Richard Metcalfe and Christopher A . Rochelle Chemical containment of waste in the geosphere

The aim of this introductory paper is to highlight those underlying chemical principles that are common to all forms of waste management by geological means, and that rely to some extent upon chemical containment. Until recently, chemical processes were usually considered mainly because they can affect the physical performance of engineered containment systems. However, in recent years, many researchers have recognized that chemical processes themselves can offer containment to wastes. Thus, it is no longer possible to view physical and chemical containment processes separately. The containment system can be optimized only if both the engineered and natural barriers are considered together, and if the engineered barrier system is designed taking the features of the geosphere into account. However, there has been relatively little reliance upon the geosphere itself as a chemical barrier. It is concluded that the potential for chemical containment should be considered in all forms of geological waste management. Even if the chemical barrier function of the geosphere is not relied upon to meet safety targets, the confidence of regulators and public alike will be enhanced if it can be demonstrated that the geosphere at the site functions as a chemical barrier. Geological disposal is an option that is taken for many different kinds of waste, ranging from domestic refuse (e.g. Christenson et al. 1994; Department of the Environment 1990, 1995) to radioactive wastes (e.g. Savage 1995; Kass et al. 1997; Anon 1999). These diverse materials are usually contained using some anthropogenic, or 'engineered' barrier system that is constructed within the geosphere. The wastes are contained by a combination of both physical and chemical processes operating within both artificial and natural parts of this system. It is important to recognize the coupling between the chemical and physical containment processes when designing any particular waste containment strategy. It is also necessary to evaluate the interactions between processes operating within any engineered barrier system and processes operating within the geosphere beyond. The lack of this will lower the confidence in evaluating the temporal variation in risk arising from any waste. Consequently, at best, possible advantageous contributions to containment from chemical processes within engineered barriers and/or the geosphere may be ignored. Conversely, at worst, there may be unforeseen increases in risk at some time in the future, owing to changing chemical conditions. For example, both chemical and physical processes may be important in containing waste early in the life of a barrier. However, later in the life of the barrier, chemical conditions might change and cause the pollutants that have accumulated within the barrier to be released. The efficacy of the barrier will then depend mainly upon the physical processes restricting contaminant transport and rather less upon chemical processes. It is critical to the development of safe containment strategies that such coupling is understood. Such an understanding is essential if we are to prevent the chemical processes governing containment early in the life of a barrier from causing the later release of much greater concentrations of pollutants than otherwise would have been the case. However, until relatively recently, most workers concerned with waste management have tended to consider chemical processes primarily because they may affect the physical containment properties of engineered barrier systems. Several texts have examined these physical aspects of containment in considerable detail (e.g. Bentley 1996). Implicitly, there has been a tendency to view 'chemical containment' as an aspect of physical containment. For example, any collapse of expandable clay minerals, such as may be caused by interactions involving polar organic molecules, will affect the physical integrity of clay barriers (e.g. Bowders & Daniele 1987; Hettirachi et al. 1988). However, this view of containment is simplistic. In reality, chemical and physical processes must be considered holistically. For example, where clay is used to confine a waste, it should be considered as a physico-chemical barrier to contaminant migration (Horseman et al. 1996). viii R. METCALFE & C. A. ROCHELLE Furthermore, the term 'containment' means different things to different people. Many members of the public require reassurance that any waste will be rendered completely immobile within the disposal environment. However, the earth scientist may view 'containment' as 'highly restricted leakage' because, over a sufficiently long time, some contaminants will leak from any containment system around any waste form. Regulators tend to be influenced by both schools of thought and the particular regulatory framework usually depends upon the timescale for which the waste will be hazardous. For example, the management of domestic wastes typically considers timescales of the order of tens of years (Department of the Environment 1995). Over such time intervals the retention of waste within a tightly defined area is often seen as a technically feasible option~ Coupled with an increasing political and social aversion to the 'dilute and disperse' or 'dilute and attenuate' approaches to waste management (Stief 1989), this view has caused renewed emphasis to be placed upon active containment technologies, rather than passive containment strategies. By definition, chemical processes are important aspects of such active containment strategies. However, active containment implicitly focuses attention upon chemical processes within engineered barriers and rather less attention is given to chemical processes within the surrounding geosphere. At the other end of the spectrum is radioactive waste management, which considers time spans greater than 10 6 years (e.g. Nagra 1994; Kass et al. 1997; Nirex 1997). While many members of the public would certainly like to see these wastes contained indefinitely, regulators recognize that containment in the strict sense of the word is impossible over these timescales. Thus, relatively more attention is given to the migration of pollutants through the geosphere. These issues present several important questions to researchers, notably: 9 What are the most important chemical processes that can be relied upon to impart chemical containment in the management of different wastes? 9 Can we predict the contribution of chemical processes to containment? 9 If so, over what timescales can we make such predictions? 9 How do we acquire the relevant data with which to evaluate the efficacy of chemical containment? 9 To what extent should engineered barrier systems be relied upon to provide chemical containment and to what extent can the geosphere itself be used as a chemical containment barrier? 9 What generic lessons about chemical containment processes can be learnt by comparing the results from different waste management