Concentrations and distributions of heavy metals in naturally growing vegetation at different distances from a zinc smelter.

Hilde-Gunn Jensen, Torill Eidhammer Sjøbakk, Eiliv Steinnes (Department of Chemistry, Norwegian University of Science and Technology, N-7491Trondheim, Norway).

E-mail: Hilde-Gunn.Jensen@fim.ntnu.no

Abstract

Concentrations of mercury, cadmium and zinc in six naturally growing plant species and corresponding surface soil were studied in Southwestern Norway at different distances from a zinc smelter. The metal concentrations in surface soil and in the moss Hylocomium splendens indicate that the heavy metals mainly originate from atmospheric emission from the smelter. Metal concentrations varied between different plant species and also between stem and leaf within the same species. Some of these variations in concentration levels may indicate different behavior of the metals depending on interaction between the metals within the plant. Correlations found between metal concentrations in the plant species indicate atmospheric deposition being the major pathway in which the metals are transferred to the plants.

Introduction

Previous studies of heavy metals in the biomonitor moss Hylocomium splendens, indicated atmospheric emission of zinc, mercury and cadmium from a zinc smelter in Odda, Southwestern Norway (Steinnes et al., 1995). Hylocomium splendens receives its nutrition mainly from the atmosphere and thus reflects atmospheric deposition well (Rühling and Tyler, 1984). Surface soil rich in organic matter retains zinc and cadmium for many years (Tyler, 1978), and is therefore a suitable indicator of atmospheric deposition (Steinnes et al., 1989). This is probably also true for mercury which binds strongly to humic substances.

Plants apparently have a higher affinity for elements essential to them, such as zinc, than for non-essential elements such as cadmium and mercury (Boardman and McGuire, 1990).When considering transport within the plant from root to stem zinc is relatively mobile, cadmium slightly mobile and mercury near immobile (Lindqvist, 1991; Cataldo et al., 1983; Boardman and McGuire, 1990). Thus, given solely root uptake of the metals no strong correlations between concentrations of the three metals should be expected.

Outline of study and methods

Sampling was carried out at different distances from the zinc smelter during June 1995. More than half of the localities were chosen at distances less than 5 km from the smelter, while the others were between 5 and 35 km from the smelter. The localities, all but two, were situated in the south-north bound line marking the dominant wind-direction in the area. Sub-samples of soil and vegetation were collected at five plots within an area of about 500 m for each locality. The plant species selected are shown in figure 2. The zinc and cadmium concentrations in soil and plant material were determined by atomic absorption spectrometry after nitric acid digestion. The mercury concentrations were determined by atomic fluorescence spectrometry after nitric acid digestion and stabilization by bromine. In addition the organic matter content in surface soil was estimated by loss-on-ignition.

Results and discussion

As shown in figure 1, concentrations of metals in surface soil generally decrease with increasing distance to the smelter, confirming the previous indications of atmospheric emissions of Cd, Zn and Hg from the zinc smelter. Concentrations recorded in the moss Hylocomium splendens indicate the same trend, although the pattern is not quite as regular as for surface soil.

Figure 1: Metal concentrations in surface soil at different distances from the smelter.

Metal concentrations varied between different plant species and also between stem and leaf within the same species. Figure 2 shows mean values of metal concentrations in tissue from the plant species investigated. In general the concentrations are higher in stem tissue than in leaf tissue. The exceptions may be explained by the impact of direct atmospheric deposition as a major pathway for the entrance of the metals into the plants.

The high mercury concentrations in the needles of Pinus sylvestris are probably partly due the relatively long life of pine needles compared to the leaves of the other higher vegetation investigated. With the exception of Salix spp. and Pinus sylvestris, the significant differences in heavy metal concentrations between any two plant species were found to be the same for mercury, cadmium, and zinc. This is not in accordance with the fact that mercury is generally known to be immobile, cadmium slightly mobile and zinc relatively mobile within plants, and indicates that the observed concentrations reflect direct atmospheric deposition rather than root uptake. This assumption is further supported by high correlations evident between the metals in the investigated plant species.

References

1. Steinnes E, Berg T, Røyset O (1997). Atmospheric Deposition of Heavy Metals in Norway. National

Survey 1995. Oslo. Report 691/97. Norwegian State Pollution Control Authority (in Norwegian).

2. Rühling Å, Tyler G (1984). Water, Air and Soil Pollution, 22:173-180.

3. Tyler G (1978). Water, Air and Soil Pollution, 9:137-148.

4. Steinnes E, Solberg W, Petersen HM, Wren CD (1989). Water, Air and Soil Pollution, 45:207-218.

5. Boardman R, McGuire DO (1990), Forest Ecology and Management, 37:167-205.

6. Lindqvist O (1991) Mercury in the Swedish enviornment: recent research on causes, consequences and

corrective methods, Dordrecht, Kluwer Academic Publishers.

7. Cataldo DA, Garland TR, Wildung RE (1983). Plant Physiol., 73:844-848.

Figure 2: Average concentrations of metals in plant tissue.