Evolution of anomalous transport following precipitation in porous media

Flow through porous media involving precipitation and dissolution reactions exhibits a unique feedback behavior between the velocity field and solute transport. In this presentation, we report the findings of a study exploring the relationship between a gradually increasing degree of precipitation and the occurrence of anomalous transport (i.e., transport that cannot be quantified by the advection-dispersion equation). Gypsum was precipitated incrementally in 60 cm long, saturated, sand-packed columns, and an inert tracer was injected between precipitation phases, yielding breakthrough curves (BTCs) as functions of an increasing degree of precipitation. Continuous time random walk particle tracking simulations were used to model these BTCs and quantify the evolution of anomalous transport. Results show an increasingly high degree of anomalous transport following precipitation, while the manner in which the increase manifested varied among duplicate experiments. Two major consistent trends were an increase in the overall BTC widths (i.e., elution time windows) and progressively heavier BTC tailing, as indicated by the steepness of the slope from each BTC peak to the point where it drops below a threshold concentration. Under the current experimental conditions, the effects of precipitation were strikingly similar to those found previously for dissolution, including early BTC onset, peak splitting, and heavier BTC tailing. Finally, the range of transport behaviors among heterogeneous natural systems might be significantly greater than that found in our work for three homogeneously-packed columns.