U.S. oil fields are spinning off thousands of tons of low-level radioactive trash as the drilling boom leads to a surge in illegal dumping and states debate how much landfills can safely take.

State regulators are caught between environmental and public health groups demanding more regulation and the industry, which said it’s already taking proper precautions. As scientists debate the impact of small amounts of radiation on cancer risks, the Environmental Protection Agency said there’s not enough evidence to say what level is safe.

Left to police the waste, state governments are increasing their scrutiny of well operators. Pennsylvania and West Virginia are revising limits for acceptable radiation levels and strengthening disposal rules. North Dakota is doing the same, after finding piles of garbage bags filled with radioactive debris in an abandoned building this year.

“We have many more wells, producing at an accelerating rate, and for each of them there’s a higher volume of waste,” said Avner Vengosh, a professor of geochemistry at Duke University in Durham, N.C., who’s studied the issue. Without proper handling, “we are actually building up a legacy of radioactivity in hundreds of points where people have had leaks or spills around the country.”

The waste is a byproduct of the drilling renaissance that has brought U.S. oil and natural gas production to its highest levels in three decades -- while also unlocking naturally occurring radium from rock formations far underground.

It’s the latest environmental challenge for an industry that’s pushed the U.S closer to energy independence while facing questions about the effects on water supplies and air pollution -- and even a surge in earthquakes.

“There’s all kinds of regulations about how to deal with waste and how to transport waste and we follow them all,” said Andrew Paterson, a vice president at the Marcellus Shale Coalition, a trade group that represents companies active in Pennsylvania drilling including Range Resources Corp. (NYSE: RRC) and Noble Energy Inc. (NYSE: NBL). “It’s very safe and the levels that we’re talking about are really low-level.”

Questions to Fort Worth, Texas-based Range, a pioneer of Marcellus drilling, were referred to the Shale Coalition. A call to Noble, based in Houston, wasn’t returned.

Studies have found higher radiation levels at oil wells since the 1970s, said Vengosh. What’s new is the volume of material being produced. “It’s just become much more abundant right now with the intensity” of drilling, he said.

The issue is shale rock, the dense formations found to hold immense reserves of gas and oil. Shale often contains higher levels of radium -- a chemical element used in industrial X-ray diagnostics and cancer treatments -- than traditional oil fields, Vengosh said.

Freeing gas and oil is a water-intensive process called hydraulic fracturing, or fracking, in which drill bits cut thousands of feet through shale fields to make way for high pressure water streams that pulverize the rock. The process displaces radium-tinged subterranean water that comes up through the wells, where it can taint soil and surface equipment. Radiation levels can build up in sludges at the bottom of tanks, pipeline scale and other material that comes in extended contact with wastewater.

Some states allow the contaminated material to be buried at the drillsite. Some is hauled away, with varying requirements for tracking the waste. Some ends up in roadside ditches, garbage dumpsters or is taken to landfills in violation of local rules, said Scott Radig, director of the North Dakota Health Department’s division of waste management.

In that state’s Bakken oil fields, “it’s a wink-and-a-nod situation,” said Darrell Dorgan, a spokesman for the North Dakota Energy Industry Waste Coalition, a group lobbying for stricter rules. “There’s hundreds of thousands of square miles in northwestern North Dakota and a lot of it is isolated. Nobody’s looking at where all of it is going.”

That’s one of the problems the state is trying to fix with rules announced last week requiring well operators to install leak-proof containers for temporary storage onsite and to use licensed waste haulers and landfills. North Dakota, the biggest oil-producing state after Texas, has commissioned a study of radiation risks that may spur further changes, Radig said.

In the meantime, North Dakota landfills have installed radiation detectors to try to catch loads exceeding the state’s current limits. Anything higher must be trucked hundreds of miles to dumps in neighboring states that have less restrictive limits.

On Feb. 28, North Dakota officials found hundreds of radioactive “filter socks” -- used to strain wastewater from wells -- dumped in an abandoned building in Noonan, just south of the Canadian border. The filters registered about 40 microrems an hour of radiation, about five times the naturally occurring “background level” in the area, Radig said.

That’s below the 1,500 microrems delivered from a typical dental X-ray. Still, it could be a danger if people got close enough to inhale or ingest radioactive dust, Radig said. Radiation levels also can increase if material accumulates in the environment.

“Just mention the word ‘radioactive’ and it scares a lot of people,” Radig said. “They’re angry about what’s going on.” His agency has received more than a dozen reports this year of oilfield debris rejected by landfills because of radiation concerns.

North Dakota wells may produce 27 tons a day of filter socks alone, Radig said, citing a private hauler’s estimate. While most material is handled properly, it’s “clearly not enough. There is definitely some illegal dumping going on.”

The state hired a contractor last week to remove the Noonan filter socks. The operation will cost about $13,000 and use money from an industry-backed fund to clean up abandoned oil and gas wells, according to a Health Department statement.

Oil and gas companies have been dealing with radioactive waste for decades, said Kari Cutting, vice president of the North Dakota Petroleum Council, an industry trade group in Bismarck. “The majority of companies that have been active in other shale plays know how to deal with it,” she said. “We do not condone illegal dumping or improper management in any form.”

In West Virginia, on the edge of the gas-rich Marcellus formation, lawmakers voted last month to require landfills to install radiation monitors and to build separate, lined cells designed to contain drilling debris. The law, signed by Gov. Earl Ray Tomblin March 31, also expanded the amount of oil and gas that waste landfills can accept.

With proper precautions, landfills are the safest place for the debris, said Thomas Aluise, a spokesman for the state’s Department of Environmental Protection. “A lot of operators were just burying them on site, unchecked, all over the state,” he said.

While it’s unclear how much drilling waste is produced nationally, state totals are rising. West Virginia landfills accepted 721,000 tons of drilling debris in 2013, a figure that doesn’t include loads rejected because they topped radiation limits. The per-month tonnage more than tripled from July 2012, when records were first kept, through last December.

In Pennsylvania, epicenter of the Marcellus boom, the oil and gas industry sent 1.3 million tons to landfills last year. That included 16,000 tons of radioactive material, according to Lisa Kasianowitz, a spokeswoman for that state’s Department of Environmental Protection.

Pennsylvania allows producers to bury some waste on site in lined pits. It’s drafting rules to discourage that as a permanent option, Kasianowitz said in a telephone interview. Further changes could come after Pennsylvania completes a study of radiation risks that’s looking at everything from worker safety at the wellhead to allowable levels in landfills. Results are due later this year, she said.

Texas last revised its rules on radioactive material in 2003 and currently has no plans to change them, said Ramona Nye, a spokeswoman for the state’s Railroad Commission, which oversees drilling. The state allows the waste to be taken to a landfill, buried on site or mixed into the soil near a well, as long as it’s treated to reduce radiation below set levels.

The rules are “more than adequate” to handle oil and gas waste safely, Nye said in an email.