Worksheet Grade 8 Science

Redmoor Treatment Plant's PFAS Report: Comparing Parts per Trillion

A grade 8 worksheet where students read a fictional plant's PFAS lab report, compare it to the real federal limit, and convert ppm, ppb, and ppt.

Grade: Grade 8
Subject: Science
Material: Worksheet
Time: 40 min
Prep: No prep
Materials: calculator (optional)

Standards

8.EE.A.3 Common Core Math

Use numbers expressed in the form of a single digit times an integer power of 10 to estimate very large or very small quantities, and to express how many times as much one is than the other.

How this material aligns: Items 3 through 6 require students to convert Redmoor's PFAS result between ppm, ppb, and ppt and express the ppm-to-ppt ratio in scientific notation.

All materials for 8.EE.A.3 →

RST.6-8.7 Common Core ELA

Integrate quantitative or technical information expressed in words in a text with a version of that information expressed visually (e.g., in a flowchart, diagram, model, graph, or table).

How this material aligns: Items 1, 2, 7, and 8 require students to integrate the plant's numeric lab results with the federal limits stated in words to judge compliance.

All materials for RST.6-8.7 →

MS-ESS3-3 NGSS

Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.

How this material aligns: Items 1, 2, 7, and 8 judge Redmoor's PFAS and lead monitoring results against the federal limits, and item 9 explains the treatment investment that would hold the contamination under a stricter future limit.

All materials for MS-ESS3-3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-16.

Objectives

Students will compare a water system's contaminant results in parts per trillion and parts per billion to real federal limits.
Students will convert concentrations between parts per million, parts per billion, and parts per trillion using powers of ten.
Students will explain why raw numbers in different concentration units cannot be compared without converting them first.

Vocabulary

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.
Parts per million (ppm): Parts per million tells how many tiny bits of a substance are mixed into a million bits of water. Scientists use it to measure very small amounts of chemicals in water.

Also: parts per billion, scientific notation

Teacher overview

The fictional Redmoor Water Treatment Plant recently began testing for PFAS, a group of manufactured chemicals sometimes called "forever chemicals" because they break down very slowly in the environment. This worksheet has students compare a laboratory report's PFAS results, measured in parts per trillion, against the real federal limit, and practice converting between parts per million, parts per billion, and parts per trillion using scientific notation. A common misconception is that parts per trillion is only a slightly smaller unit than parts per million; in fact, it takes one million parts per trillion to equal a single part per million, a difference scientific notation makes easy to see.

Redmoor Treatment Plant's PFAS Report: Comparing Parts per Trillion — Grade 8 Science

Name: ____________________________ Date: ______________

Student Worksheet

The fictional Redmoor Water Treatment Plant's lab reports contaminant levels in different units depending on how concentrated the contaminant is. Use the real facts and the plant's fictional lab results below to answer the questions.

Real facts:

One part per million (ppm) equals 1,000 parts per billion (ppb), which equals 1,000,000 parts per trillion (ppt), because each unit is a power of ten smaller than the last.
The federal maximum contaminant level for PFOA and PFOS, two common PFAS chemicals, is 4.0 parts per trillion each. Water systems must meet this limit by April 2029. In May 2026, EPA proposed letting eligible systems apply for a two-year exemption, which would give them until April 2031. The proposal would not change the 4.0 ppt limit, and it was not yet final as of September 2026.
The federal lead action level in drinking water is currently 15 parts per billion, and it is scheduled to drop to 10 parts per billion in 2027.

Redmoor's fictional lab results this month:

PFOA: 6.2 ppt
PFOS: 3.1 ppt
Lead: 8 ppb

Redmoor's PFOA result is 6.2 ppt. Does this exceed the federal maximum contaminant level of 4.0 ppt? By how many ppt?
Redmoor's PFOS result is 3.1 ppt. Does this exceed the federal maximum contaminant level of 4.0 ppt? By how many ppt?
Using the relationship 1 ppb = 1,000 ppt, convert Redmoor's PFOA result of 6.2 ppt to ppb.
Using the relationship 1 ppm = 1,000 ppb, convert your answer to item 3 to ppm.
Write the number of ppt in one ppm (1,000,000) in scientific notation.
About how many times larger is one ppm than one ppb? Use your answer to explain why a contaminant measured in ppt can still represent an extremely small concentration even when its number, like 6.2, looks larger than a number measured in ppm.
Redmoor's lead result is 8 ppb. Compare this to the current federal lead action level of 15 ppb. Is Redmoor's result above or below the action level, and by how much?
Compare Redmoor's lead result of 8 ppb to the stricter lead action level that takes effect in 2027 (10 ppb). Will Redmoor's current result still be below that level, and by how much?
Explain why a water system might need to invest in new treatment technology now, even though its current lead result is below today's action level.
A classmate says, "8 ppb of lead is a bigger number than 6.2 ppt of PFOA, so the lead result must be the bigger problem." Using what you know about parts per billion, parts per trillion, and each contaminant's limit, explain what is wrong with this reasoning.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Answer: 2.2 ppt over the limit.
6.2 ppt − 4.0 ppt = 2.2 ppt above the federal maximum contaminant level.
Answer: no, 0.9 ppt under the limit.
3.1 ppt is below the 4.0 ppt limit; 4.0 − 3.1 = 0.9 ppt of room remaining.
Answer: 0.0062 ppb.
Since 1 ppb = 1,000 ppt, divide by 1,000: 6.2 ÷ 1,000 = 0.0062 ppb.
Answer: 0.0000062 ppm.
Since 1 ppm = 1,000 ppb, divide by 1,000 again: 0.0062 ÷ 1,000 = 0.0000062 ppm.
Answer: 1 × 10^6 ppt per ppm.
1,000,000 = 1 × 10^6.
Answer: 1,000 times larger.
One ppm equals 1,000 ppb, so a ppm unit represents 1,000 times the concentration of a ppb unit. Because ppt is an even smaller unit than ppb, a number like 6.2 measured in ppt can represent a far smaller true concentration than a similar-looking number measured in ppm.
Answer: below, by 7 ppb.
15 ppb − 8 ppb = 7 ppb below the current action level.
Answer: yes, below by 2 ppb.
10 ppb − 8 ppb = 2 ppb below the stricter 2027 action level.
Answer: Treatment upgrades take years to plan and pay for.
Pipe replacement, corrosion control treatment, and monitoring programs take years to design, fund, and build, so a utility that waits until it exceeds a future limit may be unable to act in time; starting early keeps the system ahead of the stricter standard. Lead comes from pipes after the plant, the action level is not a safe level (EPA's health goal for lead is zero), and the new rules require replacing nearly all lead service lines within 10 years, which takes years to plan and fund.
Answer: The reasoning is flawed.
Ppb and ppt are different units, so the raw numbers 8 and 6.2 cannot be compared directly. Converting shows 8 ppb equals 8,000 ppt, so the lead concentration is actually larger. But each contaminant has its own limit, set by how harmful it is. PFOA at 6.2 ppt is above its 4.0 ppt limit, while lead at 8 ppb is below its 15 ppb action level, so a bigger number does not by itself mean a bigger problem.

Differentiation

Support (below grade level)

Give students the unit relationship already written as a conversion ladder (ppm → divide by 1,000 → ppb → divide by 1,000 → ppt) with arrows, and let them use a calculator for every division in items 3 and 4.

Extension (above grade level)

Ask students to find what fictional Redmoor's PFOA result of 6.2 ppt equals in ppm using one division step instead of two, by first finding the single conversion factor between ppm and ppt (1 ppm = 1,000,000 ppt), then confirm the result matches their two-step answer from items 3 and 4 (6.2 ÷ 1,000,000 = 0.0000062 ppm).

ELL supports

Pre-teach "parts per million," "parts per billion," and "parts per trillion" using a visual size comparison, such as one drop in a bathtub, one drop in a backyard swimming pool, and one drop in about 20 Olympic-size pools, matched in order to the three units. Allow students to show the conversions in items 3 and 4 as a labeled diagram of the conversion ladder instead of written sentences.

Extensions

Have students look up, with teacher guidance, their own community's most recent water quality report and identify one contaminant reported in ppb or ppt, then compare it to the appropriate federal limit using the same conversion method practiced in this worksheet.