Constitution is the supreme law of the United States. It has evolved over time. Several clauses form the basis of federal environmental policy in America. Allows federal government to regulate interstate and foreign commerce. Trade across state and national boundaries is common.
Federal law prevails over state law when they conflict.
This is the basis of the preemption doctrine. Federal law preempts state law if federal law supersedes state law, or if area is Constitutionally reserved for federal authority alone. However, federal and state authority often overlap, and questions of preemption are often decided case by case in the judiciary. Preemption doctrine is a question of balance between federal and state authority. Federal authority has grown over time.