When natural attenuation is shown to be protective of human health and the environment, and when a well-designed monitoring program is in place to document the efficiency of these processes, it can be a valuable component of site remediation strategies. In April 1999, the Office of Solid Waste and Emergency Response (OSWER) of the United States Environmental Protection Agency (USEPA) published its final policy on the use of natural attenuation, entitled Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites (USEPA, 1999). As implied by the title of this policy document, monitoring will be required to evaluate the long-term effectiveness of natural attenuation and to ensure protection of human health and the environment. According to the USEPA (1999), the monitoring program designed for each site should specify the location, frequency, and types of samples and measurements necessary to evaluate if the remedy is performing as expected, and if it is capable of attaining remediation objectives. In addition, all monitoring programs should be designed to accomplish the following goals: 1) Demonstrate that natural attenuation is occurring according to expectations; 2) Detect changes in environmental conditions (e.g., hydrogeologic, geochemical, microbiological, or other changes) that may reduce (or enhance) the efficacy of the natural attenuation processes; 3) Identify any potentially toxic and/or mobile transformation products; 4) Verify that the dissolved contaminant plume is not expanding; 5) Verify that there has been no unacceptable impact to downgradient receptors; 6) Detect new releases of contaminants to the environment that could create an unacceptable risk to receptors or impact the effectiveness of the natural attenuation remedy; 7) Demonstrate the efficacy of institutional controls that were put in place to protect potential receptors; and 8) Verify progress toward attainment of cleanup objectives. In addition to meeting all of these requirements, a site-specific contingency plan must be specified as a backup remedy in the event that natural attenuation fails to perform as anticipated. The Air Force Center for Engineering and the Environment (AFCEE), Technology Transfer Division's technical protocol for Designing Monitoring Programs to Effectively Evaluate the Performance of Natural Attenuation (AFCEE, 2000) describes how to effectively meet all of the USEPA's requirements for long-term monitoring.

According to the USEPA (1998b), there are three types of environmental monitoring including: 1) Site characterization monitoring (i.e., baseline monitoring) to describe the disposition of contamination and forecast its future behavior; 2) Validation monitoring to determine if predictions based on site characterization are accurate; and 3) Long-term monitoring to ensure that the behavior of the contaminant plume does not change.

The collection and interpretation of site characterization monitoring data for fuels and chlorinated solvents are described by Wiedemeier et al. (1995, 1997, and 1999) and USEPA (1998a). Data collection and interpretation during initial site characterization is probably the most important step in evaluating natural attenuation and in developing a viable and environmentally protective long-tem monitoring program. Without good site characterization monitoring, the design of a good long-term monitoring program is not possible. Validation monitoring consists of collecting the complete analytical suites specified by Wiedemeier et al. (1995, 1997, and 1999) and USEPA (1998a) - for one or two sampling rounds after completion of site characterization. Validation monitoring is used to ensure that the analytical results obtained from the baseline sampling events are accurate. Long-term monitoring involves collecting a subset of the parameters specified by Wiedemeier et al. (1995, 1997, and 1999) and USEPA (1998a). Ultimately the subset of parameters selected for analysis on an ongoing basis will be site-specific. The long-term monitoring protocol (AFCEE, 2000) describes how to effectively and efficiently specify the location, frequency, and types of samples and analyses required to meet the objectives of validation monitoring and long-term monitoring. In addition, guidance is provided on developing contingency remedies that will not adversely impact the natural biodegradation reactins occurring at a site, should engineered remediation be required.