Mixtures of fly ash and slugs were subjected to laboratory tests in order to estimate their influence on natural water and soil condition in order to check their possible application towards for recultivaton or landfilling. The chemical composition of the mixtures and their extracts obtained with solutions of sulphuric acid (pH 2 or 4) as eluent (simulating acid rain) were determined. Assuming the criteria to be met by drinking water specified by the European Union and WHO documents, it was established that the heavy metals content in the fly ash and slug mixtures is not expected to have a negative effect on underground water.