Fundamental properties of the selected metals

Metals in natural waters rather seldom occur as belonging to one of several well defined, distinct categories, e.g. dissolved, colloidal or particulate phases. They rather occur as intermediates between or mixtures of these categories, depending on a variety of physical, chemical and biological factors. But in most cases, only total concentrations are still measured, which means the sum of all these differently acting phases. Moreover, one should keep in mind that every measurement in the field or laboratory is just a snapshot of the dynamic reality, governing the complex interaction between metals and the environment.

Just to illustrate the difficulties met by even leading scientists today in their attempt to increasingly consider speciation aspects in monitoring and assessing the impact of metals, the "1995 Nordic Lake Survey", a remarkably comprehensive field survey including Norway, Sweden and

Finland may be mentioned here (Lydersen et al. 2002). One main objective of this study was to investigate the distribution of trace metals in lakes in relation to environmental factors, like DOC and pH (Lydersen and Lofgren 2002). Although lacking own speciation data, the authors succeeded to demonstrate that a sound evaluation and interpretation of the bulk analytical results without knowing the particular speciation of the selected trace elements (Zn, Cd, Cu, Pb, Ni, Cr, As, Al) is hardly possible. When discussing the outcome in the light of what we already know on metal speciation, the authors emphasize that their conclusions and the implications of the generated data should be treated with great care, due to the lack of own chemical speciation data. As an example, the conclusion that remobilization of metals in limed lakes, after these systems may re-acidify, may be insignificant, was only supported by data on total metal concentrations, and has to be seen for this reason with some reservation. Also the reasoning that possible ecological risks associated with Cr, Cu, Fe, Mn and Zn seem to be very low and may be only in those exceptional cases, where water concentrations are above the lowest biological risk (LBRL), was based on the use of measured total concentrations (see Table 5.2).

Table 5.2. Percentage of Swedish and Norwegian lakes below the lowest biological risk levels given as total concentrations in ^g/l according to Norwegian and Swedish criteria (from Lydersen et al., 2002)

%

Element

country

LBRLa

Norway

Sweden

Sweden

Finland

non-limed

non-limed

limed

non-limed

Zn

Norway

50

99.8

99.6

99.6

99.8

Sweden

20

99.3

99.3

98.9

99.4

Cd

Norway

0.2

99.5

98.6

98.9

99.8

Sweden

0.1

96.6

95.5

97.7

99.4

Cu

Norway

3.0

99.3

98.4

99.6

99.1

Sweden

3.0

99.3

98.4

99.6

99.1

Pb

Norway

2.5

98.4

98.2

98.1

99.8

Sweden

1.0

91.5

93.0

93.5

98.3

Ni

Norway

5

99.8

99.6

99.6

98.9

Sweden

15

100

99.8

100

99.4

Cr

Norway

10

100

99.8

100

100

Sweden

5

99.9

99.6

100

100

As

Norway

Sweden

5

99.8

99.8

100

100

aLBRL=lowest biological risk level aLBRL=lowest biological risk level

In this context, we have to be aware that assessing the environmental risk of water-borne or sediment-associated trace metals without considering their chemical speciation is today no longer state-of-the-art and may ultimately lead to wrong assumptions, decisions and predictions concerning the fate and effect of potentially toxic metals.

Just to call back to mind the complexity of interactions between metals, site characteristics and biota controlling the behaviour (speciation, bioavailability and toxicity) of metals in natural systems (soils, sediments and waters), a short overview will be given here of what we already know in this context from field studies for the metals Cr, Cu, Ni, and Zn, which have been selected for this evaluation and update. The information was mainly taken from Lydersen et al. (2002).

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