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Worksheet Grade 10 Social Studies & Civics

Reading Merribow's Annual Water Quality Report

A grade 10 worksheet on a fictional district's annual water quality report, where students convert between ppm, ppb, and ppt and compare results to limits.

Objectives

  1. Students will convert water quality results among parts per million, billion, and trillion.
  2. Students will express reported results as a percent of the federal limit that applies to them.
  3. Students will explain why lead results are reported as a 90th percentile and what a household can do with them.

Vocabulary

Parts per trillion
A unit for measuring extremely small amounts of a chemical in water. PFAS limits are set in parts per trillion because these chemicals matter even in tiny doses.
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.
Maximum contaminant level (MCL)
A maximum contaminant level, or MCL, is the highest amount of a certain substance that federal rules allow in drinking water. Water systems test regularly to prove their water stays under each limit.
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: parts per billion, 90th percentile, disinfectant residual

Teacher overview

Every community water system in the country has to deliver to its customers at least one report a year on what its testing found, and most of those reports go straight into the recycling because the units are unfamiliar. This worksheet takes the report of the fictional Merribow Water District and treats the units as the main skill: parts per million, parts per billion, and parts per trillion differ by factors of a thousand, and a reader who mixes them up will misjudge a result by orders of magnitude. The misconception to address directly is the assumption that a smaller number always signals a safer result; students routinely rank 3.1 parts per trillion as worse than 0.9 parts per million because the digits look larger, which is why every item here asks for the result in relation to its own limit.

Reading Merribow's Annual Water Quality Report — Grade 10 Social Studies & Civics
Name: ____________________________ Date: ______________

Student Worksheet

The fictional Merribow Water District serves the town of Merribow. Its most recent annual report to customers listed these results.

What was tested Merribow result Federal limit
Lead, 90th percentile of 60 sampled homes 8 ppb action level of 15 ppb now, dropping to 10 ppb in 2027
PFOA 3.1 ppt maximum contaminant level of 4.0 ppt
Turbidity, highest single reading 0.21 NTU at or below 0.3 NTU in 95 percent of monthly samples, never above 1 NTU
Chlorine residual, running annual average 0.9 mg/L maximum residual disinfectant level of 4.0 mg/L

Use these two conversions where you need them: one part per billion equals 1,000 parts per trillion, and in water 1 mg/L equals one part per million.

  1. Merribow's PFOA result is 3.1 ppt. Express that result in parts per billion.
  2. The lead action level drops to 10 ppb in 2027 and Merribow's 90th percentile is 8 ppb. What percent of the 2027 action level is Merribow's result?
  3. By how many ppb could Merribow's lead result rise before it reaches the 2027 action level?
  4. Merribow's highest single turbidity reading was 0.21 NTU against a limit of 0.3 NTU. How far below that limit was the reading?
  5. Merribow's chlorine residual averaged 0.9 mg/L against a maximum residual disinfectant level of 4.0 mg/L. What percent of the maximum was the average, to the nearest tenth of a percent?
  6. Express Merribow's chlorine residual of 0.9 mg/L in parts per million.
  7. Rank these three units from the largest to the smallest: parts per billion, parts per million, parts per trillion.
  8. Merribow's PFOA result is 3.1 ppt and the limit is 4.0 ppt. Express the result as a percent of the limit, to the nearest tenth of a percent.
  9. Merribow reports lead as the 90th percentile of the homes it sampled rather than as an average. Explain what a 90th percentile is and why a district would be required to report lead that way.
  10. The district must mail this report to every customer once a year. Name one decision a household could make with the lead figure that it could not make without the report, and explain why the mailing matters even when every result is below its limit.

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: 0.0031 ppb. Since one part per billion equals 1,000 parts per trillion, dividing 3.1 by 1,000 gives 0.0031 parts per billion.
  2. Answer: 80 percent. Dividing the 8 ppb result by the 10 ppb action level that applies from 2027 gives 0.80, or 80 percent.
  3. Answer: 2 ppb. Subtracting the 8 ppb result from the 10 ppb action level leaves 2 ppb of headroom.
  4. Answer: 0.09 NTU. Subtracting the 0.21 NTU reading from the 0.3 NTU limit leaves 0.09 NTU.
  5. Answer: 22.5 percent. Dividing 0.9 mg/L by 4.0 mg/L gives 0.225, which is 22.5 percent.
  6. Answer: 0.9 ppm. In water 1 mg/L equals one part per million, so the residual is 0.9 parts per million.
  7. Answer: Parts per million, then parts per billion, then parts per trillion. Each unit is one thousand times smaller than the one before it, so a result of a few parts per trillion is a far smaller quantity than a result of a few parts per million.
  8. Answer: 77.5 percent. Dividing 3.1 ppt by 4.0 ppt gives 0.775, which is 77.5 percent of the limit.
  9. Answer: The 90th percentile is the value that 90 percent of the sampled homes fall at or below, so only the highest tenth of the sampled homes exceeded 8 ppb. Averaging would let a few very high homes be hidden by many low ones, and lead comes from individual service lines and household plumbing rather than from the water leaving the plant, so the rule targets the worst homes rather than the typical one.
  10. Answer: Accept any decision the figure makes possible, such as asking the district whether that household's service line is lead, requesting a tap test, installing a certified filter, or running the tap before drawing water for cooking. A strong answer explains that the report also lets residents track results across years and question the district in public before a limit is exceeded rather than afterward.

Differentiation

Support

Print the two unit conversions at the top of the page in a boxed reminder and work question 1 with the class before releasing students. Allow the percent answers to be rounded to the nearest whole percent, and provide the subtraction in questions 3 and 4 already set up.

Extension

Have students rewrite the four rows of the table as a short paragraph a resident with no science background could follow, keeping every figure accurate, and then explain which unit was hardest to describe in plain language and why.

ELL supports

Pre-teach "result," "limit," "percentile," and "residual" using a simple number line showing a result sitting below a marked limit. Provide sentence frames for questions 9 and 10, such as "The district reports the 90th percentile because ___" and "With this report a household can ___."