HARRISBURG — Tests of water in Pennsylvania downriver from treatment plants that handle wastewater from natural gas drilling raised no red flags for radioactivity, the state Department of Environmental Protection said Monday.

All of the samples, taken in November through December, showed levels at or below normal, naturally occurring background levels of radioactivity, the agency said. All samples also showed levels below the federal drinking water standard for Radium 226 and 228, it said.

Concentrations of naturally occurring radium are usually low, although higher levels may exist in some areas.

Long-term exposure increases the risk of developing several diseases, such as lymphoma, bone cancer, and diseases that affect the formation of blood, according to the U.S. Environmental Protection Agency.

The state DEP, which also released what it said are the actual test results on Monday, did not say what the tests showed on other gas-drilling related pollutants. A spokeswoman said the agency released the results as soon as they came back from the laboratory.

DEP said it installed the sampling stations last fall to monitor the impact of Marcellus Shale gas drilling on water quality. The fast-growing industry in Pennsylvania produces vast amounts of chemically tainted and sometimes radioactive water that gushes from the ground when drillers use a technique known as hydraulic fracturing, or fracking.

An analysis by the Associated Press this year showed that some of that wastewater is trucked to sewage treatment plants and industrial treatment plants that discharge into rivers that also supply drinking water for many Pennsylvanians. In addition to radium, which exists naturally in Pennsylvania rock, the frackwater contains salty dissolved solids, barium, strontium, benzene and fracking chemicals.

Pennsylvania’s treatment plants are not equipped to remove all of the pollutants from the gas-drilling wastewater and the state allows the flushing of the partially treated wastewater into the rivers. But under pressure from the state and environmentalists, energy companies have begun recycling some of the frackwater and reusing it, rather than hauling it for treatment and river discharge.

John Hanger, who ran the Department of Environmental Protection last year when it installed the water monitors, said the agency’s radiation protection staff concluded that the radioactivity was not a public threat.

“Absolutely 100 percent conclusive tests are at the drinking water suppliers,” Hanger said. “What we ultimately care about is what people drink.”

Radium comes from the rock that’s underground, and is picked up by the water that is injected forcefully down the well to break up the shale deep underground and release the gas trapped inside.

When the water returns through the well to the surface, it can have many times the radiation levels that the federal government says is safe for drinking water, said John Veil, who recently retired from the Argonne National Laboratory in Washington, D.C., where he managed the water policy program.

At the industrial treatment plants that handle much of the state’s drilling wastewater, some of the radium in the wastewater is removed in processes that target particles and heavy metals, Veil said. Some also may end up discharged with the rest of the treated water into a river, where it can go downstream, he said.

A handful of municipal sewage treatment plants that handle a smaller amount of wastewater may be less adept at removing radium because they aren’t equipped to treat metals in the same fashion as the industrial plants.

Of the 10.6 million barrels of wastewater that gushed from the wells in the final six months of 2010, at least 65 percent were recycled for use in drilling another gas well, a dramatic increase.