Designing a Site Closure Strategy for Uranium in Groundwater and Surface Water - 19534
A site closure strategy was designed and mapped for residual uranium present in groundwater and surface water at the US DOE Office of Legacy Management (LM) Monticello Mill Tailings Site (MMTS) Operable Unit (OU) III, which is regulated under CERCLA. The MMTS is a former mill site that was used to process uranium and vanadium ore in southeastern Utah until 1960. OU III includes surface water in Montezuma Creek and groundwater present in a shallow alluvial aquifer along the creek valley for several kilometers downgradient of the former mill site. The current remedy consists of an operating pump-and-treat system, monitored natural attenuation (MNA), and institutional controls (ICs). Performance evaluations indicate that a prolonged restoration period is likely, as well as an asymptotic trend in uranium concentrations that still exceed the remediation goal. Multiple strategies and scenarios were evaluated before selecting a preferred site closure strategy. Recommendations were developed to support advancing the site towards closure over the next few years and guide the process of regulatory interactions and decision making at the MMTS OU III. Site closure is an important milestone that reflects successful remedial progress at LM sites. Post-closure site management would be maintained to ensure conditions are protective of human health and the environment. The site closure strategy was considered in the context of site remedial progress. The primary sources of groundwater contamination have been removed, including mill tailings and contaminated soils, sediment, and debris. Five-year reviews have regularly concluded that the remedy is protective of human health and environment and that ICs are effective in preventing exposure to contaminated groundwater. The affected aquifer is low-yielding and has no history of domestic use. In 2004, the final remedy selected in the Record of Decision (ROD) was MNA with ICs. However, a pump-and-treat system was installed and continues to operate as a contingency remedy in the area with the highest residual uranium concentrations. The need for a contingency pump-and-treat system was triggered by a departure of the long-term monitoring data from model projections that were used as the basis for remedy performance metrics specified in the ROD. In 2015, the pump-and-treat system was expanded. Average uranium concentrations have declined locally over time. However, monitoring data and column studies indicate that the system effectiveness will eventually become ineffective in reducing uranium concentrations, likely due to the reequilibration of uranium present in the solid matrix in various states (i.e., sorbed, precipitated). Currently, there are no agreed-upon metrics for discontinuing the contingency pump-and-treat system. Three closure strategies effectively applied at other CERCLA sites were considered at MMTS OU III: (1) restoring natural attenuation conditions and conducting MNA; (2) applying alternate concentration limits (ACLs); and (3) waiving a remedial goal based on technical impracticability (TI). Each closure strategy was evaluated in the following manner: (1) identifying the criteria and requirements associated with current regulations, policy, and guidance, (2) assessing the ability to meet those criteria based on the current site knowledge and the conceptual site model, and (3) identifying data gaps that need to be filled to further assess the closure strategy. Following this evaluation, a preferred closure strategy was selected: to transition to a remedy based on MNA and maintenance of ICs, without active remediation. Several scenarios for transitioning to the preferred closure strategy were identified. Each scenario consisted of 'if/then logic' associated with a larger decision diagram. The decision diagram considered the current conceptual site model (CSM) and potential outcomes of studies that will be conducted to fill data gaps, as well as the performance of the existing treatment system and long-term monitoring data. Two scenarios for transitioning to the preferred closure strategy were highlighted for future planning and were described in detail. For both scenarios, the preferred closure strategy includes demonstrating criteria for MNA acceptance, pump-and-treat termination, long-term monitoring, and eventual site delisting. Recommended actions over the next few years were also developed, including updating the CSM, conducting numerical modeling to predict remedial performance and time frames, conducting geochemical analyses to assess uranium fate and transport, and summarizing the lines of evidence and basis for MNA. This detailed example of the development of a site-specific closure strategy provides the thought process and decision logic used to assess uncertainty using an if/then framework; identify common elements and recommended actions to advance the site towards closure; and navigate the inherent uncertainty in the timing of future events, outcome of regulatory discussions, and remedial progress over time that is common to many complex US DOE facilities. In addition, the national regulations, policy, guidance, and state of practice regarding commonly used site closure strategies was summarized for consideration, including MNA, ACLs, and TI. (authors)