One of the fundamental principals of the RCRA is that dilution is not an allowable solution to pollution. Otherwise, you could just say that any amount of pollution is below applicable concentrations after it mixed into the oceans, atmosphere, whatever. And any company could emit as much as they wanted as long as they diluted it. Oil spills could simply be left alone because they’d eventually distribute throughout the earth.
Concentrations must be considered as they occur in their process streams. The process stream must meet certain requirements first and foremost, and it must be further checked to see if that could significantly affect the air or water in which it is emitted, just to make sure its good to go since water flow, temperature, and wildlife migration change throughout the year. The same is true for air emissions as well.