The noxious odor primarily affected three townhouses in Skippack after contaminated water got into the sump pumps, officials said. Firefighters were flushing those pumps with fresh water and airing out the homes with fans on Monday evening.
"We're relieved and hopeful that what they're doing will fix it," said resident Kourtnay Loughin, who called 911 about the "very, very unpleasant" smell on Sunday evening.
Officials don't know how the gas got into the groundwater, but Fire Chief Haydn Marriott said residents' health was not in danger. He also noted the substance was found in a system with sealed pipes, creating little chance of contamination to the surrounding area's water supply.
Firefighters and environmental officials used sensors Sunday night to try to identify the problem. Residents of more than 150 townhouses were asked to leave after chemical odors were detected at several units.
Metered equipment originally indicated the presence of hydrogen cyanide, a poisonous gas, in the homes, but officials later said those readings were false positives. No signs of illness were reported.
By Monday morning, Marriott said authorities had determined sump pumps as the source of the smell, but could not identify its composition. As labs began analyzing samples, officials retested some houses and allowed owners to return after finding no danger.
Loughin's unit and the two next door registered high levels of the chemical odor.
"It's more of an annoyance than anything," Marriott said.
Readings in 25 other units Monday found "negligible" levels of chemicals, likely attributable to common household items, he said. The rest of the townhomes had no detectable chemical compounds.
Marriott said his department spoke to the water authority several times Sunday. "They assured us that there was no problem with the water, that they were going to take care of the testing on their end," he said.
Officials plan to monitor the affected homes in the coming days.