Preferential and diffuse high-volume flow through an interbedded fractured-rock unsaturated zone

Abstract:

Layers of strong geologic contrast within the unsaturated zone can control recharge and contaminant transport to underlying aquifers. The importance of slow diffuse flow in certain geologic layers and rapid preferential flow in others complicates the prediction of vertical and lateral fluxes. Here we present a simple model designed to use limited geological site information to predict these critical subsurface flow processes in response to a sustained recharge source. The model is developed and tested using site-specific information from the Idaho National Laboratory (INL) in the Eastern Snake River Plain (ESRP), where confined anthropogenic sources of infiltration from spills, leaks, waste-water disposal, or retention ponds are often associated with facilities where contamination is present. In the thick unsaturated zone overlying the ESRP Aquifer, multiple sedimentary interbeds, which vary in thickness and hydraulic properties, are interspersed between massive fractured basalt units. The combination of surficial sediments, basalts, and interbeds determines the water fluxes through the variably saturated subsurface. Interbeds are often less conductive, causing perched water to collect above them, which is consistent with a hypothesis of rapid flow through the basalt being impeded from vertical percolation by the interbeds. The model successfully predicts volume and extent of perching, and approximates vertical travel times, during events that generate high fluxes from the land surface. The model is useful at sites like the INL where little hydrologic information is available about the combination of preferential and diffuse fluxes, but simple approximations of these combined flow processes can help inform operational decisions.