The effects of scale and spatial heterogeneities on diffusion in volcanic breccias and basalts: Amchitka, Alaska

Abstract

Aqueous phase diffusion of molecules in fractured crystalline rock can play a dominant role in the fate and transport of contaminants, particularly if advective flows are very slow. The importance of the role of diffusion, typically a very slow process, also becomes an important mechanism to consider if the contaminants are long-lived in the subsurface, and thus their fate and transport must be considered over very long time-scales. Owing to the inherently heterogeneous nature of the subsurface, quantification of diffusive transport in the subsurface is extremely complex. The main objective of this study was to investigate the effects of the inherent heterogeneities of the subsurface on the diffusive transport of radionuclides, with a focus on the former underground nuclear test site at Amchitka Island, Alaska. Commonly, the through-diffusion experiment is utilized to estimate transport parameters, the diffusivity and the effective porosity, for porous media samples. The available methods of mathematically deriving these parameters from these experiments are generally known to be subjective and unreliable. Thus, one phase of this study applied the results of through- diffusion experiments to investigate the applicability of the available solution methods to derive these parameters. The results indicated that a semi-analytical solution provided the most reliable parameter estimations. This knowledge was then applied to studies designed to understand the impacts of scale and spatial heterogeneities on diffusion at the study site. Multiple Amchitka Island core samples, both from the same and from various geologic layers, in varying sizes were analyzed in the laboratory via tracer-based and electrical methods, to quantify the diffusive properties and the effects of heterogeneities for Amchitka Island. The studies indicated that the transport properties for Amchitka Island are widely varying, by orders of magnitude, and are indicative of a geologically layered subsurface system; these results are extremely important for the long-term modeling of the fate and transport of radionuclides at Amchitka Island. Finally, in light of these studies, the question of long-term stewardship for the Amchitka Island test site was investigated and recommendations were provided, with the goal of providing an improved plan for the long-term monitoring and management of the site.