Mapping Groundwater Risks from Orphan Wells Across U.S. Aquifers

A new study by the U.S. Geological Survey offers the first national geospatial analysis of how orphaned oil and gas wells — unplugged and ownerless — may threaten major aquifers across the United States. Researchers examined over 117,000 documented orphan wells and found that more than half are located within aquifers that supply 94 percent of the country’s groundwater, raising serious concerns about contamination from aging, failure-prone infrastructure.

The study highlights specific regions at high risk, including Pennsylvania and the broader Appalachian basin, where many of the country’s oldest wells were drilled and where well casing failures are more likely. Gulf Coast aquifers, which include contamination-prone wetlands and open water areas, and densely drilled areas of California’s Central Valley and coastal zones also emerged as high-vulnerability zones. In central Oklahoma, the Ada-Vamoosa aquifer has the highest concentration of orphan wells per square mile.

Researchers recommend that policymakers and groundwater managers use this mapping as a tool to prioritize monitoring, remediation, and further local investigations. With tens of thousands of additional orphan wells added to federal datasets in recent years — and more being discovered — the threat to drinking water sources is growing. The findings underscore the need to accelerate well-plugging efforts and consider groundwater protection as a central concern in energy transition and land reuse strategies.