Implementing an Alternative Modeling Strategy for Closure of the Rainier Mesa Corrective Action Unit at the Nevada National Security Site - 17481
The strategy to close the Rainier Mesa/Shoshone Mountain (RMSM) Corrective Action Unit (CAU) of the Underground Test Area (UGTA) Project at the Nevada National Security Site (NNSS) is defined under a joint agreement between the State of Nevada, the U.S. Department of Energy (DOE) and the Department of Defense (DoD). That strategy required a corrective action investigation (CAI) to define the evolution of contaminated volumes of groundwater under uncertainty as a means to define a surface-projected contaminant boundary and serve as a basis for longer term monitoring. As the initial phase of the CAI was nearing completion in 2012, concerns were raised as to whether the original Federal Facility Agreement and Consent Order (FFACO) strategy was achievable. This was evident when the complexity of the system and the additional costs required to sufficiently characterize it were judged to be out of balance with the overall perceived risks (low) associated with this particular CAU. As a result, an Alternative Modeling Strategy (AMS) and New FFACO Strategy Decision Process for UGTA RM/SM CAU were approved in November 2013. The AMS removes the requirement for developing ensembles of 3D contaminant transport simulations and replaces it with a simpler one requiring the generation of multiple 1D streamline models. These are meant to encompass a range of alternative, yet plausible release and transport scenarios as a means to bound the overall lateral extent of contaminant transport. The new Decision Process is streamlined and progresses from the revised CAI stage, to an external peer review, and then, upon favorable evaluation, directly to a closure stage. Although the project has suffered some delays from implementation of the new strategy, the new process of achieving closure is expected to save several years in time and several million dollars over the original process while still protecting human health and the environment over the 1,000-year compliance period stipulated in the FFACO. (authors)