Worksheet Grade 9 Social Studies & Civics

Ospley Water District's PFAS Treatment Costs: A Cost-Benefit Worksheet

A grade 9 economics worksheet where students find the per-household cost of PFAS treatment for a fictional water district and weigh two compliance options.

Grade: Grade 9
Subject: Social Studies & Civics
Material: Worksheet
Time: 40 min
Prep: No prep
Materials: None

Standards

D2.Eco.2.9-12 C3 Framework

Use marginal benefits and marginal costs to construct an argument for or against an approach or solution to an economic issue.

How this material aligns: Items 6 and 7 have students weigh the marginal cost of building local treatment against connecting to a regional system, building the cost-and-benefit argument this indicator names.

All materials for D2.Eco.2.9-12 →

D2.Civ.13.9-12 C3 Framework

Evaluate public policies in terms of intended and unintended outcomes, and related consequences.

How this material aligns: Item 8 asks students to evaluate the intended and unintended outcomes of a proposal to let eligible systems apply for more time to meet the federal PFAS compliance date, the policy evaluation this civic indicator requires.

All materials for D2.Civ.13.9-12 →

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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 annual and per-household cost of PFAS treatment for a water district.
Students will compare two compliance options using marginal cost reasoning.
Students will evaluate a proposed change to a federal regulatory deadline.

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.

Also: compliance, marginal cost, capital cost, operating cost

Teacher overview

New federal limits on PFAS, a group of long-lasting industrial chemicals, are forcing many water systems to add expensive treatment or find another safe source. This worksheet puts students in the role of an analyst for the fictional Ospley Water District, whose wells tested above the federal PFOA limit. Students calculate what treatment would cost each household, compare building local treatment against connecting to a regional system, and evaluate a proposed change to the federal compliance deadline. The scenario keeps the mathematics grounded in one district's budget so the policy trade-offs stay concrete.

Ospley Water District's PFAS Treatment Costs: A Cost-Benefit Worksheet — Grade 9 Social Studies & Civics

Name: ____________________________ Date: ______________

Student Worksheet

The fictional Ospley Water District serves 8,000 households. Recent testing found the chemical PFOA in Ospley's drinking-water wells at 12 parts per trillion. The federal maximum contaminant level for PFOA is 4.0 parts per trillion, so Ospley must act. Use the figures below and show your work.

Building local treatment costs $6.4 million to construct, financed evenly over 20 years, plus $480,000 each year to operate.
Connecting to a regional system instead costs a one-time $9.6 million, financed evenly over 20 years, with no local operating cost.

Ospley's wells tested at 12 parts per trillion of PFOA, and the federal limit is 4.0 parts per trillion. By how many parts per trillion does Ospley exceed the limit?
The $6.4 million construction cost is financed evenly over 20 years. What is Ospley's annual capital cost for building local treatment?
Add the annual operating cost of $480,000 to your answer from item 2. What is Ospley's total annual cost to build and run local treatment?
Divide the total annual cost from item 3 among the 8,000 households. What does each household pay per year under the build-local option?
About how much is that per household each month? Round to the nearest cent.
The regional-connection option costs $9.6 million financed evenly over 20 years, with no operating cost. Find its annual cost, then its cost per household per year. Which option is cheaper per household, and by how much per year?
Explain your reasoning: Ospley's board says building local treatment gives the district more control over its own water. Using cost and benefit, give one reason a board might still choose the more expensive local option.
The federal PFOA limit was set in 2024, and water systems must comply 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 limit would not change, and the proposal was not yet final as of September 2026. Name one intended outcome and one possible unintended outcome of letting districts like Ospley apply for two more years to comply.
Over the full 20 years, what is the total cost of the build-local option? Multiply your total annual cost from item 3 by 20.

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: 8 parts per trillion. In the Ospley scenario, 12 minus 4.0 equals 8 parts per trillion above the limit.
Answer: $320,000 per year. The $6.4 million construction cost divided evenly across 20 years is $320,000 each year.
Answer: $800,000 per year. Adding the $480,000 operating cost to the $320,000 capital cost gives $800,000 per year.
Answer: $100 per household. The $800,000 total annual cost divided among 8,000 households is $100 per household each year.
Answer: About $8.33 per month. The $100 yearly cost divided by 12 months is about $8.33.
Answer: $60 per household, and connecting is cheaper by $40. The $9.6 million divided across 20 years is $480,000 a year, and divided among 8,000 households that is $60 per household per year, which is $40 less than the $100 build-local cost.
Answer: Accept any reasonable cost-benefit argument, such as: owning the treatment plant lets Ospley control its own water quality and schedule, avoids depending on another system's rates or reliability, and keeps decisions local, benefits a board might judge worth the extra $40 per household each year.
Answer: Accept one intended and one unintended outcome. Intended: eligible districts that apply get more time to plan and finance treatment, easing the immediate cost. Unintended: residents of those districts stay exposed to PFOA above the limit longer, and some districts may delay action past April 2029 while counting on an exemption that is still only a proposal and would have to be applied for.
Answer: $16 million. The $800,000 total annual cost multiplied by 20 years equals $16 million.

Differentiation

Support

Provide a worked example with different numbers (a district of 4,000 households and a $4 million cost financed over 20 years) fully solved, so students can follow the same steps for Ospley. Supply the division setups for items 2, 4, and 6 with the numbers already placed.

Extension

Suppose the regional system instead charged a flat $60 per household each year, however many households Ospley serves, while the build-local option still cost $800,000 a year in total. Ask students to find the break-even point: how many households would Ospley need before the build-local cost per household falls to $60? Have them explain what that result suggests about which option fits a growing district.

ELL supports

Pre-teach "PFAS," "capital cost," "operating cost," and "marginal cost" with simple definitions and one example each. Provide a labeled table with columns for "one-time cost," "yearly cost," and "cost per household" so students can organize each option before comparing them.