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U Geochemistry in waste-weathered Hanford sediments

Figure 1. Location of the 216-U-8 Crib waste site and geology of U-8 Crib region in the 200 West Area at Hanford Click for larger version.

This project seeks to predict reactive contaminant migration in Hanford sediments that have received acidic uranium and strontium bearing waste streams. We hypothesize that acidic waste infusion results in silicate mineral transformations that exert strong controls on uranium and strontium speciation and mobility. Our research approach involves iterative laboratory, modeling and field scale studies. By combining bench-scale simulations of sediment weathering, spectroscopic investigation of contaminant speciation, and meso-scale studies of contaminant transport, we will constrain the continued development of a mechanistic reactive transport model of coupled mineral transformation and radionuclide sequestration in the Hanford vadose zone. The current study will significantly expand the parameter space exploited by the model, which will be both applicable at the field scale and informed by molecular-scale controls, and thus strengthen its general transferability to different field scenarios.

Figure 2. Comparison of XRD results between pure meta-ankoleite, unracted parent materials (Hanford fine sediment and quartz sand), and the reacted sediments [ASCW6 (pH=3) + HS, ASCW3 (pH=3) + HS, and ASCW6 (pH=2) + quartz]. Click for larger version.

Figure 3. Becquerelite (Ca(UO2)6O4(OH)6•8(H2O)) formed in the ASCW3(pH=3)+HS system.Click for larger version.

In the proposed work, we will maintain our approach to scaling the problem of HLRW geochemical disequilibria. However, we will move the focus of our work to an emerging need to better understand Hanford sediments impacted by acidic uranium and strontium bearing waste streams, as has occurred in the 216-U-8 and 216-U-12 Crib areas.

The objectives of our research are

  1. to determine the process coupling that occurs between mineral transformation and contaminant (U and Sr) speciation in acid-uranium waste weathered Hanford sediments;
  2. to establish linkages between molecular-scale contaminant speciation and meso-scale contaminant lability, release and reactive transport; and
  3. make conjunctive use of molecular- to field-scale data to constrain the development of a mechanistic, reactive transport model that includes coupling of contaminant sorption-desorption and mineral transformation reactions.

We will investigate this new geochemical problem of emerging interest using our collaborative approach to up-scale molecular- and micro-scale information to interpret meso- and field-scale transport phenomena. In addition to experimental studies, we will pursue code development in CrunchFlow that effectively incorporates theory pertaining to mineral cannibalism, nucleation and ripening processes that we are finding affect the weathering trajectories characteristic of high level radioactive waste-sediment interaction. By focusing on common experiments and improvements to the conceptual numerical model that describes them, we will develop a more robust and generalizable reactive transport model of coupled sorption and mineral transformation processes. Our project will inform – and be informed by – a strong science connection to the near-term sediment coring and associated analyses that are planned for the U-8 and U-12 Crib sites.

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