In this ongoing study of vadose zone permeability to air and water, the focus has shifted toward comparisons of measurements in the field to those in laboratory-controlled conditions. The primary purpose of this study is to investigate how flooding along major rivers may have altered the bulk permeability of the soil. The main objective of the more recent work is to compare soil permeabilities in a field setting with those of extracted samples from the same site, but determined by laboratory testing. A relatively new technique is used to determine effective permeability in the vadose zone: evaluating pressure drawdown in the soil through time that results from applying a vacuum to a screened borehole. This report describes how field data from a drawdown test is used to estimate field permeability, and also how results from field testing for permeability compare to laboratory measurements of air and water pemeability.

A detailed discussion of the laboratory measurement program is provided, and a comparison of air to water permeability is given by calculating the so-called "intrinsic permeability." The difficulties with the use of this measure are described, and comparisons with field determinations of the same parameters are also described. A steady-flow finite-element computer model is presented and used in the determination of field air permeabilites. The framework for an extension of the steady model to a time-dependent model is presented. The ability to model time-dependent flow is necessary for the lowest permeability sites examined in this study.