As the use of natural attenuation processes for application to remediation of contaminated sites has been increasing, we need to ask whether the protocols, procedures, and capabilities for scientific evaluation of the effectiveness of natural attenuation processes have been appropriate and satisfactory. This concern is being driven by awareness of the fact that the use of monitored natural attenuation (MNA) as a remediation tool is a knowledge-based technique. In this chapter, we will be looking at reported performances of MNA at various sites, with a view to determining whether the stakeholders consider the technique to be successful and/or effective. Assessment procedures, monitoring protocols, modelling capabilities used for the reported field application cases, and field data in support of effectiveness (or otherwise) for natural attenuation as a process will be examined.

Many similarities exist between existing protocols and guidelines for NA, particularly concerning lines of evidence and data requirements. Very few consider the soil and soil gas in their protocols and most have been adapted for hydrocarbon and chlorinated solvent contamination and some for inorganic contaminants. The natural attenuation of many other contaminants has not been investigated extensively—such as PAHs, PCBs, and pesticides. Monitoring techniques will need to be substantially improved, particularly in the soil. The factors influencing the rate of NA of compounds that are not readily attenuated, including metals, chlorinated solvents, and high molecular weight organics will need to be determined. For monitoring, direct in situ indicators of bioremediation, particularly for NA (e.g., genetic markers, metabolic by-products, and techniques for characterizing the fate of contaminants, including degradation products) are being developed.