These wells once produced oil but are now unused and don't have a designated person or company responsible for maintaining or capping them. They can provide a path for pollution, like oil or brine, to contaminate underground freshwater supplies.
A new study from the U.S. Geological Survey assessed which aquifers are most at risk from orphan wells across the country.
"We wanted to get an understanding of which of these systems might have a large number of these wells and also might have some other factors that we know might be important when thinking about these orphan wells and water quality," lead study author Josh Woda said.
The researchers took aquifers that are major sources of drinking water and irrigation across the country (the study calls these "principal aquifers") and cross-referenced them with a map of known orphan wells.
"Around 50% of these documented orphan wells that we know about are located in these aquifers where 90% of the groundwater is coming from," said Karl Haase, another author on the paper.
The Ada-Vamoosa aquifer — stretching from Osage County to Seminole in Oklahoma — has more orphan wells per square mile than any other principal aquifer in the U.S. The Garber-Wellington in central Oklahoma ranks fourth. Both of these aquifers are mostly used for drinking water.
"What really stood out to us about this region was really just the sheer number of wells within a relatively small area of these aquifer systems," Woda said.
Orphan wells generally pose more risk to water the older they are.
"If these wells have been sitting there for a long time with potentially no one looking at them, there's some questions about well integrity over time," Woda said.
On average, orphan wells in the Garber-Wellington are around 50 years old — that's about middle of the pack for aquifers across the country. But none of the 1,070 wells in the Ada-Vamoosa have reported ages.
For both aquifers, the vast majority of the wells were in undeveloped areas. But all the orphan wells on the Ada-Vamoosa and more than 80% of those on the Garber-Wellington were located within communities classified as disadvantaged by (now-removed) EPA metrics. Those communities were determined by census data, like poverty and race. They also include tribal nation reservation lands and off-reservation trusts.
Researchers haven't yet surveyed for actual occurrences where orphan wells contaminate drinking aquifers. But Woda said this risk analysis points to areas where additional studies are most needed.
The study groups aquifers with similar risks and characteristics together, which Haase said could be useful for policy discussions and public awareness.
"If you are in Oklahoma, you could look at the study and say, 'Hey, look, there's some similarities to aquifers in Pennsylvania and perhaps Texas,'" Haase said. "You can find aquifers that are similar to yours and see if there's something that you could learn or develop a better understanding of what's going on."
This report was produced by the Oklahoma Public Media Exchange, a collaboration of public media organizations. Help support collaborative journalism by donating at the link at the top of this webpage.
