Nonmonotonic variations in deposition rate coefficients of microspheres in porous media under unfavorable deposition conditions.

The transport of carboxylate-modified polystyrene latex microspheres was examined in packed quartz sand under a variety of environmentally relevant ionic strength and flow conditions. The retained concentrations of microspheres in the sediment increased first, and then decreased with transport distance, indicating that the deposition rate coefficient changed nonmonotonically over the transport distance. This finding demonstrates the ubiquity of spatial variation in deposition rate coefficients under unfavorable deposition conditions, and in addition indicates that the previously recognized monotonic decrease with transport distance is not the sole form of spatial variations in deposition rate coefficients. In contrast, the deposition rate coefficients of similarly sized microspheres with different surface group densities were shown to decrease monotonically with transport distance in the same porous media, indicating that the form of spatial variation in deposition rate coefficient is highly sensitive to system conditions. The ubiquity and sensitivity of the spatial variation of deposition rate coefficients indicate that current practices that utilize log-linear extrapolation of discreet measurements of colloid attenuation to determine colloid removal with distance from source are not valid (for both biological and nonbiological colloids). The retained colloid profiles hold the promise to reveal processes governing colloid deposition under unfavorable conditions that are yet to be identified.