Monitored natural attenuation of contaminants in the subsurface: Processes

Among the alternatives considered for the remediation of soil and ground water 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 demonstrate that natural processes are effective in reaching established goals, it is necessary to determine that transformation processes are taking place 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 volatilization are discussed, aerobic and anaerobic degradation comprise the major processes for the reduction of contaminant mass in the subsurface. In discussing the mechanisms of natural attenuation, chlorinated aliphatics and petroleum hydrocarbons are used as examples because of their significant impact on subsurface contamination and the effect of their physiochemical properties on attenuation processes.