(Adds details, quotes from Ohio Environmental Council)
March 7 (Reuters) - Exxon Mobil Corp's XTO Energy unit said on Wednesday it plugged a blown out natural gas well in rural southeast Ohio that had been leaking for nearly three weeks.
The Feb. 15 blowout in Belmont County had spewed millions of cubic feet of gas into the air, triggering evacuations of nearby residences and raising concerns among environmental groups about health and environmental impacts.
Exposure to low levels of natural gas is not harmful to human health, according to the National Institutes of Health, but extremely high levels can cause loss of consciousness or death by displacing oxygen.
"We would like to press for a full accounting of the damage," said Melanie Houston, director of climate programs for the Ohio Environmental Council, an environmental advocacy group.
XTO spokeswoman Karen Matusic said the company could not immediately say how much gas leaked from the well, which was about to be put into production after being drilled and fracked.
An initial report from the Environmental Protection Agency on Feb. 17 estimated the natural gas flow rate from the well at 100 million cubic feet per day. Earthworks, an environmental group, compared the magnitude of the XTO well blowout with some of the biggest methane releases in the United States.
Matusic said the company has been taking air samples since the blowout and "never picked up anything that would harm humans or animals."
Following the well blowout, emergency responders evacuated about 30 homes within one mile of the well. Residents of all but four homes located within a half mile of the well were able to return home within a few days, Matusic said.
Ohio Department of Natural Resources (DNR) is the lead government agency at the XTO well pad. Officials at the DNR were not immediately available for comment
The U.S. EPA said it responded to a fire at the well on Feb. 15 to provide technical assistance and air monitoring at the site. Because there were no apparent release of oil or hazardous substances, the EPA said it demobilized on Feb. 21.
An unknown quantity of brine and produced water, estimated to be more than 5,000 gallons, was initially discharged to streams that flow into the Ohio River, according to the EPA.
Protected wildlife species located in proximity to or downstream from the well site are the Eastern Hellbender Salamander, Northern longeared bat, and protected fish. (Reporting by Scott DiSavino in New York and Kim Palmer in Cleveland Editing by Leslie Adler and Diane Craft)