General conclusions

One of the most popular subjects of investigation among environmental chemists and eco-toxicologists for at least three decades has been the behaviour and effects of trace metals in the environment. The upsurge of this research happened concomitantly with the rapid and widespread introduction and use of atomic absorption spectrometric (AAS)

methods. Over several decades, a huge amount of analytical data have accumulated describing total metal concentrations, e.g. of chromium, copper, nickel or zinc in water, in sediment or in soil samples (after extracting the target metal(s) in the sample). Now, most of the data has turned out to be of rather limited or no value for the understanding of trace metal interactions with biota. So, real advances in metal eco-toxicology were rather poor during that period.

It may be stated that the science of metal eco-toxicology did not really take off the ground until eco-toxicologists understood that trace metals, including the essential elements, occur in many different forms (species) in the environment and that only a few of these species, under certain circumstances, are bioavailable, i.e. can be taken up by a target plant, animal or micro-organism. Thus, it was not until people started to take a more sceptical attitude to the previously felt all-pervading blessings of the AAS and new analytical techniques were developed to consider metal speciation in environmental media that metal eco-toxicology entered its 'Golden Age'.

Today, when performing generic environmental risk assessments of trace metals (such as chromium, copper, nickel or zinc) or when evaluating the environmental impact of trace metal emissions in a defined geographical area, it is no longer acceptable to routinely start out from laboratory toxicity data and divide them by conventional safety (or application) factors to derive "predicted no-effect concentration". First, the natural background concentration of the relevant species of the assessed metal is usually fully considered. In case it is an essential element, it is state-of-the-art to consider the possibility of rectifying a situation of deficiency. An indispensable part of the assessment is to investigate (or predict) the potential distribution of bioavailable metal species under site-specific, realistic environmental conditions and to base this assessment - as far as possible - on long-term data from the field (rather than on laboratory data or short-term metal-spiking experiments), in order to take into account kinetic equilibria that may form in the real world and under realistic time scales. Since it is a highly dynamic habitat in which trace metals have to be assessed, direct measurements of the occurring metal species may only give a snapshot, and therefore, an erroneous picture of the true situation. Consequently, it may be more correct to base the assessment on a thorough knowledge of stability constants and ion activities of critical metal species, and on the use of mathematical models, such as the BLM, to calculate the potential risk of adverse effects, in view of possible, relevant combinations of site-specific environmental characteristics.

This page intentionally left blank

0 0

Post a comment