On this page
In this lesson
What you'll learn
- Explain how stormwater runoff forms and where it goes
- Name the main pollutants that leave construction sites
- Describe why sediment is so damaging to waterways
- Trace the chain from raindrop to discharge that controls must interrupt
Stormwater is rain or snowmelt that flows over the ground instead of soaking in. On natural land, most of it is absorbed by soil and plants. On a construction site, the vegetation is stripped, the soil is loosened and compacted, and runoff picks up whatever it crosses on the way to a storm drain, ditch or stream.
Where runoff goes
In most places, storm drains aren't connected to a treatment plant. A storm drain inlet on a street usually carries water through a municipal separate storm sewer system (MS4) straight to the nearest creek, river, lake or bay. Whatever runoff carries off your site, the receiving water gets.
The main construction site pollutants
| Pollutant | Common sources on site |
|---|---|
| Sediment (soil) | Graded areas, stockpiles, trenches, track-out on roads, dewatering |
| Nutrients (nitrogen, phosphorus) | Fertilizer, topsoil, sediment carrying attached nutrients |
| Concrete and cement wash water | Truck, chute and tool washout; saw cutting slurry. Highly alkaline (high pH) |
| Oil, fuel and hydraulic fluid | Equipment fueling, leaks, maintenance |
| Metals | Building materials, vehicles, paints |
| Chemicals | Paints, solvents, curing compounds, form release oils, adhesives |
| Trash and debris | Packaging, food waste, construction debris |
| Bacteria | Portable toilets |
Why sediment matters most
Sediment is the pollutant construction sites release in the largest amounts. EPA has noted that sediment runoff rates from construction sites are typically 10 to 20 times greater than from agricultural land and 1,000 to 2,000 times greater than from forest land. Once in a stream, sediment:
- Clouds the water, blocking sunlight that aquatic plants need.
- Smothers fish spawning beds and the habitat of bottom-dwelling organisms.
- Carries other pollutants, such as nutrients and metals, attached to soil particles.
- Fills channels, ponds, reservoirs and storm drains, increasing flooding and dredging costs.
The chain controls must break
Every stormwater control on a construction site interrupts one link in this chain:
- Rain falls on exposed soil. Raindrop impact dislodges soil particles. Control: keep soil covered (erosion control).
- Water runs off instead of soaking in. Control: phase grading, preserve vegetation and topsoil, minimize compaction.
- Runoff concentrates and gains speed, cutting rills and gullies. Control: slope breaks, check dams, diversions, velocity dissipation.
- Sediment-laden water leaves the disturbed area. Control: perimeter controls, sediment traps and basins, inlet protection (sediment control).
- Other pollutants join the flow. Control: good housekeeping, washout, spill prevention (pollution prevention).
Who regulates this
Discharges of stormwater from construction sites that disturb one acre or more (or less, if part of a larger common plan) are regulated under the Clean Water Act through NPDES permits. That's the subject of the next lesson.
Key takeaways
- Stormwater runoff usually flows untreated through storm drains to local waters.
- Sediment is the main construction pollutant, along with nutrients, concrete washout, fuels, chemicals and trash.
- Construction sites can release sediment at many times the rate of farmland and forest.
- Controls break the chain: cover soil, slow water, trap sediment, and keep other pollutants out of the flow.
Knowledge check
Lesson 1 quiz: 8 questions
Choose one answer for each question, then select Check my answers. Score 80% or higher to pass. Your answers stay in your browser and are never saved.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on the Clean Water Act, EPA NPDES rules and the EPA Construction General Permit. Your governing permit (EPA, state, tribal or local) controls. This is not legal advice, not EPA-approved, and not a California QSP/QSD or other state credential, and it does not issue a certificate. Nothing you enter on this page is saved or sent.