Ken Rehfeldt speaking to group in Oasis Valley
EM Nevada Program Underground Test Activity Project Manager Ken Rehfeldt addresses a team of stakeholder representatives and independent peer reviewers at the Nevada National Security Sites location some 65 miles northwest of Las Vegas.

LAS VEGAS – In 1989, EM launched an intensive groundwater investigation program to address the effects of historic nuclear testing and activities at the Nevada National Security Sites (NNSS) location some 65 miles northwest of Las Vegas.

Recently, scientists with the EM Nevada Program achieved a significant step toward closure of the final groundwater region of four on the site, known as Pahute Mesa.

“We have reached what is known as the model evaluation stage at Pahute Mesa,” said John Myers, EM Nevada Underground Test Activity (UGTA) lead. “This means the Nevada Division of Environmental Protection has sufficient confidence in the team’s work and our corrective action unit model results to proceed with model evaluation studies.”

EM Nevada has witnessed its groundwater program reach several key milestones over the past decade. Frenchman Flat was the first groundwater corrective action area to reach closure in 2016. The Yucca Flat groundwater region was brought to closure in 2020. The same year, the Rainier Mesa and Shoshone Mountain groundwater corrective action areas reached closure three years ahead of schedule to the result of $5 million in federal savings.

Pahute Mesa at NNSS
A look at a slice of the Pahute Mesa groundwater region from a distance.

The Pahute Mesa groundwater region posed unique challenges. Of the 828 underground nuclear tests that occurred at the NNSS from 1951 to 1992, only 82 occurred on Pahute Mesa land. However, that 10% or so of tests contained about 60% of the radioactivity released at the site.

One advantage has been the accumulation of data available to the program. Nearly 70 years of sampling, testing and analysis provided a baseline of information. EM Nevada drilled 11 additional wells between 2009 and 2016 and in total, EM Nevada collects groundwater samples from over 100 monitoring locations on and around the NNSS.

“The success at Pahute Mesa is built from the investment the DOE has made in sampling and modeling over the last 30 years and the contributions of a half dozen collaborating organizations,” said UGTA Project Manager Ken Rehfeldt of Navarro Research & Engineering, the EM Nevada Environmental Program Services contractor.

EM Nevada carried out years of data collection and modeling that culminated in a report that serves as a comprehensive forecast of groundwater movement and contaminant boundaries in the Pahute Mesa region.

“This path began 20-plus years ago with some hesitation as to whether it could be effectively achieved but those hurdles were overcome,” said EM Nevada Deputy Program Manager Bill Wilborn.

NNSS map
Map of Nevada National Security Sites Groundwater Characterization Areas.

What comes next is new data collection, testing and analysis of groundwater that is expected to take four years. Then, the groundwater region will ultimately be closed by EM Nevada.

“Model evaluation is a significant step to assess the reliability of flow and contaminant transport model results and further strengthen confidence that the model results can be used for regulatory decisions necessary for closure,” Myers said.

Rehfeldt echoed the importance of reaching the milestone.

“This is a big step. It was potentially a pinch point but with DOE and collaborator support we were successful,” Rehfeldt said. “We will evaluate and update the model based on new information and if that model is acceptable to the state when it’s finished, then the final stage is the closure stage.”

Collaborators on the project that made significant contributions include the Lawrence Livermore and Los Alamos national laboratories, Desert Research Institute, U.S. Geological Survey and Mission Support and Test Services, which is the NNSS management and operations contractor.

EM Nevada expects to drill additional wells in 2024 to provide additional data to aid with model evaluation. Work is on schedule to complete the corrective action process and achieve closure of the Pahute Mesa area by the end of 2028.

Click here for more information on the EM Nevada Groundwater Program.

-Contributor: Grant Johnson