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WATER RESOURCES RESEARCH, VOL. 43, W05426, doi:10.1029/2006WR005372, 2007

Simple predictions of maximum transport rate in unsaturated soil and rock

John R. Nimmo

U.S. Geological Survey, Menlo Park, California, USA


Abstract

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.

Received 24 July 2006; accepted 21 February 2007; published 22 May 2007.

Keywords: Unsaturated zone; vadose zone; preferential flow.

Index Terms: 1875 Hydrology: Vadose zone; 1866 Hydrology: Soil moisture; 1829 Hydrology: Groundwater hydrology; 5104 Physical Properties of Rocks: Fracture and flow.

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Citation: Nimmo, J. R. (2007), Simple predictions of maximum transport rate in unsaturated soil and rock, Water Resour. Res., 43, W05426, doi:10.1029/2006WR005372.