PITTSBURGH -- High levels of an ultra-salty compound that could be linked to oil and gas drilling persist in the Allegheny River's Pittsburgh-area watershed, while the levels declined in the nearby Monongahela River, recent research shows.

Officials at public water utilities in both watersheds grew concerned in 2009 and 2010 when bromide levels soared during a surge of Marcellus Shale gas drilling. Although not considered a pollutant by themselves, the bromides combine with chlorine used in water treatment to produce compounds that can threaten public health.

A recent Pittsburgh Water and Sewer Authority report found that high levels of bromides persisted this year in the Allegheny just downstream from industrial brine treatment plants. The plants accept wastewater from oil and gas drilling and other industrial activities.

Also, preliminary research by a Duke University team found a similar problem in a tributary of the Allegheny, professor Avner Vengosh told The Associated Press on Monday. Vengosh said the source there appears to be from conventional oil or gas wells, not shale wells.

But on the Monongahela River, a Carnegie Mellon University team said last week, preliminary research found that bromide levels declined significantly this year, after Marcellus Shale gas drillers responded to warnings from scientists and environmental groups and voluntarily stopped taking waste to treatment plants there. The Monongahela merges with the Allegheny at Pittsburgh to form the Ohio River.

In early 2011, the Pennsylvania Department of Environmental Protection called on shale gas drillers to voluntarily stop taking wastewater to public water treatment plants along rivers, and major companies and industry groups agreed to the request. Now, most shale wastewater is sent to deep underground waste wells in Ohio or recycled.

The DEP wastewater request doesn't apply to conventional oil and gas well wastewater, and Vengosh said that doesn't make sense.

"I think the focus on only shale gas is kind of misleading," Vengosh said, noting that all the wells produce naturally occurring brine water, which can be much saltier than seawater, and also contain heavy metals and natural radiation.

"It's all psychological," Vengosh said of the distinction between shale gas waste and other drilling waste. "That, for me, doesn't make any sense."

The Marcellus Shale lies under parts of Maryland, New York, Ohio, Pennsylvania and West Virginia, and the extraction procedure called hydraulic fracturing, or fracking, has made it possible to tap into deep reserves of oil and gas. The boom in shale gas fracking raised concerns about pollution. Large volumes of water, along with sand and hazardous chemicals, are injected underground to break rock apart and free the oil and gas.

Regulators contend that water and air pollution problems are rare, but environmental groups and some scientists say there hasn't been enough research on these issues. The industry and many federal and state officials say the practice is safe when done properly.

The Water and Sewer Authority report also noted that bromide levels rose in rivers below where some coal-fired power plants discharge wastewater, which can also include bromides.

Dave Mashek, a spokesman for the Pennsylvania Independent Oil and Gas Association, said that state regulators don't set a limit on bromide discharges and that the amount of wastewater that comes from conventional wells is decreasing.

He also noted that the Water and Sewer Authority testing only identified elevated bromide levels in the Allegheny for part of the year, during periods of low river flow.

Kevin Sunday, a spokesman for the Department of Environmental Protection, said many other sources of bromide exist beyond oil and gas wells. He said the volume of wastewater produced by conventional oil and gas wells is substantially lower than what comes from shale gas wells.