Design Guidance for Monitoring of Extended Spatial and Temporal Scale Plumes - 21035

Monitoring of passive groundwater remedies, such as monitored natural attenuation, is less frequent because plume behavior is driven by ambient conditions rather than the forcing conditions applied during active remediation. Passive remedies are generally expected to be conducted over reasonable time frames (∼30 to 50 years). However, the natural attenuation of long-lived radionuclides at U.S. Department of Energy sites may extend well beyond the 50-year time frame. Hence, passive remedy success depends on a monitoring approach that manages risk for long-duration plumes with no short-term exposure potential. In this document, a new monitoring paradigm is described for plume and source characteristics that warrants an extended-scale monitoring (ESM) approach. This includes guidance on sampling locations, frequency of sampling, and monitoring tool selection that focuses primarily on monitoring for exposure pathways using four contaminant scenarios. For the ESM approach, the primary monitoring objective is to demonstrate that the plume diminishes before reaching the receptor or point of compliance and that concentrations do not exceed compliance limits. This demonstration is initially accomplished through predictive assessments based on a conceptual site model (CSM). ESM identifies a controlled zone for the location of the plume where institutional controls restrict access during the remediation period. Fundamental criteria for applying ESM are a passive remediation approach, a remediation management duration of decades to hundreds of years, and conditions such that exposure pathways are effectively controlled during the long remediation period. ESM implementation includes verification of expected plume behavior. More importantly, regulators and stakeholders need to agree that the primary objective of the monitoring approach is to verify that plumes remain contained in areas where they meet protectiveness objectives for exposure pathways and ultimately reach remedial action objectives. Determination of the appropriate monitoring approach requires considering the site-specific characteristics, properties, receptor location, and contaminant conditions that are sensitive to plume behavior, plume remediation management timeframe, and risk. (authors)