PITTSBURGH -- Researchers at Carnegie Mellon University say a water quality problem in the Monongahela River that may have been linked to Marcellus Shale natural gas drilling is going away.

Jeanne VanBriesen said Thursday that preliminary data from tests this year showed that levels of salty bromides in the river have declined significantly when compared to 2010 and 2011. In many cases the bromides were at undetectable levels this year, and in general they returned to normal levels.

"These are very nice, low bromide levels, where we would like them to be,'' VanBriesen said of the 2012 test results, which were presented at a water quality conference in Pittsburgh.

VanBriesen said the decline appears to coincide with a voluntary ban on disposing gas drilling wastewater that took effect in the spring of 2011. The wastewater contains large amounts of naturally occurring, ultra-salty bromides, and drillers had been taking millions of barrels of it to conventional wastewater treatment plants that discharge into the Monongahela River.

But in early 2011, the Pennsylvania Department of Environmental Protection called on drillers to voluntarily stop using riverside plants to get rid of the wastewater, and major companies and industry groups agreed to the request.

The state's request was made after VanBriesen and other researchers presented evidence that the discharges were altering river chemistry in a way that had the potential to affect drinking water, and operators of municipal water supplies grew concerned. Although not considered a pollutant by themselves, the bromides combine with the chlorine used in water treatment to produce compounds that can threaten public health, and levels had soared in 2009 and 2010. Bromide levels in the river declined somewhat in 2011, but not enough for researchers to say that the river had returned to normal levels.

Another researcher at the conference said other Monongahela River water quality trends have been positive, too.

Rose Reilly, a biologist at the U.S. Army Corps of Engineers, has been doing conductivity tests on the Monongahela River. That helps scientists estimate the amount of dissolved material in the water, and lower readings are better.

"There is a general downward trend'' in conductivity levels for 2012, Reilly said. "That's good news.''

Steve Forde, a spokesman for the Marcellus Shale Coalition, an industry group, said its members had "been committed from the very outset'' to complying with the state request to stop discharging wastewater at regular treatment plants. Now, most of the brine is either recycled or sent to deep underground wells for disposal.

Still, VanBriesen and Reilly cautioned that the Monongahela River is still far from being a pristine waterway. Coal-fired power plants and old flooded coal mines also discharge water into the Monongahela River that can contain bromides and other pollutants, and many other industries are based in the region.

The Marcellus Shale lies under parts of Pennsylvania, New York, Ohio, West Virginia and Maryland. The gas drilling procedure hydraulic fracturing, or fracking, has made it possible to tap into deep reserves of oil and gas but has 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 those issues. The industry and many federal and state officials say the practice is safe when done properly.