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Lesson plan Grade 10 Engineering & Design

Alderfen PFAS Monitoring Plan: A Grade 10 Design Lesson

A 50-minute grade 10 engineering lesson where students set criteria for a PFAS sampling plan and convert and average trace results against the 4.0 ppt limit.

Objectives

  1. Students will specify the criteria and constraints of a PFAS sampling plan for a small water system.
  2. Students will convert trace PFAS concentrations between units and calculate a running annual average against the federal limit.

Vocabulary

PFAS
A large family of human-made chemicals used in things like nonstick pans, waterproof fabric, and firefighting foam. They are called forever chemicals because they take a very long time to break down.
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.

Also: running annual average, detection limit, sampling plan

Teacher overview

PFAS are measured at parts per trillion, a scale small enough that the design of the sampling plan, meaning where and how often water is tested, determines whether contamination is detected at all. The federal limit for PFOA and for PFOS is 4.0 parts per trillion (ppt) each, and the rule is still changing, because in May 2026 EPA proposed letting eligible systems apply for a two-year exemption that would move their April 2029 compliance date to April 2031; the proposal was not final as of September 2026, and the 4.0 ppt limits would not change. In this lesson students act as engineers for the fictional Alderfen Water System, first setting the criteria and constraints of a monitoring plan for its wells, then working the unit conversions and running averages the plan depends on. A common misconception this lesson addresses is that a single clean sample proves a well is safe; the rule compares a running annual average of several samples to the limit, so one low reading can hide a well whose average still exceeds 4.0 ppt.

Procedure

Hook (5 min): Tell students that a well tested clean in the spring, yet the family that depends on it is still worried. Ask what a single test does and does not tell you, and steer the discussion toward how often and where a system would have to sample to be confident, which are the questions an engineer answers when designing a monitoring plan.

Direct instruction (12 min): Introduce the fictional Alderfen Water System and its four wells. Define PFAS, the 4.0 ppt limit for PFOA and PFOS, and the running annual average the rule uses. Then lay out the criteria and constraints of a sampling plan: criteria include catching any well whose average approaches 4.0 ppt and using a lab whose detection limit is well below the limit; constraints include a fixed testing budget, the number of wells, and the quarterly schedule that a running annual average needs.

Model a worked example (10 min): Model the running-average idea with a fifth practice well whose four quarterly PFOA results are 2.0, 3.0, 3.6, and 3.4 ppt. Sum to 12.0 ppt and divide by 4 to get a 3.0 ppt running average, below the 4.0 ppt limit. Then model a unit conversion, showing that 4.0 ppt equals 0.004 ppb because one part per billion equals 1,000 parts per trillion.

Guided practice (13 min): In pairs, students use the well locations on the Student Handout to decide which two of the four wells they would sample first if the budget allowed only two tests this quarter, and to write one criterion and one constraint that drove their choice. Circulate and confirm each pair names a measurable criterion, not just "the closest well."

Independent practice (8 min): Each student completes the Student Handout, finding a running annual average, converting between units, comparing results to the 4.0 ppt limit, and counting how many wells exceed it.

Closure and exit ticket (2 min): Students write one sentence naming the biggest weakness of testing each well only once a year instead of quarterly. Collect on the way out.

Alderfen PFAS Monitoring Plan: A Grade 10 Design Lesson — Grade 10 Engineering & Design
Name: ____________________________ Date: ______________

Student Handout

Alderfen Water System is a fictional utility used for these problems. Its four wells are sampled each quarter for PFOA and PFOS. The federal limits for PFOA and PFOS are 4.0 ppt each. Well locations: Well A is beside a former fire-training area, Well B is in forested upland, Well C sits beside an airport drainage ditch, and Well D is near a closed landfill. Use 1 ppb = 1,000 ppt and 1 ppt = 1 ng/L.

  1. Well A's four quarterly PFOA results are 3.2, 4.8, 5.0, and 3.8 ppt. Find Well A's running annual average, and state whether it exceeds the 4.0 ppt limit.

  2. Express the 4.0 ppt limit in parts per billion (ppb), using 1 ppb = 1,000 ppt.

  3. Well B's running annual average is 2.5 ppt. What percent of the 4.0 ppt limit is that? Use average ÷ limit × 100.

  4. A lab reports Well C at 6 ng/L of PFOS. Using 1 ppt = 1 ng/L, write that as ppt, and state whether it exceeds the 4.0 ppt limit and by how much.

  5. The four wells have running annual averages of 4.2, 2.5, 6.0, and 3.1 ppt. How many of the four wells exceed the 4.0 ppt limit?

  6. Explain one reason sampling each well only once a year, instead of four times a year, could let a real violation go undetected.

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: 4.2 ppt.
    Running annual average = (3.2 + 4.8 + 5.0 + 3.8) ÷ 4 = 16.8 ÷ 4 = 4.2 ppt, which is above the 4.0 ppt limit.

  2. Answer: 0.004 ppb.
    4.0 ppt ÷ 1,000 ppt/ppb = 0.004 ppb.

  3. Answer: 62.5%.
    2.5 ÷ 4.0 × 100 = 62.5, which is 62.5 percent of the limit.

  4. Answer: 6 ppt.
    Because 1 ppt = 1 ng/L, 6 ng/L is 6 ppt, which is above the 4.0 ppt limit by 2.0 ppt.

  5. Answer: 2 wells.
    Only the wells averaging 4.2 ppt and 6.0 ppt are above the 4.0 ppt limit, so 2 of the 4 wells exceed it.

  6. Answer: A yearly sample can miss a spike.
    Accept any answer explaining that a running annual average needs several samples, so a single yearly test can land on a low quarter and hide a well whose true average, across seasons, still rises above 4.0 ppt.

Differentiation

Support (below grade level)

Provide the two unit relationships (1 ppb = 1,000 ppt and 1 ppt = 1 ng/L) on a reference card, and complete question 1's averaging step together so students see how the four results add and divide before working the conversions on their own.

Extension (above grade level)

Ask students to design a sampling schedule that would keep total tests to eight per year across the four wells while still catching a well whose average approaches the limit, and to justify which wells get more frequent sampling using the running-average idea.

ELL supports

Pre-teach parts per trillion, running average, detection limit, and sampling plan with a labeled number line showing how ppt, ppb, and ng/L compare. Provide a sentence frame for question 6: "Testing once a year can miss ___ because the average needs ___," and allow calculations to be shown as numbered steps.