Monitored natural attenuation of contaminants in the subsurface: Processes

Among the alternatives considered for the remediation of soil and ground wa ter at hazardous wastes sites are the use of natural processes to reduce or remove the contaminants of concern. Under favorable conditions, the use of natural attenuation can result in significant cost savings and compensate for uncertainties encountered in complex subsurface settings. In order to d emonstrate that natural processes are effective in reaching established goa ls, it is necessary to determine that transformation processes are taking p lace at a rate which is protective of human health and the environment, and that these processes will continue for an acceptable period of time. While chemical transformation, dispersion, dilution, sorption, and volatili zation are discussed, aerobic and anaerobic degradation comprise the major processes for the reduction of contaminant mass in the subsurface. In discu ssing the mechanisms of natural attenuation, chlorinated aliphatics and pet roleum hydrocarbons are used as examples because of their significant impac t on subsurface contamination and the effect of their physiochemical proper ties on attenuation processes.