Natural attenuation (abbr. NA) refers to processes in environmental compartments that occur under natural conditions and lead to the reduction and/or breakdown of impurities present. It involves microbiological, chemical and physical processes. Such processes may be selectively used and accelerated (enhanced) for the decontamination of soil and groundwater. In such a case the term of reference is "enhanced natural attenuation" (ENA). If the natural sequence of processes is selectively monitored, one refers to "monitored natural attenuation" (MNA).