High-resolution measurement of pore saturation and colloid removal efficiency in quartz sand using fluorescence imaging.

Colloid deposition in unsaturated, nonuniform porous media is poorly explained by current models and difficult to measure using breakthrough curves and retained mass profiles. We present new methods which enable time-lapse fluorescence imaging to quantify variations in pore saturation, theta, and colloid deposition in 2D, nonuniform unsaturated flow fields. Calibration experiments revealed direct proportionality between fluorescence F and theta in 20/30 mesh quartz sand. Analysis of breakthrough data in fluorescence images allows quantification of the mean mobile concentration, mean deposition rate, and hence the colloid removal efficiency eta directly from data at the pixel-scale throughoutthe flow field. We imaged carboxylate-modified latex microspheres from a point source in saturated flow and unsaturated flow across a capillary fringe at 10(-3), 10(-2), and 10(-1) M NaCl. Total numbers of colloids deposited and values of eta increased with ionic strength. We modeled the observed variations in eta with theta to estimate the partitioning of colloid deposition between air-water and solid-water interfaces. In the broad saturation range 0.2 < theta < 1, our results suggest that only at the lowest ionic strength, where deposition at solid-water interfaces was strongly unfavorable, did colloid deposition associated with air-water interfaces significantly influence the total colloid removal.