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Worksheet Grade 9 Science

Hollin Gap Water Audit: Non-Revenue Water and Pipe Replacement

A grade 9 worksheet where students audit a fictional town's water losses, price the lost water, and weigh leak detection against a faster pipe replacement program.

Objectives

  1. Students will calculate non-revenue water as a volume and as a percent of water produced.
  2. Students will convert an annual water loss into a daily loss per mile of main and into a dollar value.
  3. Students will compare a leak detection survey and a faster replacement schedule using payback period and other criteria.

Vocabulary

Non-revenue water
Water that a utility treats and pumps but never gets paid for. Most of it escapes through leaks in old pipes, and some is lost to broken meters or theft.
Water main
A big underground pipe that carries drinking water beneath the streets to a neighborhood. Smaller service lines branch off it to reach each building.
Water audit
A careful check of where water goes, either in a home or across a whole water system. Audits find leaks and waste so they can be fixed.

Also: payback period

Teacher overview

A utility knows how much water it produced and how much it billed, and the gap between those two numbers is called non-revenue water. Some of that gap is leakage from old pipes, some is water used for firefighting and main flushing, and some is meters that under-record. The ASCE 2025 Infrastructure Report Card gave United States drinking water a C-, and by ASCE's count a water main breaks somewhere in the country roughly every two minutes, so the condition of buried pipe is a standing question for most systems. This worksheet has students complete a water audit for the fictional town of Hollin Gap and then rank two ways of closing the gap. A misconception worth correcting first: students often assume every gallon of non-revenue water is leaking out of a broken pipe, when authorized unbilled use and meter error are also part of the total.

Hollin Gap Water Audit: Non-Revenue Water and Pipe Replacement — Grade 9 Science
Name: ____________________________ Date: ______________

Student Worksheet

The fictional town of Hollin Gap owns 128 miles of water main. Last year the utility produced 620 million gallons and billed customers for 471 million gallons. The utility repaired 34 main breaks during the year. Producing and delivering water costs the town $1.85 per 1,000 gallons. Round to one decimal place unless an item says otherwise.

  1. Find Hollin Gap's non-revenue water for the year, in millions of gallons.
  2. Non-revenue water is what percent of the water produced? Round to the nearest tenth of a percent.
  3. Convert your answer from item 1 into gallons per day, using 365 days. Round to the nearest gallon.
  4. Divide your answer from item 3 by the 128 miles of main to find the daily loss per mile of pipe. Round to the nearest gallon.
  5. At $1.85 per 1,000 gallons, what did the non-revenue water from item 1 cost the town over the year?
  6. The utility repaired 34 main breaks along 128 miles of pipe. Find the break rate in breaks per mile per year, rounded to the nearest hundredth.
  7. Hollin Gap currently replaces 3.5 miles of main each year. At that pace, how many years would it take to replace all 128 miles?
  8. The town council wants every mile replaced within 20 years. How many miles per year would that require?
  9. Replacement costs $1.4 million per mile. How much more would the town spend each year to move from 3.5 miles per year to the pace you found in item 8?
  10. A different option is a leak detection survey costing $210,000, which the contractor says would find and repair enough leaks to recover 40 percent of the non-revenue water from item 1. What would that recovered water be worth in a year at $1.85 per 1,000 gallons?
  11. Divide the survey cost by your answer from item 10 to find the payback period in years. Round to the nearest tenth.
  12. The council has asked for a recommendation. Rank the leak detection survey and the faster replacement schedule against three criteria: cost per year, how long the benefit lasts, and how much street disruption each causes. Use your numbers from items 9, 10, and 11 as support.
  13. Explain why a utility cannot assume that all of the non-revenue water in item 1 is leaking from pipes. Name two other places the missing water could go.

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: 149 million gallons. 620 million gallons produced minus 471 million gallons billed equals 149 million gallons of non-revenue water.
  2. Answer: 24.0 percent. 149 divided by 620 equals 0.2403, or 24.0 percent.
  3. Answer: 408,219 gallons per day. 149,000,000 divided by 365 equals 408,219.18, which rounds to 408,219 gallons per day.
  4. Answer: 3,189 gallons per mile per day. 408,219 divided by 128 miles equals 3,189.2, which rounds to 3,189 gallons per mile per day.
  5. Answer: $275,650. 149,000,000 gallons divided by 1,000 equals 149,000 units of a thousand gallons, and 149,000 times $1.85 equals $275,650.
  6. Answer: 0.27 breaks per mile per year. 34 breaks divided by 128 miles equals 0.2656, which rounds to 0.27.
  7. Answer: 36.6 years. 128 miles divided by 3.5 miles per year equals 36.57 years, which rounds to 36.6 years.
  8. Answer: 6.4 miles per year. 128 miles divided by 20 years equals 6.4 miles per year.
  9. Answer: $4.06 million per year. 6.4 minus 3.5 equals 2.9 additional miles per year, and 2.9 times $1.4 million per mile equals $4.06 million per year.
  10. Answer: $110,260. 149 million gallons times 0.40 equals 59.6 million gallons, which is 59,600 units of a thousand gallons, and 59,600 times $1.85 equals $110,260.
  11. Answer: 1.9 years. $210,000 divided by $110,260 per year equals 1.905, which rounds to 1.9 years.
  12. Answer: Accept any ranking that uses the calculated figures. A defensible answer notes that the survey costs far less, pays for itself in about 1.9 years, and disrupts almost no streets, yet its benefit fades as new leaks open in the same old pipe. Faster replacement costs about $4.06 million more each year and tears up streets, yet it fixes the cause and the benefit lasts for decades. Many students will recommend the survey first and replacement as the long-term program, which is the usual utility practice.
  13. Answer: Accept any two of the following, explained: water used for fighting fires, water used to flush mains and test hydrants, water taken through unmetered municipal connections, water lost to meters that under-record at low flow, and water lost to billing or record-keeping errors. Only some of the 149 million gallons is leakage, so the audit has to separate real losses from apparent losses before the town chooses a fix.

Differentiation

Support

Provide a partly completed audit table for the fictional town of Hollin Gap with items 1 and 3 already filled in, so students can begin at the per-mile calculation. Give a worked example of a payback period using a different, smaller purchase before students attempt item 11.

Extension

Have students find the break rate per mile that Hollin Gap would reach if replacement continued for ten years at 6.4 miles per year and each replaced mile cut its own break rate in half, then explain what assumptions their estimate depends on.

ELL supports

Pre-teach produced, billed, non-revenue, and payback with a simple flow diagram showing water leaving the plant, water arriving at meters, and the gap between them. Provide a sentence frame for item 12: "I would choose ___ first because ___, and I would plan ___ for the long term because ___."