A hybrid hydrologic-geophysical inverse technique for the assessment and monitoring of leachates in the vadose zone. 1998 annual progress report

'The objective of this study is to develop and field test a new, integrated Hybrid Hydrologic-Geophysical Inverse Technique (HHGIT) for characterization of the vadose zone at contaminated sites. This new approach to site characterization and monitoring can provide detailed maps of hydrogeological heterogeneity and the extent of contamination by combining information from electric resistivity tomography (ERT) surveys, statistical information about heterogeneity and hydrologic processes, and sparse hydrologic data. Because the electrical conductivity of the vadose zone (from the ERT measurements) can be correlated to the fluid saturation and/or contaminant concentration, the hydrologic and geophysical measurements are related. As of the 21st month of a 36-month project, a three-dimensional stochastic hydrologic inverse model for heterogeneous vadose zones has been developed. This model employs pressure and moisture content measurements under both transient and steady flow conditions to estimate unsaturated hydraulic parameters. In this model, an innovative approach to sequentially condition the estimate using temporal measurements has been incorporated. This allows us to use vast amounts of pressure and moisture content information measured at different times while keeping the computational effort manageable. Using this model the authors have found that the relative importance of the pressure and moisture content measurements in defining the different vadose zone parameters depends on whether the soil is wet or dry. They have also learned that pressure and moisture content measurements collected during steady state flow provide the best characterization of heterogeneity compared to other types of hydrologic data. These findings provide important guidance to the design of sampling scheme of the field experiment described below.