Monitored natural attenuation (MNA) is not a remediation technology per se. However, it is a viable and cost-effective option for managing the risks posed by contaminated groundwater subsequent to an active remediation scheme where naturally occurring processes in the subsurface can reduce the mass, toxicity, mobility, volume and concentration of contaminants to acceptable levels.
There are many physical, chemical, biological and geochemical parameters that may be used to demonstrate the presence of natural attenuation mechanisms. The process of selecting appropriate determinants for monitoring these mechanisms will depend upon the nature of the contaminant, the site's biochemical environment and the dominant processes under consideration.
Adoption of the MNA approach requires demonstration that natural processes are occurring at a rate that protects the wider environment and achieves remedial objectives within a reasonable time frame. Therefore, a consistent groundwater monitoring network and sampling plan is required. This should include monitoring for the contaminants of concern and its daughter products, and monitoring natural attenuation indicators such as dissolved manganese, dissolved iron, nitrate, sulphate, redox potential, carbon dioxide, dissolved methane, dissolved oxygen and pH.
In recent years, Environment Agency guidance coupled with a greater understanding of natural attenuation processes has led to a wider acceptance of the technique in isolation or conjuction with other remedial techniques.
MNA can provide a remedial solution to sites that have recently been remediated, or as a method in its own right.