11.5 Sustainable stormwater management  (Page 2/26)

Since the pattern of precipitation varies seasonally the water pollution and flooding risks posed by stormwater runoff also tend to vary seasonally. Generally, larger flood and pollution risks will occur in the spring, when rapid snowmelt can generate a lot of runoff volume (especially if the ground is still frozen), which can carry pollutants that have accumulated within the snow cover over the winter months to nearby streams and rivers. There can also be storm-related flood and pollution "spikes" when heavy rain strikes the ground at a faster rate than it can be infiltrated into the soils, or when it is prevented from infiltrating into the soils by roofs, paving, or other impermeable surfaces. This initially high volume of stormwater runoff can carry greater amounts of contaminants – a process often described as the "first flush" phenomenon    . Usually, the first half-inch of stormwater will be carrying the highest pollution load, so its capture and management becomes a priority for water quality protection.

How some of these features, especially the amount of impervious surface associated with different densities of development, affect the generation of urban runoff are illustrated in Figure Degrees of Imperviousness and its Effects on Stormwater Runoff . Research by the Center for Watershed Protection has found that stream quality becomes impaired when 10% of the stream's watershed is impervious and that an urban stream's ecology is severely impacted when more than 25% of its watershed is impervious.

When flowing downhill within a watershed, stormwater runoff can pick up pollutants from various anthropogenic sources and activities. It can also collect pollutants from the atmospheric deposition of particulates and air pollutants carried to the earth's surface by precipitation, by windblown dust, or by simply settling out of the atmosphere. Urban runoff can also dissolve or transport chemicals that may be found naturally in soil or nutrients which may have been deliberately added to lawns. Common urban pollutants can include such things as pesticides and fertilizers applied to residential lawns, parks and golf courses, enteric microbes from animal waste, industrial chemicals that may have been accidentally spilled on the ground or improperly stored, or oils and greases leaking from cars parked in lots or on driveways.

As stormwater runoff flows towards lower-lying areas of the watershed, it carries these contaminants with it and therefore contributes to the pollution of the stream, river or lake into which it is discharging. Once it reaches a river or stream, the concentrations of pollutants in the receiving waters are naturally reduced as the contaminants are carried downstream from their sources, largely through dilution but also by settlement, by uptake by posure to sunlight and oxygen, and by interactions with various chemical and physical proplants and animals (including bacteria and other microorganisms), through degradation by excesses occurring within the waterway and its streambed.