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Water pollution — What’s the flow?
Grade Level: 6–12
Duration: 1 class
Subject: Water pollution
- Students will identify urban runoff.
- Students will identify peak flows.
- Students will identify causes of difference in flow rates.
When rain falls, the water goes several places. It percolates down through the soil and ends up as groundwater. It soaks into the soil, is drawn up through plant roots and released into the atmosphere by transpiration. It also flows over the land surface into a local body of water such as the St. Johns River. When this third process occurs in urbanized areas, with water flowing over streets, parking lots, construction areas, and golf courses, it is termed urban runoff. Human-caused sources of overland flow, such as sprinklers, are also included in the definition of urban runoff.
Urban runoff is a concern to all of us because runoff is often heavily polluted. In fact, in some areas it is believed to be the main cause of water pollution. Rainwater can become contaminated in the process of flowing across the urban landscape. Sources of this contamination include the following:
- Motor oil (dripping cars, dumping of used motor oil)
- Antifreeze or coolant (dripping cars, dumping of used antifreeze or coolant)
- Other automobile fluids and greases
- Rubber dust from tires
- Pet wastes
- Fertilizers and pesticides
- Paints and thinners (disposed of improperly or dumped down a storm drain)
- Dirt from construction sites or unpaved roads
- Industrial yards
The urban pollution problem is characterized by many individuals all contributing a little bit to make a large problem.
There are several important differences between runoff in an urban setting and runoff in a more natural setting. In an urban area, cement, asphalt, and other impermeable surfaces (surfaces that water cannot pass through) cover more land area. Water that would soak into the ground in natural areas runs off in urban areas. A much greater volume of water runs off urban areas than rural areas. Because pavement moves water efficiently, urban flows tend to move much faster, peak sooner, and have more scouring power than rural flows. So not only are more contaminants (such as pesticides, fertilizers, oil, and grease) found in urban areas, but the rapidly moving water is more capable of picking up contaminants, even when attached to soil particles, and washing them into a nearby water body.
One of the major factors contributing to urban runoff pollution is simple ignorance of stormwater systems and storm drains. Storm drains, the metal grates seen along curbs and in streets, do not lead to water treatment plants. They drain directly to a natural water body, such as the St. Johns River, via ditches, streams, or drains. Storm drains are installed to prevent flooding. They are designed to drain rainwater off the streets and put the water as quickly as possible into local lakes or rivers.
The only treatment that exists for stormwater is retention/detention ponds. These ponds remove a certain amount of pollutants and sediments found in urban runoff but often do not remove pesticides or nutrients. Additional treatment of runoff would be expensive and require the construction of additional treatment facilities.
- Place students in working groups of 4–5 students.
- Distribute copies of the “flow vs. time graph” or enlarge and post it so that all students can see it. The graph compares runoff data from two imaginary streams that are similar in size, location, and amount of rainfall during a storm. The two creeks differ only in their surroundings. Countryside Stream flows in a forested or natural area, and City Stream flows through an urban area.
- Next, distribute a student worksheet to each group. Explain that when a stream is carrying the greatest amount of runoff, it has “peaked.” Ask groups to complete worksheets using information from the “flow vs. time graph”.