Biological processes, including microbial degradation, have been identified as critical mechanisms for attenuating organic contaminants during transit through the vadose zone to the groundwater. On-site soil remedial measures using biological processes can reduce or eliminate groundwater contamination, thus reducing the need for extensive groundwater monitoring and treatment requirements. On-site remedial systems that utilize the soil as the treatment system accomplish treatment by using naturally occurring microorganisms to treat the contaminants. Treatment often may be enhanced by a variety of physical/chemical methods, such as fertilization, tilling, soil Ph adjustment, moisture control, etc. The development of a bioremediation program for a specific contaminated soil system includes: (1) a thorough site/soil/waste characterization; (2) treatability studies; and (3) design and implementation of the bioremediation plan. Biological remediation of soils contaminated with organic chemicals has been demonstrated to be an alternative treatment technology that can often meet the goal of achieving a permanent clean-up remedy at hazardous waste sites, as encouraged by the U.S. Environmental Protection Agency (U.S. EPA) for implementation of the Superfund Amendments and Reauthorization Act (SARA) of 1986. Bioremediation is especially promising if it is incorporated in a remediation plan that uses an integrated approach to the cleanup of the complete site, i.e., a plan that involves the concept of a "treatment train" of physical, chemical, and/or biological processes to address remediation of all sources of contaminants at the site.