Suppose you turn this over to the states. I used to live in Charlotte, which draws it water from the Catawba river and returns it to the Catawba after treatment a few miles above the state line with South Carolina where several communities downstream draw there drinking water. In the interests of saving money, the city requests that the state agency not require them to treat the water before returning it tout the river. The state agency allows it knowing that it will only affect a few miles of river in North Carolina. What remedy would cities downstream in South Carolina have other than more expensive treatment options?
What exactly has the executive-branch EPA done to solve problems such as in your hypothetical scenario?