Water for Schools
Menu

Worksheet Grade 12 Careers & CTE

Reading Lab Results at Hollindale's Water Plant

A grade 12 careers worksheet where students act as a lab analyst, comparing fictional water quality results against EPA limits and computing removal efficiency.

Objectives

  1. Students will compare fictional water quality results against EPA drinking water limits.
  2. Students will calculate a turbidity removal efficiency and report a value to an appropriate accuracy.

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.
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.

Also: residual, removal efficiency

Teacher overview

A water quality lab analyst spends much of the day comparing test results against legal limits and deciding whether a system passes. That work rewards care: reading a value against the wrong limit, or reporting more decimal places than the instrument can measure, can turn a safe result into a false alarm or hide a real problem. This worksheet gives students a fictional set of results from one plant and asks them to judge each against the real EPA limit. The common error to watch is false precision, meaning a reported value that claims more accuracy than the meter can give.

Reading Lab Results at Hollindale's Water Plant — Grade 12 Careers & CTE
Name: ____________________________ Date: ______________

Student Worksheet

You are a lab analyst reviewing one day of results from Hollindale Water Plant. The utility name and all sample readings below are fictional, written for this exercise. Compare each reading against the real EPA limits provided.

EPA limits: Finished-water turbidity must stay at or below 0.3 NTU in 95 percent of monthly samples and never exceed 1 NTU. The maximum residual disinfectant level for chlorine is 4.0 mg/L, and systems usually keep 0.2 to 2.0 mg/L in the distribution system. The lead action level is 15 ppb now and drops to 10 ppb in 2027. The maximum contaminant level for PFOA and for PFOS is 4.0 ppt each.

Hollindale's fictional results: finished-water turbidity 0.12 NTU; raw-water turbidity 6.0 NTU; chlorine residual at the plant 1.8 mg/L; chlorine residual at the far end of distribution 0.5 mg/L; lead 90th-percentile 8 ppb; PFOA 3.2 ppt; PFOS 4.6 ppt.

  1. The finished-water turbidity reads 0.12 NTU. Does this sample meet the 0.3 NTU limit? Explain in one sentence.
  2. Raw-water turbidity is 6.0 NTU and finished-water turbidity is 0.12 NTU. Using efficiency = (influent minus effluent) divided by influent, times 100, find the turbidity removal efficiency to the nearest whole percent.
  3. The chlorine residual at the plant reads 1.8 mg/L. Is it below the 4.0 mg/L maximum residual disinfectant level? Explain in one sentence.
  4. The far end of distribution reads 0.5 mg/L chlorine residual. Is that within the usual 0.2 to 2.0 mg/L distribution range? Explain in one sentence.
  5. Hollindale's lead 90th-percentile result is 8 ppb. Does it meet both the current 15 ppb action level and the 10 ppb level that starts in 2027? Explain in one sentence.
  6. By how many ppb is Hollindale's lead result of 8 ppb below the 2027 action level of 10 ppb?
  7. PFOA measured 3.2 ppt and PFOS measured 4.6 ppt, each against a 4.0 ppt limit. Which one exceeds its limit, and by how many ppt?
  8. The turbidity meter reads to the nearest hundredth of an NTU. A trainee wrote the finished-water turbidity as 0.1200000 NTU. Rewrite the value with the number of decimal places the meter supports.
  9. Based on your answers, name the single result that would require Hollindale to act, and explain why in one sentence.

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: Yes. The finished-water turbidity of 0.12 NTU is below the 0.3 NTU limit, so this sample meets the standard.
  2. Answer: 98 percent. Efficiency = (6.0 minus 0.12) divided by 6.0, times 100 = 5.88 divided by 6.0, times 100 = 98 percent.
  3. Answer: Yes. The plant residual of 1.8 mg/L is below the 4.0 mg/L maximum residual disinfectant level.
  4. Answer: Yes. The far-end residual of 0.5 mg/L falls between 0.2 and 2.0 mg/L, so it is within the usual distribution range.
  5. Answer: Yes. The lead result of 8 ppb is below both the current 15 ppb action level and the 10 ppb level that begins in 2027.
  6. Answer: 2 ppb. The 2027 action level is 10 ppb and the result is 8 ppb, so 10 minus 8 equals 2 ppb.
  7. Answer: 0.6 ppt. PFOS at 4.6 ppt exceeds its 4.0 ppt limit, and 4.6 minus 4.0 equals 0.6 ppt over the limit; PFOA at 3.2 ppt is below its limit.
  8. Answer: 0.12 NTU. The meter reads to the nearest hundredth, so the value should be written with two decimal places, as 0.12 NTU.
  9. Answer: The PFOS result. At 4.6 ppt it is above the 4.0 ppt limit, so it is the one reading that would require Hollindale to act. One sample above 4.0 ppt is not by itself a violation, because compliance is judged on a running annual average, but it tells the utility to resample, monitor more often, and plan treatment.

Differentiation

Support

Give students a two-column checklist that pairs each result with its matching limit, so they can line up the right numbers before judging pass or fail, and provide a calculator for the efficiency step in problem 2.

Extension

Ask students to determine what raw-water turbidity would have been needed for the plant to reach exactly 99 percent removal while still finishing at the fictional 0.12 NTU, working backward through the efficiency formula, and to show the algebra.

ELL supports

Pre-teach "turbidity," "residual," "action level," and "removal efficiency" with a simple picture for each, such as clear versus cloudy water for turbidity. Allow the yes-or-no items to be answered by circling "meets" or "does not meet" with a short reason.

Extensions

Have students find their own utility's most recent Consumer Confidence Report, which every community water system in the US must publish each year, and locate the turbidity, lead, and disinfectant residual entries. Comparing those real numbers to the limits in this worksheet shows students that a lab analyst's judgment sits behind every line of that report.