The U.S. Department of Energy’s Office of Environmental Management (EM), Technology Innovation and Development (EM-32), is supporting development of the Advanced Simulation Capability for Environmental Management (ASCEM). ASCEM is a state-of-the-art scientific tool and approach for understanding and predicting contaminant fate and transport in natural and engineered systems. This modular and open-source, high-performance computing tool will facilitate integrated approaches to modeling and site characterization that enable robust and standardized assessments of performance and risk for EM cleanup and closure activities. As part of the initial development process, a series of demonstrations was defined to test ASCEM components and provide feedback to developers, engage end users in applications, and lead to an outcome that would benefit the sites. The demonstration was implemented for a sub-region of the Savannah River Site General Separations Area that includes the F-Area Seepage Basins. The physical domain included the unsaturated and saturated zones in the vicinity of the seepage basins and the Fourmile Branch. An unstructured mesh was used to fit the grid to the hydrostratigraphy and topography of the site. The calculations modeled variably saturated flow, and the resulting flow field was used in simulations of the advection of non-reactive species and the reactivetransport of uranium. As part of the demonstrations, data management, visualization, and uncertainty quantification tools were developed to analyze simulation results and existing site data. These new tools can be used to provide summary statistics, including information on which simulation parameters were most important in predicting uncertainty and visualizing the relationships between model input and output.