Multiscale source-term coupling of site-scale groundwater and watershed models for long-term tritium transport at the Savannah River Site

Study region The Fourmile Branch Creek catchment at the Savannah River Site (SRS), USA, a watershed historically impacted by tritium ( 3 H) releases from nuclear production facilities. Study focus We developed a sequential multiscale framework that transfers time-dependent 3 H discharge calculated by a process-based 2D site-scale groundwater-flow and transport model to a 3D watershed model with internally coupled surface-subsurface hydrology. The framework avoids the computational cost of resolving site-scale contaminant processes throughout the entire watershed while retaining their effects on long-term river concentrations. New hydrological insights The results show that effective precipitation is the dominant control on 3 H discharge and river concentrations, influencing both transport timing and dilution processes. The coupled model reproduces the order of magnitude and temporal trends of observed 3 H concentrations in river water under a range of parameter conditions. The study highlights the importance of linking site-scale groundwater processes with watershed-scale modeling to evaluate contaminant transport, while also demonstrating limitations associated with simplified source representation and transport pathways.