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Worksheet Grade 8 Science

Hartwell's GAC Filter and PFAS in Parts per Trillion

A grade 8 science worksheet on PFAS removal at a fictional well, using scientific notation and removal efficiency to compare readings against the 4.0 ppt limit.

Objectives

  1. Students will calculate the removal efficiency of a treatment filter from before-and-after concentration readings.
  2. Students will express very large and very small water quantities in scientific notation and compare them.

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: removal efficiency, scientific notation

Teacher overview

In the fictional town of Hartwell, the water plant added a granular activated carbon (GAC) filter to lower two PFAS chemicals in its well water, PFOA and PFOS. This worksheet has students calculate how much of each chemical the filter removed, decide whether the treated water meets the federal maximum contaminant level of 4.0 parts per trillion, and practice writing trillion-scale numbers in scientific notation. PFAS limits are still being phased in, so the worksheet also asks why an operator keeps watching for rule changes.

Hartwell's GAC Filter and PFAS in Parts per Trillion — Grade 8 Science
Name: ____________________________ Date: ______________

Student Worksheet

Suppose the fictional town of Hartwell tests its well water for two PFAS chemicals, PFOA and PFOS, before and after the water passes through a granular activated carbon (GAC) filter. All readings are in parts per trillion (ppt).

  • Raw well water: PFOA = 24 ppt, PFOS = 18 ppt
  • After the GAC filter: PFOA = 3 ppt, PFOS = 2 ppt
  • The EPA maximum contaminant level (MCL) for PFOA and for PFOS is 4.0 ppt each.
  • Removal efficiency (%) = (raw reading − treated reading) ÷ raw reading × 100.
  1. A trillion is written 1,000,000,000,000. Write one trillion as a power of ten.
  2. On a test day, Hartwell's plant treats 500,000 gallons of water. Write 500,000 as a single digit times a power of ten.
  3. Use the removal formula to find the GAC filter's removal efficiency for PFOA. Round to the nearest tenth of a percent.
  4. Use the removal formula to find the removal efficiency for PFOS. Round to the nearest tenth of a percent.
  5. Which chemical did the filter remove more effectively? Compare your two percentages to decide.
  6. Is the treated PFOA reading below the 4.0 ppt MCL? By how many ppt does it clear or miss the limit?
  7. Is the treated PFOS reading below the 4.0 ppt MCL? By how many ppt does it clear or miss the limit?
  8. How many times as much PFOA was in the raw water (24 ppt) as in the treated water (3 ppt)?
  9. How many times as much PFOA was in the raw water (24 ppt) as the 4.0 ppt MCL allows?
  10. Explain why scientists report PFAS in parts per trillion and use scientific notation instead of ordinary decimals.
  11. PFAS rules are still being phased in. In one sentence, explain why a plant operator keeps checking for new EPA deadlines.

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: 10^12. One trillion is a 1 followed by 12 zeros, so 1,000,000,000,000 = 1 × 10^12.
  2. Answer: 5 × 10^5. The number 500,000 is 5 followed by 5 zeros, so it equals 5 × 10^5 gallons.
  3. Answer: 87.5 percent. (24 − 3) ÷ 24 × 100 = 21 ÷ 24 × 100 = 0.875 × 100 = 87.5%.
  4. Answer: 88.9 percent. (18 − 2) ÷ 18 × 100 = 16 ÷ 18 × 100 = 0.8889 × 100 ≈ 88.9%.
  5. Answer: PFOS. Its removal efficiency of about 88.9% is higher than PFOA's 87.5%, so the filter removed a larger share of the PFOS.
  6. Answer: Yes, by 1 ppt. Treated PFOA is 3 ppt, and 4.0 − 3 = 1 ppt below the limit.
  7. Answer: Yes, by 2 ppt. Treated PFOS is 2 ppt, and 4.0 − 2 = 2 ppt below the limit.
  8. Answer: 8 times. 24 ÷ 3 = 8, so the raw water held eight times as much PFOA as the treated water.
  9. Answer: 6 times. 24 ÷ 4 = 6, so the raw reading was six times the amount the MCL allows.
  10. Answer: Parts per trillion describe extremely small amounts, and writing them with scientific notation keeps the zeros manageable so the numbers are easier to compare and less likely to be miscopied.
  11. Answer: Because the 4.0 ppt limits must be met by April 2029, and EPA has proposed, but not yet finalized, letting eligible systems apply for a two-year exemption to April 2031, an operator watches for new rules so the plant stays in compliance and plans upgrades in time.

Differentiation

Support

Provide the removal-efficiency formula already substituted for PFOA, so students see (24 − 3) ÷ 24 × 100 written out before they try PFOS on their own. Give a place-value chart that lines up 500,000 and 1,000,000,000,000 under labeled powers of ten.

Extension

Ask students to combine the two treated readings into a total PFAS value, then determine what single removal efficiency the filter would need to bring a hypothetical raw total of 50 ppt down to the same treated total, showing each step.

ELL supports

Pre-teach "raw," "treated," "removal," and "limit" with a simple before-and-after diagram of water entering and leaving the filter. Allow students to answer items 5, 10, and 11 with a labeled sketch plus a short phrase rather than full sentences.