PNNL

Abstract

The Integrated Field Research Challenge (IFRC) at the Hanford Site 300 Area uranium (U) plume addresses multi-scale mass transfer processes in a complex subsurface biogeochemical setting where groundwater and riverwater interact. A series of forefront science questions on reactive mass transfer motivates research. These questions relate to the effect of spatial heterogeneities; the importance of scale; coupled interactions between biogeochemical, hydrologic, and mass transfer processes; and measurements and approaches needed to characterize and model a mass-transfer dominated biogeochemical system. The project was initiated in February 2007, with CY 2007, CY 2008, CY 2009, and CY 2010 progress summarized in preceding reports. A project peer review was held in March 2010, and the IFRC project acted upon all suggestions and recommendations made in consequence by reviewers and SBR/DOE. These responses have included the development of “Modeling” and “Well-Field Mitigation” plans that are now posted on the Hanford IFRC web-site, and modifications to the IFRC well-field completed in CY 2011. The site has 35 instrumented wells, and an extensive monitoring system. It includes a deep borehole for microbiologic and biogeochemical research that sampled the entire thickness of the unconfined 300 A aquifer. Significant, impactful progress has been made in CY 2011 including: i.) well modifications to eliminate well-bore flows, ii.) hydrologic testing of the modified well-field and upper aquifer, iii.) geophysical monitoring of winter precipitation infiltration through the U-contaminated vadose zone and spring river water intrusion to the IFRC, iv.) injection experimentation to probe the lower vadose zone and to evaluate the transport behavior of high U concentrations, v.) extended passive monitoring during the period of water table rise and fall, and vi.) collaborative down-hole experimentation with the PNNL SFA on the biogeochemistry of the 300 A Hanford-Ringold contact and the underlying redox transition zone. The modified well-field has functioned superbly without any evidence for well-bore flows. Beyond these experimental efforts, our site-wide reactive transport models (PFLOTRAN and eSTOMP) have been updated to include site geostatistical models of both hydrologic properties and adsorbed U distribution; and new hydrologic characterization measurements of the upper aquifer. These increasingly robust models are being used to simulate past and recent U desorption-adsorption experiments performed under different hydrologic conditions, and heuristic modeling to understand the complex functioning of the smear zone. We continued efforts to assimilate geophysical logging and 3D ERT characterization data into our site wide geophysical model, with significant and positive progress in 2011 that will enable publication in 2012. Our increasingly comprehensive field experimental results and robust reactive transport simulators, along with the field and laboratory characterization, are leading to a new conceptual model of U(VI) flow and transport in the IFRC footprint and the 300 Area in general, and insights on the microbiological community and associated biogeochemical processes influencing N, S, C, Mn, and Fe. Collectively these findings and higher scale models are providing a unique and unparalleled system-scale understanding of the biogeochemical function of the groundwater-river interaction zone.