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Lesson plan Grade 7 Science

Ashford Mills and PFAS: Reading Parts-Per-Trillion Results

A 45-minute grade 7 science lesson on a fictional town's PFAS-tainted well, where students compare test results to the federal limit and study a rule still being changed.

Objectives

  1. Students will interpret PFAS test results in parts per trillion and compare them to the federal limit.
  2. Students will explain why PFAS are called emerging contaminants and describe how the rules for them are still changing.

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.
Emerging contaminant
A pollutant that scientists have recently learned to detect or worry about, but that may not be regulated yet. PFAS, some medicines, and microplastics are examples.
Granular activated carbon
Small black grains of specially treated carbon that grab contaminants as water flows through them. Utilities use big tanks of it to remove PFAS, bad tastes, and odors.
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.

Teacher overview

PFAS are a family of human-made chemicals used in nonstick and waterproof products, and they break down so slowly that people call them "forever chemicals." Because they are measured in parts per trillion, an extremely small amount matters, and the federal rules for them are new and still changing. This lesson uses a fictional town's contaminated well to teach students how to read a parts-per-trillion result, compare it to the limit, and understand why a rule that is still being revised is different from a settled one. A common misconception this lesson addresses is that "a tiny number must be a tiny problem," when for PFAS a reading of only a few parts per trillion can be over the legal limit.

Procedure

Hook (6 min): Ask students what "one part per trillion" might mean. Offer a comparison: one part per trillion is about one drop of water in 20 Olympic-size swimming pools. Explain that PFAS limits are set at this scale because the chemicals are harmful even in small amounts and do not break down.

Direct instruction (12 min): Introduce the federal maximum contaminant level for two common PFAS, PFOA and PFOS, which is 4.0 parts per trillion each. Explain that this rule was finalized in 2024 and gives water systems until April 2029 to comply, and that in May 2026 EPA proposed letting eligible systems apply for a two-year exemption, to April 2031. The exemption would not apply automatically to every system, and the 4.0 limit would not change. Stress that the proposal was not final as of September 2026, so the rule is still in motion and the dates can change. Explain that PFAS are called emerging contaminants because scientists and regulators have only recently begun to measure and limit them in drinking water. Model the fictional case: the town of Ashford Mills found 12 parts per trillion of PFOA in one well, and ask what that means compared to the 4.0 limit.

Guided practice (12 min): Work through the comparison together. Twelve parts per trillion divided by the 4.0 limit is three, so the well reads three times the limit. Then introduce granular activated carbon, a treatment that traps PFAS as water passes through, and show that after treatment the reading dropped to 2 parts per trillion. Ask whether that now meets the limit.

Independent practice (10 min): Students complete the handout, computing how far above the limit the well was and how much the carbon filter removed. Circulate and check that students divide by the limit for item 1 rather than subtracting.

Closure and exit ticket (5 min): Each student writes one sentence explaining what "the rule is still being changed" means for a town planning its PFAS treatment. Collect and scan; if more than a few students describe the limit as permanent and settled, revisit the point at the next class.

Ashford Mills and PFAS: Reading Parts-Per-Trillion Results — Grade 7 Science
Name: ____________________________ Date: ______________

Student Handout

Ashford Mills is a fictional town. Its Well 5 was tested for PFOA, one type of PFAS. The federal limit for PFOA is 4.0 parts per trillion.

  1. Well 5 measured 12 parts per trillion of PFOA. How many times higher than the 4.0 limit is that reading? Use the well reading divided by the limit.
  2. After a granular activated carbon filter, the PFOA dropped to 2 parts per trillion. What percent of the original 12 parts per trillion did the filter remove? Use (before minus after) divided by before, times 100.
  3. Is the treated water, at 2 parts per trillion, below the 4.0 limit? By how many parts per trillion?
  4. One part per billion equals 1,000 parts per trillion. Write the 4.0 parts-per-trillion limit as a number of parts per billion.
  5. In one or two sentences, explain what it means that EPA has proposed letting some water systems apply for two more years, from April 2029 to April 2031, to meet the PFAS limit, and why a town should keep watching for updates.

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: 3 times.
    12 divided by 4.0 = 3, so the well read three times the limit.
  2. Answer: 83%.
    (12 − 2) ÷ 12 × 100 = 10 ÷ 12 × 100 = 83.3 percent, which rounds to about 83 percent removed.
  3. Answer: Yes, by 2 parts per trillion.
    2 is below 4.0, and 4.0 − 2 = 2 parts per trillion below the limit.
  4. Answer: 0.004 parts per billion.
    4.0 parts per trillion divided by 1,000 = 0.004 parts per billion, which shows how small the limit is.
  5. Answer: EPA has proposed extra time for some systems, and the rule may change again.
    If finalized, eligible systems could apply for an exemption that gives them until April 2031 instead of April 2029 to install treatment. It would not be automatic for every system, and the 4.0 limit would stay the same. Because the proposal is not final, the town should keep watching for new dates. Accept any answer that captures the possible extra time for some systems and the ongoing change.

Differentiation

Support

Provide the two formulas (divide for "how many times," and the percent-change formula) on a reference strip, and complete item 1 together. Offer a labeled picture of a carbon filter so students can see where the PFAS is trapped.

Extension

Ask students to find what the percent removal would need to be for the carbon filter to bring the 12 parts per trillion down to exactly the 4.0 limit, and to show the calculation, then explain why a town would aim below the limit rather than exactly at it.

ELL supports

Pre-teach "contaminant," "limit," "parts per trillion," and "filter" with a simple visual of one drop in 20 Olympic-size pools. Provide a sentence frame for item 5: "Some systems may get until ___ instead of ___, so the town should ___."