This is a renewal proposal for a previous collaborative effort between the PNNL Scientific Focus Area (SFA) and EMSL.PNNL Scientific Focus Area. The PNNL SFA will develop a predictive understanding of the groundwater-surface water interaction zone (subsurface interaction zone, SIZ) and its linkages with the water cycle that incorporates hydrologic impacts on fundamental biogeochemical and ecological processes into a multiscale modeling framework that forecasts system responses and feedbacks to environmental changes. The SIZ is a critical and ubiquitous domain at the groundwater-surface water interface that regulates contaminant releases to surface waters and associated carbon and nitrogen cycling. Our research plan supports U.S. Department of Energy (DOE) goals to advance a predictive understanding of the biogeochemical structure and function of subsurface environments to enable systems-level environmental prediction and decision support. A key aspect of our plan is the performance of observational, experimental, and computational science on contaminant, carbon, and nitrogen transformation dynamics in the SIZ within the context of an integrative multiscale modeling approach that enables bi-directional (up- and down-scale) transfer of knowledge, process models, and parameters to the necessary scale of prediction. Using the Hanford Reach of the Columbia River as our field site, overarching research questions are driven down from the reach scale to focus lower-scale scientific hypotheses on essential system attributes, behaviors, or mechanisms for robust process model development. A facies-based, multiscale simulation framework will connect SFA biogeochemical transport models across scales while preserving robust process descriptions derived at the local field scale and in the laboratory. New predictive microbial ecological models will be coupled to hydro-biogeochemical models to translate microbial composition into biogeochemical process rates. Our research on the subsurface interaction zone will provide essential knowledge and relevant models for rivers worldwide that flow through glacio-fluvial aquifers and for high latitude/elevation catchments with coarse grained sediments vulnerable to climate change.