Characterization and modeling studies of sites of underground testing on the NNSS were conducted after acceptance of the FFACO agreement in the mid-1990s. However, progress was slow and multiple problems were encountered with implementing the original UGTA strategy (Marutzky et al. , 2010). The strategy assumed sequential progress through planned characterization and modeling studies and underestimated modeling uncer­tainty and the importance of unanticipated scientific discoveries in characterization and modeling work. Progress, particularly progress in modeling studies, is often non-systematic with unexpected discoveries where these discoveries require rethinking of modeling approaches. Further, much of the initial modeling work for UGTA was focused on the physical and chemical processes of flow and transport rather than gaining information required to make regulatory decisions (process-driven modeling studies instead of decision-driven modeling studies). These difficulties culminated with a negative review by an external peer review panel in 1999 (IT Corporation, 1999) of the Frenchman Flat CAU data analysis and modeling studies. The panel found the studies insufficient to conclude the corrective action investigation stage of the UGTA strategy for the CAU.

The FFACO UGTA strategy was revised in 2009 (FFACO, 1996; as amended March 2010) working with NDEP to better represent the iterative nature of modeling studies, to more fully evaluate the impact of uncertainty on modeling results and to bring risk perspectives to the strategy. The origi­nal UGTA strategy was based on a standardized approach to modeling, monitoring and closure in place at all CAUs regardless of the hydrological source term and/or the proximity of testing areas to the boundaries of the NNSS. Additionally, the original strategy identified a single key regulatory decision near the end of the site characterization and model development stage (stage two of the corrective actions). If this decision was approved for an individual CAU, the studies would proceed to a 5-year proof of modeling results, followed by closure in place with implementation of a long-term monitoring network. All modeling studies would have been concluded at the end of the second corrective action stage.

Two significant changes were made in the revised strategy. First, the strategy was redesigned to be consistent with recent guidance by the National Academy of Sciences (NRC, 2007a) and the EPA (USEPA, 2009) on the use of modeling in regulatory decisions for environmental modeling. The UGTA strategy was redefined (FFACO, 1996; as amended March 2010) where the emphasis and culmination of the second stage was based on adequacy of model development. The third stage was redefined as a model evaluation stage, where model results are tested to build confi­dence that the model results can be used for the intended regulatory deci­sion. The fourth stage of the strategy was largely unchanged, an emphasis on CAU closure in place and implementation of a long-term monitoring network.