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Preferential flow, diffuse flow, and perching in an interbedded fractured-rock unsaturated zone

February 15, 2017

Layers of strong geologic contrast within the unsaturated zone can control recharge and contaminant transport to underlying aquifers. Slow diffuse flow in certain geologic layers, and rapid preferential flow in others, complicates the prediction of vertical and lateral fluxes. A simple model is presented, designed to use limited geological site information to predict these critical subsurface processes in response to a sustained infiltration source. The model is developed and tested using site-specific information from the Idaho National Laboratory in the Eastern Snake River Plain (ESRP), USA, where there are natural and anthropogenic sources of high-volume infiltration from floods, spills, leaks, wastewater disposal, retention ponds, and hydrologic field experiments. The thick unsaturated zone overlying the ESRP aquifer is a good example of a sharply stratified unsaturated zone. Sedimentary interbeds are interspersed between massive and fractured basalt units. The combination of surficial sediments, basalts, and interbeds determines the water fluxes through the variably saturated subsurface. Interbeds are generally less conductive, sometimes causing perched water to collect above them. The model successfully predicts the volume and extent of perching and approximates vertical travel times during events that generate high fluxes from the land surface. These developments are applicable to sites having a thick, geologically complex unsaturated zone of substantial thickness in which preferential and diffuse flow, and perching of percolated water, are important to contaminant transport or aquifer recharge.

Citation Information

Publication Year 2017
Title Preferential flow, diffuse flow, and perching in an interbedded fractured-rock unsaturated zone
DOI 10.1007/s10040-016-1496-6
Authors John R. Nimmo, Kaitlyn M Creasey, Kimberlie Perkins, Benjamin B. Mirus
Publication Type Article
Publication Subtype Journal Article
Series Title Hydrogeology Journal
Series Number
Index ID 70178565
Record Source USGS Publications Warehouse
USGS Organization National Research Program - Western Branch

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