USE OF RESISTIVITY SOUNDING DATA IN A HYDROGEOLOGIC CONCEPTUAL SITE MODEL

The work described here is part of a research project to develop a strategic approach to ground-water monitoring applicable to sites of interest to the U.S. Nuclear Regulatory Commission. The overall strategy involves a structured approach beginning with a detailed analysis of site data and local to regional geologic setting and history; development of a detailed conceptual site model (CSM) integrating geological, geophysical and hydrological data, and computer simulation of flow and transport for this model. This strategy will identify specific locations and times where monitoring is needed. We are currently testing parts of the strategy with field data from the U.S. Geological Survey's (USGS) Amargosa Desert Research Site (ADRS) (http://nevada.usgs.gov/adrs/.)

Numerical simulation of tritium migration at the ADRS (Mayers et al., 2005, doi:10.2136/vzj2004.0179) proposed a vertical transport path together with diffusion along gravel layers, but none of these simulations were able to reproduce the observed tritium profiles. Our analysis uses electrical resistivity soundings in and near the ADRS as published by Bisdorf, 2002 (USGS Open-File Report 02-0140), to constrain a CSM with a fault for the ADRS area. We believe this model supplies the needed vertical pathway referred to in Mayer, op. cit., and also explains a steep gradient in the water table map as published by Walvoord, et al., 2004 (doi:10.1029/2004WR003599).

This analysis illustrates the importance of developing an integrated CSM for waste site selection, and monitoring network design. The observed vertical and lateral migration of tritium at the ADRS seems to be strongly influenced by geologic structures that were unknown prior to the USGS research efforts. These structural controls should be the focus of future monitoring design.

Data from the ADRS were provided through the courtesy of Brian J. Andraski (USGS ADRS Coordinator, Carson City, Nevada) and David A. Stonestrom (USGS National Research Program, Menlo Park, California). Their assistance is gratefully acknowledged.