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Water: Middle School

Exercise III. A Suburban Bacterial Dilemma


On a map of the United States, locate Washington D.C. To the west of the city are suburbs in the state of Virginia. Four Mile Run stream runs through the suburbs. Four Mile Run drains a watershed made up of an urban area with closely packed houses, streets, and shopping centers. The watershed is less than 20 square miles, but almost 183,000 people live there (according to the U.S. Census of 2000).


Nearly 40 percent of the watershed is covered with impervious surfaces (surfaces that do not absorb water) such as buildings, parking lots, pavement, and roads.

Urban and suburban areas like the Four Mile Run watershed often face problems with pollution. Agencies that have been monitoring Four Mile Run have found that it contains high amounts of bacteria. It does not meet minimum state standards and therefore has been labeled "unsafe for fishing or swimming."

Population Density

A way of measuring how closely people live together. It is measured in people per square mile.

Bacteria Pollution

Information on bacteria pollution in waterways can be found on the Four Mile Run Web site. Exit EPA Disclaimer

Where do all the bacteria come from? As the Four Mile Run watershed mostly has houses, offices, malls and shops, it is easy to rule out industries and factories as the source of bacteria. Scientists discovered that along with some waste generated by animals such as raccoon, geese, and deer that survive in suburban areas, domestic pet waste is also a significant source. At approximately 800 per square mile they estimated that dogs contribute more than 5,000 pounds of pet droppings every day in the 20 square mile watershed.

How do they know it's pet waste?

Scientists have been studying bacteria in the watershed's streams and ponds to try to find out the exact sources of bacteria. They are trying to find the sources by doing DNA fingerprinting studies. These studies show scientists what type of animal created the bacteria. The studies examine Escherichia coli, also known as E. coli. This strain of bacteria lives in the intestines of humans and warm-blooded animals, mammals, and many birds. Because each warm-blooded species has a unique DNA fingerprint, scientists can examine DNA from the E. coli and link it to the animal that produced it. Then they will know which animals are responsible for the bacteria in the waterways.

Hint:To find the population density of an area, divide the area's population by the area's size.

Population Density = population / size

If the population of the Four Mile Run watershed is 160,000 and its size is 20 square miles, what is its population density?

What is the population density of your area? Get population figures: Density Using Land Area For States, Counties, Metropolitan Areas, and Places.

Reading a Pie Chart

Look at the pie chart below. What are the top three sources of bacteria in the Four Mile Run watershed?


Sources of Bacteria in the Four Mile Run Watershed.
Source: Northern Virginia Regional Commission, Don Waye, March 25, 2002

Using the Internet

There are many ways to reduce the amount of bacteria that enters streams and lakes. They are called best management practices. In the space provided, write a paragraph on the best management practices that could be used to reduce the amount of animal-related bacteria that enters waterways in urban and suburban areas.

Hint:Best management practices are described on the Internet. Two good sources that discuss best management practices for bacteria are Nonpoint Source News-Notes: "Removing Bacteria from Runoff: An Overview of Strategies" (PDF) (32 pp, 864K, About PDF) (see pages 17-22) and Reducing Bacteria with Best Management Practices (3 pp, 319K, About PDF) Exit EPA Disclaimer

Group Activity

In a group of three, come up with a public planning strategy for reducing animal-related bacteria. For each part of your strategy, list the possible objections and roadblocks you might run into and the steps you would take to address them.

Public Planning Strategies

To reduce bacteria pollution, many cities have come up with a combination of best management practices they would like to use. This combination of ideas is known as a public planning strategy.


Middle Schools  |  Stop Pointless Personal Pollution  |    Exercise: I  |  II  |  III  |  IV  | 

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