A Groundwater Modeling Case Study to Evaluate Dewatering at a Uranium Mill Tailings disposal Site

Описание: Pete Schillig is a Senior Hydrogeologist with RSI EnTech support contractor to the Department of Energy’s Office of Legacy Management. Pete has 15 years of practicing professional experience, specializing in site characterization and numerical flow, fate and transport modeling for CERCLA and UMTRCA regulated sites. Pete holds M.S. and Ph.D. degrees in geology from the University of Kansas, a P.G. license from the Commonwealth of Pennsylvania, and serves as an associate editor for Groundwater Monitoring and Remediation.

A Groundwater Modeling Case Study to Evaluate Dewatering at a Uranium Mill Tailings Disposal Site

The Department of Energy's Office of Legacy Management (DOE-LM) manages over 100 sites nationwide related to past radiological and nuclear material production. One such site is the Rifle, Colorado, Disposal Site, which features a 71-acre engineered disposal cell built in 1996 to contain uranium mill tailings from the Old and New Rifle Processing sites.

Since 2012, pore fluids within the tailings at the southern, lower elevation extent of the disposal cell have been accumulating at a rate exceeding the removal capability of the existing leachate collection system. Pore fluid levels approaching the top elevation of the high-density polyethylene liner located at the toe of the cell, necessitated the design and implementation of additional dewatering capability.

This case study demonstrates how an ensemble of calibrated groundwater flow models was necessary in the design process to overcome significant data limitations for model configuration and calibration. By using an ensemble of groundwater flow models, the quantified uncertainty in calibrated parameters was applied throughout the evaluation of the dewatering design to provide a broader and more robust understanding of the potential outcomes. The design was further improved by a geophysical survey that located buried mill debris within the disposal cell, which could have impeded drilling operations through the uranium mill tailings. With the final design completed, the new extraction system was installed and became operational in December 2024.