Solid waste: terminological and long-term environmental risk assessment problems exemplified in a power plant fly ash study

Legal definitions exert a significant impact on the waste management strategy. Waste that is technically suitable for recovery does not automatically become a raw material if there is no market for it, or its use is not commercially effective and, hence, they should be disposed of. The majority of disposed wastes, including recyclable waste, are not environmentally safe. Waste as a freshly generated anthropogenic material is not geochemically stable. Przezchlebie fly ash surface pond (Upper Silesia, Poland) in the post-closure stage was subject to field validation of the results of laboratory leaching/extraction tests and long-term column experiments on fly ash (FA) leaching behaviour under controlled conditions for environmental risk assessment. The study showed: (i) the possibility of a discontinuous non-linear time delayed increase of pollution potential of disused 'non-hazardous' large-volume waste in the dumping sites to the hazardous level; (ii) inconsistency of the laboratory leaching tests and the actual leaching behaviour of trace metals, particularly when equilibria conditions are dictated by kinetically determined reactions where the test results reflected entirely wash-out (I) and dissolution (II) phases, but did not comprise delayed release (III) phase; and (iii) necessity of life-cycle screening/monitoring of 'non-hazardous' dumping sites for contaminant release as a function of the primary (pH-Eh, ionic strength, ionic composition of solute) and secondary controlling factors (L/S-liquid to solid ratio, water flow conditions) along the vertical profile of an anthropogenic or natural vadose zone. These data are to be used to develop long-term predictive hydrogeochemical models and their field validation, and for providing an early warning and remedial actions with respect to the particular site. The formation of pH (and Eh) as a function of time-dependent (kinetically defined) processes appeared to be a key issue for a correct prediction of the leaching behaviour of waste. The presented case study on FA shows that waste, even those considered non-hazardous and fit for use in a commercially proven applications, should not be treated the same way as a natural raw material. To facilitate waste utilisation and disposal in environmentally safe way and to prioritise its use, reliable environmental risk assessment prediction models and testing procedures, as well as special enforcement strategy and regulations, should be developed with respect to waste and not 'materials.'