Since 1972, implementation of the Clean Water Act (CWA) has shown success in controlling water pollution from point sources such as municipal waste water treatment plants and industrial discharges. This progress is overshadowed, however, by the emergence of nonpoint source pollution as a main contributor to water quality problems. The National Water Quality Inventory: 2000 Report to Congress identified urban runoff as one of the leading sources of water quality impairment in surface waters (USEPA 2002b in EPA 2005). Of the 11 pollution source categories listed in the report, urban runoff/storm sewers was ranked as the fourth leading source of impairment in rivers, third in lakes, and second in estuaries.
Nonpoint source (NPS) pollution comes from many diffuse sources. NPS pollution originates when rainfall or snowmelt moves over and through the ground. As the runoff moves, it picks up and carries away natural and human-made pollutants, finally depositing them into lakes, rivers, wetlands, coastal waters, and even underground sources of drinking water called aquifers. These pollutants include:
• Excess fertilizers, herbicides and insecticides from agricultural lands and residential areas.
• Oil, grease, and toxic chemicals from urban runoff.
• Sediment from improperly managed construction sites, crop and forest lands, and eroding stream banks.
• Bacteria and nutrients from livestock, pet wastes, wildlife, and faulty septic systems.
Inadequately controlled or treated runoff causes problems for water bodies including changes in flow, increased sedimentation, higher water temperature, lower dissolved oxygen, degradation of aquatic habitat structure, loss of fish and other aquatic populations, and decreased water quality due to increased levels of nutrients, metals, hydrocarbons, bacteria, and other constituents.
For urban and suburban areas, these problems can largely be traced to activities that occur on the land. Whether the problem arises from lawn care chemicals, pet wastes, or motor oil and toxic metals from parking lots and streets, stormwater plays a major role in transporting pollutants to streams, drinking water sources, and other receiving water bodies.
The marriage of land and water is the watershed. A watershed is a land area that drains to a given body of water. Precipitation that falls in the watershed will either infiltrate into the ground, evapotranspirate back into the air, or run off into streams, lakes, or coastal waters. A watershed may be large or small. The Connecticut River, for example, drains 11,260 square miles in four states (Connecticut, Massachusetts, New Hampshire, and Vermont). Along its 410 miles, from its headwaters at the Canadian border to the confluence with the Atlantic Ocean in Long Island Sound, this huge watershed includes 149 tributaries, 38 major rivers, and numerous lakes and ponds. Each of the smaller water bodies also has its own watershed, or subwatershed. Fitting together like the pieces of a puzzle, each of these smaller
watersheds continue to drain downstream, one into the other, until reaching the Connecticut River.
Rivers and streams support diverse aquatic communities and perform vital ecological roles of processing carbon sediments and nutrients upon which downstream ecosystems depend. Healthy, functioning watersheds naturally filter pollutants and moderate water quality by slowing surface runoff and increasing the infiltration of water into soil. The result is less flooding and soil erosion, cleaner water downstream, and greater ground water reserves.
Land development directly affects watershed functions. The U.S. Census Bureau projects that the U.S. population will grow by 50 million people, or 18 percent, between 2000 and 2020. Many communities are asking where and how they can accommodate this growth while maintaining and improving their water quality. Residential and commercial development create impervious surfaces and compacted soils that filter less water, which increases surface runoff and decreases groundwater infiltration. These changes can increase the volume and velocity of runoff, the frequency and severity of flooding, and peak storm flows. Studies have shown that at 10 percent imperviousness, a watershed is likely to become impaired and grows more so as imperviousness increases (Arnold, 1996; Shueler, 1994 in EPA 2006).
While land development necessarily involves the creation of impervious surfaces, how and where development takes place can influence the ultimate degree of environmental impact from streets, rooftops, and yards. Where development has occurred on forest and undeveloped land, critical areas for infiltration and aquifer recharge that soaked up rainwater prior to development now export runoff to lower lying areas and local receiving bodies. Water flowing over pavement absorbs heat, which impacts waterways that support cold water species. It also flows faster, thus delivering water in pulses. The faster flows can scour stream banks and accelerate erosion, while increased temperatures can spur excessive algal growth. The higher rate of algal growth can interfere with a variety of ecological, industrial, recreational, and water filtration processes. Conventional construction practices have relied on mass clearing and grading. This practice compacts the soil surface and further prevents infiltration, even on lots overlaid with turf. Thus, the generation of stormwater volume, as well as the pollutant load carried in that volume, is very much tied to how and where land is developed.
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