The transport of polystyrene microspheres was examined in packed glass beads under a variety of environmentally relevant ionic strength and flow conditions. The observed profiles of numbers of retained microspheres versus distance from the column entrance were much steeper than expected based on a constant rate coefficient of deposition acrossthe length of the column, indicating apparent decreases in deposition rate coefficients with transport distance. Deviation in the profile from log-linear decreases with distance was greatest under highly unfavorable conditions (low ionic strength), relatively reduced under mildly unfavorable conditions (high ionic strength), and was eliminated under favorable conditions. The generality of apparent decreases in deposition rate coefficients with distance of transport among microspheres, bacteria, and viruses leads to the conclusion that such effects reflect processes that are fundamental to filtration under unfavorable conditions. Numerical simulations of experiments that were performed under unfavorable conditions utilized a log-normal distribution of deposition rate coefficients among the colloid population in orderto simulate the effluent curves and retained profiles simultaneously. It is shown that while straining could be a significant contributor to the steep retained profiles at low ionic strength, where overall retention is low, distribution in interaction potentials among the population was a viable mechanism that can yield apparent decreases in deposition rate coefficients with distance of transport.

[1] Laboratory experiments, pore-scale simulations, and continuum (Darcy) -scale simulations were used to study mixing-induced precipitation in porous media. In the experimental investigation, solutions containing Na 2 CO 3 and CaCl 2 were each injected in different halves of a quasi two-dimensional flow cell filled with quartz sand. As a result of the in situ mixing between the two solutions, a narrow calcite precipitate layer (less than 5 mm wide) of more or less uniform width was formed between the individual solutions. Pore-scale simulations were conducted to help understand the mechanism of precipitation layer formation. The effect of the Peclet number, Pe, and the Damköhler number, Da, on mixing induced precipitation was also investigated. Pore-scale simulations revealed the presence of large pore-scale concentration gradients. This, and the presence of features, such as the precipitation layer, with characteristic lengths on the order of the average sand grain diameter, indicate the absence of a clear scale separation required for the strict derivation of Darcy-scale advection-dispersion equations. Nevertheless, we found that an adaptive high-resolution model based on advection-dispersion equations with grid sizes in the mixing zone smaller than the size of the sand grains can qualitatively reproduce the essential features of the experiment. As an alternative to computationally expensive high-resolution simulations, we proposed new forms for the homogeneous and heterogeneous reaction terms in Darcy-scale advection dispersion equations. These terms involve transport and mixing indices that account for highly nonuniform pore-scale concentration distributions and highly localized reactions. The proposed model accurately estimates the changes in solute concentrations due to homogenous and heterogeneous reactions during precipitation of minerals, observed in the pore-scale simulations, while conventional low-resolution advective-dispersion equations produced erroneous results.

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[1] A numerical model based on smoothed particle hydrodynamics (SPH) was used to simulate reactive transport and mineral precipitation in porous and fractured porous media. The stability and numerical accuracy of the SPH-based model was verified by comparing its results with analytical results and finite element numerical solutions. The numerical stability of the model was also verified by performing simulations with different time steps and different number of particles (different resolutions). The model was used to study the effects of the Damkohler and Peclet numbers and pore-scale heterogeneity on reactive transport and the character of mineral precipitation and to estimate effective reaction coefficients and mass transfer coefficients. Depending on the combination of Damkohler and Peclet numbers the precipitation may be uniform throughout the porous domain or concentrated mainly at the boundaries where the solute is injected and along preferential flow paths. The effective reaction rate coefficient and mass transfer coefficient exhibited hysteretic behavior during the precipitation process as a result of changing pore geometry and solute distribution. The changes in porosity and fluid fluxes resulting from mineral precipitation were also investigated. It was found that the reduction in the fluid flux increases with increasing Damkohler number for any particular reduction in the porosity. The simulation results show that the SPH, Lagrangian particle method is an effective tool for studying pore-scale flow and transport. The particle nature of SPH models allows complex physical processes such as diffusion, reaction, and mineral precipitation to be modeled with relative ease.