HIGH-RESOLUTION MODELING STUDIES OF VADOSE ZONE REACTIVE TRANSPORT

An exposed section of the Hanford vadose zone was characterized for hydrogeological and geochemical properties at the millimeter scale using high-resolution visible and infrared imaging, in situ measurements (air and water permeability), cation exchange capacity, grain size sampling, and statistical correlation. A key feature of sediments from this area of the Hanford Site is the clear inverse functional dependence of cation exchange capacity on grain size. This correlation was used in conjunction with the highly resolved spatial distribution of sediment textures for the exposed face at the Army Loop Road Field Site, to provide an equivalently detailed distribution of cation exchange capacity.

The focus of study was the reactive transport of strontium-90 in physically and geochemically heterogeneous unsaturated sediments. In this system, the mobility of strontium-90 is controlled by multicomponent ion exchange, and mineral precipitation and dissolution. Two-dimensional reactive transport simulations for an experimental release of strontium and magnesium at the field site were performed at 1-mm spatial resolution to identify the impact of multiscale property variability on the behavior of strontium in the vadose zone under a range of recharge conditions. Parallel processing subsurface simulators were used to accommodate the large number of grid cells and components as well as the comprehensive coupling of flow, transport, and reaction processes. The simulations were designed to test the sensitivity of strontium mobility to small-scale recharge-dependent transport pathways and the preferential distribution of exchange sites with respect to those pathways. Of particular interest were exclusion effects that lead to the bypassing of reactive surfaces at different flow regimes.

(The relationships used to transform the digital images into high-resolution distributions for predictive flow and transport models are discussed in the companion paper by Ward et al., “Integration of Multi-scale Physical and Chemical Heterogeneities Using High-Resolution Digital Images.")