Water pollution in urban areas is typically thought of as coming from a particular source, such as a factory, malfunctioning sewage treatment system, or business. When the pollution can be identified with a specific location, it is described as point source pollution. The Federal Water Pollution Control Act (1972), also known as the Clean Water Act, and the Oil Pollution Act (1990) have resulted in changes in common practices at point sources, which have significantly reduced the amount of pollution emitted from those sources.

When the source of the pollution is not clear, or when pollution results from the accumulation of many everyday acts of individuals, it is described as non-point source pollution. Efforts are being made to reduce water pollution from diffuse, non-point sources such as residential lawns, impervious surfaces like asphalt and concrete, building sites, and small businesses. Pollution from these sources generally occurs when rainwater flows across the surfaces and picks up dirt, motor oil, fertilizers, pesticides, bacteria, and animal waste, and then transports them to adjacent waterways. While individual activities may produce relatively small quantities of pollutants, there are so many of these non-point sources that their collective effect makes non-point source pollution the greatest current threat to America's water quality.

One effective way to reduce non-point source water pollution in urban and suburban areas is to plant rain gardens. A rain garden is a shallow depression in the ground that is designed to retain runoff from rainfall or snowmelt for up to three days. Instead of flowing into a storm sewer or natural water body, the runoff soaks into the ground. Once in the ground, plant roots, soil, and bacteria act as a filter for the water, trapping or absorbing pollutants as the water passes through. Rain gardens also improve water quality by reducing the volume of flooding during melt events. Flooding can cause erosion, filling waterways with sediments that can be as damaging as fertilizer and motor oil.

There is no standard format for building a rain garden. In general, the depression should be four to eight inches but may be deeper if retention for longer than three days is desired. The garden can be any shape or size. If designed to catch runoff from a particular roof or parking lot, the garden should be one-third the size of the impervious surface area it drains. Gutter downspouts can be extended underground to empty directly into the rain garden, which should be at least ten feet from buildings to prevent foundation damage. Planting native wildflowers and grasses is the key to creating a low-maintenance rain garden. The U.S. Fish and Wildlife Service can provide advice about which species are native to a particular area.

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