Prior to the early 1980s, there was a common view that once contaminated, an aquifer was lost as a water resource for the foreseeable future. In response to highly publicized instances of groundwater contamination, such as the Love Canal, and a growing awareness of the high incidence of chlorinated organic chemicals in groundwater supplies, in 1980 the US Congress passed the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA, or better known as Superfund), and in 1984, amended the Resource Conservation and Recovery Act (RCRA), in both cases, legislating the cleanup of contaminated groundwater. A new industry was spawned: groundwater remediation.

With little experience and technical background, it is not surprising that hydrologists and engineers brought their traditional knowledge to bear on this new challenge. In particular, the primary method of groundwater remediation was to pump the contaminated water to the ground surface, remove the contaminants by some conventional treatment process and discharge the water (pump-and-treat). By the late 1980s, with few sites cleaned up, much longer times required for cleanup than predicted and escalating costs, one might have expected a radically different approach to remediation to be welcomed with open arms.

The use of granular iron, in a permeable reactive barrier (PRB) configuration, for removal of chlorinated organic contaminants, was first proposed in the early 1990s. The initial submission to a refereed journal was rejected and the proposed approach was generally greeted with a high degree of suspicion and scepticism. Nevertheless, through the interest of the scientific community and the results of early field demonstrations, by the mid 1990s, the technology had gained considerable credibility. Currently more than 100 PRBs containing granular iron have been installed for removal of chlorinated organic contaminants from groundwater, and perhaps an equal number containing other media have been installed for removing other contaminants. These numbers are commonly cited as evidence of the great success and acceptance of the PRB technology. On the other hand, relative to the magnitude of the problem, with contaminated sites numbering many hundreds of thousands, the influence of the technology on global environmental quality is, as yet, little more than miniscule.

The contributions in this volume, and the conference from which they were derived, provide considerable insight concerning the evolution of the PRB technology. First, it is clear that the technology has gained global interest and acceptance. Perhaps of greater importance, the nature of the questions being addressed has evolved from "if" to "how". Indeed, the reactions of iron with chlorinated solvents are generally accepted and attention has now turned to more effective means of applying the technology. Active areas of research reflected in this volume include alternative reactants for other types of contaminants, configurations and reactants suitable for mixed contaminant plumes, emplacement alternatives and long-term performances. With the maturation of the technology, as reflected in these topics, it appears certain that PRBs will continue as a significant remediation technology and that their stature and acceptance is likely to increase in the future.