Environmental Legislation was passed in 1970, known as the National Environmental Policy Act, along with another law called the Environmental Quality Improvement Act.  The laws exist to keep the environment safe from public or private actions that could cause damage. 
Environmental law is a combination of three things. The first is the law that is passed through Congress. The second is the list of regulations set by the Environmental Protection Agency in relation to that particular law. And the third is the legal interpretation of those regulations by the federal courts, in the event of an issue.