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.

Nathalie A. Wall, Sara A. Mathews

Sandia National Laboratories - Carlsbad Programs Group, 4100 National Parks Highway, Carlsbad, NM 88220, USA

The Waste Isolation Pilot Plant (WIPP) is a repository for transuranic wastes from the US Department of Energy (DOE) defense programs, located approximately 650 meters below the surface, in the bedded salt of the Salado Formation of Permian age. DOE has demonstrated in the 1996 Compliance Certification Application (CCA) that the repository performance will meet compliance standards for the 10,000 year regulatory period. Performance assessment (PA) calculations show that migration of radionuclides will be insignificant, in the absence of human intrusion. One of the factors taken into account in PA is the presence of humic acids (HA), which are strong metal complexing materials of colloidal size, found in all natural waters and resulting from biodegradation of animal and plant matter. HA might be present in the WIPP disposal room brines, or may form in situ, due to the presence of soils, nutrients, and cellulosic substrates for microbial action in the waste. Although soluble humic material might not be present in appreciable concentrations during the 10,000-year regulatory period, as humics are limited by their solubilities due to rapid coagulation by brine constituents, the project has bounded the humic effects by assuming that soluble humic materials are present during the 10,000-year WIPP performance period. The latest experimental results show that HA are stable for several months in WIPP brines; however, when MgO, the WIPP engineered barrier, is present, the humic material precipitates completely.

Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.