The determination of Cd, Cu, Fe, Ni, Pb and Zn in Baltic Sea water

The determination of Cd, Cu, Fe, Ni, Pb and Zn in Baltic Sea water

231 Marine Chemistry, 8(1980) 231--244 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands T H E D E T E R M I N A T I ...

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231 Marine Chemistry, 8(1980) 231--244 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

T H E D E T E R M I N A T I O N O F Cd, Cu, Fe, Ni, Pb A N D Z n I N B A L T I C S E A WATER BERTIL MAGNUSSON and STIG WESTERLUND Department of Analytical and Marine Chemistry, University of Gbteborg, S-412 96 GSteborg (Sweden) (Received February 26, 1979) ABSTRACT

Magnusson, B. and Westerlund, S., 1980. The determination of Cd, Cu, Fe, Ni, Pb and Zn in Baltic Sea water. Mar. Chem., 8: 231--244. Cd, Cu, Fe, Ni, Pb and Zn were determined in 123 samples from the Baltic Sea proper. The trace metals were extracted directly on board the vessel, using a dithiocarbamate-Freon procedure. Final analyses of the extracts are performed onshore by atomic absorption spectrometry. Similar trace-metal concentrations are found in different areas of the Baltic proper. Most values fall in the following ranges: Cd, 30--60 ng 1-1 ; Cu, 0.6--1.0/lgll-l ; Fe, 0.3-0.9 pg 1-1 ; Ni, 0.6 -0.9 pg 1-1 ; Pb, 0.05--0.2 pg l-l ; and Zn, 1.5--3.5 pg l- . The metalconcentrations are generally independent of depth. However, copper exhibits a small but significent decrease in concentration below 80 m. Filtration did not affect trace-metal concentrations, with the exception of iron in waters from lower layers. Similarly, storage under acid conditions was shown to affect only the concentration of iron. An electro-chemical technique was also used to determine Cu in some samples.

INTRODUCTION T h e Baltic Sea acts b o t h as a p r i m a r y a n d a s e c o n d a r y r e c i p i e n t f o r large a m o u n t s o f u r b a n w a s t e originating f r o m t h e b o r d e r i n g c o u n t r i e s . T h e i m p a c t o f this i n p u t a n d in p a r t i c u l a r o f t r a c e m e t a l s , can b e o b s e r v e d in Baltic s e d i m e n t s . N i e m i s t 5 a n d T e r v u ( 1 9 7 8 ) f o u n d an increase in t h e a m o u n t s o f Cd, Pb a n d Zn in s e d i m e n t laid d o w n d u r i n g t h e last 40 years. Metals e n t e r i n g t h e Baltic as p a r t o f f r e s h w a t e r discharges are m a i n l y t r a p p e d in t h e coastal zone. Higher m e t a l - c o n c e n t r a t i o n s w e r e f o u n d in n e a r s h o r e surface s e d i m e n t s t h a n in s e d i m e n t s f r o m t h e o p e n Baltic. H o w e v e r , e v e n in t h e o p e n Baltic, t h e t r a c e - m e t a l c o n c e n t r a t i o n s o f s u r f a c e s e d i m e n t s are elevated a b o v e b a c k g r o u n d levels ( O l a u s s o n et al., 1 9 7 7 ) . R e m o v a l o f t r a c e m e t a l s f r o m t h e Baltic c a n also t a k e place, in a d d i t i o n t o r e m o v a l b y s e d i m e n t a t i o n , t h r o u g h w a t e r e x c h a n g e w i t h t h e K a t t e g a t t . S t u d i e s o f tracem e t a l c o n c e n t r a t i o n s in r e l e v a n t p a r t s o f t h e Baltic a n d K a t t e g a t t will t h u s m a k e it possible t o e s t i m a t e t h e d e g r e e o f t r a c e - m e t a l r e m o v a l b y e x c h a n g e o f w a t e r mass.

232

In order to assess the ecological risks involved in trace-metal pollution, a knowledge of the various forms in which the metals exist in the water is mandatory. Unfortunately, there is no fast and reliable method for achieving this. The results presented here cover only the "dithiocarbamate-reactive" metals. In order to estimate the fraction of the total which these represent, some samples were analyzed after storage under acid conditions. This method of storage serves to release most adsorbed metals, and also dissolves most of the colloidal iron hydroxide. The investigation was conducted during the second joint Soviet--Swedish expedition in the Baltic, on the R.V. "Ernst Krenkel" in March 1978. Samples were taken at 21 stations in the Baltic proper (see Fig. 1). At one station (no. 4), samples were collected over a five
.

.

.

.

.

~2 o

16 o

2O*

Fig.1. Cruise track of the second Soviet--Swedish expedition. The long-term station is station No. 4.

EXPERIMENTAL

Samples were collected with 1.8-TPN samplers (Hydrobios, Kiel, FRG):

in addition, a 20-1 Niskin sampler and a 5-1 sampler were tested on a few occasions. Water was transferred from the samplers to polypropylene bottles,

and then extracted on board within half an hour of collection. Some of the samples were pressure-filtered (0.5 kg) through 0.45-ttm filters, with a Millipore plastic filtration system. The filters were pre-cleaned with 1 M HCI and water. For the storage experiment, polypropylene bottles were filled with samples acidified with 1 ml concentrated Merck Suprapur HNO3 per liter of seawater. The bottles were then stored for a b o u t two months at r o o m temperature before extraction.

233 Glass- and plastic-ware were washed in 7 M HNO3 and then rinsed six times in ultrapure water from a Milli-Q water purification system (Millipore Company). "Reactive" metals were extracted using the dithiocarbamate-Freon method described by Danielsson et al. (1978). Starting with 400 ml seawater, the metals were extracted as dithiocarbamate complexes at pH 5 into Freon 113. After phase-separation, the metals were back-extracted into 10 ml of 0.3 M nitric acid. Calibration was carried out for one depth at each station by standard addition. The extracts were stored in glass-stoppered test-tubes (Pyrex), or in 100-ml polyethylene bottles. The stored samples were neutalized with concentrated Suprapur ammonia before extraction. The detenninations were performed onshore by atomic absorption spectrometry (AAS). A Perkin-Elmer 370 spectrometer equipped with a graphite furnace (HGA 2100) was used for the determinations of Cd, Cu, Fe, Ni and Pb: Zn was analyzed by flame atomisation, using a Perkin-Elmer 403 spectrometer. The recovery, as evaluated by standard addition, was 85-100% The precision of the method was estimated by duplicate on-board extractions of aliquots from the same sample. From these duplicate

TABLE I Estimated precision from duplicate measurement, and blank values from 9 blank determinations: S.D. = standard deviation Metal

Cd Cu Fe Ni Pb Zn

Duplicate measurements

Blank determinations

Mean value (~g1-1 )

S.D. (%)

Mean value ( ~ g l -I )

S.D. (%)

0.044 0.76 0.73* 0.77 0.11 2.7

21 18 24* 8 30 22

0.005 0.098 0.28 0.021 0.043 0.87

77 52 38 15 25 38

* Values taken from 10 samples at a depth of 25 m on the long-term station.

measurements (12), the standard deviation was calculated according to Kaiser (1970) (Table I). Blank determinations following the method of Danielsson et al. (1978) were done nine times during the cruise to determine the metal contents of the reagents as well as metals introduced during extraction.

~3,t

The blank-contribution to a 400-ml sample is shown in Table I. With the exception of iron, blank-values did not depend oll whether the extracts were stored in polyethylene bottles or ill glass test-tubes. For iron, the polyethylene bottles gave a high blank-value, usually 1--4 pg 1 - ~: m this case, it was obvious that the normal washing procedure with nitric acid was insufficient. The ironvalues determined for extracts stored in polyethylene bottles have therefore been discarded. For copper, in addition to determination by AAS, in some samples analysis was also done using an electrochemical technique similar to anodicstripping voltammetry (ASV) potentiometric-stripping analysis (PSA), as described by Jagner and/~r6n (1978). The analyses were performed at pH 2.

RESULTS AND DISCUSSION

Hy drochemical parameters Salinity, temperature, oxygen, nutrients, pH and alkalinity were also determined during the cruise: these data will be present in detail elsewhere. From the determinations the following observations can be made, Winter conditions prevailed, with a colder upper water mass, showing relatively constant salinity and temperature down to a thermo- and halocline normally at a depth between 40 and 60 m. Below this depth, salinity and temperature increased slowly towards the bottom. For t h e shallower waters of the Bornholm basin, halocline was at a depth of 20--30 m. During observations at the long-term station, a short-term inflow o f Kattegatt-water was observed in the b o t t o m layer. Oxygen was present in all samples; however, in the b o t t o m layer of water the concentration decreased to a minimum of 3% of the saturation value.

Initial approach The results for unfiltered, directly-extracted water are presented in Table II. No significant differences m metal concentrations between different areas of the Baltic could be detected from these data. Therefore, the whole open Baltic has been treated as one water-mass, and values falling outside o f the range mean +3 standard deviations (calculated for the remaining values) have been excluded in the statistical treatment. These values are marked with asterisks in Table II. The inflow of Kattegatt water during observations at the long-term station could not be correlated with trace-metal concentrations. Therefore, data for the long-term station have been treated as replicate analyses of the same water mass (Fig.2). The mean and standard deviation for various depths a~ all stations are given in Figs.3--5, together with the relevant histograms.

235

Cadmium The Cd-concentration ranges from 20 to 150 ng 1-] , with 64% of the values lying between 30 and 50 ng 1-t (Fig.3). In the upper layer (0--50 m) of low salinity the Cd-concentration is apparently constant for the open Baltic. The Cd-concentration was also found to be constant during the entire period of observation at the long-term station (Fig.2). Cd-

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Mean -* st.dev. Estimated precision

Fig.2. Profiles of the concentrations of Cd, Cu, Fe, Ni, Pb and Zn at the long-term station. Sampling at each depth was performed 10 times, at 12 hour intervals. The figures in brackets indicate the number of samples on which the mean + S.D. is calculated. The estimated precision in the analytical procedure is also indicated, as specified in Table II.

concentration decreases slightly with depth, to a mean of 34 ng V 1 for the more stagnant b o t t o m layer of water (110--400 m). The average concentration found here is similar to that determined b y Kremling (1978), b u t it is somewhat lower than that found by Briigmann (1977).

Copper The concentration of Cu found by direct extraction varies between 0.3 and 1.2/~g l-I , with most values (70%) lying in the range 0.6--1.0 pg l-I (Fig.3). In the upper layer (0.50 m), the variation in concentration is within the precision of the analytical method; the mean concentration was

236 T A B L E II C o n c e n t r a t i o n s o f d i r e c t l y e x t r a c t a b l e Cd, Cu, Fe, Ni, Pb and Z n •

(l

Station, Total d e p t h , Date

Depth (m)

Cd (ngl ~ )

Cu (ug1-1 )

Fe (ttgl ~

Ni ( t t g l -~ )

Pb ( t t g l -~ )

Zn ( ~ g l ~)

1 -35 m 78-03-17

10 20 b 30

62 100 62

0.60 0.52 0.38

0.4 0.7 0.8

0.56 0.40 0.43

0.15 0.16 0.08

2.1 2.9 ] .5

3--BY1 44 m 78-03-18

10 20 30 405

75 84 81 75

0.88 0.86 1.08 0.88

0.6 0.4 0.7 0.7

0.86 0.89 0.85 0.73

0.14 0.11 0.11 0.11

3.9 2.9 4.5 3.7

4--BY2 c 46 m 78-03-22

10 25 40

45 44 43

0.84 0.92 0.86

0.6 0.7 0.8

0,77 0.77 0.72

0.14 0.16 0.14

3.3 4.2 3.8

5 -BY3 45 m 78-03-23

10 25 405

46 33 30

0.77 0.84 0.87

0.9 1.1 1.6"

0.75 0.70 0,67

0.16 0.29 0.18

2.5 1.9 2.3

6 75 m 78-03-23

10 25 40 70

1 4 0 *d 65 61 80

1.18 1.05 1.07 0.92

0.8 0.5 0.5 2.3*

1.14" 1.08 0.80 0.71

0.40* 0.20 0.15 0.28*

7.0 3,7 2.6 3.0

7---BY4 90 m 78-03-24

10 30 50 80

63 86 68 72

0.95 0.80 0.95 0.65

0.2 0.3 0.3 0.9

0.85 0.68 0.72 0.71

0.26 0.17 0.20 0.12

2.6 2.0 2.0 2,6

8--BY5 90 m 78-03-24

10 30 50 80

46 43 55 70

0.84 0.92 0.83 0.65

0.4 0.4 0.5 2.5

0.73 0.85 0.68 0.73

0.13 0.17 0.13 0.14

2.0 2.4 3.4 3.1

9 60 m 78-03-24

10 25 50

39 39 49

0.71 0.74 0.83

0,4 0.2 0.4

0.64 0.76 0.68

0.31 0.23 0.18

1.5 1:5 1.5

10--BY7 94 m 78-03-24

10 30 60 80

57 62 59 59

0.98 0.97 0.96 0.93

0.4 0.3 0.4 0.8

0.72 0.69 0.66 0.70

0.19 0.17 0.17 0.18

2.3 3.0 2.4 4.0

11 106 m 78-03-24

10 30 60 100

45 79 40 40

0.83 1.14 0.78 0.64

0.3 0.4 0.4 2.9

0.60 0,73 0.56 0.66

0.12 0.24 0.09 0.15

1.8 5.4 1.8 3.4

12 - B Y 8 86 m 78-03-25

10 30 60 80

40 49 41 34

0.68 0.89 0.76 0.55

0.3 0.3 0.3 1.6

0.71 0.77 0.63 0.66

0.12 0.25 0.13 0.08

1.9 5.2 1,8 1.9

237 Station, a Total depth, Date

Depth (m)

Cd (ng I-I )

C

u

(~g 1-')

13--BY9 130 m 78-03-25

10 30 70 90 120

40 38 35 35 37

0.71 0.94 0.64 0.63 0.63

14--BY10 150 m 78-03-25

10 30 50 70 100 130 b

37 62 41 37 34 53

0.76 ~3" 0.89 0.73 0.76 1.03

15~-BY15 226 m 78-03-26

10 30 60 100 150 200

42 44 38 32 33 27

16--BY20 175 m 78-03-27

10 30 70 100 130 160

17--BY32 176 m 78-03-27

Fe ( ~ g l -~)

Ni ( ~ g l -~)

Pb ( ~ g l -~)

Zn ( ~ g l -~)

0.2 0.4 0.4 1.1 1.6

0.79 0.85 0.81 0.70 0.78

>0.6* 0.14 0.11 0.10 0.07

1.7 2.3 1.9 2.0 1.9

-------

0.75 0.92 0.74 0.77 0.82 0.88

0.12 >0.6* 0.17 0.10 0.11 0.27

2.5 5.3 2.1 2.8 3.3 5.6

2.21" 0.85 0.76 0.61 0.45 0.38

0.78 0.71 0.68 0.67 0.67 0.54

0.28 0.15 0.13 >0.60* 0.09 0.15

2.5 4.3 2.3 4.9 2.5 2.4

42 42 35 45 30 30

0.99 0.79 0.61 1.04 0.41 0.52

O.77 0.72 O.76 0.75 0.66 0.84

O.22 0.13 0.10 0.29 0.09 0.18

3.2 2.8 3.3 4.2 3.1 2.4

10 30 60 90 130 160

44 48 46 33 33 36

1.19 0.89 0.84 0.62 0.53 0.74

0.88 0.82 0.75 0.70 0.59 0.54

0.25 0.31 0.20 0.25 0.27 0.25

3.4 7.4* 3.8 3.4 1.8 3.0

18--BY34 105 m 78-03-27

10 30 70 90

44 59 38 34

0.98 0.94 0.88 0.77

0.69 0.72 0.63 0.61

0.12 0.18 0.10 0.16

2.6 5.7 2.3 3.7

19--BY38 106 m 78-03-27

10 30 70 90

43 46 38 27

0.93 0.92 1.06 0.58

1.13" 0.66 0.68 0.65

0.09 0.12 0.19 0.05

3.0 2.9 2.5 2.4

10 30 100 200 300 400

46 66 25 27 26 32

0.88 0.99 0.48 0.48 0.51 0.35

0.76 0.71 0.55 0.49 0.67 0.66

0.08 0.20 0.04 0.08 O.O8 0.09

3.0 4.2 1.7 1.8 2.1 1.6

20--BY31 440 m 78-03-28

D

m

M

m

m

(cont. next page)

238 T A B L E II ( ( ? o n t . ) Station," Total depth, Date

Depth ~m)

('d (rig 1-1)

Cu (/jg 1- ;)

10 30 70 9O 110

36 33 31 24 30

0.78 0.86 0.81 0.39 0.44

22--BY29 173 m 78-03-28

10 30 65 100 160

39 36 30 30 29

0.73 0.70 0.51 0.43 0.35

23 45 m 78-03-29

10 30 40

55 45 40

0.80 0.75 0.67

21 - B Y 3 0 120 m 78-03-28

Fe (/l~ 1- 1)

Ni (pg 1--1)

Pb ipg 1 1

Zn

0.75 0.69 0.68 0.70 0.75

0.08 0.09 0.15 0.03 0,10

1.8 2.8 1.6 1,4 2,4

--

0.76 0.68 0.68 0.72 0.69

0.11 0.06 0.04 0.07 0.03

2,5 ] ,7 1.8 2,0 ] ,6

-----

0.88 0.78 0.87

0,09 0.07 0.07

2.5 2.1 1.4

-

a F o r s t a t i o n s t h a t " b e l o n g " t o t h e B a l t i c s t a n d a r d s t a t i o n s , t h e n u m b e r is f o l l o w e d b y a Baltic Year (BY) number. b 5-1 T P N s a m p l e r , c F o r t h e l o n g - t e r m s t a t i o n , t h e m e a n m e t a l - c o n c e n t r a t i o n is given. d , E x c l u d e d in t h e s t a t i s t i c a l e v a l u a t i o n .

determined as 0.88 #g 1-1 . The electrochemical method yielded a mean of 1.1 pg 1-1 for this upper layer (Table III). These values are in fair agreement with those found by other investigators: 1.2 #g 1-1 in the Bornholm Sea (Kremling and Petersen, 1978); 0.9--1.2 pg 1-1 in the southern Baltic (Danielsson and Westerlund, 1978}; and 0.9 pg 1-1 in the southern Baltic (Brzezinska, 1978). T A B L E III

Comparison between the directly-extractable Cu determined by AAS, and Cu determined by the electrochemical method PSA: S.D. = standard deviation Depth (m)

AAS Cu (ug l-' )

S.D. (#g l-' )

PSA Cu (#g l" )

S.D. (/~g l-I )

No. of samples

10 20--30 40--50 70 100-400

0.83 0.88 0.81 0.78 0.44

0.13 0.20 0.15 0.14 0.09

1.18 0.87 1.05 1.17 0.65

0.23 0.31 0.21 0.11 0,10

10 7 8 3 4

239

The Cu-concentration decreases with depth, to a mean of 0.47 pg 1-1 for water at 110--400 m depths (Fig.3). Kremling (1978) found a similar profile for the Landsort deep. No change in Cu-concentration was observed at the long-term station, even though there was an inflow of Kattegatt-water. This indicates that the lower Cu-concentration in the bottom layer of water (110--400 m) is not due to inflowing Kattegatt-water. Intermittent anoxic conditions in the bottom layers of water may be responsible for the lower

Cd 5O

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Fig.3. (a) Profiles of the concentrations of Cd and Cu. Samples were divided according to sampling depth into 6 groups; 10 m, 2(}--30 m, 40--50 m, 60--70 m, 80--100 m and 110--400 m, The means -+ S.D. are plotted for samples from these depth ranges. Number of samples are indicated in brackets. The estimated precision in the analytical procedure as specified in Table II is also indicated. (b) Histogram of the concentration-values of Cd and Cu.

concentrations. Under such conditions, insoluble CuS(solid) may be formed, thus depositing Cu from the bottom layers of water as sediment. Of the sulfides (Zn, Cd, Cu, Pb and Ni), CuS probably has the lowest solubility (Dyrssen and Wedborg, 1979); therefore, this precipitation effect would be most likely to occur in the case of Cu. It is also possible that Cu could be transformed into a more stable form, which is not directly extractable nor acid-leachable at pH 2; however, we think this is less likely.

240

Iro n

For the directly-extractable fraction, a mean concentration of 0.48 pg Fe 1-] (Fig.4) was obtained for the upper water-mass. The mean value increased to 1.6/~g 1-~ towards the bottom. A comparison between concentrations in unfiltered and filtered water (Table IV) revealed no significant difference for the upper water-layer. For water at the b o t t o m layers, extractable iron decreased after filtration, to the

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Fig.4. Profiles o f t h e c o n c e n t r a t i o n (a) a n d h i s t o g r a m of t h e c o n c e n t r a t i o n - v a l u e s (b) o f F e a n d Ni. F o r e x p l a n a t i o n see t e x t t o Fig. 3.

TABLE IV C o m p a r i s o n o f m e t a l - c o n c e n t r a t i o n s in u n f i l t e r e d a n d filtered w a t e r (21 samples) Cd ( n g 1-1 ) Unfiltered Filtered

50 52

Cu

(ug 1-1 ) 0.76 0.82

Fe a

(ug 1-1 ) 1.4 0.4

Fe b

(ug 1-~ ) 0.5 0.4

Ni

(ug 1-1 ) 0.71 0.73

a 5 s a m p l e s t a k e n 5 . 1 5 m a b o v e t h e w a t e r - s e d i m e n t interface. b 11 samples t a k e n at d e p t h s b e t w e e n 10 a n d 5 0 m.

Pb

(ug 1-~ ) 0.15 0.13

Zn

(ug 1-~ ) 2.4 2.6

241

same level of concentration as in the upper water-mass. This result may be explained by the conclusion that there is a higher load o f suspended particulate material in the b o t t o m layer of water. It appears that extractable iron is readily adsorbed onto this material. The mean value for dissolved Fe is thus 0.4--0.5 pg 1-1 , independent of depth. This is about 10% of the total iron found in stored acidified samples (Table V). The majority of Fe is

TABLE V C o m p a r i s o n of m e t a l c o n c e n t r a t i o n s o b t a i n e d f o l l o w i n g d i r e c t e x t r a c t i o n a n d f o l l o w i n g e x t r a c t i o n a f t e r 2 m o n t h s ' storage at p H ~ 2 ( 1 8 s a m p l e s ) Cd

Direct Stored

Cu

Fe a

Ni

Zn

(ng l-' )

(/zg l-' )

( . g l -~ )

(/~g l-' )

(/zgl -~ )

45 58

0.82 0.85

0.6 5.4

0.80 0.99

3.1 3.4

a 10 samples.

0

0.1 ,

Pb p g o I ; t

i

Zn pg I"~ 0

i

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242

therefore in an inex~ractable, colloidal or particulate form. The concentrationlevel of dissolved Fe is consistent with the results of solubility studies by Byrne and Kester (1977), which predict a maximum concentration of about 1 pg 1-~ for Fe(III) dissolved in seawater. Nickel

Ni has a relatively homogeneous distribution in the Baltic, with 86% of the samples showing concentrations in the range 0.6--0.9 #g 1-1 (Fig.4). The variation with depth for Ni is close to our limits of analytical precision; the horizontal variation is less than +14%. The mean varies from 0.77 in the upper water-layer (0--50 m), to 0.68 pg 1-1 at the b o t t o m (110--400 m). The Ni-concentration determined here is in good agreement with that found under summer conditions in the southern Baltic by Danielsson and Westerlund (1978). Thus there are apparently no seasonal variations in the Ni-concentration. Lead

Lead is one of the most difficult elements to determine, partly because of its low concentration, and partly because of severe contamination problems. According to Schaule and Patterson (1978), most commercial water-samplers contaminate the sample with Pb. Water collected at the long-term station gave approximately the same Pb-values for each of the three different water samplers used in this investigation. However, the precision is p o o r for all samples, and the blank-value is high relative to the sample-concentration (see Table I). The profile for Pb (Fig,5) shows no significant trends with depth, and the means for various depths vary from 0.11 to 0.17 pg 1-1 . The histogram demonstrates that most Pb-values (68%) are in the range 0.05-0 . 2 p g 1-1 . Other investigators have reported higher Pb,concentrations in the Baltic; for example, Brilgmann ( t 9 7 7 ) gives 0.3--0.5 ttg 1-1 . However, our Pbvalues for the Baltic are still more than two orders of magnitude higher than the values recently found for oceanic water (Schaule and Patterson, 1978). Sediment-analyses reveal an increasing input of Pb- to the Baltic: Niemist5 and Tervu (1978) found that the concentration of Pb in the sediments has risen from 1 0 - - 2 0 g g g-1 at the turn of the century ~o 60--80 pg g-I in 1970. These analyses imply that the Baltic is severely contaminated, and our results may therefore give a reasonable indication o f the level of Pbcontamination. Zinc

The histogram (Fig.5) shows that most of the Zn-values {72%) a r e i n the

243

range 1.5--3.5 gg l-~ . No measurable variation with depth is observed in Znconcentration. The Zn-concentration at the long4erm station is slightly higher than at the other stations; but, considering the rather large scatter in the data, the magnitude of the actual difference which this represents is difficult to ascertain. The relatively high blank-value found here (Table I), compared to blankvalues obtained for Zn in an onshore laboratory (Danielsson et al., 1978), suggests Zn-contamination from the on-board laboratory environment. Slightly higher Zn-concentrations have been reported from other investigations in the Baltic: e.g. Briigmann (1977), 4--7 /~g 1-1 ; Kremling (1978), 4--6 pg 1 - 1 . During the Bosex expedition, Danielsson and Westerlund (1978) found a mean Zn-concentration in surface water of 3.5 pg r I , and in the bottom layers 5.5 pg 1 - 1 .

Filtering and storage experiments The values determined for several of the filtered samples are considerably higher than those for the corresponding unfiltered samples. This indicates that contamination during filtration is clearly a risk, especially if no clean laboratory facilities are available. However, the mean values (Table IV) for unfiltered water do not differ significantly from the mean values for filtered water except in the case of iron. This indicates that our data for the other metals can be compared with the equivalent data from other investigators which are based on filtered samples. The results for stored acidified samples, when compared with directlyextracted samples (Table V), show a minor increase in metal concentrations (from 4 to 19%) for all elements except for iron, where the increase is 90%. These increases may be due to leaching from particulate and colloidal matter. This is most probable in the case of iron; however, for the other elements, such small increases in concentration could well result from contamination introduced during storage. CONCLUSIONS

The concentrations of trace metals found are in fair agreement with those determined by other investigators. There seems to be no horizontal variation in trace-metal concentrations in the open Baltic, nor does there seem to be any marked correlation between these concentrations and salinity. However, the salinity range encountered in the Baltic (normally 8 - - 1 2 ~ ) is too small for the observation of estuarine removal of river-borne trace metals (cf. Burton and Liss, 1976). Comparing samples directly extracted with samples stored prior to extraction under acid conditions, we conclude that the main species of Cd, Cu, Ni and Zn react with dithiocarbamate to form extractable complexes. Fe, on the other hand, appears to be present dominantly in a non-extractable form.

244 ACKNOWLEDGEMENT The authors with to thank Ms. Kerstin Ardn for doing the PSA analyses, and Prof. David Dyrssen and Mr. Lars-GSran Danielsson for their helpful criticism. Thanks are also due to Dr. Ken Mopper for revising the English text, and to Mr. Torgny Johansson for preparing the drawings.

REFERENCES Briigmann, L., 1977. Zur Verteilung einiger Schwermetalle in der Ostsee--eine Ubersicht. Acta Hydrochem. Hydrobiol., 5: 3--21. Brzezinska, A., 1978. Trace metals in the waters of the Gdansk basin. Proc. Conf. Baltic Oceanographers, 11th. Rostok, April 1978. Burton, I. D. and Liss, P. S. (Editors), 1976. Estuarine Chemistry. Academic Press, London. Bryne, R. H. and Kester, D. R., 1976. Solubility of hydrous ferric oxide and iron speciation in seawater. Mar. Chem., 4: 255--274. Danielsson, L-G. and Westerlund, S., 1978. S o m e results of trace metal analysis from Bosex 77. Proc. Con. Baltic Oceanographers, 11th. Rostok, April 1978. Danielsson, L-G., Magnusson, B. and Westerlund, S., 1978. A n improved metal extraction procedure for the determination of trace metals in sea water by atomic absorption spectrometry with electrothermal atomization. Anal. Chim. Acta, 98: 47--57. Dyrssen, D. and Wedborg, M., 1979. Major and minor elements, chemical speciation in estuarine waters. In: E. Olausson and I: Cato (Editors), Chemistry and Biochemistry of Estuaries. Wiley, N e w York. Jagner, D. and Ar6n, K. 1978. Derivate potentiometric stripping analysis with a thin film of mercury on a glassy carbon electrode. Anal. Chim. Acta, 100: 375--388. Kaiser, K., 1970. Quantitation in elemental analysis. Anal. Chem., 42: 26A--58A. Kremling, K., 1978. The distribution of selected trace metals in Baltic waters. A study on the basis of two anchor stations. Proc. Conf. Baltic Oceanographers, 11th. Rostok, April 1968. Kremling, K. and Petersen, H., 1978. The distribution of Mn, Fe, Zn, Cd and Cu in Baltic seawater; a study on the basis of one anchor station. Mar. Chem., 6: 155--170. Niemist6, L. and Tervu, V., 1978. Preliminary results of heavy metal contents in some sediment cores in the northern Baltic Sea. Proc. Conf. Baltic Oceanographers, 11th. Rostok, April 1978. Olausson, E., Gustafsson, O., Melin, T. and Svensson, R., 1977. The current level of heavy-metal pollution and eutrophication in the Baltic proper. Medd. Maringeol. Lab.

G6teborg, 9 : 1 - 2 8 . Schaule, B. and Patterson, C,, 1978. The occurrence of lead in the Northeast Pacific and the effects of anthropogenic inputs. In: M. Branica (Editor), Proc. Int. Experts Discussion on Lead. Pergamon Press, Oxford, in preparation.