Urbanareasimpactwaterqualit.cfm Urban areas impact water quality
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Urban areas impact water quality

Do not ask for whom the water flows; it flows for thee. With apologies to John Donne, this takeoff on a phrase he and Ernest Hemingway made famous applies to the responsibility everyone has to protect the quality of water.

Protecting water quality requires cooperation from everyone in a given watershed, including those in urban areas as well as rural areas, said DeAnn Presley, Kansas State University Research and Extension environmental soil science specialist.

Those who manage property in and near urban areas may not be entirely aware of the significant impact they can have on water quality, and what they can do to protect that valuable resource, Presley said.

In any environment, one of the keys to protecting water quality is to make sure there are plenty of land areas that allow water to slow down and soak in during a hard rainstorm, she said.

In urban areas, this means special attention must be paid to land management in development sites, said Stacy Hutchinson, who is an environmental engineer with K-State Research and Extension.

"When an area is developed and houses are built, often much of the existing vegetation on the land area is initially destroyed in the process. When permeable, vegetated land cover is replaced with impermeable structures and surfaces such as buildings, roads, walks, and driveways, the result is an increase in stormwater runoff because less water infiltrates," Hutchinson said.

For example, she said:

A house on a fifth of an acre (8,750 square feet) receives one inch of rain:

--Before the impermeable surfaces are added, there could be about 380 gallons of water leaving the site. For visualization purposes, this would be about seven oil drums.

--After adding an average three-bedroom house, driveway, walkway, and garage (2,900 square feet), the amount of water leaving could be around 1,880 gallons (about 34 drums).

--This is an increase of 1,500 gallons of stormwater.

"About five times as much runoff can be expected when a site is converted to residential development. For a development with 100 houses, this increase would be about 150,000 gallons of runoff, which could have a definite impact on water volume and velocity entering streams and lakes," the environmental engineer said.

Stormwater runoff includes urban pollutants, such as car fluids, pet waste, and lawn fertilizer, which are dissolved and carried off of driveways, roads, and yards, Hutchinson said. Soil may also be eroding, in which case the water runoff will contain soil sediments.

One way to improve rainfall infiltration and reduce runoff in a developed area is to limit foot and vehicle traffic. The most critical time to avoid traffic is when the soil is moist, as moist soil is much easier to compact than soil that is dry.

High traffic also leads to vegetation damage or loss, which can increase erosion, she said.

"A dense stand of deeply rooted vegetation helps promote good water infiltration in the soil. Vegetation will also help reduce surface sealing," she added.

Urban developers wanting to control storm water runoff and erosion have several options. For example, a bioswale or rain garden can be used to increase infiltration.

A bioswale is a storm water runoff conveyance system that provides an alternative to storm sewers, Hutchinson said. It can absorb low flows or carry runoff from heavy rains to storm sewer inlets or directly to surface waters. Bioswales improve water quality by infiltrating the first flush of storm water runoff and filtering the large storm flows they convey.

A rain garden is a small garden which is designed to withstand the extremes of moisture and concentrations of soil sediments, pesticides, and nutrients (particularly nitrogen and phosphorus) in storm water runoff, Hutchinson said. Rain gardens are often located close to the source of the runoff. They slow storm water movement, giving the water more time to infiltrate and less opportunity to gain momentum and erosive power.

More information on the topic is available in the K-State Research and Extension publication MF-22732, "Urban Water Quality," available online at: www.oznet.ksu.edu/library/h20ql2/mf2732.pdf.

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Urban stormwater runoff structures require good maintenance

The performance of a bioswale or rain garden can decline with time, possibly due to soil sealing, meaning that the surface pores have become plugged with smaller particles, said DeAnn Presley, K-State Research and Extension environmental soil science specialist. When that happens, water may remain standing in the area for up to two days.

Performance of these systems can be improved by removing surface deposits of fine materials and reseeding bare areas, Presley said.

"The buildup of fine sediment should be periodically removed, typically in late March or early April when the site is mowed or burned. If the native grasses fail to grow after sediment removal, replant the area," she added.

If there has been compaction or deposition of fine sediments, the soil surface may have to be loosened by tilling; modified by incorporation of more permeable material such as sand, compost, or other organic material; and then reseeded. Aeration is also an option.

Another approach, which would be less invasive but would reduce the pollution removal efficiency and water retention capacity, would be to lower the overflow pipe or berm and thus reduce water levels, Presley said.

More information is available at county and district K-State Research and Extension offices and in the Extension publication MF-2814, "Stormwater Best Management Practice Maintenance," online at: www.oznet.ksu.edu/library/h20ql2/mf2814.pdf.

Date: 1/30/09

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