Pennsylvania state regulators have fined natural gas drilling company Chesapeake Energy Corp. more than $1 million, which officials said Tuesday is the single largest fine for an oil or gas operator in the state.

The state Department of Environmental Protection said the fine stems from Chesapeake's contamination of private water supplies in northern Pennsylvania's Bradford County and a February tank fire at a drilling site in southwestern Pennsylvania's Washington County.

"It is important to me and to this administration that natural gas drillers are stewards of the environment, take very seriously their responsibilities to comply with our regulations, and that their actions do not risk public health and safety or the environment," DEP Secretary Mike Krancer said in a statement.

A Chesapeake spokesman said the Oklahoma City-based company will issue a statement shortly.

Chesapeake is one of the most active companies pursuing the lucrative Marcellus Shale natural gas formation, considered the nation's largest natural gas reservoir. Since the beginning of 2008, more than 3,000 Marcellus Shale wells have been drilled in Pennsylvania, raising fears of river and groundwater pollution.

In Bradford County, the department said that improper well casing and cementing allowed natural gas to seep into groundwater and contaminate 16 families' drinking water wells. The department began investigating the complaints last year.

In Avella in southwestern Pennsylvania, three condensate separator tanks caught fire on Feb. 23, injuring three subcontractors working on-site, the DEP said.

In December, the agency announced a settlement with Houston-based Cabot Oil & Gas Corp. to pay $4.1 million to residents in the northern Pennsylvania town of Dimock where private water wells were also contaminated with methane gas.