Given the complexities and challenges of remedial efforts for contaminated sediments, a range of technology options are needed to improve sediment assessment and remediation. One such option for sediments is monitored natural recovery (MNR), which relies on natural processes to reduce ecological and human health risks to acceptable levels, while monitoring recovery over time to verify remedial success. At some sites, a combined approach can optimize remedy effectiveness by integrating MNR with capping or dredging, or with innovative technologies that promise to accelerate natural recovery processes. While MNR has been affirmed by several regulatory authorities, there remains some concern regarding exposure to contaminants remaining at a site and uncertainty regarding the time required for recovery. Greater guidance was needed to effectively evaluate and implement MNR at contaminated sediment sites to address these uncertainties and improve the cost-effectiveness of sediment remediation.
The objective of this project was to develop a technical guide for MNR at contaminated sediment sites to promote consistency among practitioners by developing lines of evidence to evaluate the site-specific suitability of MNR and in applying MNR to sediment sites.
Monitored natural recovery generally calls for a weight-of-evidence approach that includes such metrics as sediment deposition, reduced contaminant mobility, chemical or biological transformation, erosion and dispersion of particle-bound contaminants, and reduction in contaminant mobility and bioavailability to higher level trophic organisms. The technical guide discusses how to assess and validate the sufficiency of natural recovery processes to achieve acceptable risk reduction within a desired timeframe while presenting a framework for their evaluation in support of MNR. It provides a systematic approach for comparing MNR to other remedial options, incorporating a process-oriented method for using these lines of evidence to make inferences about exposure and risk reduction rates over time. Case studies and generic examples were included to demonstrate concepts at work in real-world situations.
The guidance also addressed costs associated with site characterization, interpretation, modeling, and assessment in support of MNR. Notably, MNR investigations can be very cost effective because information gained also supports the evaluation of more aggressive technologies such as capping, dredging, and using novel amendments. As in the case of terrestrial groundwater contamination where natural attenuation plays a role at virtually all petroleum and chlorinated solvent release sites, one can expect some form of natural recovery to be included in the mix of technologies used at most contaminated sediment sites.
The technical guide established the principles and evaluation criteria for a comprehensive and cost-effective evaluation of MNR as a remedial option. In addition, this guidance provides a formalized framework for properly designing and implementing MNR and for predicting long-term MNR performance and potential human health and ecological risk reductions, providing remedial project managers and site owners with improved tools for more cost-effective sediment characterization and remediation.
The primary audience of this technical guide for MNR at contaminated sediment sites are DoD project managers and management teams evaluating and implementing MNR at contaminated sediment sites. The technical guide also is intended for environmental scientists and engineers, private site owners, and regulators. However, MNR will be most successful when the regulatory community and stake holders are involved early in the remedy selection process and fully support the remedy decision.