Update 1:
From the comments, MGross has found the entire ruling, here:
http://www.pacourts.us/...
Update 2:
In the comments, RLMiller tells us the ruling is thanks largely to a special provision in Pennsylvania's Constitution that guarantees all residents the right to clean air and pure water.Adam B found said provision, enacted in 1971:
The people have a right to clean air, pure water, and to the preservation of the natural, scenic, historic and esthetic values of the environment. Pennsylvania's public natural resources are the common property of all the people, including generations yet to come. As trustee of these resources, the Commonwealth shall conserve and maintain them for the benefit of all the people.