Water-soluble fraction of mercury, arsenic and other potentially toxic elements in highly contaminated sediments and soils.

The water-soluble contents of mercury, arsenic and other potentially toxic elements in highly contaminated sediment and soil samples from Portugal were determined. Mercury and arsenic concentrations were detectable and reproducible among replicate experiments. Despite the acidic pH, the low organic carbon content and the exceptionally high levels of contamination of certain samples (total mercury contents varied between 0.15 and 3180 mg kg(-1) while total arsenic concentrations ranged from 11 to 6365 mg kg(-1)), the water-soluble percentages of both mercury (<1.2%) and arsenic (<4.6%) were generally low. The variability of the water-soluble fractions of these two elements among these samples and at the occurring pH conditions seems not to be associated with the release of other potentially toxic elements. The highest water-soluble concentrations of the remaining potentially toxic elements were generally observed in the 15-25 cm depth layer of sediments from areas colonised with plants (Halimione portulacoides) and in mining soil samples. Zinc, cobalt, copper and cadmium showed the highest water-soluble percentages of elements in relation to total metal contents. Given the high contamination levels, the availability of potentially toxic elements in these areas as well as possible risks to the environment and humans should be further investigated. The presence of plants (H. portulacoides) appears to cause significant changes in the sediment matrix that increase the mobility of several potentially toxic elements, particularly in the 15-25 cm depth layer. The effects of vegetation on the fractionation of potentially toxic elements on these sediments should be further studied.