It’s changed over the years, but here is what I found from The United States Government Manual 1991/92:

The purpose of the Environmental Protection Agency is to “protect and enhance our environment today and for future generations to the fullest extent possible under the laws enacted by Congress.  The Agency’s mission is to control and abate pollution in the areas of air, water, solid waste, pesticides, radiation, and toxic substances.  Its man date is to mount an integrated, coordinated attack on environmental pollution in cooperation with State and local governments.”