Runoff from storm events is part of the natural hydrologic process. Rainwater that does not infiltrate into the ground, evaporate, or that is not used by plants will flow into lakes, streams, rivers, and washes. As runoff moves downhill, vegetated depressions and other surface features slow the water and remove some pollutants and sediments. However, in urban settings, existing vegetation and topography often have been altered, graded, or paved and stormwater is diverted into storm drainpipes or other diversionary features. When the drainage pattern of a watershed is so altered, flows increase in volume and velocity and can pick up sediments and pollutants from land surfaces at an increased rate. Stormwater that flows through urbanized areas to receiving waters is called “urban runoff”.
Urban runoff may carry a wide range of pollutants including nutrients, trash and debris, sediments, heavy metals, bacteria, petroleum products, and synthetic chemicals, such as pesticides. Because urban runoff does not originate from a distinct “point source” (e.g. a chemical plant), it is also often referred to as “non-point source” pollution. These contaminants in urban runoff have the potential to pollute water bodies within Yavapai County. Urban runoff can degrade the physical, chemical, and biological characteristics of water bodies and desert wash ecosystems.
In addition to the pollutants picked up by runoff before it enters a storm drain, some stormwater includes wastes and wastewater from non-stormwater sources. These non-stormwater sources are referred to as “illicit discharges”. These discharges are “illicit” because law usually prohibits such releases; municipal storm drainage systems normally are not designed to treat wastewater. Illicit discharges can include sanitary wastewater flowing to the storm drainage system through illegal connections; effluent from septic systems; car wash, laundry, and other industrial waste waters; improper disposal of automotive and household wastes, such as used motor oil and pesticides; and spills from roadways. Illicit discharges enter the system either through direct connections (e.g. wastewater piping either mistakenly or deliberately connected to the storm drains) or indirect connections (e.g. infiltration through cracked piping, spills collected by drain outlets, and paint or oil dumped directly into the drain). The result is contaminated stormwater that may contribute pollutants, including heavy metals, toxics, oil and grease, solvents, nutrients, viruses, and bacteria into area washes, streams, and lakes. These same sources of pollution also may move downward into ground water.
In response to the impact of non-point source discharges on water quality, the U.S. Environmental Protection Agency (EPA) and the State of Arizona have developed specific sets of regulations to address the problem.