Advantages and Disadvantages

In comparison to conventional engineered remediation technologies, natural attenuation offers a number of advantages, especially if intrinsic bioremediation is occurring:

1. During intrinsic bioremediation, contaminants can be ultimately transformed to innocuous byproducts (e.g., carbon dioxide, ethene, chloride, and water in the case of chlorinated solvents; and carbon dioxide and water in the case of fuel hydrocarbons), not just transferred to another phase or location within the environment;
   
   
2. Natural attenuation is non-intrusive and allows continuing use of infrastructure during remediation;
   
   
3. Natural attenuation does not involve generation or transfer of wastes;
   
   
4. Natural attenuation is often less costly than other currently available remediation technologies;
   
   
5. Natural attenuation can be used in conjunction with, or as a follow-up to, other intrusive remedial measures;
   
   
6. Natural attenuation is not subject to limitations imposed by the use of mechanized remediation equipment (e.g., no equipment downtime).
   

Natural attenuation has the following potential limitations:

1. Time frames for complete remediation may be long;
   
   
2. Responsibility must be assumed for long-term monitoring and its associated cost, and the implementation of institutional controls;
   
   
3. Natural attenuation is subject to natural and anthropogenic changes in local hydrogeologic conditions, including changes in groundwater flow
   direction or velocity, electron acceptor and donor concentrations, and potential future releases;
   
   
4. The hydrologic and geochemical conditions amenable to natural attenuation are likely to change over time and could result in renewed mobility of previously stabilized contaminants (e.g., manganese and arsenic) and may adversely impact remedial effectiveness;
   
   
5. Aquifer heterogeneity may complicate site characterization, as it will with any remedial approach; and
   
   
6. Intermediate products of biodegradation (e.g., vinyl chloride [VC]) can be more toxic than the original compound (e.g., trichloroethene [TCE])

Regulatory Issues

Starting in 1995, EPA and many states began to develop policies regarding the use of natural attenuation for groundwater remediation. This process is still on-going, and the following discussion provides only a brief snapshot of developing regulatory policy.

The regulatory community was initially very skeptical of monitored natural attenuation, often viewing it as the do-nothing alternative. Regulators were unwilling to accept a perceived risk of not implementing a highly engineered remediation process in lieu of monitored natural attenuation. However, as evidence accumulated of the successful implementation of natural attenuation, the Air Force in conjunction with the Robert S. Kerr Laboratory of the US EPA was able to demonstrate that monitored natural attenuation can be protective of human health and the environment. This helped to eradicate the regulatory barriers to implementation. As a result, monitored natural attenuation is now viewed as an integral part (or all) of a groundwater remediation process when supported by the field evidence.

Community Acceptance

The U. S. EPA Directive 9200.4-17 USEPA (1999) provides the following guidelines regarding public acceptance of the monitored natural attenuation.

• Remedial actions in general should include opportunity(ies) for public involvement that serve to both educate interested parties and to solicit feedback concerning the decision making process.
• Whether a particular remediation timeframe is appropriate and reasonable for a given site is determined by balancing tradeoffs among many factors which include public acceptance of the timeframe required to reach remediation objectives.

Natural attenuation requires that the proponent support that this alternative is sufficiently protective of human health and the environment. Thus, a strong effort must be made to cultivate a level of public confidence in the natural attenuation treatment option at each site. This will involve a combination of education regarding natural attenuation processes and the compilation of site-specific evidence in support of this option.