During any construction project, the term “offsite discharge” will make even the most experienced superintendent cringe. Most cases of offsite discharges are completely legal and not considered illicit by the Environmental Protection Agency (EPA) if a NPDES permit is in place. Per the EPA:

The Clean Water Act prohibits anybody from discharging "pollutants" through a "point source" into a "water of the United States" unless they have an NPDES permit.

What many do not realize is that applying and receiving an NPDES Permit or Notice of Intent, allows for stormwater to legally be discharged offsite under the conditions of the permit. Though most offsite discharges are allowed under the NPDES permit, the Clean Water Act (CWA) identifies multiple pollutants that are considered illicit if discharged. Per the CWA:  

Some examples are dredged soil, solid waste, incinerator residue, sewage, garbage, sewage sludge, munitions, chemical wastes, biological materials, radioactive materials, heat, wrecked or discarded equipment, rock, sand, cellar dirt and industrial, municipal, and agricultural waste.

The most common and perhaps most recognizable illicit discharge is sediment laden or turbid water. Most states set an amount of allowable turbid water discharge to a measurable 29 NTU (Nephelometric Turbidity Units) over background conditions. Properly installing and maintaining appropriate erosion and sediment control devices or Best Management Practices, BMP’s, will greatly decrease the chances of illicit offsite discharges. It is important to inspect and maintain perimeter BMPs and point source BMPs such as inlet protection and outfall erosion control. Stormwater runoff should be directed to outfall locations that are designed in a project’s engineered construction plans. Outfall locations should be inspected routinely as well as during rain event inspections. As shown in the pictures, the outfall structure should be free of sediment and debris. If the outfall is discharging, the outfall structure should not exceed the allowable NTU over background conditions. Though offsite stormwater discharges are allowed under an NPDES Permit, it is important to always monitor offsite discharges and outfall locations.

References

“NPDES Permit Basics.” EPA, Environmental Protection Agency, 3 Aug. 2020, www.epa.gov/npdes/npdes-permit-basics.