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Lab / experiment Grade 7 Health & Consumer Science

Lake Tamsett Beach Clarity: A Lifeguard Visibility Test

Students cloud water with measured soil, record the depth where a marked disk disappears, and use the data to judge when a lake beach is too murky to guard.

Objectives

  1. Students will measure the depth at which a marked disk disappears in water clouded with known amounts of soil.
  2. Students will compare mean depths across four samples and find the percent decrease between two of them.
  3. Students will use their data to explain a beach visibility rule and what that rule does not tell a swimmer.

Vocabulary

Turbidity
Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.

Also: sediment, runoff, mean, advisory

Teacher overview

Guards at the fictional town of Sanderly's beach on Lake Tamsett cannot rescue a swimmer they cannot see, so the town checks how far down the water stays clear before it opens the swim area each morning. Storms wash soil off streets and fields into the lake, which makes the water cloudy for a day or two afterward. In this lab students add measured amounts of soil to water, record the depth at which a black and white disk disappears, and use their results to explain the beach rule. Teaching note: say early and plainly that this test measures whether a guard can see a swimmer on the bottom rather than whether the water is clean, because bacteria carried in by the same storm cannot be seen at any clarity.

Lake Tamsett Beach Clarity: A Lifeguard Visibility Test — Grade 7 Health & Consumer Science
Name: ____________________________ Date: ______________

Procedure

  • Set out four labeled jars. Fill each one with about 800 mL of tap water, which leaves room to shake it.
  • Add dry soil to three of the jars. The jar labeled 0 gets no soil. The jar labeled 1 gets one teaspoon. The jar labeled 2 gets two teaspoons. The jar labeled 4 gets four teaspoons.
  • Put the lids on and shake each jar for ten seconds so the soil spreads through the water.
  • Pour the water from the jar labeled 0 into the clear tube. Pour in the same amount of water for every test.
  • Hold the marked string at the top of the tube. Lower the disk slowly into the water.
  • Stop as soon as you can no longer tell the black part from the white part. Read the depth on the string and write it down in centimeters.
  • If you can still see the pattern when the disk rests on the bottom, record the full water depth and mark it with a plus sign.
  • Raise the disk until you can see the pattern again, then lower it once more to check your reading.
  • Test each jar three times. Shake the jar again before every test so the soil does not settle.
  • Rinse the tube between jars. Leftover soil in the tube will change the next reading.
  • Safety: do not drink or taste any sample. Wipe up spills right away so the floor does not get slippery. Wash your hands when the lab is finished.

Data & Analysis

Record the depth in centimeters at which the disk disappeared. Deeper readings mean clearer water.

Jar (teaspoons of soil) Trial 1 (cm) Trial 2 (cm) Trial 3 (cm) Mean depth (cm)
0
1
2
4

  1. Which jar had the greatest mean depth in your data, and what does that tell you about the water in it?

  2. Subtract the mean depth of the 4-teaspoon jar from the mean depth of the 0-teaspoon jar. How many centimeters did the depth drop?

  3. Suppose a class working with a much taller tube recorded a mean depth of 96 cm with no soil and 24 cm with four teaspoons. By what percent did the depth drop for that class?

  4. The fictional Sanderly beach rule says a guard must be able to see the disk at 120 cm before the swim area opens. Suppose the reading at the beach this morning is 84 cm. How many centimeters short of the rule is that reading?

  5. Does doubling the soil cut the mean depth in half in your data? Use two of your means to support your answer.

  6. A storm washes both soil and animal waste into the lake. Explain why the water can look clear again two days later and still be unsafe for swimming.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

  1. Answer: The jar with no added soil. Accept the student's own greatest mean, which in nearly every group is the 0-teaspoon jar, with an explanation that less soil lets more light through, so the disk stays visible deeper.

  2. Answer: Accept any difference that matches the student's own two means. The 4-teaspoon mean should be well below the 0-teaspoon mean, so the difference should be a positive number of centimeters.

  3. Answer: 75 percent. The drop is 96 cm minus 24 cm, which is 72 cm. Then 72 divided by 96 is 0.75, or 75 percent.

  4. Answer: 36 cm. 120 cm minus 84 cm is 36 cm, so the water is 36 cm short of the depth the rule requires.

  5. Answer: Answers vary. Visibility depth is roughly inversely related to the amount of suspended soil, so doubling the soil should come close to halving the depth, but settling, the limited depth of the tube, and reading error often pull real data away from that pattern. Accept either conclusion when it is supported by two of the student's own means.

  6. Answer: Sediment settles out in a day or two, so the water looks clear again, but bacteria from animal waste are far too small to see and can stay in the water after the soil has settled. Clear water tells a guard that a swimmer is visible, and it does not tell anyone that the water is free of germs.

Differentiation

Support

Give these groups two jars instead of four, using the 0-teaspoon and 4-teaspoon samples only, and provide a string already marked every 2 cm with large printed numbers. Work one mean through with them on the board before they compute their own.

Extension

Have students repeat one trial with flour or powdered clay in place of soil and compare how quickly each sample clears when it stands still for ten minutes. Ask them to decide which material would be the bigger problem for a beach after a storm and to justify the choice with their readings.

ELL supports

Pre-teach "clarity," "sediment," and "runoff" with a photo of muddy water flowing into a lake. Provide a picture of the marked disk with the depth labels, and give a sentence frame for question 6: "The water looks clear because ___, and it can still be unsafe because ___."

Extensions

Have students graph mean depth against teaspoons of soil and describe the shape of the graph in words. Groups can then draft a one-page morning checking routine for the fictional Sanderly beach staff that states who takes the reading, how often it is taken, and what the staff does when the reading falls short of the rule. A parks department or aquatics director can be invited to describe how their own swim area decides to open or close after heavy rain.