development of the heavy metal-pollution in the sediments of the lower Neckar river during the past 25 years

A. Yahya1, B. Pilz2

¹Institute of Environmental Geochemistry, University of Heidelberg, INF 236, D- 69120 Heidelberg

e-mail: ayahya@ugc.uni-heidelberg.de

²Institute of Geography, University of Heidelberg, INF 348, D-69120 Heidelberg

Abstract

In order to follow the development of one of Germany´s formerly most polluted rivers, the Neckar river, an investigation was carried out in January 1998. Thereby, the sediments of 14 dams between Lauffen and Mannheim-Feudenheim were examined for their present heavy metal content.

It was found that none of the highly contaminated "old sediments", which were deposited before the seventies and eighties, were still exposed at any of the sites. Nevertheless, a slight increase in the concentration of Fe, Mn, Cd, Cu and Zn was registered at some dams since the last investigation in 1990. As explanation, the erosion of contaminated "old sediments" at a location further upstream may be a reason for this increase. As no further decrease in the concentration of heavy metals (except for the elements Cr and Pb) was observed, it might be presumed that the present low heavy metal content reflects pollution derived from non-point sources of a densely populated and industrialized region.

Introduction

The investigation of aquatic sediments has become a major topic in the research of aquatic systems, as they reflect the current quality of the system and provide information about the impact of human activities. Since the industrial revolution human activities (urbanization, industrialization, mining, agriculture, etc.) led to an increased input of heavy metals into the environment and an accumulation in the nearby river systems. Here, the organic and inorganic contaminants were absorbed at the surface of sediment particles and deposited in areas with low velocity rates such as dams.

Since the seventies many investigations (Banat et al.1972, Förstner & Müller 1974, Müller 1997, 1980, 1991, Pilz 1999) were carried out in order to assess the pollution level of heavy metals in the sediments of the Neckar river, in Baden-Württemberg, Germany. Thereby the first investigation in 1972 revealed, that the sediments of the Neckar river were heavily contaminated, especially with Cd, Cr and Ni.

This high degree of pollution led to the installation of many sewage treatment plants in the seventies. These reduced the amount of heavily contaminated, harmful substances discharged into the river system and led to a continuous decrease in the heavy metal concentration of the sediments.

Although there is a continuous decrease in the input of heavy metals in the Neckar river, older, high contaminated sediments remain in the dams as a danger source of heavy metal pollution. There is always a chance that previously deposited heavy metals can be remobilized, e.g. through the erosion of sediments during flooding events or by the excavation of the navigation ways. Remobilization of older sediments by flooding was already detected in 1994 at the dam of Lauffen. Therefore, regular observations are still necessary.

For this reason and in order to follow the development of the heavy metal pollution of the sediments of the lower Neckar river, an investigation of the recent heavy metal concentrations between the dams of Lauffen and Mannheim-Feudenheim was carried out in January 1998 (Fig. 1).

Materials and Methods

The sediments were collected at 14 dams along the last 137 km of the Neckar River using a van Veen grab sampler. In order to be able to compare the results with those of previous investigations and with the "average shale" value of Turekian & Wedepohl (1961), which serves as the pre-civilizational "geogenic background" for fine grained sediments, the fractions < 20 µm and < 2 µm were examined. The sediment particles <20 µm were separated by sieve analysis, the particles < 2 µm by repeated suspension (in distilled water) and settlement cycles using an "Atterberg-cylinder".

The heavy metals were dissolved using a mix of HCl and HNO₃ (3:1), their contractions were measured by flame- and flameless AAS.

Fig. 1: Study area. The Neckar river between Lauffen and Feudenheim

Results and Discussion

The results of the investigation in 1998 (Pilz, 1999) can be seen in table 1. As expected, the concentrations in the samples with particles < 2 µm are higher than those in particles < 20 µm.

A closer look at the locations where the highest and lowest heavy metal concentrations were found reveals, that the highest concentrations of heavy metals with typical anthropogenic background such as Cd, Hg, Cu, Cr and Zn were located in the highest industrialized regions of the investigated river section.

[%]

[mg/kg]

x < 20µm

mean < 2 µm

3.0

4.0

828

987

13,2

17,2

0,98

1,43

240

480

107

320

463

min. < 20 µm

< 2 µm

2.7

3.5

492

541

11,1

14,0

0,62

0,89

238

361

260

343

max. < 20 µm

< 2 µm

3.2

4.2

1004

1325

17,1

20,4

1,7

2,09

353

788

100

127

399

634

s < 20 µm

< 2 µm

0.11

0.14

125

240

1,47

1,88

0,28

0,35

Tab. 1: Statistical parameters of heavy metals in the sediments of the dams between Lauffen and Mannheim-Feudenheim

The comparison of the average metal concentration of the 1998 sediments (fraction <2 mm, fraction <20µm) with those of sediments from earlier investigations (1972, 1979, 1985, 1988, 1990), revealed that the concentrations of Cd and Hg were found within the range of the 1990 concentration levels. Therefore, the continuous reduction of the Cd and Hg content since the mid seventies has now come to an end. In contrast, the elements Cr an Pb still show a further reduction in their concentrations.

Elements which are not typically influenced by human activity like Fe, Ni, Co and Mn showed an increased concentration since the last investigation in 1990. The average concentration of Fe for example increased from 2.8% to 3.95%. A reason for that can be the higher content of O₂ in the water during the winter time. In 1990 the samples were taken during the winter time, while in 1990 the samples were take during the summer time. The increase in the concentrations of Ni and Co may be linked to this seasonal increase of Fe because both elements are absorbed at the surface of iron and manganese oxides. Therefore, the solution of iron and manganese oxides in the anaerobic zones, their transport to the surface and their precipitation at the O₂-rich water-sediment border may also lead to an increase in the content of Ni and Co.

The most significant increase of concentration can be seen by the elements of Cu and Zn. A closer look at the longitudinal concentration profile of these elements reveals, that the highest concentrations and increases can be found in the more populated and industrialized regions of the study area.

Fig. 2: Development of the heavy metal concentration in the sediments of the lower Neckar river between Lauffen and Feudenheim from 1979-1998. Concentrations in mg/kg.

Of special interest was the sharp increase of the content of Zn in the sediments of the dam of Mannheim-Feudenheim compared with the result of the 1990 investigation. The concentration rose from an average of 186 mg/kg in 1990 to an average of 634 mg/kg in 1998. In addition the content of Cd and Mn was doubled. As explanation might be a new point source of pollution or the erosion of older, highly contaminated sediments at this location.

Further research should also be carried out at the dam of Kochendorf, where an increase for the elements of Cu, Pb and Zn was observed at this location.

Conclusions

The investigations in 1998 revealed a further reduction of point sources and a much more balanced longitudinal profile of the heavy metal concentration in the sediments of the Neckar river. As no further decrease in the average heavy metal concentration was detected in the 1998 survey (Fig. 2), except for the elements Cr and Pb, one can presume that the recent heavy

metal content possibly represents the minimum of pollution of a densely populated and industrialized region.

This fact does not preclude the merit of future investigations of the sediments at dams along the Neckar river. Only by taking a periodic look at the site of each dam can one detect possible increases in the concentration of some heavy metals there. Although no direct exposure of older, more contaminated sediments was found during the investigation in 1998, the erosion and deposition of contaminated "old sediments" at a location further up the river system may have led to an increase of the heavy metal concentration at some dams of the Neckar.

Special interest should be put on the further research of the heavy metal content of the sediments of the dams of Mannheim-Feudenheim and Kochendorf, where the concentrations of Cd, Mn, Zn and Cu, Pb, Zn increased.

References

Banat,K., Förstner, U., Müller, G. (1972), Naturwissenschaften 59, 525-528

Förstner, U. & Müller, G. (1974), Schwermetalle in Flüssen und Seen. Springer Verlag Heidelberg, 203 pp.

Müller, G. (1979), Umschau in Wissenschaft und Technik 79, pp. 778-783.

Müller, G. (1980), Naturwissenschaften 67, pp. 308-309.

Müller, G. (1991), IN: Müller, G. (Hrsg.): 3. Neckar-Umwelt-Symposium 7.-8. Oktober 1991. Heidelberger Geowiss. Abh. 48. pp. 1-15.

Pilz, B. (1999), Diplomarbeit, Heidelberg University, 118 pp.

Turekian, K. & Wedephol, K.H. (1961), Bull. Geol. Soc. Am. 72, pp. 175-192.