Water for Schools
Menu

Quiz & test questions Grade 12 Science

Halbrook Water District: A Drinking-Water Standards Unit Test

A grade 12 science unit test on drinking-water standards, mixing multiple choice, short answer, and chlorine-dose computation for a fictional water district.

Objectives

  1. Students will identify the regulatory limits for pH, turbidity, chlorine residual, lead, and PFAS.
  2. Students will compute chlorine feed rate, chlorine demand, and a turbidity average and compare them to standards.
  3. Students will explain why selected water-quality limits exist and what an exceedance requires.

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.
Chlorine residual
Chlorine residual is the small amount of chlorine still left in the water after it leaves the treatment plant. It keeps protecting the water from germs all the way to your faucet.
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.

Also: pounds formula

Teacher overview

This 40-minute unit test is worth 30 points and checks whether students know the main drinking-water quality standards and can apply the operator math behind them. It is set at the fictional Halbrook Water District and mixes four multiple-choice items, three short-answer items, and three computation items. The computation items use the pounds formula and a turbidity average, while the short-answer items ask students to explain why limits exist and what an exceedance requires.

Halbrook Water District: A Drinking-Water Standards Unit Test — Grade 12 Science
Name: ____________________________ Date: ______________

Assessment

All items refer to the fictional Halbrook Water District.

  1. (2 points) The secondary, aesthetic range for drinking-water pH is:

    • A) 0 to 7
    • B) 6.5 to 8.5
    • C) 7 to 14
    • D) exactly 7.0

  2. (2 points) Finished water at a conventional filtration plant must stay at or below what turbidity in 95 percent of monthly samples?

    • A) 0.3 NTU
    • B) 5 NTU
    • C) 30 NTU
    • D) 100 NTU

  3. (2 points) The maximum residual disinfectant level for chlorine in a distribution system is:

    • A) 0.2 mg/L
    • B) 4.0 mg/L
    • C) 15 mg/L
    • D) 40 mg/L

  4. (3 points) Define turbidity and explain why high turbidity can interfere with disinfection.

  5. (4 points) Halbrook doses chlorine at 2.2 mg/L into a flow of 3.0 MGD. Using pounds per day = dose (mg/L) x flow (MGD) x 8.34, calculate the chlorine feed rate in pounds per day.

  6. (4 points) With chlorine dosed at 2.2 mg/L, a grab sample shows a residual of 0.5 mg/L. Using chlorine demand = dose minus residual, find the chlorine demand in mg/L.

  7. (3 points) Most fish require dissolved oxygen above about:

    • A) 0.3 mg/L
    • B) 2 mg/L
    • C) 5 mg/L
    • D) 30 mg/L

  8. (3 points) The lead action level is currently 15 ppb and drops to 10 ppb in 2027. Explain what an action level requires a utility to do when the level is exceeded.

  9. (4 points) Halbrook's four filters report finished-water turbidity of 0.12, 0.20, 0.08, and 0.16 NTU. Calculate the mean turbidity, then state whether each reading is at or below the 0.3 NTU limit.

  10. (3 points) PFOA and PFOS each have a maximum contaminant level of 4.0 ppt, a rule whose compliance deadlines are still shifting. Explain why regulation that is still in motion makes compliance planning harder for a utility.

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: B) 6.5 to 8.5.
    The secondary drinking-water standard for pH is the 6.5 to 8.5 range, an aesthetic guideline rather than a health limit.

  2. Answer: A) 0.3 NTU.
    Conventional filtration plants must keep finished-water turbidity at or below 0.3 NTU in 95 percent of monthly samples, never exceeding 1 NTU.

  3. Answer: B) 4.0 mg/L.
    The maximum residual disinfectant level for chlorine is 4.0 mg/L; option A's 0.2 mg/L is the low end of the typical 0.2 to 2.0 mg/L distribution residual, not the maximum.

  4. Answer: Turbidity is the cloudiness of water from suspended particles; high turbidity shields microbes from disinfection.
    Turbidity measures how many fine particles are suspended in water. Those particles can physically shelter pathogens from chlorine and increase disinfectant demand, so high turbidity makes disinfection less reliable. Award full credit for a definition plus the disinfection link.

  5. Answer: 55.0 lb/day.
    2.2 mg/L x 3.0 MGD x 8.34 = 6.6 x 8.34 = 55.04, which rounds to 55.0 pounds per day.

  6. Answer: 1.7 mg/L.
    Chlorine demand = dose minus residual = 2.2 mg/L - 0.5 mg/L = 1.7 mg/L.

  7. Answer: C) 5 mg/L.
    Most fish need dissolved oxygen above about 5 mg/L; below that, many species cannot survive.

  8. Answer: It triggers required actions such as corrosion-control treatment and lead service line steps.
    When more than 10 percent of sampled taps exceed the action level, the utility must take steps such as optimizing corrosion control, expanding public education, and moving ahead on replacing lead service lines. An action level is a treatment-and-action trigger rather than a pass-or-fail health limit.

  9. Answer: 0.14 NTU; all four readings are at or below 0.3 NTU.
    Mean = (0.12 + 0.20 + 0.08 + 0.16) / 4 = 0.56 / 4 = 0.14 NTU. The highest reading, 0.20 NTU, is at or below 0.3 NTU, so every reading meets the limit. The rule is applied to individual samples: at least 95 percent of monthly samples must be at or below 0.3 NTU, and none may exceed 1 NTU, so a low average by itself would not show compliance.

  10. Answer: Shifting deadlines make it hard to schedule and fund the required treatment.
    When a limit is set but its compliance deadline keeps moving, a utility cannot be sure when new treatment must be running or how to time large investments. Planning, financing, and public communication all depend on a firm deadline, so a rule still in motion adds uncertainty and risk.

Differentiation

Support

Provide a one-page reference listing the pH, turbidity, chlorine, oxygen, lead, and PFAS limits, and set up item 5 as 2.2 x 3.0 x 8.34 so students complete only the arithmetic. Allow a calculator on all computation items.

Extension

Ask students to write a fourth computation item of their own for a classmate, using the pounds formula with a different dose and flow, and to provide a full worked answer key for it.

ELL supports

Pre-teach "turbidity," "residual," and "action level" with simple definitions and icons, and provide sentence frames for the short-answer items 4, 8, and 10. Allow the multiple-choice items to be answered by circling rather than writing.

Extensions

Have students find the pH, turbidity, and any lead results published in a local water quality report, then compare those measured values with the standards on this test and note which had the widest safety margin.