Chances are the quality of your drinking water was improved by filtration through porous media at some point before it reached your tap, perhaps naturally by transport through the subsurface, or purposefully by passage through an engineered sand filter. Engineered filtration processes have been utilized for decades, and these processes are monitored to ensure the removal of a required degree of particles, e.g., colloids (biological and non-biological particles ranging between a few tens of nanometers to ten microns), from water. Filtration is manifest in both natural and engineered contexts, e.g., by the relatively high quality of spring water, and by the difficulty of targeting the delivery of microbes, zero-valent iron, and other colloids with novel properties to contaminated locales in the subsurface for the purpose of remediation.