Background
The 300 Area U plume represents a valuable natural laboratory where the hydrologic, mass transfer, and biogeochemical processes controlling contaminant fate and transport can be investigated in a unique manner across the entire span (i.e., a transect) of vadose zone-aquifer-river effluence.
The 300 Area Field Site includes the aquifer and the near-shore zone of groundwater-river water interaction. The groundwater flow path moves toward the river shore through a zone of pervasive and recalcitrant U(VI) contamination, where the bounding components - the vadose zone, aquifer sediments, groundwater, and the Columbia River - interact in a complex and dynamic manner. The aquifer sediments are an open framework matrix of river cobble with significant infilling of U(VI)-sorbing fines whose mass and composition varies spatially. Lenses of silt and sand create facies-scale heterogeneities. Pore, interfacies, and inter-formation mass transfer processes are important. Seasonal infiltration of river water into the aquifer causes water quality gradients, head reversals, and nutrient and microorganism intrusion into the near-shore aquifer.
The site is at the very south end of the Hanford Site, with easy access by investigators and proximity to PNNL's science and analytical infrastructure. Its hydrologic and biogeochemical characteristics are representative of numerous other contaminated sites at Hanford. The site is also being used for DOE EM-22 supported feasibility studies of U(VI) groundwater remediation that can result in mutually beneficial collaborations and infrastructure sharing with this project. DOE-RL, State and Federal regulators, and local stakeholders seek additional scientific information on this visible site upon which to base the best long-term decision for site remediation and closure.