Use of ²³⁴U/ ²³⁸U Ratios to Measure In Situ Weathering Rates in the Hanford Vadose Zone

Katharine Maher, Donald J. DePaolo, and John N. Christensen

Contact: Katharine Maher, 510/642-9524, katem@eps.berkeley.edu

Research Objectives
Weathering rates of subsurface soil and rocks are difficult to quantify because of the difficulties in assessing the amount of reactive surface area and the thermodynamic potentials driving the chemical reactions.   Mineral dissolution rates measured in the laboratory typically predict rates that are 2 to 4 orders of magnitude faster than estimates based on field measurements (White et al., 1996).   Few field measurements are available, and most of those are on silicate soils.   Almost no field measurements exist for deep vadose zone materials or for rocks in the saturated zone.

The uranium-series (U-series) isotope system can be used to measure reaction rates in aquifers and thick vadose zone environments. This approach is based on a-recoil of ²³⁴Th atoms across grain boundaries, which enriches the pore fluid in ²³⁴U.   Dissolution of the solid phase releases mainly 238 U to the pore fluid, so that the ²³⁴U/ ²³⁸U ratio of the pore fluid is a measure of the local ratio of the dissolution uranium flux to the a-recoil flux (Tricca et al., 2001; DePaolo et al., 2003 submitted).   The in situ reaction rate can be calculated from measurements of the ²³⁴U/ ²³⁸U isotopic ratio of interstitial fluids and solid phases, if the a-recoil flux can be estimated independently.

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