Results of Sequential Leaching

The applied leaching procedure [TESSIER et al., 1979] shows some lack of precision resulting from nonselective extraction, nonexactly definable phases in the environmental river compartment, and possible readsorption after the leaching steps [KHEBOLAN and BAUER, 1987 SAGER et al., 1990 TESSIER and CAMPBELL, 1990], No leaching sequence yet exists for definite discrimination of different metal species. 

Lead can be found almost exclusively in the easily-reducible and in the residual fraction (Fig. 8-11). Its main binding forms are obviously the adsorption on iron and manganese hydroxides and mineral binding. The large increase in the lead concentration from Elsterberg to Greiz is not characterized by an increase of an individual binding form. 

Zinc can be found in all investigated sediments, 25% in the carbonate fraction and 40% in the easily-reducible fraction (Fig. 8-12). Solely the content of the organic and resi- 

These investigations demonstrate that different anthropogenic and geogenic sources of the pollutant loads of river sediments cannot be unambiguously classified by investigation of the binding forms of heavy metals. 

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The results of sequential leaching experiments and the bioaccumulation studies reveal a selective assimilation of certain chemical forms of some trace metals by molluscs. There seems to be a relationship between seawater-soluble chromium levels and accumulation of the element in the kidneys of exposed bivalves. Particulate forms of chromium, while present in the digestive glands, do not appear to have been assimilated by the organisms, even though much of the total chromium was dissolved by a relatively mild extractant -25% acetic acid. Thus, an estimation of the bioavailability of particulate metals based on their solubility in dilute acetic acid can be erroneously high, at least for herbivorous filter feeders. 

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