Introduction

In recent years it has become obvious how important knowledge of natural attenuation processes is with regard to the management of contaminated sites. In Germany, there are three reasons why these processes should be investigated in more detail:

First, more and more contaminated sites have been identified in Germany. Moreover, the size of known contaminated sites had to be increased, often in all three dimensions due to the use of new methods of site investigation
Secondly, national and international experience has shown that often conventional remediation technologies merely shift the problem (e.g. "dig and dump") or are not effective within reasonable periods of time (e.g. "pump-and-treat"). Additionally, the expenses of implementing these technologies often are in no relation to the actual value of the site.
Thirdly, new investigations have come up with evidence of intrinsic degradation and retention processes at a site being more effective than artificial processes induced by technical remediation measures. Among others, this is true for bioaugmentation: Specialised microorganisms are only effective in raising intrinsic degradation rates in the short term.

The objectives and legitimacy of the KORA funding priority are based on these three areas of experience. Naturally occurring degradation and retention processes in the subsurface are able to slow down the propagation of contaminants in soil and groundwater. In the plume they can, under favourable conditions, lead to a reduction in contaminant concentrations. The processes are summarised by the US EPA under the term of "natural attenuation". The long-term monitoring of intrinsic processes ("monitored natural attenuation" - MNA) is the US EPA’s prerequisite for the acceptance of natural attenuation as (part of) a remediation strategy. The transition from natural attenuation to active remediation measures is smooth, the US EPA defines the borderline by the exclusion of human intervention. Measures which are suitable for supporting intrinsic processes and enforcing their efficiency are known as "enhanced natural attenuation

From this, society’s reaction to natural attenuation results. Although the catch phrase "qualified idleness" now seems to be obsolete, technical criteria and legal evaluations are still lacking: Which processes are responsible for intrinsic retardation and degradation? How can they be detected and evaluated by predictions based on mathematical models? Evaluation refers to risk assessment, the necessity of remediation, and the possibility of integrating them in active remediation measures.