Simple predictions of maximum transport rate in unsaturated soil and rock

[1] In contrast with the extreme variability expected for water and contaminant fluxes in the unsaturated zone, evidence from 64 field tests of preferential flow indicates that the maximum transport speed V_max, adjusted for episodicity of infiltration, deviates little from a geometric mean of 13 m/d. A model based on constant-speed travel during infiltration pulses of actual or estimated duration can predict V_max with approximate order-of-magnitude accuracy, irrespective of medium or travel distance, thereby facilitating such problems as the prediction of worst-case contaminant traveltimes. The lesser variability suggests that preferential flow is subject to rate-limiting mechanisms analogous to those that impose a terminal velocity on objects in free fall and to rate-compensating mechanisms analogous to Le Chatlier's principle. A critical feature allowing such mechanisms to dominate may be the presence of interfacial boundaries confined by neither solid material nor capillary forces.