Nonpoint-Source Pollution—Urban Style

Linda Benedict  |  6/2/2005 6:37:41 PM

Margaret Frey

Water quality monitoring of urban environments indicates high pollutant loading in the first runoff of rain known as “first flush.” As precipitation falls, it picks up contaminants from littered streets, drainage ditches, petroleum residues from cars, heavy metals and tar residues from roads, and chemicals applied for fertilization and insect control.

Lawn and garden care practices have the potential to significantly and adversely affect the water quality of urban waterways. The major pollutants found in runoff from urban areas include sediment eroded from bare-soil areas, nutrients from over-fertilization and oxygen-demanding substances such as leaf and grass clippings. In addition, excessive pesticide use, along with over-watering, can contribute to water quality degradation.

Other household activities that may generate pollutants include use of paints, solvents, detergents, cleaners and automotive products such as oil and antifreeze.

Proper management techniques can reduce and possibly eliminate the need for fertilization and pesticide use in the urban setting. These include: (1) mowing the lawn when grass blades are at an appropriate height, (2) leaving grass clippings on lawn (if mowed at proper height) to provide nutrients, (3) testing soils to determine necessary amount of fertilizer, (4) using slow-releasing fertilizers, (5) determining pest problems for appropriate selection and application of pesticides and (6) composting yard wastes for future source of nutrients. Most of these practices will also reduce the cost of lawn maintenance.

Impervious surfaces surrounding homes and businesses in the form of driveways, sidewalks and patios force rainwater to run off. Cities experience nine times more runoff than wooded areas. Runoff from city streets is generally channeled into storm drains and drainage ditches. A recent study indicated that E. coli bacteria regrow through cloning within storm drains and sediments of urban streams. The study also concluded that nonhuman species are the dominant sources of E. coli to the stream and its tributaries. The major nonhuman mammal contributors are raccoons, dogs, deer and rats.

Reducing the amount of excessive runoff is possible by using more porous paving surfaces that allow rainwater to soak into the ground. Significant strides have been made in developing porous asphalt pavement in the last three decades. The material is similar to conventional asphalt in durability, but it contains a much smaller percentage of fine particles. As a result, the asphalt allows water to soak through to the base material and into the soil below.

Roof downspouts spill onto driveways that are graded down to the street gutters, which, in turn, lead to storm drains that dump accumulated rainwater into drainage canals. The destructive deluges of this collected rain erode streambanks. In places with good soil drainage, it is recommended to capture and then spread and infiltrate the rainwater runoff from paved areas and roofs.

Oil stains and outdoor spills of antifreeze, brake fluid and other automotive fluids on driveways and in parking lots are easily carried away by a rainstorm. One quart of oil can contaminate up to two million gallons of drinking water. It has been estimated that every three weeks more oil is deposited on driveways and streets in the United States than was deposited in the Valdez oil spill in Alaska. Routine maintenance can prevent cars from leaking. Pans, carpet scraps and matting can catch drips when the car is parked at home.

The LSU AgCenter has educational programs that address nonpoint-source pollution in urban areas:

A series of one-page fact sheets on “You Can Help Protect Our Waters” is available free from any parish extension office and online at the LSU AgCenter’s website.

Extension agents give demonstrations and teach water-saving techniques with a traveling exhibit called the “Splashmobile.”

Extension agents provide workshops for teachers as requested.

Margaret Frey, Extension Specialist, LSU AgCenter

(This article appeared in the spring 2001 issue of Louisiana Agriculture.)

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