 I
FIG. 6. Dissolvedand suspended Mn concentrations in outlet water from Lake Vettasjarvi during the period April 22, 1985, to August 30, 1985, (a) and Lake KSlntiijlrvi (b). (c) The suspended Mn/AI
ratio in outlet water from Vettasjtirvi and Wnt6jlrvi.
This supports the conclusion that the peak of dissolved Mn in June in the river was the result of spring overturn in the lakes. However, data clearly showed that large and relatively deep lakes of the type studied did not contribute significantly to the high Mn concentration in the river during flood in May. Hence, there must be some other source(s) for Mn during spring flood. Major elements, Ca, Si, Na, S, and Mg were diluted by the spring flood in the river. This indicates that melt-water or surface run-off contributed to the discharge peaks during flood in May. Mixed mires of the type found in the Kalix River drainage basin are effluent areas for groundwater where little infiltration can occur. It is therefore reasonable to assume that much of the melt-water run-off observed during May in the Kalix River originated from peatland.
Seasonal effectson the geochemistry of Mn
k -
0 100
....* ....
1491
suspsndsd disbud
“,““‘,““‘,I”“’
150 200 250 300 350 400 450 IMI JIJ~AIS~O~NIDIJIFIMIA~M~
500
550
600
RG.7.Dailyloadof dissolved and suspended Mn in lake outlet waters from (a) Lake Vettasjti and in the Kalix River (c ) .
An average Mn concentration of 0.7 pg L-’ can be estimated for snow in the drainage area (ROSS and GRANAT, 1986). The total precipitation was 189 mm during the period from October 1984 to April 1985. From these values (and
Table 1.
dissolved SlJSpCNld8d
0
(b) Lake Wntiijiiwi,
with a total drainage area of 23600 km2), the total amount of Mn in snow can be calculated
to be 3 122 kg. Twenty
percent, or 624 kg, of this was deposited on peatland. This is 0.9% of the total amount of Mn transported in the river
Measured Mn load in the Kalix River during late spring and summer 1965 compared with the calculated quantity of Mn discharged from lakes within the drainage basin during the same period.
May Mn (kg)
f
JI JIAISI
June
July
river
lakes
river
lakes
river
lakes
45800 29500
4800 1700
14600 10900
18000 8500
3200 15600
3800 7000
August river lakes
4800 23800
3100 6700
C. Ponter, J. Ingri, and K. Bostriim
1492
25
250 l
yr: 200
Mn diss kg(24h).
& 2
9 _
150
15 z
100
10
50
5z
1" g +j 5
$ g3 r' 0 110 120 130 140 150 160 170 160190
1
May
1
June
1
200 210 220 230 240'
July
1 August
1
FIG. 8. Dissolvedconcentrations and daily dissolvedload of Mn in the Skordtjlmsbacken stream draining a peatland
area. during May 1985. Hence, the Mn load in snow cannot explain the high Mn load during May in the river. Analyses of peat cores from poor mires and from ombrotrophic Scandinavian bogs have shown an enrichment of Mn in the uppermost living part and a pronounced loss in the peat below the low water level ( SONESSON, 1970; DAMMAN, 1978). Accumulation of dissolved Mn in mire waters has been known for a long time, but the mechanism causing this enrichment relative to groundwater is incompletely understood ( SHOTYK, 1988). The Skordtjlmsbacken stream drains a peatland area with small and shallow lakes (Fig. 1, E). Measurements showed high dissolved Mn concentrations throughout the sampling period, especially in late April and early May (Fig. 8). Shortly before maximum melt water discharge the dissolved Mn concentration was 238 pg L-‘. Mn was diluted by melt-water discharge in mid-May and on June 6 the dissolved concentration was 20.9 fig L-’ . This increased to approximately 35 pg L-’ between June 13 and June 25. A new local maximum was observed in early August. The average discharge of dissolved Mn was 225 kg/day during the maximum melt-water discharge and decreased rapidly down to 50 kg/day in late May. It seems reasonable to assume that the enrichment of dissolved Mn in runoff during meltwater discharge in the Skiirdtjlmsbacken stream was caused by melt-water mixed with Mn-rich mire water. The high concentration of dissolved Mn shortly before melt-water maximum in May in the Kalix River could therefore be explained by an increased supply of Mn-rich mire water. COSSAet al. ( 1990) observed a transient peak of dissolved Mn in the St. Lawrence River during snowmelt. This was very similar to the peak observed in the Kalix River, but they did not explain the origin of the peak. The increase in outflowing Mn in early August from Skiirdtjlmsblcken was probably related to heavy rains (the rainfall in this area was 116 mm during August in the area compared with 54 mm during July). Rain water might displace Mn-rich water from mires and shallow lakes in the same way that it is displaced by melting snow during flood in spring. This source could supply Mn to the in periods of heavy rainfall. However, we do not believe that this would explain the almost constant increase in concentration of dissolved Mn from September to November in the Kalix River (Fig. 3a).
The Si/Al ratio measured in lakes has been shown to reflect the growth of diatoms ( STABEL,1985 ). In the Kalix River a decrease in dissolved Si and a simultaneous increase in the suspended B/Al ratio was measured during the growth season (Fig. 9). This probably reflects the uptake of dissolved Si into the suspended biomass. The demand for Mg in metabolism is small compared with that generally available in fresh water. Because of the solubility characteristics of Mg and its minor biotic demand, concentrations of Mg are maintained at a relatively uniform level ( WETZEL, 1983) and Mg has been used to calculate groundwater influxes into a lake ( WETZEL and OTSUKI, 1974). We have used Mg-normal&d data for dissolved Si and Mn in the Kalix River to display temporal variations unrelated to changes in discharge. The constant slow increase of normal&d dissolved Si during autumn most likely reflects mineralisation of diatoms in river and lake sediments (Fig. 10). The correlation between normalised dissolved Mn and Si concentrations is striking (Fig. 10). The slow and constant increase of dissolved Mn during autumn can be explained analogously to the increase of dissolved Si. Settling Mn-rich particles were dissolved during early diagenesis in river and lake sediments, and dissolved Mn was
5.5
5.5
--c suspended ......o...... dissolved 150
200
250
300
350
I JIJIAIS~OINIDI
FIG. 9. The dissolved Si concentration and suspended Si/~l ratio duringsummer and autumn 1985 in the Kalix River.
1493
Seasonal effectson the geochemistry of Mn
100
150
200
250
300
350
400
450
500
550
600
65
FIG. 10. The Mg-nonnalised dissolved Si and Mn concentrations in the Kalix River from May 1985 to September 1986.
added continuously to the river water in September, October, November, and December from this source. There are several possible explanations to the observed phase change of Mn in the Kalix River during the summer. Laboratory studies suggest that for the pH values and temperatures found in lake water the oxidation rate of dissolved Mn should be very slow. However, in real lake waters Mn can precipitate within a few days, at pH values down to 6.5. The rapid oxidation of dissolved Mn in the lake waters depends on the presence of natural particulate matter, implying either that the reaction is biologically mediated or that it is catalysed by particles ( CHAPNICKet al., 1982; TIPPING et al., 1984). Biological oxidation of Mn generally occurs between pH values of 6.5 to 7.5 (BROMRELDand DAVID, 1976; UREN and LEEPER, 1978). Several workers have emphasized the importance of microbially mediated Mn oxidation in fresh water ( CHAPNICKet al., 1982; DIEM and STUMM, 1984; TIPPING, 1984). GRAHAM et al. (1976) concluded that the speciation of Mn in rivers is a function primarily of pH-dependent oxidation processes. LAXENet al. ( 1984), on the other hand, did not find any coupling between pH and soluble (co.0 15 pm) Mn in their investigation of fifteen rivers and streams. The rapid decrease of dissolved Mn and increase of suspended Mn in June, in the Kalix River, was correlated in time with an increase in temperature and pH value (Fig. 3b). High suspended Mn/Al ratios were observed during the summer and early autumn months, when the water temperature and pH value were high and when large quantities of biogenic particles were present in the suspended load, as indicated by high Si/ Al ratios. Data in Fig. 6 showed that Mn-rich particles were formed in lakes and exported to the river. The oxidation rate of dissolved Mn may be increased by suspended Mn-Fe oxyhydroxide surfaces ( STUMM and MORGAN, 198 1). These par-
ticles may have catalysed the oxidation of dissolved Mn in the river. The export of these particles ceased when the lakes were stratified in the autumn, thus explaining the low sus-
pended/dissolved ratio in the river in late autumn and winter. However, comparable high suspended Fe/ Al and Mn/ Al ratios have been measured also during autumn and winter in the river. Approximately half of the suspended load during winter consisted of Fe in a non-detrital form, most likely Feoxyhydroxides. Scavenging of dissolved Mn onto Mn-Feoxyhydroxides thus appears to be of minor importance for the phase change of Mn during summer in the river. The rapid increase of the suspended Mn / Al ratio started at a pH of approximately 6.7 both sampling years. However, during the winter months (December-March ) a constant pH of ap proximately 6.7 was measured. During this period, low suspended Mn / Al ratios were observed and the total Mn transport was dominated by the dissolved phase. Furthermore, the pH was higher than 6.7 during autumn (September-November) but the suspended Mn/ Al ratio was low. The Mn/Al ratio started to increase when the water temperature in the river reached approximately 15°C. This was measured both sampling years. The Mn/AI ratio remained high so long as the temperature remained above 15°C and decreased when the temperature fell below 15“C. It has been shown that the oxidation rate ofdissolved Mn in English lake waters has a well-defined optimum temperature, varying from 15°C to 3O”C, in the pH range 7-8. This is a strong indication of biological mediation of the oxidation (TIPPING, 1984). It is plausible that increased activity of Mn-oxidising bacteria catalysed the precipitation of dissolved Mn in the Kalix River. This could explain the rapid decrease of dissolved Mn in late June, the high suspended Mn/Al ratios during summer, and the relatively low suspended Mn/ Al ratios during winter. Acknowledgments-The study was financed by grants from NFR. This support is greatfully acknowledged. We also wish to thank W. Davison, E. Tipping, and an anonymous reviewer for their constructive comments on the manuscript and P. Digby for his improvements of the English. Editorial handling: J. I. Drever
C. Ponter, J. Ingri, and K. Bostriim
1494 REFERENCES
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