Practising sustainable remediation of contaminated sites and groundwater requires a complete understanding of the real costs and benefits of action. A remedial option which is sustainable will result in a net improvement in human welfare, or a net benefit to society. This can be measured explicitly in economic terms by including all of the wider benefits of remediation (to the problem holder and to the rest of society), and all of the costs of remediation, including hidden external costs not normally considered in the remedial decision-making process. If a remedial option is economic in the widest social sense, then it is inherently sustainable, because its result is beneficial to all. Currently, the vast majority of remediation projects undertaken do not consider these wider benefits, and very few explicitly consider the external or hidden costs of remediation – costs which accrue to society as a result of remedial action, intended or unintended. This external damage could include any-thing from the off-gases produced by aeration-based water-treatment systems (such as air-strippers), the greenhouse gases produced during energy-intensive remediation, and the chlorinated organic compounds generated as daughter products during biodegradation, to the waste tipped in landfills during a typical ‘dig-and-dump’ remediation. In each case, the act of remediating the site has created a secondary effect, which in itself has some effect on the environment, or carries with it some potential for future damage. The costs to society of bearing these secondary effects and liabilities are considered ‘external’ to the problem holder’s own analysis, and consequently have hitherto not been considered in most ‘economic’ analyses. Indeed, the economic impacts of these ‘external costs of remediation’, and their implications for remedial decision-making, have rarely been considered in the literature, and can be considerable. However, a rigorous economic analysis of any remediation option is not complete without considering these oft-neglected external costs. Without considering these costs explicitly, there is every likelihood that the burden of these costs is being shifted unknowingly on to others. Achieving sustainability in remediation requires a like-for-like inclusion of these external costs in an overall economic analysis.

Key words: contaminated site management, cost–benefit analysis, decision support tools, environmental economics, remediation, sustainability