Measuring Diffusion Coefficients Using LA-ICP-MS
The movement of solutes in groundwater is of great interest to researchers because of issues such as groundwater contamination, the movement of chemical tracers, and developing methods for pollutant remediation. In my project, solute transport resulting from diffusion was analyzed using Laser Ablation Inductively Coupled Plasma Mass Spectroscopy, or LA-ICP-MS. Diffusion of solutes into rock matrices is characterized by ion and rock specific diffusion coefficients (Deff). Knowing these values is necessary for quantifying transport. The effect of fracture skins, which commonly are present between fracture conduits and rock matrices, on diffusion is also examined. Values found for Deff for two sandstones, the Breathitt Sandstone and the Hinckley Sandstone, as well as for the fracture skin on the Hinckley Sandstone, allowed for interesting observations on sandstone aquifers to be made. The average values of Deff for the Breathitt and Hinckley
Sandstones are 5.57 × 10-7 cm2/sand 5.735 × 10-7 cm2/s, respectively. The average Deff for a Hinckley sandstone fracture skin is 3.06 × 10-7 cm2/s, which is lower than through the matrix, suggesting the fracture skin impedes diffusion.
Understanding diffusion through sandstones involves many factors, including the time of diffusion, the particle path tortuosity, the ion tracer used, and chemical processes such as sorption. After addressing each of these, my method worked in finding effective diffusion coefficients for sandstones and their fracture skin surfaces. Use of the LA-ICP-MS uniquely allows the spatial variability of diffusion to be determined. This study adds to past studies in showing this method is efficient and accurate in determining diffusion coefficients in a wide variety of porous media.