CHRONOLOGY OF ANTHROPOGENIC HEAVY METALS AND PAHs IN LAKE CONSTANCE SEDIMENTS: BACK TO THE BACKGROUND!

German Müller,University of Heidelberg, Institute of Environmental Geochemistry

e-mail gmueller@ugc.uni-heidelberg.de

ABSTRACT

The results based on dated sediment cores from Lake Constance collected in 1975 (MÜLLER et al. 1977, Müller 1981) and 1995 (Müller 1997) revealed a very strong positive correlation between certain heavy metals and PAHs, especially between cadmium and benzo(a)pyrene. Both groups of pollutants began to rise from background concentrations (1880 - 1890) reaching their maxima between 1960 and 1970.

The development of the heavy metals Pb, Hg, Cd, Zn and of PAHs ran more or less parallel with coal consumption in Europe.

Sediment cores collected in 1995 showed a further continuation of a strong decrease of both heavy metals and PAHs, indicated already in the uppermost layers of the 1975 core. Concentrations of heavy metals in the uppermost (1995) sediment layer are now again about the same as before 1895.

In the 1995 cores silver was measured for the first time and a parallel development with Cd, Zn, and PAHs could be observed (Müller, 1997).

Specific PAH contents were found more than twenty times less than during their peak concentration.

INTRODUCTION

„Less fear for Lake Constance Water“. This was the headline of a press release from the 1997 assembly of the „International Study Group Waterworks Lake Constance-Rhine“ following comments, that the final threat of Lake Constance as a drinking water rerservoir (Fig. 1A) no longer exists: the oil pipeline of the Italian ENI company that ran between Genoa in Italy and Ingolstadt in Germany, and part of which was embedded directly under the beach of the lake near Bregenz in Austria, had been closed in January 1997.

This event can in fact be seen to represent the end of a process that began almost fifty years ago when, faced with signs that the lake was becoming eutrophic, the communities and countries sharing the lake set out to stop the flow of pollutants and, ultimately, restore the water to ist former quality.

The significance of all measures taken lies in the fact that Lake Constance provides drinking water for about five million inhabitants of Germany, Austria, Switzerland and Liechtenstein.Through history the lake has served as a water supply to those living nearby.Larger scale abstraction came in 1885, when the Swiss city Sankt Gallen began pumping for its drinking water supply, a trend which has continued so that another 17 communities now do likewise

Today, the total annual abstraction for drinking water is about 180 Mio m³, of which aboot 140 Mio m³ is abstracted by the „Bodensee Wasserversorgung Baden Württemberg“ from the north-western section of the lake known as „Überlinger See“ and transferred via long distance transport systems into south-west Germany, reaching as far as the city limits of Heidelberg, some 180 km north..

Lake Constance covers an area of 539 km² and contains 449 km³ water, so that the water balance is virtually uneffected by the withdrawal of drinking water. The average annual outflow from the lake (11.51 km³) nearly equals ist inflow (11.47 m³), while precipitation (0.50 km³) is nearly balanced by evaporation (0.33 m³) and removal of drinking water.

The lake consists of two basins, the deep „Obersee“ (the western finger-shaped part of which is the „Überlinger See“) with an average depth if 100 m and a maximum depth of 254 m, and the shallow „Untersee“ with an average depth of 13 m.

The two basins are connected by the 4 km long „Seerhein“.

Aprroximately 70% of the catchment lies in the Swiss and Austrian Alps („Northern Alps“), with 73% of the total water infdlow to the lake entering by the Rhine river, this upper portion of the Rhine being known as the „Alpenrhein“.

The river also brings about 90% of the lake’s solid load - between 2 and 4.5 M tonnes of fine-grained suspended material become sediments in the Obersee basin.To prevent heavy floods in the lowest course of the Alpenrhein, its course was shortened and straightened by shifting it eastward into the Fussach Bay. However the new mouth had to be deviated westwards since 1966 by a huge dam, still be extended, as the sediments deposited in front of the new mouth threatened to cut off the Bay of Bregenz from normal water circulation.

The „Bodensee-Projekt“

In 1960 the „Deutsche Forschungsgemeinschaft“ (German Research Association) establihed the „Bodensee-Projekt“ after first signs of eutrophication were revealed by an increase in the phosphorus concentration in the lake water in combination with changes in the amount and diversity of the phytoplankton towards the end of the 1950s.

According to the categories of Vollenweider (1979), Lake Constance was an oligotropzhic lake until the beginning of the 1960s.

Thereafter, an a lmost exponential phosphorus (PO₄-P) concentration occurred until the end of the 1970s (Fig. 1HB) along with a marked increase of plankton growth.

Phosphorus, mainly derived from municipal wastewater, was recognised as the key factor limiting primary organic production.

According to Wagner (1976), in 1974 about 59% of the total annual phosphorus load of 1930 tonnes derived from polyphosphates in detergents, while 20% were attributed to untreated domestic sewage, the remainig 12% to agricultrural runoff and atmospheric deposition.

Only about 1o tonnes were derived from the natural geochemical load.

In response to the signs of eutrophication Germany, Austria and Swizzerland in 1961 signed a contract on the Purity of Lake Constance. The newly established International Commission for Water Protection of Lake Constance then forced the construction of new or extension of already existing sewage treatment plants to provide phosphate removal throughout the catchment. About 6000 M Swiss Francs had to be spent for this purpose.In addition, the phosphorus content of detergents was reduced or totally restricted in Germany and Swizzerland. As a consquence, the PO₄-P content of the lake water decreased from a maximum of 93 µg/l in 1980 to only 11 µg/l in 1999 (Fig. 1B).

The present phosphorus concentration, indicating oligotrophic conditions, is about the same as some 40 years ago.

The phosphorus concentration in the lake sediments follows more or less the P-concentration curve of the lake water Müller (1997b).

Close relationship also exists between P-concentration in the water body/ biomass production / carbonate precipitation and ist accumulation in the sediment.

HEAVY METALS AND OTHER CONTAMINANTS IN THE SEDIMENTS

In his benchmark publication „Sedimente als Ausdruck des Zustandes eines Gewässers“ (Sediments as indicators of the condition of an aquatic system), published in 1956, the Swiss limnologist Hans Züllig opened the door for a new insight into the role of aquatic sytems as sinks and sources of inorganic (e.g heavy metals, radionuclides, nutrients) and organic (e.g. polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins and furans, large groups of pesticides, such as DDT) contaminants.

The study of dated sediments not only permits characterization of the present condition of an aquatic system but also enables the historical reconstruction of the contamination of a water body.

Fig 1C represents results of an investigation carried out in Sept. 1995 of selected heavy metals in a sediment core collected in the central and deepest part of Lake Constance from a water depth of 244 m (Müller, 1997a) The sediment layers consist of fine grained mud with a median grain size of < 20µm.

With the exception of the uppermost 2.5 cm reddish-brown sediment layer, all deeper sediments are grayish to grayish black.

Analyses were carried out after a digestion with aqua regiae with flame- and flameless AAS.

It is clearly to be recognized, that within the past 100-105 years the concentration of lead, zinc, cadmium, and silver follow a distinct pattern: in 1890, at the beginning of the „industrial revolution“ in the lake area, the metals have concentrations representing their natural geogenic background, which is also found in the deeper and much older sediments.

Due to the growing influence of industrialisation and groth of population, the anthropogenic component grew until the maximum concentration was reached in 1961 for lead, 1963 for zinc, 1969 for cadmium,and 1970 for silver. The corresponding enrichment facctors are 5.2 - 3.7- 3.4 and 6.1, respectively.

After their maxima in the 1960s, the concentrations of the metals decreased sharply and reached their lowest contents in the youngest srediment layer, with concentrations being about the same as one hundred years ago.

The historical development of selected PAHs, Fig. 1D (data from Hagenmaier, published in Müller (1988), between 1915 and 1985 shows the up and down even more impressive.

Fig. 1D depicts data of dioxins and furans in cores collected 1995 and 1996 (Hagenmaier and Walczok,1996) with maxima around 1970 - 1975. All isomer patterns in the sediments after 1945 show „thermal“ distribution patterrns, whereas in the early sediments a pattern is found which at least for the furans resembles a „chlorine“ pattern.

CONCLUSIONS

There are clear indications that Lake Constance, within an incredibly short span of time, has once again become one of the cleanest lakes in Central Europe, meaning that it may serve as drinking water reservoir for many future generations.

However this success has only been made possible by a combined effort of the participating countries after a very early recognition of the perils arising from increasing phosphorus concentration in the lake water.

The complete network of sewage treatment plants provided with phosphate elimination facilities in the catchment area not only reduced the phosphorus input into the lake drastically, but is also responsible for the sharp decrease of other contaminants found in the sediments.

This runs parallel with a general reduction of coal use and - at the same time- the improvement of coal fired electricity plants with pressurized fluidized bed firing and the application of „clean energy techniques“.

From the results of their earlier studies on the geochemistry of Lake Constance sediments Müller et al. (1977) already concluded: „Coals are assumed to be the main source of heavy metal enrichment, incomplete combustion (pyrolysis) of coal also seems to be responsible for the very sharp increase of PAH“.

However, only after 1995 we know, that the group of coal-derived elements has to completed

with Silver (Ag).

REFERENCES

Hagenmaier, HP., Walczok, M.(1996), Organohalogen Compounds, vol. 28, 16.Symposium on Chlorinated Dioxins and Related Compounds, Amsterdam, August 12 - 16 1996, 101 - 104

Müller, G, (1988), In: Gefährdung und Schutz von Gewässern (Kohler,H., Rahmann H.editors) Hohenheim

19 - 34.

Müller, G. (1997a),Naturwissenschaften 84,37

Müller, G.(1997b), In: Geochemie und Umwelt (Matschullat, J., Tobschall, H.J., Voigt, H.-J. editors, Berlin Springer , 317 - 341

Müller, G.(1999), Extended abstract, 6 th Internat. Argentum Conference, Madison, USA August 1999.

Müller, G., Grimmer, G., Böhnke, H. (1977), Naturwissenschaften 64, 427 - 431

Stabel, H.H., Kleiner, J.(1995), gwf Wasser-Abwasser 136, 601 - 607

Vollenweider,R.A. (1979), gwf Zeitschr. Wasser Abwasserforsch. 12, 46 - 66

Wagner, G. (1976), Arch. Hydrobiol. 78, 1 - 41

Züllig, H. (1956), Schweiz. Z. Hydrol. 47, 5 -143

Captions for Fig. 1

1 A Lake Constance in the centre of its drainage area

1 B Soluble reactive phosphate (P-PO₄) in the lake water. From Stabel & Kleiner (1995).

2000 content 9 µg/l (Dr. Stabel, oral contribution)

1 C Heavy metals in the 1995 core (Müller 1947a, 1999)

1 D Selected PAHs in sediments within a time frame 1915 - 1985 (Hagenmaier in Müller,1988)

1 D Dioxins and furans in the sediments. Circles core 1994, squares core 1995 (Hagenmaier % Walczok,1996