MERCURY

IN BALTIC AND NORTH

SEA WATERS

DIETHER SCHMIDT Deutsches Hydrographisches lnstitut, Laboratorium Si~lldorf, Wiistland 2, D-2000 Hamburg 55, Germany

(Received August 21, 1990; revised April 23, 1991)

Abstract. Improved determination methods for very low Hg concentrations in sea water using cold vapor AAS with pre-enrichment and purification on gold as well as a new sampling system ('MERCOS' PTFE water samplers) have been applied to extensive investigations for reactive and total Hg levels. The first research cruises concentrated on the southwestern Baltic Sea. Concentrations found in the surface water were around 2 ng L -~. Since 1979, Hg has been regularly monitored in the German Bight of the North Sea as a part of German and international monitoring programs. Two investigations have been performed in the lower course of the river Elbe, from Hamburg to the sea. Results show that the Elbe is the predominant 'point source' for Hg in the German Bight. In a large multidisciplinary research project 'ZISCH', a closely spaced station network has been sampled twice, in 1986 and 1987, to survey the entire North Sea, i.a. for Hg. Over most of the area, the median of Hg values in the surface water was 0.5 ng L-t; in a distinct field, however, extremely high levels were found, of more than 200 ng L -~. 1. Introduction Since the M i n a m a t a disaster in J a p a n b e c a m e k n o w n a r o u n d 1956, H g has m o v e d to the t o p o f the list o f h e a v y metals in sea water c o n s i d e r e d to be d a n g e r o u s . T o g e t h e r with Cd it was p u t on the ' b l a c k list' o f all c o n v e n t i o n s for the p r o t e c t i o n o f the m a r i n e e n v i r o n m e n t . This resulted in the i n i t i a t i o n o f a n u m b e r o f m o n i t o r i n g p r o g r a m s at n a t i o n a l a n d i n t e r n a t i o n a l levels, in o r d e r to o b t a i n a general view o f the c u r r e n t state o f H g p o l l u t i o n o f sea water, to start m o n i t o r i n g the t r e n d o f m e r c u r y c o n c e n t r a t i o n s over l o n g e r p e r i o d s , a n d to c o n t r o l the p o s s i b l e success o f c o - o r d i n a t e d p r o g r a m s for r e d u c t i o n o f the i n p u t o f this m e t a l into the m a r i n e environment. I n v e s t i g a t i o n s with the m o s t m o d e r n m e t h o d s in v a r i o u s i n s t i t u t i o n s a n d c o u n t r i e s have s h o w n in recent years, t h a t even in c o a s t a l waters c o n s i d e r e d to be p o l l u t e d , low H g c o n c e n t r a t i o n s are often m e a s u r e d . H i g h l y sensitive d e t e c t i o n m e t h o d s were t h e r e f o r e a b s o l u t e l y necessary for m o n i t o r i n g .

2. Methods and Materials NEW SAMPLING TECHNIQUE A very simple s a m p l i n g m e t h o d was d e v e l o p e d for the u l t r a t r a c e analysis o f h e a v y metals in s h a l l o w c o a s t a l waters a n d the surface waters o f the o c e a n to a m a x i m u m d e p t h o f 100 m. T h e s a m p l i n g system is easy to use at sea ( F r e i m a n n et aL, 1983). To a v o i d c o n t a m i n a t i o n , i n t e r c h a n g e a b l e 500 m L P T F E ('Teflon') bottles are Water, Air, and Soil Pollution 62: 43-55, 1992. 9 1992 Kluwer Academic Publishers. Printed in the Netherlands.

44

DIE'I'HER SCHMIDT

used both for sampling and storage of samples. The sampler passes the contaminated surface layer around the research vessel in a closed configuration and is opened at the desired depth in the usual manner by plastic messengers. The sea water samples are stabilized immediately in situ by purified HNO3 in the sample bottles. The samplers are cleaned and handled in clean benches, as are all chemical operations with the water samples, both on the research vessel and in the laboratory on land. In 1985, a new type of clean room laboratory container was developed and built. It permits processing and conserving water samples and cleaning and servicing sampling equipment under strictly controlled clean room conditions aboard research vessels. There is a deep freezer in an anteroom. In the clean laboratory there are filtration equipment, bench space for chemists and 2 clean benches. The air within the laboratory and the clean benches is processed through HEPA filters to remove dust particles. US Federal standard 209a Class 100 is attained in the clean laboratory, Class 10 inside the clean benches. The container laboratory has proved to be very successful since its introduction. To obtain uncontaminated samples from the sea surface, a rubber raft (dinghy) is sometimes used when weather permits. It moves away from the research vessel a few hundred meters against the wind and surface current. This is done to preclude the possibility of contamination from the research vessel's immediate vicinity. During the slow speed of the rubber dinghy, the water is filled from the bow with the aid of a plastic arm directly gripping onto the sample bottles.

[ 200 mL min -l N z 50 MIN

I > |

i

400 mL SEAWATER SAMPLE pH - 0.9

0R NG 0F

GAS

AMALGAMATION i00 mg Au/SILICA-WOOL

+ 500 mL min-I N 2 .~ I

I

I

RELEASING OF Hg

COLD VAPOUR AAS COLEMAN MAS 50

4

3 mL SnClz

I

1 I

~

TEMPERATURE ABOUT 780 ~

1 I

Fig. 1. Schematic diagram showing the analytical method for trace determination of Hg in sea water.

MERCURY IN BALTIC AND NORTH SEA WATERS

ANALYTICAL

45

METHOD

The determination of very low Hg concentrations in sea water was made possible by the development (since 1979) of a greatly improved method, which consists of an integrated system for sampling and analysis. In order to prevent systematic errors either through contamination or the loss of the trace metal the same PTFE('Teflon'-)bottle is used as a sampling vessel at sea, as a storage bottle, and as a reaction vessel for the analysis. The cold Vapor atomic absorption spectrometry is used, with N 2 a s purging gas, reduction by SnCI2, and enrichment as well as purification through the formation of amalgam on finely dispersed gold, which was produced by high-vacuum-sputtering on pure silica glass wool. In an additional step, after 2 hr of UV-irradiation, the total Hg content is determined, which contains the organically bound Hg together with the inorganic reactive Hg, which was measured before sample irradiation. An 150 W immersion UV lamp is mounted vertically in the center of the original teflon sampling bottle for irradiation under constant stirring and cooling. The limit of detection of this method was found to be 0.5 ng L -I and the coefficient of variation 4% for 1,5 ng L -1. Freimann and Schmidt (1982) published a detailed description of this analytical method. Figure 1 shows a schematic diagram of the procedure. 3. Results

MERCURY DETERMINATIONSIN THE BALTICSEA One of the first applications of the newly developed method for Hg was on a cruise with RV GAUSS in September 1980 to the southwestern Baltic Sea. The stations and the Hg values are displayed in Figure 2. In this case, surface water was sampled from a slowly moving rubber raft as described earlier. In the upper part of the Figure, the bars above the station numbers represent Hg concentrations. The empty portion of the bars gives the 'labile', inorganic reactive Hg, the hatched portion, the 'organic' Hg after UV irradiation. Both together represent the total mercury value. The Hg levels found proved to be very uniform and surprisingly low as compared with the general expectations at that time (DHI, 1981). The complete set of data from this study as well as continuing investigations during the following years, both in the Baltic and in the North Sea, has been published in a series of annual monitoring reports by DHI, beginning with the data from 1980 (DHI, 1982 ff). The low surface water Hg concentrations have been confirmed by our own work as well as by Brfigmann (1986) who reported a mean content of 3.3 ng L -1 for 'dissolved' Hg in Baltic Sea water.

9

9 706

701

703

704

705 706 707 708 7090 710

711

712

713 717 718

F S "GaUB ", S e p t e m b e r D H I / M 312

714 715 716

9 STATION NR.

Fig. 2. Mercury in the surface layer of the south-western Baltic Sea (see text for detailed explanation).

~'?07~~

D707

0705

3

1980

720

721

,--t

=

rn

rn

M E R C U R Y 1N BALTIC AND N O R T H SEA WATERS

47

Fig. 3. Distribution of total Hg in the German Bight in July 1981.

MONITORING OF THE NORTH SEA (GERMANBIGHT) Since 1980, the G e r m a n Bight has been monitored, inter alia, for Hg once a year. In the past few years, the outer estuaries of the rivers Elbe, Weser, and Ems have also been investigated. It is thus hoped not only to survey the Hg content in this probably contaminated area, but also to fulfill the obligations of the Oslo and Paris Conventions for the prevention of marine pollution as well as of the national 'Joint Bund/L/inder Monitoring Program for the Coastal Waters of the North Sea' (BLMP). A particularly extensive investigation was carried out in 1981. Figure 3 shows the data in a simplified way. The numbered dots represent the monitoring stations of the Deutsches Hydrographisches tnstitut. For each station, the total Hg content (after UV irradiation) is shown in the abscissa over the depth at which the sample was taken. (See side diagram for numerical values.) The results show that the concentrations in the open sea are relatively low, with higher values in coastal waters, especially in samples taken from near the bottom of the sea. The large rivers are therefore the source of much of the Hg present in the North Sea. The high level of particulate material containing Hg in the outer estuary of the Elbe is particularly striking. Figure 4 summarizes the total Hg content data from the G e r m a n Bight monitoring, 1980 to 1985. The distribution shows an accumulation of concentrations towards values between 5 and approximately 25 ng L -1.

48

DIETHER SCHMIDT

25 N 20

15

10

ill 0

I

. . . . . . . . . . . . . . . . .

. .F].

..

NG HG-TOTAL

L- I

Fig. 4. Histogram of the frequencydistribution of total Hg content in the German Bight from monitoring during 1980 to 1985.

INPUT FROM THE ELBE RIVER

The German Bight monitoring showed very high Hg levels in the outer estuary of the Elbe. In 1983, an investigation (repeated in 1985) was carried out to determine the changes in concentration of Hg (together with Cd, Cu, Fe, Mn, and Ni) from the fresh-water region of the Elbe to the sea water of the open G e r m a n Bight. The methods of sampling and analysis were identical to enable direct comparison of the results. Figure 5 gives the positions of the stations. Six are situated at regular intervals between the Port of H a m b u r g (El) and Cuxhaven (E6), followed by stations on a section through the G e r m a n Bight that were chosen from the monitoring stations already in operation. They start at station 6 northwest of Scharh6rn island, go past Helgoland to station 19 which is situated close to the research platform N O R D S E E and which gives typical values for the outer G e r m a n Bight. The samples were taken in August 1983 by the D H I ' s research vessel GAUSS

MERCURY IN BALTICAND NORTH SEA WATERS

49

19 S4o4s, *

GERMAN

54oa~,

BIGHT

i1

5aols

54o~e

.

4

3

5a~

E2 --('

Bo~.

IgoB,~

~

E1 H A M BURG

] 9o3~,

I II o 9

Fig. 5. Geographical location of the stations of the Elbe river investigation: lower course and estuary of the Elbe from Hamburg to Cuxhaven and section across the German Bight.

ELBE RIVER - NORTH SEA - PROFILE 1983 10 4.

--

I

I

I

I i

I

I

~

I

I

z~\ J

i

I

/ //., / .../

10~

g

I

/+

........ X#84184

....~ .....

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*......

+ ...... +

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100

*

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I 12

I gh

2,; BELOW SURFACE

..

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5

-

~ B

STATION

E6

I E5

- ~ H ' - E4

2m ABOVE BOTTOM

I E3

-~l----I E2

E1

NO.

Fig. 6. Mercury concentrations found at 2 different depths at all stations along the Elbe section (see Figure 5 for stations).

50

DIETHER SCHMIDT

on a cruise from Hamburg to the German Bight. Salinity was found to lie between 32 in the North Sea and almost 0 in the Elbe beyond Glfickstadt. The results for Hg are given below. The concentrations of Hg in unfiltered water samples (Figure 6) along the section span two orders of magnitude: from more than 1000 ng L -1 in the Port of Hamburg to less than 10 ng L -1 at the research platform. Generally speaking, water samples from 2 m above the river/sea bed show higher Hg concentrations than the samples taken 3 m below the surface; this is due to the higher level of suspended matter found there. Salinity is the most important parameter in the chemistry of an estuary, Therefore, Hg values of all water samples from all depths and stations were pooled and plotted against their salinity. The logarithm of Hg concentrations was then applied to the salinity values (Figure 7). A regression line and the Pearson correlation coefficient r were calculated for various possible distribution functions. A frequently used model describing a mixing of sea water with highly contaminated fresh water moving downstream through the estuary would be expected to show a linear decline in the concentration of metal with increasing salinity. This is not the case, however, when chemical reactions occur. In our case, the correlation coefficient for the linear decrease is so small that no significant correlation can be seen. This is also valid for other generally accepted functions. The best correlation, however, applied to an exponential decline in Hg concentration with increasing salinity. Function,

Elbe RiveP - North Sea - Profile 104

I

[

i

1983 I

Y = A * oxp(B* X) A = 431.4

B = -0. 102 r = B. 94 v CO

4->

0 (_ 4J C (D 0 (-

IB2.

0 (-3

li~1

ED ZIZ

&

ig ~

0

7

I

[

14

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proct. F i g . 7. H g c o n c e n t r a t i o n s

Sol init}/ in r e l a t i o n to salinity.

MERCURY IN BALTIC AND NORTH SEA WATERS

51

coefficients and the correlation coefficient r are displayed in the diagram. This shows that Hg reaction is more intensive at increased salinity, due probably to the precipitation and scavenging with particulate matter (Schmidt e t al. 1986; Schmidt and Wendlandt, 1987). F r o m the diagram of Hg concentration v s salinity (Figure 7) it is obvious that most values accumulate in the low and high salinity regions, i.e. in river and open sea water. Relatively few data were acquired for brackish water because in the first investigation the river stations were selected according to geographical considerations. In order to enhance the representation of the intermediate brackish water, where the mixing occurs, additional sampling was obtained during a repetition of the cruise in June 1985. An evaluation of the data from the second investigation yielded very similar results. Again, Hg in unfiltered samples shows a pronounced exponential decrease over more than two orders of magnitude, from fresh to sea water (Schmidt, 1990). All our investigations show the Elbe being the predominant 'point source' for Hg in the G e r m a n Bight. About 99.5% of the Hg originates in the German Democratic Republic and Czechoslovakia. The other heavy metals examined show a different behavior. S U R V E Y OF T H E E N T I R E N O R T H S E A

In 1984, the University of H a m b u r g and other research institutes in the north of G e r m a n y organized a large, multidisciplinary research project to study the effects of 'Circulation and Pollutant Transfer in the North Sea' (acronym ZISCH). Two extensive, quasi-synoptic surveys of the whole North Sea formed an important part of the project. Physical, chemical, and biological measurements were carried out on FS G A U S S (and WFS P L A N E T ) and FS V A L D I V I A parallel over a dense network of stations. The author was responsible for trace heavy metals in the water column. The first cruise was carried out in M a y / J u n e 1986 to determine the situation at the end of the spring phytoplankton bloom. The second survey took place between J a n u a r y and March 1987 to enable the winter values to be determined at a time when biological activity in the sea is low.

TABLE I Summary of Hg concentrations (reactive, unfiltered) at the 10 m depth from the second ZISCH-Survey of the entire North Sea, January to March, 1987.

Number of stations (n) Mean (ng L-I Hg) Median (ng L ~Hg) Minimum (ng L -l Hg) Maximum (ng L -1 Hg)

All stations

Extremastations East of Scotland

Remaining stations

138 5.2 0.5 < 0.5 >200

10 63 21 6.4 > 200

128 0.65 0.5 < 0.5 3.9

52

D[ETHER SCHMIDT

62-

= s0 ,~ L-~ 9

0

= 2D ng L-I = 10~L

61

9

missing value ~ i)~

60

" ~d

59-

s+

k-/

~-,

: ; ~/~

=

9

o

=

o

,

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.

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9 .

.

,.

.

.

L++ =

9

"

9

"\

.kO'+t A

+ \ +

,

-.+

9

". 51 '

.~+.,r

9

55-

".~.

. 9 - ~.'t'4"_ " -

we. /--o

value exactly equivalent to ~ (no cl~ir~:ation)

9

.o"

9

,~

56-

-~

Mercury (CVAAS)

r-,

"re~r

unfiltered

02.05+- 15.06.1986

++

- + - + + + '

i

-5-4-3-2-1

t

0

1

I .....

2

l'",'.~b

3

4

....

t'""l

5

.....

6

I .....

7

I .....

g

l'"r+'l+"'"l

9

+ .....

t'+"q

....

10 11 12 1.5

Fig. 8. Distribution of reactive Hg concentrations at the 10 m depth from the first survey of the entire North Sea, M a y / J u n e 1986.

M E R C U R Y IN B A L T I C AND N O R T H SEA W A T E R S

53

sco=e:

4;

= 20 ng L-I

t7~

= lo ~ L-1

!;

r~ missing value

-~

value exactly .,~ClUJvotent

L-"

0

.qli

:i = 50 r~ L-1

to oreo (no

30-

~l~ssificotion) i :i

59 f 58

.:i

/

57-

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9 e.-"

. 9

54.

.r

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.

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!:i

. " .

..

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9

. .

9

.

I

-

"..

53-

52-

51

50

~- -

'

. . . . . ~

28.01.-06,03.1987

=i

49.... I ..... I ..... l ' r ' " P ' " ; ' l ..... I ..... r ..... I..... I ..... I ..... I ..... 1..... 1 ' " " I ..... I ..... I ..... l . . . . . I . . . . , I . . . . . -5-4-.3-2-1 @ 1 2 3 4 .5 6 7 8 9 1 0 11 1 2 1.3

~-r

Fig. 9. Distribution of reactive Hg concentrations at the 10 m depth from the second survey of the entire North Sea, J a n u a r y / M a r c h 1987.

54

DIETHER SCHMIDT

Trace metal samples were taken on the cruise in 1986 at 129 stations and on 150 stations in 1987. This was presumably the first heavy-metal survey of such magnitude in the North Sea in which a number of different, up-to-date trace analysis methods were applied in one multidisciplinary project. Figure 8 shows summarized results of Hg concentrations selected from a larger data set. They are based on water samples from a depth of 10 m, taken at every station. They show a two-dimensional, quasi-synoptic picture of Hg concentrations in the surface waters of the North Sea including results from a few samples taken in the North Atlantic between the Shetland, Faroe and Orkney Islands. Only the results for 'total' (dissolved and particulate), 'reactive' Hg, i.e. unfiltered and not irradiated, are selected. The area of the filled circles is equivalent to the concentration of Hg, as stated in the legend. It can be seen that in most regions of the open North Sea very low Hg levels around 1 ng L -1 exist. In the southwest North Sea, in some coastal areas of the German Bight and to the east of Scotland as well as in parts of the Skagerrak the values lie between 1 and 10 ng L -1. Our earlier results from measurements in similar areas of the open sea are thus confirmed (Schmidt and Freimann, 1984). Higher Hg levels can be seen in the outer German Bight, probably due to the Elbe. Why similarly high levels are present in the Southern Bight of the North Sea and around the Shetlands is as yet unexplained. The highest concentrations, however, are found in the offshore regions of the northwest quadrant of the North Sea, east of Scotland. The results of samples taken at other depths at the same and neighboring stations have to be regarded for comparison (Schmidt and Dicke, 1987, 1990; Schmidt, 1988; Kersten et al., 1988). The second (winter) survey from January to March 1987 resulted in a very surprising picture for the Hg concentration in the North Sea (Figure 9). The data are summarized in Table I. Almost the entire area of the North Sea showed Hg levels in the surface waters at or below our detection limit of 0.5 ng L -1. A few values in the Southern Bight and the Central North Sea are around 10 ng L -1. Extreme maximum values of more than 200 ng L -1 are found at only 10 stations (again) in the offshore areas east of Scotland. Preliminary speculations concerning the origin of these very enhanced Hg levels concentrate on a possible connection to the natural gas production in the North Sea. At least in some fields, natural gas is known to contain high amounts of gaseous elemental Hg. The data, resulting from analyses finished only very recently, have to be scrutinized interdisciplinarily in comparison to complimentary results from other laboratories participating in the cruises.

Acknowledgments The author is especially grateful to Dr. Monika Dicke, Peter Freimann, Wolfgang Gerwinski, Dr. Michael Haarich, Birgit Hussel, Andreas Michel, Karin Przygodda, and Ute Wendlandt, who participated each in some phase of the 10 ys of Hg research

MERCURY IN BALTICAND NORTHSEA WATERS summarized GAUSS

here. T h a n k s

55

are d u e t o t h e c a p t a i n s a n d c r e w s o f r e s e a r c h v e s s e l s

and PLANET.

References Brfigmann, L.: 1986, Rapp. P-v. Rdun. Cons. int. Explor. Mer 186, 329. DHI: 1981, 'Jahresbericht 1980', Deutsches Hydrographisches Institut, Hamburg. DHI: 1982 ff., @berwachung des Meeres. Bericht ftir das Jahr 1980 ft.', Deutsches Hydrographisches Institut, Hamburg. Freimann, E and Schmidt, D.: 1982, Fresenius' Z. Anal. Chem. 313,200. Freimann, E, Schmidt, D., and Schomaker, K.: 1983, Marine Chem. 14, 43. Kersten, M., Dicke, M., Kriews, M., Naumann, L., Schmidt, D., Schulz, M., Schwikowski, M., Steiger, M.: 1988, 'Distribution and Fate of Heavy Metals in the North Sea', in: W. Salomons, B. L., Bayne, E. K., Duursma, U , F6rstner (eds.), Pollution of the North Sea. An Assessment, Springer-Verlag, Berlin, Heidelberg, New York etc., pp. 300-347. Schmidt, D.: 1988, 'Trace Metal Distributions in Sea Water: Two Surveys Covering the Entire North Sea', International Council for the Exploration of the Sea, C.M. 1988/E:35, 14 p. Schmidt, D.: 1990, 'Gradients of Trace Heavy Metal Concentrations in the Elbe Estuary', in: W. Michaelis (ed.), Estuarine Water Quality Management. Monitoring, Modelling, andResearch, Coastal and Estuarine Studies, Springer-Verlag, Berlin, Heidelberg, New York etc., pp. 443-448. Schmidt, D. and Dicke, M.: 1987, 'Trace Determination of Mercury in the Water Column: Two Surveys Covering the Entire North Sea', Proceedings, International Conference Heavy Metals in the Environment, New Orleans, Sept. 1987, Vol. 2, 315-317, Edinburgh. Schmidt, D. and Dicke, M., 1990, 'Schwermetalle im Wasser', in: J. L. Lozfin, W. Lenz, E. Rachor, B. Watermann, H. von Westernhagen [Hrsg], Warnsignale aus der Nordsee. Wissenschaftliche Fakten, Verlag Paul Parey, Berlin und Hamburg, pp. 30-41. Schmidt, D. and Freimann, E: 1984, Fresenius' Z. Anal. Chem. 317, 385. Schmidt, D., Freimann, E, and Zehle, H.: 1986, Rapp. P-v.Rdun. Cons. int. Explor. Mer. 186, 32l. Schmidt, D. and Wendlandt, U.: 1987, 'Spurenbestimmung yon Quecksilber im Meerwasser der Nordsee', in 4. Kolloquium Atomspektrometrisehe Spurenanalytik, Konstanz, April 1987, B. Welz [Hsg.], Uberlingen, pp. 617-628.