Computation of porosity and saturation in the vadose zone at the Savannah River Site

Philip H. Nelson
U. S. Geological Survey
Denver, Colorado


Geophysical logs, penetrometer runs, and core data have been acquired to characterize the shallow sediments of the A/M area at the Savannah River Site, where chlorinated solvents were released to the subsurface. Solvents have migrated through the vadose zone into a shallow aquifer. Logs in the vadose zone, the focus of this paper, have been analyzed to determine porosity, water content, and to flag the clay zones.

The fresh, resistive groundwater complicates the usage of Archie's laws. We use the density and resistivity logs to compute an apparent water resistivity in a sandy unit in the saturated zone. This value is used in the vadose zone, along with the density and guard resistivity logs, to determine saturation. Saturation and the density log are then used to compute porosity and water content in the vadose zone. In addition, the neutron and resitivity logs are used to compute a clay flag.

The calculated logs suggest a three-way partition of vadose-zone lithologies into clay-rich, high-silt/low-clay, and partially saturated sand zones. Clay-rich zones are delineated by low resistivity and/or high neutron porosity. High-silt/low-clay zones occur where resistivity values are moderate and where the formation is saturated or nearly so. Partially saturated sand occurs where moisture content is less than vadose-zone porosity. This three-way division of the vadose zone is believed to be important in terms of contaminant, water, and gas transport.

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