Federal and state regulators need to assess a new public-health threat from natural-gas drilling - radiation in Pennsylvania's water.

Wastewater samples from 116 of 179 Marcellus Shale gas wells have been found to contain high levels of radiation. But wastewater discharges into rivers and streams haven't been tested for radiation, the New York Times reported, even though environmental regulators and industry officials have known about the risks.

Sen. Bob Casey (D., Pa.) is calling on the U.S. Environmental Protection Agency and the state Department of Environmental Protection to inspect Pennsylvania's drinking water for radiation. He also wants the agencies to explain "why sufficient inspections haven't taken place." It's a great question.

The drilling boom in Pennsylvania clearly has put a strain on state inspectors, and on the state's water-treatment infrastructure. Drilling companies were issued more than 3,300 Marcellus gas-well permits in 2010, up from 117 in 2007. There are now 2,815 shale-gas wells in Pennsylvania, with thousands more to come.

Drillers use millions of gallons of fresh water to drill each well. In a technique called hydraulic fracturing or "fracking," they pump the water under high pressure more than a mile underground to break apart the shale and release the methane trapped within.

Much of the water returns to the surface, in highly salty concentrations and mixed with industrial chemicals. The drilling companies have been hauling wastewater to municipal treatment plants, where it is eventually returned to rivers and streams.

But until now, it wasn't widely known that the wastewater also contains radioactive material. The shale has high levels of naturally occurring elements such as radium and uranium, which can be released during fracking. In some cases the levels of radioactivity in the wastewater have been hundreds or thousands of times higher than what is allowed in drinking water.

Obviously, the public doesn't drink wastewater. But sewage treatment plants generally aren't capable of removing radioactive material from water.

And tests are not being conducted to see whether the treated water is radioactive. The Times article said no testing had occurred since 2008 at more than 65 water-intake sites around the state. Most of these plants haven't been tested since 2005, well before the Marcellus gold rush began.

Two companies, Pittsburgh Water and Sewer Authority and Pennsylvania American Water Co., plan to start testing immediately. That's good news. But regulators should not have allowed these firms to be caught off guard from the potential threat.

The drilling industry says it increasingly recycles its wastewater rather than trucking it away for treatment. But the burden on water treatment plants will remain as thousands more wells are drilled.