Radiotracer Method in Study of Reactive Transport Across Chemical Gradients in Porous Media
Los Alamos National Laboratory, Chemistry Division, Isotope and Nuclear Chemistry, MS J514, Los Alamos, NM 87545, USA
Transport of reacting solutes in porous media has been receiving increasingly more attention over the last decade due to concerns about the fate of contaminants in the environment. Of the very large number of studies reported in the literature, few have studied reactive transport in heterogeneous environments where physical and chemical discontinuities often occur across the interface between the matrixes. In this study, radiotracer 3H+, 22Na+, and 59Fe3+ diffusion tube experiments on the layered acidic jarosite/alkaline coal fly ash system have been carried out to study the effect of chemical gradients in porous media on transport and mobility of chemical constituents. The results of this study indicate that discontinuities in porous media caused by chemical gradient will not only affect significantly the transport properties but also the mobility of chemical constituents. In the particular case of precipitation, a new reaction zone may form with properties differing in transport and thermodynamics relative to the original two source media due to pore filling by precipitates. Radiotracer diffusion tube experiments may provide modelers with reliable information that incorporates some of the complexities such as local heterogeneities observed in soils.