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Worksheet Grade 7 Social Studies & Civics

Is One Tap Enough? Sampling Rowan Springs' Water Supply

A grade 7 worksheet on representative sampling and federal testing rules, using a fictional city's lead and turbidity monitoring program and real EPA thresholds.

Objectives

  1. Students will calculate what percent of a population a water-testing sample represents.
  2. Students will evaluate whether a sampling plan is representative of the risk it is meant to detect.
  3. Students will explain why federal water-testing rules require public reporting of results.

Vocabulary

Action level
A trigger number in lead and copper testing. If too many homes test above this level, the utility must take steps like improving treatment and telling the public.
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: representative sample, transparency

Teacher overview

A water utility cannot test every glass of water in a city, so it relies on a sampling plan, a deliberate choice about which taps to test and how often, to stand in for the whole system. This worksheet uses the fictional city of Rowan Springs to walk through two real federal testing requirements: sampling homes for lead and sampling treated water for turbidity, or cloudiness. A common misconception this worksheet is built to correct is the idea that "representative" always means "randomly chosen"; for lead testing, federal rules deliberately target the homes most likely to have a lead problem rather than choosing homes at random, because the goal is to find the highest-risk taps, not to describe the average tap.

Is One Tap Enough? Sampling Rowan Springs' Water Supply — Grade 7 Social Studies & Civics
Name: ____________________________ Date: ______________

Student Worksheet

Suppose the fictional city of Rowan Springs serves 20,000 homes through its water system. Use these facts for the questions below.

  • This testing period, Rowan Springs is required to collect a lead sample from 100 homes.
  • The lead action level is currently 15 parts per billion (ppb), and it is scheduled to drop to 10 ppb in 2027 under an updated federal rule.
  • Rowan Springs' treatment plant must keep turbidity, a measure of cloudiness, at or below 0.3 NTU in at least 95 percent of its monthly samples.

  1. What percent of Rowan Springs' 20,000 homes does the 100-home lead sample represent?

  2. Federal rules require Rowan Springs to choose its 100 sample homes from the properties most likely to have a lead service line, such as older homes built before lead pipes were banned, rather than choosing 100 homes at random from across the whole city. Explain why targeting the highest-risk homes makes sense for finding lead, even though it is not a simple random sample of the entire city.

  3. Suppose instead that Rowan Springs chose all 100 sample homes from one recently built neighborhood where no home has a lead service line. Would that sample be useful for detecting lead risk citywide? Explain your reasoning.

  4. Of Rowan Springs' 100 sampled homes, 6 test above the current lead action level of 15 ppb. What percent of the sample exceeded the action level?

  5. Now suppose the 6 homes had tested lower. Each tested above 10 ppb but not above 15 ppb. Would they count as exceeding the action level under the current rule (15 ppb)? Would they count as exceeding the action level under the 2027 rule (10 ppb)? Explain the difference.

  6. Rowan Springs collects 40 turbidity samples from its treated water in one month, and 39 of them meet the 0.3 NTU standard. What percent of the samples met the standard?

  7. Does Rowan Springs' plant meet the requirement that at least 95 percent of monthly turbidity samples stay at or below 0.3 NTU? Use your answer to item 6.

  8. Rowan Springs must publish its test results, including any homes above the lead action level, in a public report available to every resident. Explain one reason this public-reporting requirement matters for the community's trust in its water system.

  9. Explain the difference in purpose between the lead-sampling program, which targets specific high-risk homes, and the turbidity-sampling program, which repeatedly tests the plant's overall output through the month.

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: 0.5 percent.
    100 ÷ 20,000 = 0.005 = 0.5%.

  2. Answer: Targeting the highest-risk homes first finds the worst contamination faster than a random sample would, because a random sample of the whole city could easily miss the smaller number of homes that actually have lead pipes, while a risk-based sample concentrates testing exactly where the danger is most likely.

  3. Answer: No, that sample would not be useful for detecting citywide lead risk, because it deliberately excludes the type of home, older properties with lead service lines, that the testing program exists to find; a sample drawn only from low-risk homes cannot represent the risk faced by higher-risk homes elsewhere in the city.

  4. Answer: 6 percent.
    6 ÷ 100 = 0.06 = 6%.

  5. Answer: Under the current 15 ppb action level, homes testing above 10 but not above 15 ppb would not count as exceeding it, since their results sit at or below 15 ppb. Under the 2027 rule's 10 ppb action level, those same results (above 10 ppb) would count as exceeding it, since they sit above the new, stricter threshold.

  6. Answer: 97.5 percent.
    39 ÷ 40 = 0.975 = 97.5%.

  7. Answer: Yes.
    97.5% (item 6) is greater than the required 95%, so Rowan Springs' plant meets the turbidity requirement for that month.

  8. Answer: Public reporting lets residents see the same information the utility and regulators see, rather than having to trust an unverified assurance that the water is safe, which builds confidence that problems will be disclosed rather than hidden.

  9. Answer: Lead sampling targets a small set of specific homes believed to carry the highest risk, aiming to catch the worst individual cases, while turbidity sampling repeatedly tests the plant's overall treated water throughout the month, aiming to confirm that the whole system is consistently performing within its standard.

Differentiation

Support (below grade level)

Provide the percent calculations for items 1, 4, and 6 as a labeled fraction ("100 out of 20,000") before students convert to a percent, and allow a calculator for every division step. Offer a two-column chart labeling "targeted sampling" and "random sampling" for items 2 and 3.

Extension (above grade level)

Ask students to design a lead-sampling plan for a fictional second city of 50,000 homes, choosing what fraction of homes to sample and explaining, in writing, what information they would use to decide which homes count as highest-risk.

ELL supports

Pre-teach "representative," "action level," and "turbidity" with a simple visual (a small handful of marbles chosen from a large jar to represent a sample). Allow items 8 and 9 to be answered with a short phrase or labeled diagram instead of a full paragraph, where the standard permits.