Reactive transport modeling of column experiments on the evolution of saline-alkaline waste solutions.

Leakage of saline-alkaline tank waste solutions often creates a serious environmental contamination problem. To better understand the mechanisms controlling the fate of such waste solutions in the Hanford vadose zone, we simulated reactive transport in columns designed to represent local site conditions. The Pitzer ion interaction module was used, with principal geochemical processes considered in the simulation including quartz dissolution, precipitation of brucite, calcite, and portlandite, multi-component cation exchange, and aqueous complexation reactions. Good matches were observed between the simulated and measured column data at ambient temperature ( approximately 21 degrees C). Relatively good agreement was also obtained at high temperature ( approximately 70 degrees C). The decrease of pH at the plume front is examined through formation of secondary mineral phases and/or quartz dissolution. Substantial formation of secondary mineral phases resulting from multi-component cation exchange suggests that these phases are responsible for a decrease in pH within the plume front. In addition, a sensitivity analysis was conducted with respect to cation exchange capacity, selectivity coefficient, mineral assemblage, temperature, and ionic strength. This study could serve as a useful guide to subsequent experimental work, to thermodynamic models developed for the concentrated solutions at high ionic strength and to other types of waste plume studies.