<br />Appendix - Arden Hills Water Quality Task Force - June 2000
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<br />development. At their best, TNDs provide stores, offices, schools, day-care centers, recreational opportunities,
<br />and mass-transit facilities all within a short walk of residents'
<br />homes, This gives residents the option to walk rather than drive and thereby eliminate some
<br />of the 12 automobile trips that the members of an average suburban household make each
<br />day.38
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<br />Other Non-Structural Methods- Water Quality Education Programs
<br />The most successful of the education/outreach programs accomplish three goals. First, they educate people about the nature
<br />of the urban stonnwater pollution problem -- its causes and consequences. Second, they infonn people about what they can
<br />do to solve the problem, Finally, since hands-on education is often the best, the program achieves either pollution reduction
<br />or restoration targets.
<br />
<br />Post Construction Stonnwater Management - The management and planning methods used to reduce the overall impact of
<br />stonnwater ftom developed, or developing, areas, The primary activity of most municipal and Watershed control
<br />organizations.
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<br />Pre-Construction Stonnwater management- The identification of issues and planning for water quality management prior to
<br />construction, Includes public education, construction site controls, elimination of illegal and harmful discharges, and
<br />improved municipal housecleaning practices, Studies and case histories have shown these methods to be more effective and
<br />more cost effective than post construction management.
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<br />Structnral BMP's-
<br />Detention practices
<br />Temporarily store runoff, then discharge it through a pipe or other outlet structure into streams or other waterbodies.43 Use
<br />of wet and dry detention ponds are the most common such practices, Dry ponds release all of their water within a specified
<br />time period (generally up to 48 hours), while wet ponds keep some water at all times and retain excess water for a longer
<br />period than dry ponds, When dcsigned to be such, wet ponds can be an aesthetic or recreational amenity. Another less
<br />common detention practice is the use of cisterns. Detention ponds allow warm stormwater to enter directly into receiving
<br />waters, Stonnwater collected in ponds may heat to 950 F or more on a hot summer aftemoon.5l Since aquatic ecosystems
<br />are sensitive to changes in water temperature, these warm waters can cause a loss of fish and other aquatic organisms
<br />
<br />Bioretention & Biofiltration
<br />These practices filter stormwater to reduce contaminant loading using plants as an additional mter
<br />medium. Plants absorb nutrients and metals to a certain extent and facilitate microbial breakdown,
<br />but most of the pollutant removal from these practices occurs when the presence of the plants
<br />physically blocks the storrnwater flow, slowing the flow and allowing contaminants to settle out. One
<br />example of this structure are called raingardens. Municipalities may use several types of
<br />biofiltration, Constructed wetlands are artificial versions of natural wetlands, using shallow bodies of
<br />standing water filled with reeds and other wetland vegetation to filter the water. Engineered filter
<br />strips are slightly sloped, flat land planted with grasses, trees or other vegetation which remove
<br />stormwater pollutants as the storrnwater moves gently across the strip in a even, sheet-like flow.
<br />Swales are wide ditches with moderate sloped banks and bottoms covered with filtering turf; in many
<br />cases swales also permit infiltration as well as clean the water. Bioretention areas are constructed
<br />forested or vegetated beds, usually composed of gravel, soil, trees and shrubs, a sand layer, and a
<br />grassed swale.
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<br />Infiltration practices
<br />Temporarily store runoff in basins from which the water percolates slowly into the soil below. Like detention practices, they
<br />reduce peak flows. However, infiltration practices also recreate, to a greater or lesser extent, the natural pattern of water
<br />infiltration into the ground that existed before increased imperviousness covered the land. When designed and installed
<br />correctly and maintained regularly, infiltration practices are among the most effective structural BMPs, and often address
<br />most of the stonnwater impacts previously discussed, Studies have shown that infiltration can get 98 percent of stonnwater
<br />into the earth, cool stonnwater to 550F, remove up to 83 percent of nitrogen, and remove up to 98 percent of copper.45
<br />The principal reason why infiltration is often a preferred method is that runoff is cooled as it flows though the ground,
<br />thereby reducing the detrimental thennal effects that runoff has on aquatic ecosystems, "Retention" basins look like dry
<br />detention ponds, but have no outlet, forcing water to infiltrate through the bottom of the basin.46 Infiltration trenches are
<br />generally filled with rocks and gravel to surface level, creating a reservoir that holds water until it passes into the
<br />surrounding soil. Dry wells are deeper, narrower versions of infiltration trenches. Some of the most successful are wide-
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