Numerical Evaluation of Multicomponent Cation Exchange Reactive Transport in Physically and Geochemically Heterogeneous Porous Media

Sophisticated deterministic numerical models have been developed during the last two decades for the analysis of multicomponent reactive solute transport. Most of these models account for spatial heterogeneity by parameter zonation. Most stochastic analyses of reactive transport in physically and geochemically heterogeneous porous media consider a single reactive species. Attempts have been made recently for the stochastic analysis of multicomponent reactive species which rely on simplifying assumptions. Given the lack of theoretical stochastic analyses of multicomponent cation exchange reactive transport in physically and geochemically heterogeneous porous media here we use Montecarlo techniques. We analyze cation exchange reactive transport through a two-dimensional vertical domain 40 m wide and 10 m deep. The 2-D domain, initially filled with 1 mM NaNO3 and 0.2 mM KNO3, is flushed by 0.6 mM CaCl2 solution from left to right. This case illustrates the chromatographic separation of Na and K. Na is weakly adsorbed and is eluted first. K is more tenaciously held than Na, and it appears retarded in the effluent. Both permeability and cation exchange capacity are assumed to be random Gaussian functions with spherical semivariograms. Flow and reactive transport equations are solved with a general purpose reactive transport code (CORE2D). Spatial moments are calculated to characterize the longitudinal features of the reactive plume assuming a single modal shape of the spatial derivative of depth- averaged concentrations of Na+, Ca+2 and Cl-. Numerical results show that the greater the variance of log-K, the larger the displacement of the center of the reactive plume while the larger the variance of log-CEC, the smaller the displacement of the reactive plume. The second order spatial moments of the plume increase with increasing variances of log-K and log-CEC. Effective retardation factor of Ca+2 is greater than that of Na+. Both of them increase with increasing variances of log-K and Log-CEC and depend on the correlation structure between log- K and log-CEC. Na+ and Ca+2 retardation factors for a negative correlation structure between log-K and log-CEC are smaller than those for uncorrelated log-K and log-CEC.Presenters: name: Samper, Javier affiliation: Universidad de Coruña, Spai