Pennsylvania environmental regulators are pursuing a record $4.5 million fine against a gas driller over what they describe as a major case of pollution from a leaking waste pit.

The Department of Environmental Protection filed a civil complaint against EQT Corp. on Tuesday, accusing the Pittsburgh-based company of polluting streams and groundwater and harming trees and other vegetation around its impoundment in Duncan Township, Tioga County.

The company has been uncooperative during the investigation and “fails to recognize the ongoing environmental harm” from its impoundment, Acting DEP Secretary Dana Aunkst said in a statement.

EQT, one of the biggest drillers in the Marcellus Shale natural gas field with more than 400 active wells in Pennsylvania, responded aggressively, noting the agency filed its administrative complaint less than a month after EQT took regulators to court over their interpretation of the state’s Clean Streams Law.

“The timing is suspect, and the exorbitant proposed penalty is inconsistent with both past and recent agency practice,” said Lewis Gardner, EQT’s general counsel and vice president.

The penalty would be the largest the state has ever imposed on the shale gas industry. Last month, Fort Worth, Texas-based Range Resources agreed to pay a $4.15 million fine for environmental violations at a Western Pennsylvania site that handled natural gas drilling waste.

EQT’s impoundment held wastewater from the hydraulic fracturing process. Problems began cropping up in April 2012, and more than 200 holes were ultimately found in the pit’s lining, according to DEP. An unknown but significant amount of wastewater fouled streams, springs and groundwater, the DEP said.