Worksheet Grade 7 Science

Dellwood's PFAS Cleanup: Percent Removal and Well Blending

A grade 7 science worksheet where students read a town's PFAS results in parts per trillion, find percent removal by a carbon filter, and blend wells to meet the limit.

Grade: Grade 7
Subject: Science
Material: Worksheet
Time: 35 min
Prep: No prep
Materials: Calculator

Standards

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 through 7 have students evaluate a carbon-filter and well-blending plan that monitors and lowers PFAS pollution in a town's drinking water.

All materials for MS-ESS3-3 →

7.RP.A.3 Common Core Math

Use proportional relationships to solve multistep ratio and percent problems. Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.

How this material aligns: Items 1, 2, and 9 use proportional reasoning to compute the percent of PFAS removed by treatment, the multistep percent work this standard targets.

All materials for 7.RP.A.3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will calculate the percent of a contaminant removed by a treatment step.
Students will compare treated water results against a regulatory limit and decide whether they pass.
Students will use blending ratios to bring a contaminant below a set 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.
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.

Also: blending

Teacher overview

PFAS are a group of long-lasting chemicals sometimes called "forever chemicals" because they break down very slowly. Federal rules set the maximum contaminant level for two of them, PFOA and PFOS, at 4.0 parts per trillion each. Water systems must meet these limits by April 2029. In May 2026 EPA proposed letting eligible systems apply for a two-year exemption, to April 2031. The 4.0 parts per trillion limits would not change, and the proposal was not yet final as of September 2026, so the rule is still changing. This worksheet follows the fictional town of Dellwood, which found PFAS in one of its wells and installed a granular activated carbon filter to remove them. Students read the before-and-after test numbers, find the percent removed, and work out how to blend two wells so the water meets the limit.

Dellwood's PFAS Cleanup: Percent Removal and Well Blending — Grade 7 Science

Name: ____________________________ Date: ______________

Student Worksheet

Suppose the town of Dellwood (a fictional town) tests the water from Well 5 before and after its new carbon filter. All results are in parts per trillion (ppt). The limit for PFOA and for PFOS is 4.0 ppt each.

Well 5 raw water, before the filter: PFOA 32 ppt, PFOS 20 ppt.
Well 5 water, after the filter: PFOA 8 ppt, PFOS 2 ppt.
Well 9 is clean water with 0 ppt PFOA.

Find the percent of PFOA removed by the carbon filter as the water went from 32 ppt down to 8 ppt.
Find the percent of PFOS removed as the water went from 20 ppt down to 2 ppt.
The limit for PFOA is 4.0 ppt. The treated Well 5 water is 8 ppt PFOA. Does it meet the limit? By how many ppt is it over or under?
The treated Well 5 water is 2 ppt PFOS. Does that meet the 4.0 ppt limit? By how many ppt is it over or under?
The operator mixes the treated Well 5 water (8 ppt PFOA) with an equal amount of clean Well 9 water (0 ppt PFOA). What is the PFOA level of the 50-50 mix?
Does the mix from item 5 meet the 4.0 ppt PFOA limit? Explain in one sentence how mixing changed the result.
Six months later the filter wears out and the treated Well 5 water rises to 12 ppt PFOA. The operator has 100 gallons of this water. How many gallons of clean Well 9 water (0 ppt) must be added to bring the mix down to 4.0 ppt PFOA?
Explain in your own words why mixing clean water with contaminated water lowers the parts-per-trillion number, even though no PFAS is destroyed.
A nearby town has raw water at 40 ppt PFOA. If its filter also removes 75 percent, like the filter in item 1, what will the treated PFOA level be?
Using what you found in item 7, explain why Dellwood must keep testing its treated water on a regular schedule instead of testing only once.

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: 75 percent.
The filter lowered PFOA from 32 ppt to 8 ppt, a drop of 24 ppt. 24 divided by 32 equals 0.75, which is 75 percent removed.
Answer: 90 percent.
PFOS dropped from 20 ppt to 2 ppt, a drop of 18 ppt. 18 divided by 20 equals 0.90, which is 90 percent removed.
Answer: 4 ppt over.
The treated PFOA is 8 ppt and the limit is 4.0 ppt, so it does not meet the limit. 8 minus 4 equals 4 ppt over the limit.
Answer: 2 ppt under.
The treated PFOS is 2 ppt and the limit is 4.0 ppt, so it meets the limit. 4 minus 2 equals 2 ppt under the limit.
Answer: 4.0 ppt.
Mixing equal amounts averages the two numbers: (8 ppt plus 0 ppt) divided by 2 equals 4.0 ppt PFOA.
Answer: Yes, it meets the limit.
The mix is exactly 4.0 ppt, which is at the limit. Adding clean water spreads the same PFAS through more water, so the concentration drops.
Answer: 200 gallons.
The 100 gallons at 12 ppt hold a fixed amount of PFAS. To reach 4.0 ppt, the total must be large enough that 100 times 12 divided by the total equals 4. That gives a total of 300 gallons, so 300 minus 100 equals 200 gallons of clean water added.
Answer: The PFAS spreads through more water.
Blending does not remove any PFAS, but the same small amount is now shared across a larger volume, so the parts-per-trillion count for any sample of the mix is lower.
Answer: 10 ppt.
Removing 75 percent leaves 25 percent behind. 40 ppt times 0.25 equals 10 ppt PFOA in the treated water.
Answer: Filters lose strength over time.
Item 7 shows the treated level rose from 8 ppt to 12 ppt as the carbon wore out. Regular testing catches that rise so the town can replace the carbon before the water passes the limit.

Differentiation

Support

Give students a worked example of one percent-removal calculation (for instance, 50 ppt down to 20 ppt) before they begin, and provide the removal formula on a printed strip: percent removed equals the drop divided by the starting amount, times 100. Allow calculators for every item.

Extension

Ask students to find the blend ratio needed in item 7 if Well 9 were not perfectly clean but held 1 ppt PFOA, and to explain why a slightly contaminated blending source changes how much clean water is required.

ELL supports

Pre-teach "removed," "limit," "blend," and "sample" with a simple picture showing a full glass of dark water poured into a larger clear pitcher. Let students answer items 6, 8, and 10 with a labeled sketch plus a short phrase rather than a full sentence.