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Webinar – Importance of Conceptual Model Development for the Pahute Mesa Groundwater Model, Nevada National Security Site (NNSS) August 2, 2023

Importance of Conceptual Model Development for the Pahute Mesa Groundwater Model, Nevada National Security Site (NNSS)

Ken Rehfeldt
Navarro Research and Engineering, Inc.

Charles Russell
Desert Research Institute

Wednesday, August 2, 2023
1:00-2:30 p.m. Eastern Time

Webinar Slides

Abstract:  Underground nuclear testing at the Nevada National Security Site (formerly the Nevada Test Site) ended in 1992 but has left a legacy of radionuclide contamination in the subsurface, including the groundwater.  A tri-party agreement, the Federal Facility Agreement and Consent Order (FFACO), between the U.S. Department of Energy (DOE), the U.S. Department of Defense, and the Nevada Division of Environmental Protection was finalized in 1996 and defines the process for reaching the end state for groundwater contamination, which is closure in place with monitoring and institutional control. The FFACO process requires the development of groundwater flow and transport models to forecast the maximum migration of radionuclides (at the 95% level) for a 1,000-year period.  The large area of investigation and the high cost of data acquisition emphasize the importance of the conceptual model for guiding the numerical model development.  An iterative process of data collection, analysis, and modeling utilizing scientists from multiple organizations led to the development of a robust conceptual model and successful Corrective Action Unit (CAU) model for Pahute Mesa.

About the speakers:

Ken Rehfeldt is the Navarro Research and Engineering Project Manager for the Underground Test Area activity for the Department of Energy Environmental Management Nevada Program’s restoration activities on the Nevada National Security Site.  He began work on the NNSS in 1992 and has been involved in environmental restoration activities at numerous sites across the NNSS.  He participated in the Yucca Mountain Project for five years while employed by Los Alamos National Laboratory.  He holds degrees in Geological Sciences, Hydrology, and Civil Engineering, the latter a Ph.D. from the Massachusetts Institute of Technology.

Charles Russell is a hydrogeologist at the Desert Research Institute in Las Vegas, Nevada.  His area of expertise combines groundwater hydraulics with aqueous geochemistry to solve hydrogeologic problems.  Mr. Russell has conducted hydrologic research on the Nevada National Security Site for over thirty-five years and is one of the initial investigators who started the Underground Test Area activity.  He currently serves as the Deputy Directory of the Division of Hydrologic Sciences at the Desert Research Institute.  He also serves as Science Advisor to DOE’s Office of Environmental Management Nevada Program.

The Performance and Risk Assessment Community of Practice (PRACOP) focuses on sharing knowledge, best practices, and lessons learned related to performance assessments and risk assessments, particularly for DOE sites.  These assessments are critical for:

  • management (design, operation, and closure) of disposal facilities, including landfills or near-surface disposal facilities
  • waste form development and implementation
  • closure of tanks and waste sites
  • in situ deactivation and decommissioning
  • soil and groundwater remediation.

PRACOP meetings and webinars are open to interested parties from federal, state, tribal, contractor, stakeholder, and academic organizations involved in DOE performance assessment or risk assessment activities.  Those who would like to be added to or removed from the distribution list should send their name and email address to  The PRACOP website is at, where archived materials are available.