What is construction site runoff?

Starting work on a fresh construction site often involves moving land around and grading soil so that building can begin. Whether it is a housing development or a high-rise building, moving the soil around can loosen contaminants that are buried within. In addition to any soil contamination that is on a given construction site, any sort of chemicals or debris on the site may get washed away during a rain event. This is known as construction site runoff.

What is construction site runoff?

Construction site runoff is polluted stormwater that carries sediment, chemicals and other debris from a construction site into local waterways.

Sediment from construction sites is one of the most significant pollutants, often caused by exposed soil and excavation. Chemicals included oils, fuels, solvents and paints from construction equipment and materials. Debris in the runoff can include trash and litter from the site, and nutrients in the runoff can be from fertilizers and other materials on site.

Why it matters

Construction sites represent a unique source of stormwater-related pollution: when soil is disturbed, vegetation removed and surfaces left bare, rainfall transforms ground disturbance into mobile sediment and stormwater becomes the transport vehicle.

According to the U.S. Environmental Protection Agency (EPA), “sediment is the primary pollutant of concern associated with construction site stormwater runoff.”

Beyond sediment, construction site runoff often contains oils, grease, concrete washout, nutrients and other chemicals – all of which can degrade waterways, fill channels, increase turbidity and stress aquatic life.

What is runoff in simple terms?

In simple terms, runoff is the portion of rainfall or snowmelt that flows over the land surface because it cannot soak into the ground or evaporate.

When water falls on saturated soils, paved surfaces, roofs or driveways, it moves downhill under gravity and often enters storm drains, streams or channels rather than filtering into the ground. For stormwater professionals, runoff is a key concept because it carries sediment, pollutants, debris and excess flow into receiving waters – posing risks for flooding, erosion and water quality degradation.

How is construction site runoff regulated?

Construction site runoff is identified in the municipal separate storm sewer system (MS4) permitting framework as a minimum control measure required for permittees to manage. The federal Construction General Permit (CGP) sets out requirements for projects disturbing once acre or more: develop a Stormwater Pollution Prevention Plan (SWPPP), install and maintain Best Management Practices (BMPs), minimize exposure of soils and stabilize disturbed areas when activity ceases.

Key management elements

Effective construction site runoff management hinges on a few core practices:

• Site planning and sequencing: Minimizing the footprint of disturbance at any one time, protecting adjacent waterways and phasing construction to limit exposed soils.
• Erosion control: Measures such as mats, mulch, temporary seeding or geotextiles applied to prevent soil detachment.
• Sediment control: Devices such as silt fences, sediment basins, check dams or fiber rolls that intercept sediment-laden runoff before it leaves the site.
• Conveyance management: Diverting clean flows around disturbed areas, installing temporary swales or inlets and maintaining storm drain cleanliness.
• Stabilization: Final site restoration with vegetation or cover as early as possible to reduce exposure time.
• Inspections and maintenance: Regular checks of BMPs and prompt repairs or replacements where they fail.

Why is runoff a problem?

Unchecked construction site runoff can lead to multiple problems: sediment accumulation in receiving water that harms aquatic habitat; increased turbidity and reduced light penetration; clogged storm drains and water infrastructure; heightened flood risk; and possible non-compliance with permits – leading to enforcement actions.