Which conclusions from the study are justified

This quite large-scale study of metals and organic pollutants, conducted by Lithner et al. (2003) in Stockholm's inner waterways, has provided some results, which are scientifically reliable and represent new and important knowledge. Among these, it is pertinent to mention:

• A certain number of "new" metals (not previously studied as environmental metals) were confirmed as pollutants in Stockholm. The pollution was, in relative terms, dominated by tungsten, lead, zinc, antimony, copper, gold, silver and tin in the order mentioned.

• Strong positive co-variation was demonstrated to occur between lead, tungsten, zinc copper and antimony in water, which was interpreted as an indication of a common source, supposed to be related to road traffic. (However, the authors do not mention that there used to be an industry producing electric light bulbs, containing tungsten, close to the sampling and mussel exposure site called "Saltsjon".)

• Traffic pollution was furthermore assumed to be confirmed by the strong co-variation between tungsten, lead and certain PAHs in the transplanted zebra mussels. (However, the authors failed to explain how tungsten from the very stable tungsten carbide used in the tyre studs can be released and taken up by the mussels.)

• Since the authors pointed out that pollutant levels in living organisms reflect exposure better than do ambient concentrations in water and sediment, and that the presence in biota clearly shows that a toxicant is biologically available, one of the main objectives of the study was to use data on metals and organic pollutants in biota as a basis for an environmental risk assessment.

• The only (possibly) reliable data on Cr, Cu, Ni and Zn in biota presented was the data series showing levels of metals in zebra mussels transplanted to 9 different sites in the waters around Stockholm, where the mussels had been exposed to a mixture of present and historical pollution for 6 weeks before analysis.

• No enrichment of Ni and a very slight enrichment of Cu was noted in the mussles when moving from reference sites in Lake Malaren to the sites in central Stockholm, while the same comparison for Cr and Zn indicated a certain enrichment (4.3 times and 2.3 times increase, respectively).

• When comparing the body burden of metals in the mussels exposed in the brackish water downstream of Stockholm with the reference mussels exposed in Lake Malaren water upstream of Stockholm, threre was no clear indication of increased Cr, Cu or Ni contamination, and a very slight increase in Zn concentration, possibly caused by the combined emissions from Greater Stockholm.

• A comparison of the levels of Cr, Cu, Ni and Zn in transplanted zebra mussels in Stockholm with the levels of the same metals in some other, comparable, lakes and rivers in the world, leads to the conclusion that the Cr and Cu levels in the Stockholm material did not differ (or was lower) compared to the ranges obtained for the reference areas. Ni levels in Stockholm tended to be higher than elsewhere, explained by the high natural background for Ni in the region of Lake Malaren. Zn, finally, was slightly higher in the mussels exposed in Stockholm, compared to some - but not all - of the reference materials.

• The small enrichments of the here discussed metals in biota may not be directly related to any current emissions from road traffic - or any other current emissions from activities in Stockholm, because the mussels reflect the combined impact of current emissions and historical pollution, to which the mussels are exposed by means of sediment resuspension.

• Thus, although the study by Lithner et al. (2003) might be used to assess the environmental risk of W, Pb, Sb, Au, Ag and Sn, as well as of various PAHs and chlorinated organics, it is not scientifically justified to

164use the presented data for any risk assessment, nor any proposals on strategic measures, regarding Cr, Cu, Ni or Zn.

The authors have dedicated much effort to data presentation, discussion and conclusions based on the results of their attempts to calculate BAFs or on the whole section dealing with Asellus aquaticus and Chironomus plumosus. These parts of their report are not scientifically reliable and should be left without consideration.

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