The legal and scientific definitions of the term “point source” are not synonymous.

Scientifically, a point source is any discharge where the pollution is localized and highly concentrated. As a result, significant environmental damage might be observed at the point of discharge, becoming increasingly diluted the further from the source of pollution one travels.

The CWA has a more limited and specific definition. According to Section 502 of the Act, a “point source” means “any discernible, confined and discrete conveyance, including but not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock, concentrated animal feeding operation, or vessel or other floating craft, from which pollutants are or may be discharged.”

Importantly, the CWA specifically excludes agricultural stormwater discharges and return flows from irrigated agriculture from the definition of a point source. As a result, agricultural stormwater discharges (including discharges from agricultural piped systems like tile drains) are typically regulated as non-point sources. See Bd. of Water Works Trs. of Des Moines v. Sac Cnty. Bd. of Supervisors of Drainage Districts 32, 42, 65, 79, 81, 83, 2017. On the other hand, urban stormwater—a scientific non-point source—is usually regulated as a point source under the CWA. Systems where stormwater is routed into sewer systems separate from the sanitary network (known as municipal separate storm sewer systems or MS4s) must apply for an NPDES permit under the CWA.

Scientifically, point sources are best treated using a centralized treatment, or a prevention technology that targets the specific discharge location, while non-point sources are best treated using dispersed treatment and best management practices.

Under the Clean Water Act, point sources require NPDES permits and are targeted using technology-based effluent limitations (TBELs). TBELs require a minimum level of treatment of pollutants for point source discharges based on available treatment technologies, while allowing the discharger to use any available control technique to meet the limits.

Water quality-based effluent limitations (WQBELs) are intended as a pollutant back-stop, tackling further water quality issues that are not already addressed by TBELs on the point sources draining into a given waterbody. WQBELs are determined at a state level using protocols laid out in Section 303(d) of the CWA. States determine desired uses for all U.S. waters within their boundaries, then identify any waters that fail to meet those uses due to water quality concerns. These waters are then classified as impaired, and states must develop strategies to reduce the Total Maximum Daily Load (TMDL) of the pollutants of concern into these waters to achieve water quality goals. TMDLs include target allocations for both point sources (consistent with their NPDES permits) and non-point sources draining into the waterbody.