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Storm water runoff is a huge source of pollution in the nation's waterways. In most cities, storm water is not treated, unlike the sewer system. This means everything storm water picks up before entering storm drains — litter, silts, oils off the streets, fertilizers off of lawns — is deposited straight into a community's rivers, lakes, and streams. Aware of the threat storm water poses to the country's water system, the Environmental Protection Agency (EPA) set up the National Pollutant Discharge Elimination System (NPDES) Storm Water Program to protect the country's waterways from pollution.

Since 1990, the EPA has mandated that construction sites larger than 5 acres and communities with storm water conveyances serving populations of more than 100,000 need to apply for an NPDES permit. Technically, these community storm water conveyance systems are called municipal separate storm sewer systems (MS4s). MS4s are required to develop a storm water quality management program.

On March 10, 2004, Phase II of the NPDES rule went into effect, expanding the regulations to include smaller communities and lowering the construction site threshold to 1 acre and greater. Construction sites smaller than 1 acre but part of a larger common plan of development must also abide by these rules.

Operators of construction sites subject to these rules must have a Storm Water Pollution Prevention Plan (SWPPP). The SWPPP must include a set of best management practices, or BMPs, specific for the site. Some BMPs are general housekeeping items, such as protecting garbage dumpsters from rain. Other BMPs are more directly targeted at minimizing the amount of exposed earth, helping storm water infiltrate the soil, or keeping sediment on site. In addition, BMPs can be temporary or structural.

The storm water rules set forth by the EPA are not just empty threats. In May 2004, the EPA came down on Wal-Mart with a $3.1 million fine for having inefficient storm water controls on 24 construction sites in nine states.