Tuesday, 20 August, 2019


It’s been 25 years since the Environmental Protection Agency began the Phase I Municipal Separate Storm Sewer System (MS4) permit program in November of 1990. SEH's April Ryan, PE, an expert in water resources and stormwater engineering, shares 25 Stormwater Facts and a bit of her own insight.

 1. Stormwater is precious and powerful.
Stormwater is precipitation (rain or snowmelt) that falls to the ground then flows over land instead of percolating into the ground. In essence, it is the combination of earth’s most precious resource, water, and its most ubiquitous force, gravity. An inch of rain on an acre of land is equivalent to 27,154 gallons of water. It weighs about 113 tons. That’s powerful.

2. Stormwater affects the water you drink.
When precipitation hits the ground, some water trickles down into groundwater aquifers, while the rest flows on the surface to lakes, streams and rivers. Depending on where you live, your drinking water comes from either surface water (lakes and rivers) or a groundwater aquifer.

3. Today’s stormwater moves faster than ever.
Before urbanization, the majority of precipitation was naturally and slowly filtered by soil and grasses. As development occurred and pavements became widely used, stormwater conveyance systems (ditches, gutters, storm sewers) were constructed to quickly transport water from streets to surface waters, like lakes, rivers, wetlands and streams. The NPDES permit program controls water pollution by requiring MS4s to treat stormwater before discharging into surface waters.

4. Polluted stormwater runoff is the number one water pollutant.
As stormwater moves across developed areas, it picks up garbage, debris, sediment, chemicals, automotive fluids, fertilizers, leaves and other pollutants from parking lots, yards, streets, roofs and other hard surfaces. If untreated, these pollutants enter our waterways.

5. Stormwater pollution is classified in two ways.
Water pollution comes either from “point” or “nonpoint” sources. A point source is identifiable, such as a pipe or drain discharging a pollutant directly into a body of water. A nonpoint source is more difficult to define. It includes all the polluted stormwater within a certain geography, like a watershed, for example.

6. An MS4 battles against stormwater pollution.
MS4 refers to conveyance or system of conveyances (including roads with drainage systems, streets, catch basins, curbs, gutters, ditches, man-made channels, and storm drains) which is owned or operated by a state, city, town, county, district, association, or other public body (created by or pursuant to state law).

7. There are many stormwater pollution culprits.
Significant stormwater pollutants include:
    Oil, grease, and toxic chemicals from motor vehicles
    Fertilizers and pesticides used on lawns, gardens, roadways, etc.
    Viruses and bacteria from pet waste and failing septic systems
    Litter and trash from motorists, pedestrians and businesses

8. MS4s were born out of the Clean Water Act.
MS4s are regulated by the National Pollutant Discharge Elimination System/State Disposal System (NPDES/SDS) permit program. The NPDES permit was enacted in 1972 as part of the pivotal Clean Water Act.

9. There are over 7,450 MS4s in the country.
As populations continue to grow, so will the number of MS4s.

10. Population and location determine the regulated MS4s.
Although large communities are generally associated with the MS4 program, a regulated MS4 includes smaller public entities located in urbanized areas and/or located near specific water resources.

Reposted with permission from SEH InSight. Read 15 additional facts at www.sehinc.com/stormwaterfacts.