Three geochemical concepts demonstrate the advantage of a long-term strategy compared to the traditional approaches in waste management: (i) the mobility concept, (ii) the concept of capacity controlling properties, and
(iii) the concept of final storage quality, the first being relevant for process studies, the second for effect evaluations and the third for problem solutions.
Mobility Concept. Among the criteria to assess which element or elemental species, beside its toxic potential, may be of major concern in ecological evaluations, one question deserved primary attention: "Is the element mobile in
geochemical processes, mainly because of its volatility or solubility in water, so that the effect of geochemical perbations can propagate through the environment?" Typically for systems involving solution/solid interactions, "mobility" is given by accelerating and retarding factors and processes.
Storage Capacity Controlling Properties. This conceptual approach has been developed in the framework of the concept of "chemical time bombs" (Stigliani, 1991). To make the scientific objectives clearer, it is useful to distinguish between two different mechanisms: The first is direct saturation, by which the capacity of a soil or sediment for toxic chemicals becomes exhausted. The second way to "trigger" a time bomb is through a fundamental change in a chemical property of the substrate that reduces its capacity to adsorb (or keep adsorbed) toxic materials.
Final Storage Quality (Baccini, 1989). Final storage properties can be achieved by using with typical "geochemical engineering" techniques such as: Selection of favourable milieu conditions for the deposition of large-volume wastes such as dredged materials, selection of additives for the solidification and stabilization of hazardous waste materials, optimization of elemental distribution at high-temperature processes, e.g. incineration of solid waste materials, and chemical leaching procedures.
Interlinkage of Controlling Parameters. Analytical and experimental procedures as well as technologies related to final storage can be deduced from Salomons' (1993) concept of interlinkage between capacity controlling parameters and major biogeochemical cycles: Gradients ("driving forces") typically involve degradation of organic matter, which in turn will affect changes in redox and pH-conditions as well as release of dissolved organic carbon and salt ions. Typical capacity controlling properties are buffer and sorption capacities. In the long term these parameters are not constant and are influenced by internal interactions (e.g., carbonate dissolution) and external factors, such as acid precipitation Not only are these capacity controlling properties not constant in the dynamic landfill system, but also the changes often exhibit a non-linear behaviour.
System descriptions include chemical-physical, solid phase and geochemical characterizations. Most common of the former are determinations of elution and desorption rates of critical pollutants from solid substances ("elutriate tests"), while diffusion studies, e.g., on solidified waste residues, are predominantly based on the experience with radioactive material. Geochemical investigations are mostly aimed for long-term prediction of typical borderline conditions, such as redox, acid producing and complexation potential. Key information on the behaviour of critical elements under changing pH-conditions - the master-variable for metal-related problems - can be gained from so-called "pH-stat"-experiments, where buffer capacity, acid production potential and metal release from a sample is simultaneously determined over a certain period of time. Regarding the solid phase major emphasis will be posed on the long-term capacities of minerals to carry and store critical pollutants, mainly metals, under changing environmental conditions and to form mineral barrier systems for solid waste disposal. In this respect, the presence of solid organic substances still pose considerable problems. Different from mineralogical systems, long-term interactions with pollutants cannot sufficiently be described, modelled and predicted. This is one of the reasons, why new regulations on landfilling will impose strict limitations on the content of organic matter. At the actual state of knowledge, this usually means application of thermal processes.
Baccini, P. (ed.), The landfill - reactor and final storage. (Springer Berlin, 1989).
Salomons, W., In Contaminated soil '93. (Arendt, F. et al., eds.) 225-238 (Kluwer Dordrecht, 1993).
Stigliani, W. M., Chemical time bombs. Definition, concepts and examples. IIASA Laxenburg (1991).