Stormwater -
National Pollution Discharge Elimination System
Definitions

Stormwater Management Terminology

Creation of a stormwater management program has resulted in a vocabulary of terms and words which may be unfamiliar. If you were told to improve your BMP's, to be careful of what is discharged to the MS4, or that the highest percentage of pollutants are conveyed with something called the First Flush... would you know what it meant?  The following may help you become more accustomed to frequently encountered phrases found in NPDES stormwater-related articles and materials.

BMPs (Best Management Practices): In environmental regulation, Best Management Practices are defined as "source control" and "that practice which is used for a given set of conditions to achieve satisfactory water quality and quantity enhancement at a minimum cost." More simply put, BMPs refers to many practices or types of controls, for example: a silt fence or staked hay bales during construction to prevent erosion; limiting the removal of vegetation and limiting the time bare land is exposed to rainfall; sweeping streets to keep pollutants and litter from being washed into the storm sewer system; fertilizer and pesticide application control; and structural controls such as physical containment of stormwater in ponds to allow settling, filtration and percolation.

Ditch Blocks, Cross Blocks or Check Dams: These blocks or dams are defined as a small temporary dam constructed of filter fence material, stone or hay bales. They are installed across a swale or stormwater conveyance channel to reduce the velocity and prevent erosion of the swale. It also serves to settle out the sediment, rather than being carried to a water body.

First Flush: Describes the washing action that stormwater has on accumulated pollutants. The first runoff, especially off streets and parking lots, washes them clean and carries pollutants with it. The first one-inch of runoff carries 90 percent of the pollution.

Illegal Dumping: Is disposal of waste in a non-permitted area, such as a back area of a yard, a stream bank, or some other off-road area. Illegal dumping can also be the pouring of liquid wastes or disposing of trash down storm drains. It is often called "open dumping," "fly dumping," and "midnight dumping" because materials are often dumped in open areas, from vehicles along roadsides, and late at night. Illegally dumped wastes are primarily non hazardous materials that are dumped to avoid paying disposal fees or expending the time and effort required for proper disposal.

Illicit Connection: Illicit connections are defined as illegal and/or improper connections to storm drainage systems and receiving waters.

Illicit Discharge: The discharge of anything other than stormwater to a municipal separate storm sewer system. No debris or waste should be dumped into the MS4 since these materials are quickly carried to nearby waters.

Impervious: Any covering which prevents stormwater from entering the ground; a building, parking lot, driveway, patio, pool.

Infiltration (Retention) Practices: Retaining stormwater on site; allowing it to infiltrate into the ground or to evaporate are most effective at reducing pollution since the first flush is not discharged to surface waters.

LUF (Land Use Factor): Land use factor is a methodology used in calculating a stormwater utility fee to a parcel of property. Displacement of stormwater from the site would increase the fee, whereas retaining the stormwater would decrease the fee.

MS4 (Municipal Separate Storm Sewer System): Refers to the city's underground storm sewers, manholes, catch basins; M for municipal and four words beginning with S was shortened to become MS4.

Pervious: A surface which allow the penetration of water into the ground.

Point sources: A point source is the origination of a pollutant in a water body.

Pollutant: Any dredged spoil, solid waste, incinerator residue, filter backwash, sewage, sewage sludge, garbage, munitions, chemical wastes, biological materials, radioactive materials, wrecked or discarded equipment, rock, sand, cellar dirt and industrial, municipal, and agricultural waste discharged into water.

Sediment: Material carried with stormwater runoff; sediment fills drainage ditches, rivers and lakes, degrading water quality.

Stormwater Runoff: Water flowing over the land during and immediately following a rainstorm. Stormwater carries nutrient laden sediment, heavy metals, oils, and other materials which have accumulated on the land between rain events and flushes them into streams, rivers, and lakes.

Swale: Depression or ditch designed to convey or retain stormwater; typically contains standing or flowing water only after a rainfall, the intent being to filter stormwater through the ground. Swales should not be filled in. Leaves, vegetation, limbs and debris should not be disposed of in a swale. Pollutants in these materials will be carried with the stormwater and the benefit of the swale will be lost.

Total Maximum Daily Load (TMDL): A TMDL or Total Maximum Daily Load is a calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards, and an allocation of that amount to the pollutant's sources.

Water quality standards are set by States, Territories, and Tribes. They identify the uses for each water body, for example, drinking water supply, contact recreation (swimming), and aquatic life support (fishing), and the scientific criteria to support that use.

A TMDL is the sum of the allowable loads of a single pollutant from all contributing point and non point sources. The calculation must include a margin of safety to ensure that the water body can be used for the purposes the State has designated. The calculation must also account for seasonal variation in water quality.

The Clean Water Act, section 303, establishes the water quality standards and TMDL programs.