In the waterways of central Stockholm compared to other waters

The investigation conducted by Lithner et al. (2003) provides a general idea about the degree of contamination with bioavailable trace metals in the inner waterways of the city of Stockholm, based on the analysis of zebra mussels (Dreissena polymorpha) exposed for 6 weeks to the local water quality in cages placed in the water column at a distance of 1 m above the sediment surface. It should be clearly recognized that the mussels were exposed to an unspecified mixture of water with suspended solids originating from current (autumn 2001) effluents, drainage water and natural runoff from the catchment, together with various contributions of "older material", first deposited in the sediments of each of the investigated sites and then - to a smaller or greater extent - resuspended into the water column during the period of exposure of the mussels. According to Broman et al. (2001), it is reasonable to assume, on the basis of earlier estimates, that roughly about 50% of the material settling to a sedimentation trap in the eastern part of Lake Malaren during a given period is in reality resuspended bottom sediment. The sedimentation traps are usually placed at a greater distance from the sediment surface than the baskets containing mussels in Lithner's et al. (2003) investigation. As a consequence, the mussels in the baskets are exposed to suspended particles in the water originating to at least 50% (probably more) from previously settled material.

The resulting levels of contamination of the mussels with metals, representing a mixture of present-day and historical metal loading, has been compared with the metal loading of zebra mussels from some other water bodies in the world (Lithner et al., 2003), see Table 4.5.

Table 4.5. Concentrations of some metals (^g/g DM )in soft tissues of Dreissena polymorpha after exposure for 6 weeks in central Stockholm compared to metal concentration in mussels caught in some other lakes and rivers. Ref. data compiled by Lithner et al., 2003.

Water body

Pb

Cr

Cu

Ni

Zn

Central Stockholm

0.2-0.9

0.7-5.9

10-18

25-47

170-380

Great Lakes, N.America

0.8-4.6

4.2-10

14-34

18-33

98-160

(15th and 85th percentiles)

Lake Ontario, S. part

1.8-6.5

3.3-5.3

5-26

5-13

160-310

Lake Geneva

0.5-1.2

0.8-3.2

16-42

20-45

120-190

Subalpine lakes, Italy

1.5-6.1

1.9-5.3

11-36

9-26

130-370

River Moselle

4-20

1-4

22-50

20-45

120-240

River Po estuary

2.3-3.9

3.5

18-21

14-16

130-160

As mentioned above, it should be remembered that the study of the level of contamination by road traffic-generated bioavailable trace metals in the waters of central Stockholm, carried out by Lithner et al. (2003), does not specifically describe the current situation, as it was in the autumn of 2001, but some mixed, partly historical situation, influenced to an unknown degree by older, most probably greater, discharges of metals and organic pollutants. These pollutants were incorporated in the bottom sediments over several years. Later on, the contaminated particles may have been resuspended in the water column and absorbed by the mussels transplanted to the site in question.

Taking this into consideration, it is interesting to note that - according to the data in Table 4.5 - the road traffic origin of the studied metals, as shown by the low concentration range of the traffic indicator lead, obviously is not confirmed. As a matter of fact, the lead concentrations found in the mussels, exposed to water and suspended solids in central Stockholm waterways, are the lowest in all of the identified water bodies where metal data on zebra mussels were given.

Chromium in mussels exposed in Stockholm showed a relatively broad concentration range, rather much corresponding to the levels found in lakes in southern Europe and in the southern part of Lake Ontario, but the Swedish data were lower than what was recorded in the Great Lakes over the period 1992-98 (O'Connor, 2002).

The levels of nickel in Stockholm tended to be on the higher side, compared to most other areas shown in Table 4.5 and, although they showed good correspondence with the data from Lake Geneva and River Moselle, they were clearly higher than in the Great Lakes in North America and in northern Italy. Lithner et al. (2003) explains the high nickel values in the mussels in Lake Malaren by geological factors (Ni-rich glacial and postglacial clays in the region).

Copper concentrations in the Stockholm material of mussels were low, compared to almost all the other water bodies, with the exception of some very low values in North American lakes. Even a comparison with the 15th percentile of the mussel data from the Great Lakes survey, 14 ^g/g DM, supports the conclusion that copper concentrations were not enhanced in the mussels exposed in the central Stockholm area, in spite of the partial impacts of older emissions of copper from industrial facilities and STPs in central Stockholm. The relatively narrow concentration ranges for copper in mussels from most of the shown water bodies and, in particular, the rather uniform upper levels, is also an indication of the capacity of the mussels to regulate copper accumulation in their soft tissues.

On the other hand, zebra mussels may have a weaker capacity to regulate their body burdens of zinc, as shown by the relatively broad concentration range and by the strong positive relationship between total zinc in water and zinc in mussel tissue (Lithner et al., 2003). These authors assume that the high zinc concentrations in the mussels exposed in eastern Lake Malaren is due to the high levels of zinc in sediments and suspended matter in this part of the Stockholm aquatic environment. This is most likely an effect of historical zinc emissions from industrial sources as well as zinc runoff from unpainted galvanized metal constructions.

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