Monitoring in the vadose zone at two inactive uranium mill tailings sites

The Uranium Mill Tailings Radiation Control Act of 1978, Public Law 95-604, authorized the U.S. Department of Energy (DOE) to perform remedial action at designated sites. One of these sites is the Monument Valley, Arizona, inactive uranium mill tailings site. The Monument Valley tailings are currently being studied by the Technical Assistance Contractor (TAC) to DOE to characterize the site and to make recommendations for remedial action. The study includes characterization of contaminant migration in the vadose zone. Computer modeling was initiated to predict seepage rates through the tailings and underlying alluvium. A monitoring network of suction water samplers and tensiometers was set up to measure field conditions. Darcy's Law was used to calculate the flux through the pile. These seepage values were then compared to the computer estimated seepages. The monitoring program was designed on the basis of a successful vadose zone study completed at the Riverton, Wyoming, tailings site, which is similar both climatically and geologically to the Monument Valley site. Measurements obtained from the Riverton instruments were successfully used to define the rate of vertical movement of residual pore water, infiltrating rainfall, and dissolved solutes through the Riverton pile and into the underlying aquifer. This paper summarizes the results of the Riverton study and describes the vadose zone monitoring network that was installed at Monument Valley based on that study