EPA Announces Final Cleanup Plan for Drinking Water Aquifer on Tohono O’odham Nation | US EPA

Cleanup will pave the way to beneficial groundwater use, ensuring future drinking water security

August 3, 2023

TUCSON – The U.S. Environmental Protection Agency (EPA) has finalized its plan for the cleanup of the Cyprus Tohono Mine Site Basin Fill Aquifer under the Comprehensive Environmental Response Compensation and Liability Act (CERCLA), also known as the Superfund law. The newly signed Record of Decision outlines a cleanup plan which will use reverse osmosis treatment for groundwater contaminated with perchlorate, uranium, and sulfate, and will restore groundwater as a drinking water resource.

The Cyprus Tohono Mine Site is located on the Tohono O’odham Nation near the Village of North Komelik, about 30 miles south of Casa Grande, Arizona. Mine waste which caused the groundwater contamination was removed in 2008, but an approximate 4-mile-long plume of groundwater polluted by perchlorate, sulfate, and uranium remains in what is known as the Basin Fill Aquifer. This aquifer was formerly a drinking water source, but alternative drinking water is currently being supplied to nearby residents due to its contamination.

“Ensuring access to clean and safe drinking water is one of EPA’s most important missions,” said EPA Director of Region 9 Superfund and Emergency Management Division Michael Montgomery. “Today’s announcement is an essential step in reducing the burden on communities in the region, including the Tohono O’odham Nation.”

The selected cleanup plan will pump out and treat contaminated groundwater with a technology called reverse osmosis, a water treatment process that removes contaminants from water using pressure to force water through a semipermeable membrane where the contaminants are filtered out. Treated water will then be reinjected back into the aquifer or made available for other beneficial uses. Additionally, wells will be installed to monitor the site contamination to ensure it is cleaned up as intended.

While the cleanup plan selected cleans up the site in the shortest timeframe, EPA estimates it will take about 30 years to complete, with an additional 20 years of monitoring. This site is being cleaned up under what’s known as the Superfund Alternative Approach program.

The Cyprus Tohono Mine site’s contamination originally stems from copper sulfide and oxide ore mining operations in the 1880s. During the 1950s and 60s, the site included a small open pit copper oxide mine, which eventually was enlarged to remove 350,000 tons of ore. From 1975-1997, large-scale copper mining produced about 25 million tons of ore. Cyprus Tohono Corporation began operating the property in 1987. Since 2009, no active mining has occurred at the site.

Learn more by visiting EPA’s Cyprus Tohono Mine Site webpage.

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