About the Site
Concept: PNNL and collaborators are using the Hanford 300 Area uranium (U) plume in waste management area 300-FF-5 as a site for an Integrated Field-Scale Subsurface Research Challenge (IFRC). This 5-year research project, which began February 1, 2007, is supported by DOE's Office of Biological and Environmental Research (OBER) through its Environmental Remediation Sciences Division (ERSD), and with the concurrence and support of DOE-RL, DOE Pacific Northwest Site Office, and Hanford's groundwater remediation contractor CH2M Hill Plateau Remediation Company, LLC (CPRC).
As the basis for this project, the scientific team has carefully reviewed the literature and identified the scientific process of contaminant mass transfer from immobile to mobile domains as a major uncertainty in predicting field-scale reactive contaminant migration rates, and in effectively remediating DOE lands.
Consequently, we selected multi-scale mass transfer processes as our primary scientific theme. Hanford's 300 Area U(VI) plume is the ideal DOE site to investigate this theme. Monitored natural attention (MNA) was initially selected as the site remedy, but slow, mass-transfer limited desorption has compromised the ability of MNA to reach regulatory standards. Mass transfer limitations occur at this site from the pore, to the macroscopic, and to the facies scale. Resolving of scientific issues associated with field-scale mass transfer processes will have a major positive impact on remediating contaminated sites at Hanford and countless other locations nationally.
Research performed at this site will redefine scientific understanding of field-scale mass transfer processes for a speciation-controlled inorganic ion (UO22+(aq)), while simultaneously providing new knowledge and models needed to make effective and defensible remedial decisions to protect Columbia River water quality. Two additional project goals are to: 1) provide relevant materials and field experimental opportunities for other ERSD researchers, and 2) generate a lasting, accessible, and high-quality field experimental data base that can be used by the scientific community for testing and validation of new conceptual and numeric models of subsurface transport.
The IFRC will focus on field-scale mass transfer issues and related phenomena controlling kinetic and time variant U geochemical behavior and migration in the 300 Area vadose zone and groundwater plume. This scope is complemented by other ongoing laboratory studies at PNNL and collaborating universities and labs as supported by the ERSP and other EM programs. This research is not being supported by the IFRC budget, but is nonetheless aligned with IFRC project objectives and goals. For example, this research has determined: 1) the complex geochemical speciation of U(VI) in many 300 Area subsurface sediments, 2) laboratory mass transfer rates for desorbing U and the controlling effects of various geochemical properties and scale, 3) the adsorptivity of U(VI) on various 300 Area vadose zone and aquifer sediments and a preliminary surface complexation model, and 4) the sensitivities of various surface geophysical measurement techniques to facies change and subsurface materials properties variations at the site. This complementary research enriches the IFRC effort and is essential for its future success.
The 300 Area plume is at the south end of the Hanford Site with easy access and proximity to PNNL's laboratory and EMSL facilities. Four generic science questions on mass transfer are posed to motivate research. These relate to the effect of spatial heterogeneities; the importance of scale; coupled interactions between biogeochemical, hydrologic, and mass transfer processes; and measurements/approaches needed to characterize and model a mass transfer dominated system. These questions are resolved through the evaluation of three site-specific, field hypotheses that take advantage of the unique hydrogeologic attributes and considerable background information base for the site. The hypotheses focus on different physical mass transfer processes in the vadose zone and saturated zone; the coupling of abiotic and biotic processes and their regulation by and influence on mass transfer; and the importance of mass transfer to natural attenuation and engineered remediation (e.g., the polyphosphate concept under evaluation at the site). The hypotheses will be evaluated using infiltration, injection, and down-well experiments in the field using 300 Area site waters of variable U concentration as required to evaluate adsorption and desorption processes, or as spiked with remediation reagents. Monitoring arrays aligned along natural gradients that develop at the groundwater-river interface and that propagate into the U-plume will also be established and instrumented for continuous monitoring.
An innovative, integrated field research site has been designed that transects waste source areas in the vadose zone of the historic processes ponds (specifically the South Process Pond), through contaminated aquifer regions, to final discharge to the Columbia River. The site has been designed to take advantage of seasonal head reversals and flow direction changes in the aquifer that are driven by river stage. These head reversals allow for scientific exploration of different groundwater flow paths with different heterogeneities, properties, and biogeochemical intensities. The site will be well characterized by various geophysical techniques and by direct sampling during installation of the monitoring arrays for both vadose zone and saturated zone injection sites. Extensive correlations between geophysical measurements of different type and direct sediment analyses (hydrophysical, geochemical, total U, and microbiology) will allow development of a 3-dimensional geostatistical model of the experimental domain that will enable estimation of sediment facies distributions and properties along inaccessible hydrologic flow-paths between wells. Various surface and down-hole geophysical measurements along with direct water sampling and analyses will be used to monitor the migration and evolution of injected or naturally developing plumes involved in hypothesis testing.
A significant modeling effort is included for rigorous and robust site characterization and experiment interpretation. Several different stochastic and deterministic approaches, and associated models, will be used to facilitate: 1) estimation of spatially variable hydrogeologic and geochemical reaction parameters, 2) robust, state of the art field experiment interpretation, and 3) scientific understanding and hypothesis resolution, and to demonstrate the most feasible mass transfer modeling approaches for transfer to DOE lands. These modeling activities will be further enhanced by linkages with a SciDAC funded project that will use the 300 Area U plume as its example field site. The project will deliver a parameterized reactive transport simulator to the site as a final product that is based on the STOMP code.
Science and experimental collaborations, as well as leveraged facilities use and sharing, are planned with a DOE EM-22 project at the 300 Area to evaluate the feasibility of polyphosphate-induced autunite {Ca[(UO2)(PO4)]2(H2O)10-12} precipitation to mitigate U(VI) discharge to the Columbia River. The EM-22 project injected polyphosphate and calcium chloride solutions into the saturated zone (in 6/07) to induce autunite and apatite precipitation, and our collaborative experimental plans will follow a careful review of their results and discussions with ERSD and EM-22 sponsors. The IFRC will proactively publish results in the best scientific journals, and transfer data, knowledge, and coupled models to the Hanford Site during and after the term of the project.