The EPA found Exxon Mobil stored more than 1 billion gallons of hazardous waste illegally at a fertilizer plant in Pasadena, Texas.
The company in a settlement agreed to spend more than $150 million to clean the site and monitor groundwater for the next 50 years.
Agrifos Fertilizer bought the 509-acre facility from the Texas oil company in 1998. The site includes a processing facility to produce phosphoric acid from mineral ores.
"Mining and mineral processing facilities generate more toxic and hazardous waste than any other industrial sector, based on EPA's toxic release inventory," the environmental regulator said in a statement. "If not properly managed, these facilities pose a high risk to human health and the environment."
Exxon Mobil as the prior owner of the site retained responsibility for waste issues at the plant.