MASONTOWN, Pa.  For years, residents here complained about the yellow smoke pouring from the tall chimneys of the nearby coal-fired power plant, which left a film on their cars and pebbles of coal waste in their yards. Five states  including New York and New Jersey  sued the plant’s owner, Allegheny Energy, claiming the air pollution was causing respiratory diseases and acid rain.

So three years ago, when Allegheny Energy decided to install scrubbers to clean the plant’s air emissions, environmentalists were overjoyed. The technology would spray water and chemicals through the plant’s chimneys, trapping more than 150,000 tons of pollutants each year before they escaped into the sky.

But the cleaner air has come at a cost. Each day since the equipment was switched on in June, the company has dumped tens of thousands of gallons of wastewater containing chemicals from the scrubbing process into the Monongahela River, which provides drinking water to 350,000 people and flows into Pittsburgh, 40 miles to the north.

“It’s like they decided to spare us having to breathe in these poisons, but now we have to drink them instead,” said Philip Coleman, who lives about 15 miles from the plant and has asked a state judge to toughen the facility’s pollution regulations. “We can’t escape.”