Advective and diffusive contaminant transport through heterogeneous sandy-clay formation

Abstract:

Experimental Site and data: Since 1963, radioactive waste has been injected in relatively deep (300-400 m) aquifers of Cretaceous terrigenous deposits in western Siberia nearby Tomsk city. Injection and overlaying zones are sandy-clay formation with complex internal architecture with about 50-50% volumetric proportion of low and high permeable units. The main concern for future post-injection period relates with waste lateral and upward migration to shallow aquifers and discharge zone. Due to radioactivity of the wastes the predictive time-scale on orders of magnitude large than observed up to day injection period. The field-scale site characterization data were used for development of 3-D heterogeneity model. The hydraulic, diffusive and retardation properties of low permeable units were studied at the lab scale.

Simulation: The numerical flow and transport experiments simulated long-term transport study were performed to study the effect of hydraulic heterogeneity, the role of diffusion into low permeable units and sorption of migrants on overall contaminant subsurface spreading.

Results: Simulated breakthrough curves exhibit long tails and appear to be non-Fickian. For the subvertical transport the diffusion into low permeable units do not change significantly the shape of breakthrough curves due to formation of preferable flow pattern due to connectivity of high permeable units. The sorption on low permeable units plays important role changing the shape and mean arriving time of contaminants. For use ADE with effective parameters in such a medium, a full anisotropic-media dispersion model should be tested.