EPA sets the minimum standards, the states have the authority to decide on how to meet those standards. It is also likely that those states located in EcoTopia will set standards more strict than EPA.
Over the next 20-30 years these new pollution regs will have a significant impact on the country. They will mainly affect agriculture, mining, and silvaculture. To some degree they will affect citizen behavoir in the form of more stringent zoning laws and in certain cases affect an individual with a run-off problem.
Primarily to be used for water pollution, it is unknown how it will be used to control air pollution in that rain washes the air pollution into the rivers.
Some states have set up seperate agencies to handle this while others have split it among existing agencies.
Much info is available on the internet.
These are usually "sweetheart suits" between NRDC and EPA that lead to pre-concluded consent decrees, hardly what I would call settled law. The nonpoint regs are "put in place" by fraudulent mechanics, often ignoring the data submitted in support of it. Santa Cruz County, one of the two complicit local jurisdictions in California, was just such a case.