Field-scale model for the natural attenuation of uranium at the Hanford 300 area using high performance computing
- Los Alamos National Laboratory
- PNNL
Three-dimensional reactive flow and transport simulations are carried out to better understand the persistence of uranium [U(VI)] at the Hanford 300 Area bordering the Columbia River. The massively parallel code PFLOTRAN developed under a DOE SciDAC-2 project is employed in the simulations. The calculations were carried out on 4096 processor cores on ORNL's Jaguar XT4 & 5 Cray supercomputers with run times on the order of 6 hours, equivalent to several years if performed on a single processor with sufficient memory. A new conceptual model is presented for understanding present-day and future attenuation rates of U(VI) at the 300 Area site. Unique to the conceptual model is the recognition of three distinct phases in the evolution of the site corresponding to: (I) initial emplacement of waste; (II) present-day conditions of slow leaching of U(VI) from the Hanford sediments; and (III) the complete removal of non-labile U(VI) from the source region. This work focuses on Phase II. Both labile and non-labile forms of U(VI) are included in the model as sorbed and mineralized forms of U(VI), respectively. The non-labile form plays an important role in providing a long-term source of U(VI) as it slowly leaches out of the Hanford sediment. Rapid fluctuations in the Columbia River stage on hourly, weekly and seasonal time scales are found to' playa major role in determining the migration behavior of U(VI). The calculations demonstrate that U(VI) is released into the Columbia River at a highly fluctuating rate in a ratchet-like behavior with nonzero U(VI) flux occurring only during flow from contaminated sediment into the river. The cumulative flux, however, is found to increase approximately linearly with time. The flow rate and U(VI) flux into the Columbia River predicted by the model is highly sensitive to the value used in the conductance boundary condition at the river-sediment interface. By fitting the conductance to the measured piezometric head at well 399-2-1, good agreement was obtained with observation for both the mean flux of water and U(VI) at the river-aquifer boundary with no other adjustable parameters. It was found that a multirate sorption model developed to account for long tails observed in U(VI) breakthrough curves obtained from column experiments with contaminated Hanford sediments, gave similar results for Phase II as an equilibrium surface complexation model for the discharge rate of U(VI) into the Columbia River.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC52-06NA25396
- OSTI ID:
- 971643
- Report Number(s):
- LA-UR-09-06408; LA-UR-09-6408; WRERAQ; TRN: US201004%%226
- Journal Information:
- Water Resources Research, Journal Name: Water Resources Research; ISSN 0043-1397
- Country of Publication:
- United States
- Language:
- English
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