Contaminant transport in the groundwater results in dilution of the contaminants. In a certain distance from the contaminant source, concentration ranges can be reached in which available nutrients and electron acceptors are sufficient to allow complete degradation of the residual contaminants, resulting in a steady state of contaminant spreading and degradation or even in shrinkage of the plume if the source is removed or exhausted. The use of this site feature as 'remediation' is possible only if the extent of the plume in time and space can be forecasted reliably. Furthermore, it is necessary that sensitive receptors (targets) are not affected.
An essential part of natural attenuation is detailed knowledge of the hydrogeology and the use of reactive groundwater modeling. The geological and hydrogeological parameters of the model (e.g., sorption coefficients, hydraulic conductivity, ground-
water flow velocity), as well as the biological degradation rates within individual redox zones, have to be determined by site-specific and/or laboratory investigations. These parameters and also the determined distribution of the contaminants are fed into the model. With the model the future extension of the plume can be estimated, allowing one to decide whether natural attenuation will be sufficiently effective. Experience has shown that natural attenuation is applicable in only a limited number of situations. However, it might be suitable as a post-treatment phase after active remediation. Natural attenuation already has been used to remove chlorinated ethenes, monoaromatic hydrocarbons (BTEX), and petroleum hydrocarbons .
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