SILVER IN SEDIMENTS OF THE ELBE RIVER, ITS ESTUARY AND IN THE
NORTH SEA
German Müller,
Institute of Environmental Geochemistry Heidelberg University, INF 236, D-69120
Heidelberg, Germany. E-mail gmueller@ugc.uni-heidelberg.de
ABSTRACT
In fine grained sediments of
the Elbe River and its tributaries not only cadmium and mercury, but also
silver was found in very high concentrations excceedig the geogenic background
concentration by a factor of up to 400. Two major contributors (point sources)
were found to be responsible for this enrichment: Production of photographic
material in the lowermost part of the Elbe near Pardubice (Czech Republic) and
in the lower course of the Mulde tributary near Wolfen (former German
Democratic Republic).
As a second source silver was and still to-day is derived from relics
of the old silver mining district, the Silber-Erzgebirge („Silver Ore
Mountains“) in the catchment area of the Mulde tributary. Millions of tons of
tailings still deposited close to the river banks are partially eroded at each
high water level and transported downstream into the Elbe river.
INTRODUCTION
|
Fig. 1 Catchment area of the Elbe (Czech Labe) River. |
After the German reunification in 1989 the
sediments of the Elbe River (Czech Labe),
draining about 90% of the Czech Republic and large parts of eastern Germany
(Fig.1) known for their extreme concentrations of cadmium and mercury,
underwent thorough investigations
(Müller & Furrer 1994,1998; Schindler & Lochovsky 1996; Prange et al.
1997) leading to the point sources of heavy metals in the catchment area.
Additional analyses for silver led to the surprising result, that
this metal occurs in concentrations up to 500 times higher than in the „average
shale“ concentration (0.07 mg/kg), considered as geogenic background (Turekian
& Wedepohl, 1961), representing fine grained (normally with a mean grain size of < 20µm) sediments of all geological ages not
affected by anthropogenic influences.
Main tributaries of the Elbe from the left
side are the Moldau (Czech Vltava) in
the Czech Republic, Mulde and Saale in Germany draining the Erzgebirge, and
from the right side Schwarze Elster and Havel in Germany.
The limnic part of the Elbe extends until
Geesthacht. The city of Hamburg with its large harbour system is already part
of the estuary, strongly influencend by the North Sea.
ANALYTICAL METHODS
In this report, Ag concentrations in sediments are based on the
analysis of their < 20 µm grain fraction. This fraction with its high
specific surface area is assumed to represent more or less the original fine
grained suspended load of a water
body, resulting in mud after
deposition, in contrast to the coarser material, > 20 µm, the bed load, resulting prerferentially in sand
(with a very low specific surface
area) when deposited.
Analyses were carried out with flame- and flameless Atomic Absorption
Spectroscopy standard techniques in the aqua regia extracts.
RESULTS and DISCUSSION
|
Fig. 2 Results of sediment analyses
(fraction <
20 µm) from two sampling campaigns in 1992
and 1995 (after Prange et al., 1997). |
Fig. 2 depicts the Ag concentration of the
Elbe sediments in its total length and of major tributaries during two sampling
campaigns (Prange et al., 1997). It is clearly to be seen, that between October
1992 and October 1995 a considerable decrease of the Ag concentration took
place. The sharpest decrease was found in the Czech part of the Elbe near
Hradec Králové and in the mouth of the Moldau river. In both campaigns the
Mulde river, although with an already strongly reduced Ag concentration,
remained the main polluter of the Elbe river.
Our campaign, carried out one year later in
October 1996 resulted in a similar picture as in 1995 , the mouth of the Mulde
river with about 11 mg/kg Ag representing still the highest Ag source.
In 1992 the „Synthesia“ industrial center in the lower Elbe near
Hradec Králové with the center of photographic industry in the Czech Republic,
and the mouth of the Moldau (Vltava) river draining the Czech capital Prague,
contributed the highest Ag concentrations.
In a detailed sediment study of the Czech part
of the Elbe between Hradec Králové and the Czech-German border Borovec (1995)
could clearly trace a silver point source between Hradec Králové and Pardubice.
The Ag content of the suspended matter of the
Vltava/Moldau river (Schindler & Lochovsky 1997) showed very high values (5
- 15 mg/kg) betweeen Prague and its mouth into the Elbe river causing a high Ag
import into the main river. Prague is
well known as the city of gold and silver jewellery, photographic industry and
film consumption might also have played an important role.
|
Fig. 3 Ag concentration in sediments (< 20 µm) of two
sedimentary cores collected in Jessipek near Hradec Králové and Bucher Brack near
Tangermünde. After Prange et al., 1997 |
In a dated sediment core „Jessipek“ (near
Hradec Králové, Czech Rep.) Prange et al. (1977) found the maximum silver
concentration in sediments deposited in 1992 with 45 mg/kg (Fig. 3 ), at
„Bucher Brack“ (near Tangermünde), 129 km downstream from the mouth of the
Mulde river, the maximum was dated to 1981 with 25 mg/kg. Both concentration
curves are related to photographic industry: Core „Jessipek“ is situated
directly below the largest Czech producer of photographic materials, core
„Bucher Brack“ contains at least a large part of the silver freight from the
biggest producer of photographic materials at Wolfen (former „German Democratic
Republic“) which was closed only in
1989.
As a result of all measures applied during the
past decade to minimize pollution in the Elbe river the Ag concentration of the
sediment at Geesthacht, lowermost limnic station of the Elbe, between 1989 and
1996 was reduced continuously from 11.6 to 3.3 mg/kg (Fig. 4, data from
Ackermann, 1999).
|
Fig. 4 Ag concentration in sediments
(< 20 µm) at Geesthacht, lowermost limnic
section of the Elbe river, between January 1980 and January 1997 (from Ackermann, 1999) |
The present still high Ag content of suspended material transported from the
Mulde into the Elbe river near Dessau
(between Febr. 1998 and Jan.1999, a mean Ag concentration of 5.6 mg/kg was measured, Müller, in prepn.) - is
believed to be related to the former large mining district, still „active“ with
millions of tons of tailings deposited along the river in the hinterland of the
Mulde river - the „Silber-Erzgebirge“ („Silver-Ore Mountains“) and no longer to
the producer of photographic material at Wolfen.
|
Fig. 5 Ag concentration in sediments
of the North Sea.
Analyses carried out by the Bundesanstalt
für Geowissenschaften, Hannover
on material studied by Irion & Müller(1988) |
In the tidal area of the Elbe a strong
dilution of all heavy metal contents in the sediments takes place by mixing
with relatively „clean“ marine sediments (Müller & Förstner, 1975). This
process is enforced by the fact, that
about two Mio tons of contaminated Elbe sludge have to be dredged
annually from the Hamburg harbor basins and deposited on land.
Within the North Sea still a clear zonation
with decreasing heavy metal concentrations (except for Pb) from the mouths of
the Elbe and Rhine rivers into the open sea
is to be observed (Irion & Müller, 1988). Analyses of silver carried out from identical samples by the
„Bundesanstalt für Geowissenschaften“in Hannover, not included in their
publication, are now presented for the first time in Fig. 5. Ag concentrations
decrease seawards from the mouth of the Elbe (and of the Rhine) from around 1 mg/kg to < 0.4 mg/kg.
Sequential extractions with a six-step
technique slightly modified after Kersten & Förstner (1987) carried out on
sediments of the Czech part of the Elbe sediments by Borovec (1996) led to the
result that the largest amount of Ag (82%) is bound to non- or
partially-crystalline manganese oxide or hydrous oxide phases and
only about 18% are extracted in the organics/oxidizable sulfides extraction
step.
Our own measurements (under work) indicate
silver sulfide to be the major species in downstream Elbe sediments.
Literature
cited
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