In Zacatecas, Mexico, four plants are operating to extract Ag, Au, and Hg using CaS₂O₃solution from surface soil containing tailings from the amalgamation method used during 1550–1900. The metal ions extracted are cemented by scrap Cu wires. Hg is separated by evaporation from the cemented amalgam and Ag and Au are obtained from the residue. A part of the soil to be leached was separated and leached as in the industrial process. Only 121 ppm of Hg was freed from 168 ppm of extractable Hg. About a half of the remaining Hg in the soil evaporated during 18 months. This confirms that the Hg in the soil is metallic. Pb and As are also freed in the same process. It is estimated that 13 000–34 000 t of Hg had been discarded in the extraction of Ag.

Six soil samples taken from three contaminated sites have been assessed using chemical, ecotoxicological and genotoxicological methods. From two of these sites, samples were characterised after remediation. An overall assessment of the soil samples was done with a scheme consisting of two phases. Remediation of main pollutants in these samples, did in general not lead to a risk reduction of the water-extractable ecotoxic and genotoxic potential. It was concluded, that the assessment and remediation of contaminated sites without consideration of biological test systems is not sufficient. Focussing on single hazardous compounds does in many cases not lead to a reduction of the ecotoxic or genotoxic potential. Consequently, effects of accompanying hazardous compounds, which are not routinely quantified, might be underestimated. It was pointed out, that the assessment of contaminated soil samples on the basis of LID-values applying dual dilution series seems to be sufficient for the evaluation, if there is no need for a quantitative comparison of toxic potentials.