The National Pollutant Discharge Elimination System (NPDES) was established to protect natural waters by ensuring that direct discharges to water bodies meet certain criteria. NPDES permits apply to point sources of pollution, such as effluent from a wastewater treatment plant or storm drainage piped into a stream, but since 1987 do not apply to “nonpoint” sources of pollution such as agricultural runoff. There are only a few leverage points, mainly hinging on economic incentivization, for controlling nonpoint source pollution.
Most major industrial and municipal wastewaters are regulated with permits administered by the state, which set distinct chemical and physical water quality parameters for discharge (e.g. pH 6.5-9.0). Some permits are general and incorporate entire categories of sources, while individual permits take specific situations into consideration. Individual permits are open to public comment before they are finalized but can be quite difficult to modify once they are set. Permittees are usually responsible for monitoring and reporting their discharge water chemistry to demonstrate compliance with their NPDES permit.
Most sources that discharge stormwater, such as construction sites or a city’s stormwater system, are subject to general permits that set expectations of best management practices “to the maximum extent practicable.” These include aspects like public education but do not set discharge water quality criteria. If, however, the receiving water doesn’t meet federal water quality recommendations, then each source contributing to it may be subject to a waste load allocation (WLA) goal. Note that there is no enforcement authority for nonpoint source WLAs. Water Quality Trading is a way to economically incentivize nonpoint sources to use better conservation practices, and are developed and administered on a local level.