Marine Chemistry, 21 (1987) 213-227 213 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

DISSOLVED URANIUM IN THE BALTIC SEA

R LOFVENDAHL

University of Stockholm, Department of Geology, S-106 91 Stockholm (Sweden)

(Received October 9, 1986; revision accepted March 25, 1987)

ABSTRACT

LSfvendahl, R., 1987. Dissolved uranium in the Baltic Sea. Mar. Chem., 21: 213-227.

Filtered water from the Baltic Sea was analysed for uranium concentration and 234U/2~U activity ratio with alpha-ray spectrometry. The uranium concentration shows a strong correlation to salinity, the correlation coefficient being close to 0.98. Consequently, the uranium concentration increases from 0.15ggkg -1 in the northern part, dominated by fresh water, to above 1.0t~gkg 1 in the Belt Sea. However, the data also show that dissolved uranium is not strictly conservative in the Baltic. In deeper intermittently anoxic basins of the Baltic Proper, the element is removed from the water phase and incorporated into the sediment. This is most evident in the Gotland Deep, which has been anoxic below 200m depth since 1979.

INTRODUCTION

The Balt ic Sea cons t i tu tes a large es tuar ine system, with a surface area of 393 000 km 2, and a volume of 21 200 km 3. I t is divided into six sub-basins, namely (from north); the Bo thn ian Bay, the Bo thn ian Sea, the Gulf of Finland, the Balt ic Proper, the Gulf of Riga and the Belt Sea (Fig. 1). I t is connec ted to K a t t e g a t t - S k a g e r a k k and the Nor th Sea t h r o u g h the Danish Sounds (Great and Lit t le Belt) and Oresund (Fig. 1). Dense saline water enters t h rough Great Belt, Oresund and Lit t le Belt in the approximate propor t ions 6:3:1 (Jacobsen, 1980) and moves into the Balt ic over different sills, as shown in Fig. 1. The year ly addi t ion of saline water is h ighly variable, but measurements and theore t ica l cons idera t ions (Pedersen, 1977) demons t ra te tha t the addi t ion is cont inuous . I r regular ly , dur ing meteoro logica l ly suitable condit ions, large inflows of saline wate r replace the 'old ' bo t tom water in the deep basins of the cent ra l Bal t ic Proper. The inflow of dense bo t tom water is guided by the bot tom topography. Its c i rcu la t ion coincides with the main t r anspor t of surface water, with both moving ant ic lockwise.

A s t rong halocl ine is formed in the Balt ic Proper (Fig. 1) and border ing areas. It is s i tua ted at 20m depth in the sou thwes te rn part , and sinks con- t inuous ly to 80 m in the nor th . This halocl ine is an effective bar r ie r aga ins t ver t ica l wa te r exchange. A thermocl ine also forms, which as a maximum reaches a depth of 30 m in late summer. A year ly t u rnove r of the whole water co lumn takes place in the Bo thn ian Bay, in large par ts of the Bo thn ian Sea and

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214

t~- ..... Border, suboreras

~ Contour l in~ 100m depth

~:~ 20Omdepth

Direction, bottom current

+ Sampling station -64 o

~'o.

N

~°

6 / , ° _

- 6 f f

Sothnion ~ ~, sea ! )~

i ,

\ ...¢:

"~ Gulf of Fintor~d

V.¢. ¢,~ / ' " Baltic ~ , / / Proper . j ,'7

~ S f

56 °

12 ~ ~ 20 ° 28 °

Fig. 1. The Baltic Sea. Subdivision, depth curves (contour lines at 100 and 200 m depth), bottom current paths and sampling stations.

in shallow coastal areas. The Baltic Sea has a positive water balance, i.e. more water is lost to than added from the North Sea; the yearly difference is ~ 470 km 3 (Jacobsen, 1980), which approximately equals the yearly river influx (Mikulski, 1970). The total outflux through the Belt Sea is approximately twice this figure, approaching the discharge of the Orinoco River.

Studies of uranium in the water of the Baltic Sea have been scarce until quite recently. Koczy et al. (1957) analysed about 15 unfiltered samples collec- ted from all over the Baltic. Their surface samples contained about 1 pg kg-1 (0.7-2.2), and the deep-water ones ~ 2 pg kg 1 (1.3-5.9) of uranium. Later dis- cussions on uranium in waters of the Baltic Sea (cf. Manheim, 1961; Baturin, 1968; Baturin and Kochenov, 1969) have been based on these data. Bojanowski and Szefer (1979) sampled the southern Baltic Proper and Skagerakk, and showed that the uranium concentration was strongly correlated with salinity. Lately, Gellermann et al. (1983) analysed about 10 samples from southern and southwestern Baltic, Duniec et al. (1984) reported data on 28 samples collected from all over the Baltic, while Prange and Kremling (1985) examined a number of depth profiles. These three works show the strong correlation between

215

dissolved uranium and salinity. Baturin and Kochenov (1969) estimated that the total content of dissolved uranium in the Baltic Sea was 30 x 103tons, while Gellermann et al. (1983) considered 25 x 103tons as a more accurate figure. Prange and Kremling (1985) examined the possibility of uranium loss from anoxic bottom waters, but found no conclusive evidence for this.

The Black Sea, which shows many similarities to the Baltic Sea, is better known as far as the distribution and concentration of uranium in the deep waters are concerned. According to Zhorov et al. (1982), there is a distinct vertical variation in uranium concentration. While surface water contains 1.8-2.0 pg kg ' of uranium, the intermediate, anoxic, H2S-rich section contains 1.3pgkg 1. The bottom water has concentrations similar to the surface (all these data seem to refer to unfiltered samples). Zhorov et al. (1982) also reported seasonal variation in uranium concentration. The bottom sediments in the central part of the Black Sea are enriched in uranium, with values of 10-12 ppm (Nikolayev et al., 1977).

SAMPLING AND ANALYSIS

The sampling was done on board R/V "Strombus", University of Stockholm and R/V "Argos", Swedish Meteorological and Hydrological Institute (SMHI). Water was sampled with 601 GO FLO-bottles, internally lined with teflon. At all stations except two, temperature, salinity, pH and total alkalinity were measured on board; temperature and salinity using a YSI model 33 S-C-T-meter, pH with a Radiometer 29b pH meter and total alkalinity following an acid t i trat ion procedure given by Denny (1977). The pH values were uncorrected (not in-situ corrected as corresponding SMHI-data). For stations BY 28 and BY 15 (Tables I and II), data from SMHI were used, which were measured according to the New Baltic Manual (Carlberg, 1972). This means that salinity was calculated from conductivity, pH determined with a pH meter furnished with a glass electrode and recalculated to an in situ value, 02 by a modified Winkler t i tration and H2S photometrically with the methylene blue method. Finally temperature was measured in situ using a CTD-probe.

Within a few hours, the samples were forced through 0.45pm membrane filters using peristaltic pumps. In most cases about 101 of water were collected for uranium analysis. After filtering the samples were acidified with con- centrated nitric acid to a pH of about 1.5. Besides, three unfiltered samples were collected from Gotland Deep anoxic water. Additional samples (water, suspen- ded material) were collected for analysis of dissolved and suspended major elements and some transit ion metals with inductively coupled plasma-optical emission spectroscopy (ICP-OES). The results from these analyses are not discussed here.

After addition of 232U tracer and 10mg Fe 3÷, the samples were boiled and Fe-hydroxides precipitated by adding ammonia. Overlying water phase was decanted, the final hydroxide slurry centrifuged and anew decanted. The re- maining hydroxide with adsorbed uranium was dissolved in 8MHC1, and

216

TABLE I

A n a l y t i c a l da ta , w a t e r samples , n o r t h of 58°N

S t a t i o n ~ t e p t h Da te Sample pH S a l i n i t y Tot. a lk . U r a n i u m ~4U/2~U (m) coo rd ina t e s dep th (m) (%o) (mmol l - l ) (pg kg 1) (AR)

BB 1 2 04 08 85 0 7.50 1.0 0.38 0.13 i 0.01 1.52 + 0.08 65045'/23048 '

BB 2-106 15 06 83 3 8.17 3.6* 0.36 ± 0.02 1.30 ± 0.06

64°42'/22°09 ' 50 7.76 3.7* 0.33 + 0.02 1.22 ± 0.07

80 7.59 3.9* 0.41 _+ 0.02 1.16 ± 0.05

100 7.45 4.1" 0.41 ± 0.02 1.25 ± 0.05 BB 3-45 19 06 83 5 3.8* 0.41 ± 0.02 1.20 ± 0.05 63020'/19047 ' 35 5.5* 0.55 _+ 0.03 1.20 ± 0.05

BS 1-212 19 06 83 5 4.6* 0.47 _+ 0.02 1.19 ± 0.05 62°55'/18°53 ' 100 6.3* 0.68 ± 0.03 1.16 ± 0.05

150 6.5* 0.68 ± 0.03 1.09 + 0.05

190 6.5* 0.60 + 0.03 1.19 ± 0.05 BS 2-275 02 09 85 5 8.25 5.1 1.18 0.50 +_ 0.02 1.16 + 0.04

60010'/19010 ' 50 7.95 6.8 1.47 0.65 + 0.03 1.14 ± 0.04

100 7.85 6.9 1.47 0.65 ± 0.03 1.13 ± 0.04 150 7.80 7.0 1.48 0.66 + 0.03 1.14 ± 0.04

200 7.75 7.0 1.48 0.67 _+ 0.03 1.25 ± 0.04 260 7.70 7.0 1.53 0.67 _+ 0.03 1.21 ± 0.04

BS ~ 5 6 02 09 85 5 8.20 5.1 1.20 0.51 + 0.02 1.15 ± 0.04

59°40'/19°53 ' 25 8.00 6.1 1.35 0.61 + 0.02 1.16 _+ 0.04

45 7.75 7.0 1.47 0.71 _+ 0.03 1.15 +_ 0.04 BY 2 ~ 2 0 2 10 06 85 5 8.60 7.3 1.51 0.64 + 0.02 1.17 ± 0.03 59002'/21005 ' 50 8.10 7.9 1.56 0.67 _+ 0.02 1.16 _+ 0.03

100 7.37 10.0 1.61 0.74 i 0.03 1.15 _+ 0.03

150 7.32 10.6 1.68 0.77 + 0.03 1.15 ± 0.03

175 7.31 10.6 1.69 0.77 + 0.03 1.17 +_ 0.03

195 0.81 _+ 0.03 1.10 + 0.03 BY 31-459 14 08 84 5 8.49 6.2* 1.41 0.61 _+ 0.03 1.14 + 0.04

58°35~/18°14 ' 50 8.00 7.1" 1.37 0.67 + 0.03 1.16 + 0.04

75 7.42 8.0* 1.35 0.67 + 0.03 1.17 ± 0.04

100 7.21 9.2* 1.40 0.70 + 0.03 1.18 +_ 0.05 125 7.08 9.8* 1.38 0.81 ± 0.04 1.14 + 0.04 150 7.07 9.9* 1.40 0.83 + 0.04 1.14 ± 0.04 200 7.06 9.9* 1.47 0.81 ± 0.03 1.17 + 0.03

300 7.05 10.2" 1.53 0.79 + 0.04 1.17 ± 0.04 400 7.05 10.2" 1.69 0.79 + 0.04 1.17 _+ 0.05

430 7.04 10.4" 1.72 0.80 _+ 0.04 1.19 ± 0.04 BY 3 ~ 2 0 6 14 08 84 5 8.59 6.5 1.35 0.66 + 0.03 1.16 + 0.05 58002'/17059 ' 50 7.90 7.2 1.41 0.70 _+ 0.04 1.09 + 0.05

100 7.14 8.9 1.52 0.67 _+ 0.03 1.20 ± 0.04

150 7.02 9.2 1.54 0.76 + 0.04 1.20 _+ 0.05

190 7.03 9.2 1.54 0.75 _+ 0.03 1.22 + 0.04

Date g iven w i t h f igures in order day m o n t h - y e a r (two f inal figures).

* C a l c u l a t e d va lues , based on the conc. of Na (ICP OES analys is ) , a s s u m i n g c o n s e r v a t i v e mixing .

217

T A B L E II

A n a l y t i c a l data , wa t e r s amp l e s f rom the Go t l and Deep (249 m depth, coo rd ina t e s 57°20'N/20°03'E, s amp l ing da te 11.06.85)

Sample pH Sa l in i ty Tot .a lk . 02 H2S U r a n i u m 234U/23s dep th (m) (%o) (mmol l 1 ) (pmol 1 1) (pmol 1-1) (pg kg 1 ) (AR)

5 8.86 7.67 1.56 288 0 0.68 + 0.03 1.14 + 0.04 50 8.03 8.01 1.53 270 0 0.66 _+ 0.03 1.15 + 0.03

100 7.47 10.67 1.62 80 0 0.80 + 0.03 1.18 + 0.03 125 7.39 11.17 1.62 51 0 0.86 + 0.03 1.18 + 0.03 150 7.35 11.81 1.77 0 11.6 0.80 +_ 0.03 1.17 + 0.04

*0.88 + 0.03 "1.15 + 0.03 200 7.35 12.27 1.71 0 44.0 0.74 + 0.03 1.14 + 0.03

*0.74 _+ 0.03 "1.17 + 0.03 240 7.35 12.40 1.75 0 68.7 0.72 + 0.03 1.17 + 0.03

*0.82 + 0.03 "1.19 + 0.03

* Unf i l t e red samples .

passed through an anion exchange column. Uranium and iron remained in the column and were subsequently collected by elution with 0.1MHC1. After evaporation to dryness, the sample was dissoved in concentrated HNO3. A saturated solution of M g(NO3)2 was added for liquid extraction against methyl- isobutylketone (MIBK). Uranium was concentrated in the organic phase, while iron remained in the water phase. Uranium was then back-extracted into distilled water and taken to dryness. The remaining material was dissolved in 7 M HNO3 and run through an anion exchange column. Uranium stuck to the resin and was finally eluted with 0.1 M HC1. The uranium was electrodeposited onto stainless steel planchetts, mainly following a procedure developed by Talvitie (1972). The uranium yield was 40-95%, with yield anticorrelated to salinity. The planchetts were counted on 300mm 2 Si-detectors until at least 2000 impulses had accumulated for each of the isotopes 23su, 234U and 232 U. The resolution of the system (FWHM) was ~ 40keV. Uranium concentration and 234U/23su AR (activity ratio) are given with an error of one sigma; this includes the propagating errors in counting statistics and in the activity of the tracer. Blank runs showed negligible amounts of the three uranium isotopes, so no background correction was called for, except for a slight cumulative con- tamination of 22STh, of which the minor peak partly coincides with 232U.

R E S U L T S A N D DIS C US S ION

In the ensuing discussion only alpha-spectrometry data will be used, as they seem internally consistent. Alongside my own data, those of Gellermann et al. (1983) and Duniec et al. (1984) are available. The latter refer to unfiltered samples, but are comparable to the filtered ones, because in Baltic oxic water more than 99% of the total uranium content is in the dissolved state (Geller- mann et al., 1983). However, this relation is applicable neither to anoxic water

218

TABLE III

A n a l y t i c a l da ta , w a t e r samples , s o u t h of 56°35'N

S t a t i on~dep th Date Sample pH S a l i n i t y Tot. alk. U r a n i u m 2~4U/Z~s

(m) coo rd ina t e s dep th (m) ( % 0 ) ( m m o l l 1) (pg k g - 1) (AR)

BP 1 27 20 08 84 5 8.52 6.8 1.38 0.67 + 0.03 1.19 ± 0.05

56°17'/16°18 ' 24 8.34 6.9 1.47 0.69 + 0.03 1.17 + 0.05 BP 2-48 16 08 84 5 8.54 6.5 1.52 0.74 ± 0.04 1.11 _+ 0.05

56°07'/16°32 ' 25 8.16 7.0 1.46 0.69 + 0.03 1.14 + 0.04 45 7.69 7.8 1.54 0.77 + 0.04 t.09 ± 0.05

BP 3-77 21 08 84 5 8.62 7.2 1.32 0.76 ± 0.04 1.16 + 0.05

55°31'/15°35 ' 40 7.94 7.9 1.60 0.79 + 0.04 1.19 ± 0.05

70 7.48 12.5 1.86 1.24 + 0.06 1.18 ± 0.04

BP 4-47 22 08 84 5 8.43 7.0 1.45 0.75 + 0.04 1.17 ± 0.04 55°28'/14°26 ' 44 7.79 8.2 1.54 0.83 ± 0.04 1.12 ± 0.04

BP ~ 2 5 24 09 85 3 8.10 7.4 1.42 0.73 ± 0.03 1.16 ± 0.04

55°17'/12°35 ' 20 8.05 8.4 1.45 0.78 + 0.03 1.16 + 0.04 BP 6-45 24 09 85 3 8.00 7.1 1.50 0.70 ± 0.03 1.20 + 0.04

54°53'/13°10 ' 20 7.95 7.5 1.48 0.72 ± 0.03 1.16 + 0.04

40 7.70 12.0 1.65 1.11 + 0.04 1.12 ± 0.04 BP 7-18 24 09 85 3 8.20 9.0 1.48 0.91 ± 0.03 1.15 + 0.03

54040'/12035 ' 14 8.00 12.0 1.60 1.09 ± 0.04 1.14 + 0.04

BP 8-20 24 09 85 3 8.15 9.2 1.51 0.88 + 0.03 1.15 + 0.03 54°29'/12°18" 16 8.00 12.8 1.67 1.08 ± 0.04 1.17 + 0.03

BP ~ 2 4 24 09 85 3 8.30 13.0 1.71 1.19 ± 0.04 1.14 + 0.03 54035'/11020 , 20 8.20 14.8 1.68 1.30 ± 0.05 1.16 _+ 0.03

BS 1 25 22 08 84 5 8.43 7.1 1.58 0.76 ± 0.04 1.16 + 0.04 55014'/12048 ' 22 7.62 7.5 1.57 0.78 ± 0.04 1.13 + 0.04

BS 2-50 23 08 84 5 8.14 8.2 1.58 0.85 ± 0.04 1.15 + 0.05 55052'/12045 ' 25 7.67 25.8 2.10 2.55 + 0.12 1.13 ± 0.03

47 7.54 28.5 2.25 2.60 + 0.12 1.16 + 0.03

BS 3-26 24 09 85 3 8.30 18.5 1.83 1.75 ± 0.06 1.14 ± 0.03 55°25'/10°55 ' 22 8.15 19.5 1.89 1.80 + 0.06 1.15 ± 0.03

BS 4 24 23 09 85 3 8.25 19.8 1.88 1.80 + 0.07 1.17 + 0.03 55°42'/10°45 ' 20 8.15 20.3 1.89 1.83 + 0.06 1.13 ± 0.02

BS ~ 3 5 25 09 85 3 8.20 12.1 1.60 1.18 + 0.05 1.10 + 0.04

56°04'/12°39 ' 30 8.02 25.6 2.03 2.46 + 0.08 1.14 + 0.02

KG 1 27 23 08 84 5 8.28 11.1 1.65 1.17 ± 0.05 1.13 ± 0.04 56017'/12019 ' 15 8.18 15.0 1.80 1.54 + 0.07 1.10 ± 0.03

25 7.75 24.4 2.16 2.53 + 0.09 1.16 ± 0.02

KG 2 21 23 09 85 3 8.28 18.5 1.83 1.64 _+ 0.06 1.13 ± 0.03 56014'/11000 , 17 8.12 21.0 1.92 1.78 + 0.07 1.14 ± 0.03

KG 3-20 23 09 85 3 8.30 19.2 1.86 1.61 ± 0.05 1.17 ± 0.03 56018'/11020 ' 16 8.12 22.1 1.93 1.93 + 0.07 1.11 + 0.03

KG 4 28 23 09 85 3 8.30 19.5 1.83 1.64 + 0.06 1.14 ± 0.03

56°22"/11°48 ' 23 8.10 26.4 2.06 2.27 ± 0.08 1.14 ± 0.03 KG 5~35 23 09 85 3 8.28 20.1 1.86 1.82 + 0.07 1.13 + 0.03 56°29"/12°10 ' 30 8.05 26.0 2.09 2.33 ± 0.07 1.18 ± 0.02 KG 6-25 23 09 85 6 8.28 19.2 1.82 1.70 _+ 0.06 1.15 _+ 0.03 56031'/12031 , 20 8.10 25.7 2.04 2.24 _+ 0.09 1.15 ± 0.03

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(cf. Table II), and probably not to water close to river mouths with plenty of suspended material.

The uranium concentration continuously increases in surface waters (depths 0-5m), from ~ 0.15#gkg ' in the archipelago of the Bothnian Bay to 1.0 pg kg ' in the northern Belt Sea (Tables I~II). The rivers draining into the Gulf of Bothnia (the Bothnian Bay and the Bothnian Sea) contribute to more than 40% of the total river input to the Baltic (Mikulski, 1970). In the Gulf of Finland only two stations have been sampled (data from Duniec et al., 1984). The Neva, debouching into the eastern part of this gulf, is the single largest river draining into the Baltic, accounting for more than 18% of the total river supply (Mikulski, 1970). Various concentration values of uranium have been reported for this river. Visual inspection of Fig. 1 in Baturin and Kochenov (1969) gives a uranium concentration of 0.3 ttg kg ~, while Vaganov et al. (1976) by use of NAA find a value of 0.16 pg kg '. Other important rivers emptying into this gulf, such as Narva and Luga, seem to have similar or a little higher uranium concentrations than Neva (Fig. 1 in Baturin and Kochenov, 1969). No 234U/238U AR data seem to be available for these rivers. For the Gulf of Riga there is no information on uranium; nor is there any on the important river Dvina, discharging into this gulf.

The 234U/23su AR (Fig. 2) is high, 1.21-1.60, in the northernmost part of the Bothnian Bay. The rest of the Baltic gives a rather homogeneous picture, with values of 1.15 _+ 0.10, demonstrating the dominant influence of seawater, with a mean ratio of 1.14 (Ku et al., 1977). There appears to be a slight variation in the 234U/23su AR in some of the depth profiles. In stations north of 62°N there seem to be higher values at the surface (Table I). It is apparent that the larger rivers emptying into the Bothnian Bay must have quite high 2~U/238U ARs (cf. Fig. 2), although the exact magnitudes are unknown. High surface values seem

15

_o o ×

>,

13

S a { i n i t y , per m i l

,'~ Data, Ge l l e rmann et a l (1983) N = 11

Data. D u m e c et a l ( lg8/~) N : 28

+ Data, t h i s w o r k N - 92

o Two c o i n c i d i n g s a m p l e s

. ÷

++ . + ~ 2 ,

~p 2~

1

Fig. 2.2~U/2~U activity ratio versus salinity (%0).

220

logical , as sur face w a t e r has lower sa l in i ty and consequen t ly a h ighe r propor- t ion of r ive r water .

The c o n c e n t r a t i o n of some elements , N a and Mg, for example , or some p roper t i e s r e l a t ed to e l ementa l c o n c e n t r a t i o n s such as sa l in i ty or conduc t iv i ty , can b e h a v e c o n s e r v a t i v e l y w h e n mix ing two di f ferent w a t e r sources. In th is r espec t mix ing be tween r ive r and s e a w a t e r en te r ing the Bal t ic can t e n t a t i v e l y be r ega rded as a b i na ry mix ture . Hence (cf. Briel, 1976)

cM v~ = CR VR + C~ Vs (1)

where C is c o n c e n t r a t i o n of a g iven e l emen t and V is the volume; the subsc r ip t M rep resen t s mix ture , R is r ive r w a t e r and S is seawate r . Gras sho f f and Voipio (1981) have discussed the b e h a v i o u r of d i f ferent d issolved e lements in the Bal t ic Sea in c o m p a r i s o n wi th the i r b e h a v i o u r in seawate r . They showed t h a t the r a t ios Na/C1 and Mg/C1 in Bal t ic wa te r s a re a p p r o x i m a t e l y the same as in seawate r . Hence , the c o n c e n t r a t i o n s of Na, Mg and also sa l in i ty will he re be r ega rded as c o n s e r v a t i v e in the wa t e r s of the Bal t ic Sea. Consider , for example , sal ini ty; in th is case C R in eq. 1 a p p r o a c h e s zero, i.e. the sa l in i ty of r ive r w a t e r is p r ac t i ca l l y zero. By neg lec t ing the t e rm CR V~ and r e a r r a n g i n g , eq. 1 gives

V~/Ys = CslC~ (2)

Fur the r , our ma in in te res t he re is the change in u r a n i u m c o n c e n t r a t i o n when mix ing s e a w a t e r and r ive r water , to tes t the a s s u m p t i o n t h a t u r a n i u m also behaves conse rva t ive ly . Cons ider the a lmos t f resh w a t e r of some of the s t a t ions examined by Duniec et al. (1984) ( the sa l ien t da t a r ep roduced he re in Table IV). Wi th a sa l in i ty of s e a w a t e r of 35% and a sa l in i ty of s t a t i on 92 of 1.5%, eq. 2 gives VM/Vs = 23.3, and accord ing ly in eq. 1 CR = 0.21. An analo- gous ca l cu l a t i on for s t a t i on 91 (Table IV) yields a CR va lue of 0.25. However , a ca l cu l a t i on based on sample BB 1 (Table IV) gives CR = 0.04. Thus, the sparse da t a ind ica te u r a n i u m c o n c e n t r a t i o n s in the d i scha rg ing r ivers of 0.20- 0.25 pg kg 1, (or, a l t e r n a t i v e l y below 0.05). This co r responds to the lower va lues in the r a n g e 0.20-O.60pg kg 1 g iven by Koczy et al. (1957) for Torne and Kemi r ivers , two of the m a i n d i schargers . Similar ly , by the use of s t a t ion 63 f rom Duniec et al. (1984) (Table IV) in the Gu l f of Finland, but us ing a m e a n r ive r c o n c e n t r a t i o n of 0 . 3 p g k g 1, i.e. N e v a r ive r w a t e r as g iven by B a t u r i n and

TABLE IV

Salinity and dissolved uranium. From Duniec et al. (1984) and own data

Station Coordinates Salinity Uranium 234U/23sU (%0) (pg kg 1) (AR)

63 60o00'/27o00 ' 4.4 0.51 1.12 91 65030'/24o10 ' 2.0 0.42 1.42 92 65o40'/24o20 ' 1.5 0.34 1.60 BB 1 65%5']23%81 1.0 0.13 1.52

221

Kochenov (1969), a CM-value of 0.68 pg kg 1 is obtained. However, the measured value is 0,51~g kg 1. By inserting the latter value in eq. 1 a mean river con- centration of 0.11 is the result; this is distinctly less than the two published values of 0.30 and 0.16 pg kg- 1 for Neva water (cf. Baturin and Kochenov, 1969; Vaganov et al., 1976). There is no point in pursuing this discussion further, until pertinent river-water data have been gathered. However, the calculated uranium concentrations seem rather low in comparison with published river data.

The hypothesis of conservative mixing needs further testing. Figure 3 ex- hibits a vertical sampling profile of the Gotland Deep. The bottom water in this basin has been continuously anoxic since 1979 according to data from SMHI (Hydrographical Data Reports and unpublished). Figure 3 shows tha the ura- nium concentration increases down to 125 m, but then decreases with depth. The salinity, on the other hand, increases down to the bottom. The correlation coefficient for uranium concentration versus salinity for the seven Gotland Deep samples is low, 0.59, with the regression equation

U = (0.524,_+ 0.319) + (0.0215 _+ 0.0133) x S (3)

where U represents the uranium concentration in pg kg 1 and S the salinity in %0, intercept values and S values are given with one sigma errors. The data presented strongly indicate that uranium has been removed from the lower anoxic part of the water column, suggesting a nonconservative behaviour. Similarly, irregularities in uranium concentration in the depth profiles of the Landsort Deep (BY 31) and the Norrk6ping Deep (BY 32) (cf. Table I), might give a hint of memory effects of the anoxic episode in 1982-1983, registered by SMHI. Accordingly, the simplest explanation for these deviations in uranium concentration with depth seems to be that oxygen deficiency in the deeper basins of the Baltic (cf. Fonselius, 1969, 1974) promotes removal of uranium from the water phase.

Both Gellermann et al. (1983), Duniec et al. (1984) and Prange and Kremling (1985) have discussed the correlation between salinity and uranium concentra- tion; the former authors calculated the regression equation

U = (0.20 _+ 0.04) + (0.080 _+ 0.003) x S (4)

with symbols as eq. 3. The correlation coefficient for this equation is 0.996 (N = 9). In Fig. 4 all accessible uranium concentrations (N = 131) are plotted versus salinity. The correlation coefficient is 0.979, with the regression equa- tion (variables as earlier)

U = (0.049 _+ 0.018) + (0.087 _+ 0.002) x S. (5)

The intercept with the ordinate is quite low, challenging the hypothesis of conservative binary mixing. With conservative mixing, the constant term represents the average concentration of river water emptying into the (nor- thern) Baltic, while the salinity term represents uranium supplied by seawater.

222

6 7 8 9 10 11 + : p H

~ 0 = 0 2 ~1 mol/I ,/~" ' A ~ H ~ /1

t ./ i [:.01:~::~%'

,150

-200

g~ O

/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / Bottorn

Fig. 3. Ver{ical sampling profile for the Gotland Deep. One sigma error bar for uranium concentra- tion. All data except uranium concentration are from SMHI.

For comparison, a conse rva t ive mixing l ine is inser ted in Fig. 4, assuming a u ran ium c o n c e n t r a t i o n of 3.3 #g kg- 1 in seawate r wi th a sa l in i ty of 35%o (Ku et al., 1977) as one source, and r iver wa te r wi th zero sa l in i ty and a u r an iu m conc e n t r a t i on of 0.20 pg k g - ' as the other . The weighed average co n cen t r a t i o n of dissolved u r a n ium in world r ivers is es t imated to be 0 .2~ .3 #g kg- ~ ; Borole et al. (1982) prefer a va lue of 0.22 pg kg '. Consequent ly , conse rva t ive mixing would give the equa t ion

U = 0.20 + 0.089 x S (6)

All but th ree samples fall below this l ine (Fig. 4), a l though the samples from the Ka t t ega t t , the Bel t Sea and the sou the rn Bal t ic are s i tua ted qui te close to it.

c . , i \ \ \ ' , ,

\

ii [i ii

z z z

E

~, ,:7, ~= ~ u

n o D ~-

<1 × * 0

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E ~>

>~

Z

Z

2~

ii

z~2 o

o _

uJ LIJ

c~ od

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223

221¸i

A ca lcu la t ion based on 45 samples col lected south of the per iodical ly anoxic basins (except the Bornho lm Deep), i.e. south of 56°35'N. gives the equat ion

(7 = (0.099 ;- 0.031~ ~ (0.087 ~ (i.002) 4 ,¢/ .~i

with the cor re la t ion coefficient 0.989. Al though this line represents statto~-~ with in te rmed ia te to high salinity, it follows eq. 5 qui te closely. A ca~( ulat lo based on samples col lected in and nor th of the cent ra l basin (N ...... t7) gives ih¢ equa t ion

I: =: (0.247 ~-0.030)~ (0052 ! 0.004):~, ~' : ;

with a co r re la t ion coefficient of 0.906. This line devia tes markedly !:tom the:, former ones. The samples represent ing it give a sca t t e r of points in a restricte,~ sa l in i ty in terva l (Fig. 4). In re la t ion to the former lines this one has t~ pivotai point at ~ 6.09&)

The most na tu ra l mechanisms tbr wi thdrawal of dissolved u ran ium from th( wate r phase are reduc t ion of u ran ium (VI) to u ran ium (IV), followed by pr,~. c ip i ta t ion as UO~, or adsorpt ion on reac t ive phases, for example organic nmterial . These possibil i t ies were discussed by P range and Kreml ing ~ 9 ~ Al though they did not detect any dis t inct deplet ion of dissolved uranium ~ the i r Got land Deep profile, they considered possible mechanisms for remow~l ,)i u ran ium and molybdenum from the wate r phase. They favoured tbe idea c,' r educ t ion followed by adsorp t ion onto organic mater ia l . It is evident ~:ha~ u ran ium is effect ively adsorbed onto Fe(III) and Mn(IV) oxyhydro×ides; th~ nodules formed in the no r the rn Balt ic have uran ium concen t ra t ions above 20 ppm (LSfvendahl and Ingri, unpublished). However , these phases cann()t exist in an anoxic envi ronment . The first ques t ion is whe the r u ran ium will be reduced in the Got land Deep. The redox potent ia l of this profile was noi measured, but can be deduced indirect ly (cf. Stumm and Morgan, 1981i. Th(: d i sappearance of O~ below 125 m depth (Table II; Fig. 3) shows tha t the E, has dropped to ~ 0.80 V, while the appea rance of H2S at depths g rea te r than 150 ~ indicates a nega t ive Eh. The drop in dissolved u ran ium seems to s ta r t in t h ( in terva l 125 150 m (Table I I ) According to Langmui r (1978), this would mea~ reduc t ion of hexava l en t u r a n y l c a r b o n a t e to a c rys ta l l ine phase, e i the r U~ (), o~" UO.2. At low t empera tu res sorpt ion processes usual ly grow in impor tan t ( ~ (Langmuir , 1978). Thus it seems na tu ra l to suggest tha t in the deep anox~( waters, the steps in wi thdrawal of dissolved u ran ium are reduc t ion folh)wed by adsorp t ion onto (organic) par t icula tes . That. u r an ium is associa ted with p a t t i cles below the redoxcl ine is suppor ted by the increase in u ran ium concentra- t ion in unf i l tered bot tom wate r samples as compared to fi l tered ones (Table ll)°

To i l luminate the re la t ion between the s tanding crop of dissolved u ran ium and u ran ium incorpora ted into the sediments, a quan t i t a t i ve es t imate seems appropr ia te . It is suggested tha t dissolved u ran ium is mainly removed from the wate r phase in the Got land Basin. Per iodical ly , the deep wa te r below 150 m is anoxic, con ta in ing H~S. This volume of wa te r will be - - l l 0 k m :~ (Ehlin and Matt isson, 1976). The total amoun t of dissolved u ran ium in this volume, if i!

225

was oxygenated would be at least 111 tons (calculated with eq. 5). This value is a minimum figure, as the equation in question includes three values from anoxic water. If, on the other hand, this volume is anoxic, and 0.75 #g kg 1 (cf. Table II) is regarded as the mean concentra t ion of dissolved uranium, the water contains 83 tons instead. The difference, 28 tons, is a conservative figure. The uranium removed from the water phase must be incorporated into the bottom sediments of the area. The bottom surface of the Gotland Basin below 150 m depth is ~ 6600 km 2 (Fonselius, 1969; calculated as a flat surface and hence a minimum figure). Given a mean concentra t ion of uranium in the bottom sedi- ments of 10mg kg 1 (estimated from Baturin, 1968), of which the authigenic phase makes up 7mgkg 1 ; a density of fresh sediments of 1.1gcm 3, and a sedimentation rate of 1 mm a- 1 (cf. BostrSm et al., 1983), 50 tons a 1 of nonde- tr i tal uranium will be incorporated into the sediments. The main uncer ta in ty in this calculat ion is probably the concentra t ion of authigenic uranium, which might be too high. However, the other parameters are also crude estimates.

This hypothet ical calculat ion shows that the total amount of dissolved uranium which will be removed from the water phase, and the amount incor- porated into the sediments per annum are of similar size. It must be remem- bered that the volume of anoxic bottom water fluctuates strongly between zero and a volume twice the hypothet ical ly discussed one (Fonselius, 1969; 1974). At long intervals the Landsort Deep is also anoxic, for example during the period 1982 1983 (SMHI, Hydrological Data Reports). The renewal of oxygenated saline bottom water in the central Baltic does not take place continuously; larger saline inflows are distinctly stochastic. Judging from salinity changes in the Gotland Deep (Fonselius, 1969, 1974), addition of larger volumes of saline water is irregular, with intervals of 1-10 years.

The uranium balance for the Baltic was discussed by Koczy et al. (1957) and Gellermann et al. (1983). Currently, lack of quant i ta t ive data, especially for the exchange through the Belt Sea, river influx and sedimentation will not advance this discussion any further. However, it is obvious that the calculated standing crop of dissolved uranium in the Baltic Sea, given as 30 × 103 tons by Batur in and Kochenov (1969) and 25 × 103 tons by Gellermann et al. (1983), is grossly overestimated. Based on the continuous salinity data presented by Fonselius (1969, 1984) it is clear tha t total dissolved uranium in the Baltic Sea has never exceeded 15 × 103 tons during this century. The reason why the mentioned authors overestimated the amount of uranium is different. In the case of Batur in and Kochenov (1969) they used too high a uranium concentration. Gellermann et al. (1983) on the other hand, gave a mean salinity in the Baltic Sea of 12%o while 6 - 7 0 would be a more accurate figure. A peak value was probably reached after the great saline intrusion in November December 1951. Wyrtki (1954) showed that this intrusion gave an approximate overall increase in salinity of the Baltic Sea of 0.1%o. This corresponds to an increase in total dissolved uranium of 1%. Of course, there are large variations in salinity at the entrance of the Baltic, i.e. the Belt Sea. As the volume of this is only 1.4% of the total, its overall influence is limited. Thus, there is a substantial stability

226

in the salinity of the Baltic Sea, although a slow and continuous increase has been registered this century (Fonselius, 1969). For dissolved uranium, th~ variation in the volume of anoxic water in the central Baltic is the main factor of uncertainty, causing deviation from conservative behaviour.

ACKNOWLEDGEMENTS

SMHI (Swedish Meteorological and Hydrological Institute) permitted use oi their chemical data. SR. Carlberg, SMHI, supplied additional information ,m this material. J. Ingri. C Pont6r and the crews on board R/V ~'Argos'" ~md R/V "Strombus" helped in sample collection. K. Bostr6m, J. Ingri, E.L. SjSberg and F.E. Wickman constructively critized earlier versions of this article. The view~ of P.J. Wangersky and two anonymous reviewers led to further improvements. E. Szpringer translated Russian and Polish articles anJ S. Jewall drew the figures. Financial support by grant no. 4962-104 from NFR (Swedish Natural Science Research Council) is acknowledged.

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