Storm water is defined as the flow of water that results from precipitation which occurs immediately following a rainfall event or results from a snowmelt.
When a rainfall event occurs, the precipitation is "absorbed" by the environment. The water may soak into the soil recharging groundwater supplies, while some is taken up by plants, and some is evaporated into the atmosphere. All the other water that flows or runs off the land is considered storm water. Unfortunately, as development increases, the ability of our environment to perform its natural process of "absorption" decreases. This is because the natural landscape that was once able to absorb and clean storm water is covered by impervious surface. Impervious surface is simply a surface that water is unable to penetrate. Rooftops, driveways, and roads are all examples of impervious surfaces.
Increased impervious surface results in both an increased amount of storm water runoff and an increased chance for pollution to enter our waterways through our storm sewer systems. This type of pollution that results from storm water runoff is called non point source pollution.