A New (old) Method to Measure Layered Vadose Zone Permeability at Field Scale for Managed Aquifer Recharge

Knowing the field scale vertical hydraulic conductivity of the vadose zone is a challenge, especially in arid settings with a thick vadose zone. Vertical hydraulic conductivity at the field scale can be determined by directly measuring the air permeability of the vadose zone. This approach to measuring the vadose zone permeability was first developed by Weeks (1978), prior to the availability of current numerical modeling capability. By measuring vadose zone pressure responses to changes in barometric pressure at various depths in nested probes, a numerical model can be calibrated. Probe depths can be used to bracket and quantify suspected low permeability horizons at depth. The natural, time-variable, barometric pressure is used as a model surface boundary and calibration is achieved by matching observed lag and attenuation of the atmospheric pressure ‘signal’ in each layer or depth as it propagates through the subsurface. The vertical gas permeability for each layer is determined using the calibrated model and, in conjunction with soil porosity and saturation, is used to compute the vertical hydraulic conductivity of vadose zone soils. This method has the advantage of measuring permeability over a large area and volume and avoiding problems with local heterogeneity. In arid settings where the soil water content is low, the vertical gas permeability obtained from the method essentially represents the intrinsic vertical permeability of the materials and thus is representative of the vertical permeability to water.

HGC originally adapted this method to directly determine landfill gas generation rates. We have used this method to measure vertical vadose zone gas permeabilities at over 50 sites to depths of up to 300 feet. Actual and theoretical examples of measuring vadose zone permeability using this method are presented.