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Worksheet Grade 9 Social Studies & Civics

Spruce County Turbidity: A Water-Quality Compliance Worksheet

Grade 9 worksheet where students calculate a fictional rural water system's monthly turbidity compliance percentage against the real federal 95 percent standard.

Objectives

  1. Students will calculate the percent of water samples meeting a stated regulatory standard from a data table.
  2. Students will compare a calculated compliance rate to the required federal threshold and determine the size of any shortfall.
  3. Students will evaluate an intended benefit and an unintended cost of a water-quality regulation for a small water system.

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: NTU, compliance, corrective action

Teacher overview

Federal regulation sets a specific bar for how clear treated drinking water must be: finished water must measure at or below 0.3 NTU (a unit of cloudiness called turbidity) in at least 95 percent of a system's monthly samples, and no single sample may ever exceed 1 NTU. This worksheet gives students a fictional rural water authority's monthly sample record and asks them to calculate whether that system met the standard, a task that mirrors the compliance reviews a real water board must complete every month. A common misconception this worksheet addresses is that meeting the 95 percent rule is the whole standard; in fact, a system can hit 95 percent and still violate the regulation if even one sample exceeds the absolute 1 NTU ceiling, which item 5 is built to surface directly.

Spruce County Turbidity: A Water-Quality Compliance Worksheet — Grade 9 Social Studies & Civics
Name: ____________________________ Date: ______________

Student Worksheet

The fictional Spruce County Rural Water Authority collected 20 turbidity samples this month. Use the table below and the federal standard (0.3 NTU or below in at least 95 percent of samples; no sample may ever exceed 1 NTU) to answer the items that follow.

Sample NTU Sample NTU
1 0.18 11 0.21
2 0.22 12 0.26
3 0.25 13 0.23
4 0.19 14 0.28
5 0.27 15 1.10
6 0.31 16 0.19
7 0.24 17 0.24
8 0.20 18 0.22
9 0.29 19 0.30
10 0.33 20 0.25
  1. According to federal regulation, what turbidity level must be met in at least 95 percent of a system's monthly samples, and what is the absolute maximum a single sample may ever reach?
  2. Count how many of Spruce County's 20 samples are at or below 0.3 NTU.
  3. Calculate the percent of Spruce County's samples that were at or below 0.3 NTU this month. Round to the nearest whole percent.
  4. Is Spruce County meeting the 95 percent standard this month? By how many percentage points does it fall short of, or exceed, the standard?
  5. Identify any sample(s) in the table that exceed the absolute 1 NTU ceiling. Why does a single sample above 1 NTU matter even if a system still meets the 95 percent rule overall?
  6. If Spruce County wants to reach the 95 percent standard next month while still collecting 20 samples, how many of those 20 samples would need to test at or below 0.3 NTU?
  7. Compare your answer in item 6 to the actual count from item 2. How many additional samples would need to test at or below 0.3 NTU for Spruce County to meet the standard?
  8. In two to three sentences, describe two steps the Spruce County Rural Water Authority board should take at its next public meeting after seeing this month's results, addressing both the treatment problem and the county's responsibility to inform residents.
  9. Evaluate the turbidity regulation itself. Name one likely benefit and one likely unintended cost of the rule for a small rural system like Spruce County's, in one to two sentences each.

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: 0.3 NTU in 95 percent of samples; 1 NTU absolute maximum.
    The federal standard requires finished water at or below 0.3 NTU in at least 95 percent of a system's monthly samples, and no single sample may ever exceed 1 NTU, regardless of the monthly average.
  2. Answer: 17 samples.
    Samples 1-5, 7-9, 11-14, and 16-20 are all at or below 0.3 NTU; samples 6 (0.31), 10 (0.33), and 15 (1.10) are not, leaving 20 - 3 = 17.
  3. Answer: 85 percent.
    17 / 20 = 0.85, or 85 percent.
  4. Answer: No, Spruce County falls short by 10 percentage points.
    95 percent - 85 percent = 10 percentage points below the required standard.
  5. Answer: Sample 15, at 1.10 NTU.
    A single sample above 1 NTU is an automatic violation of the never-exceed rule, separate from and in addition to the 95 percent calculation, because the ceiling exists to catch any single moment when treatment fails, not just the monthly pattern.
  6. Answer: 19 samples.
    95 percent of 20 samples is 0.95 x 20 = 19 samples.
  7. Answer: 2 additional samples.
    19 (the number needed) - 17 (the number currently at or below 0.3 NTU) = 2 additional samples.
  8. Answer: Answers will vary but should name a treatment step and a public-notification step.
    A reasonable answer directs the plant operator to inspect and likely replace filter media or adjust the coagulant dose that controls turbidity, and separately schedules a public meeting or notice explaining the exceedance and the county's corrective action plan to residents.
  9. Answer: Answers will vary; a reasonable response names one benefit and one unintended cost with a clear reason.
    A likely benefit: the standard protects public health by limiting the cloudy particles that can shield disease-causing organisms from disinfection. A likely unintended cost: meeting the standard consistently may require filtration upgrades that a small rural system with few ratepayers has difficulty affording without raising bills sharply.

Differentiation

Support (below grade level)

Provide a version of the table with a third column already marked "meets 0.3 NTU" or "does not meet," so students focus on counting and calculating the percent rather than scanning 20 rows for each item. Work through item 3's percent calculation together as a class before students attempt items 4 through 7 independently.

Extension (above grade level)

Ask students to decide, using only the rules and numbers on this worksheet, whether a month in which 19 of 20 samples are at or below 0.3 NTU and the twentieth sample is above 1 NTU would still put the county out of compliance, and to explain why in a short paragraph.

ELL supports

Pre-teach "turbidity," "NTU," and "compliance" with a simple visual showing two glasses of water, one clear (low NTU) and one cloudy (high NTU), labeled with their approximate NTU values. Allow items 8 and 9 to be answered with short bulleted phrases instead of full sentences.