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Lesson plan Grade 10 Science

Rilbeck's Partial Lead Line Replacements: A Corrosion Lesson

A grade 10 lesson on why cutting out only part of a lead service line can raise tap lead, using sequential sampling data from two homes to evaluate a utility's policy.

Objectives

  1. Students will calculate average lead concentrations from sequential tap samples at two homes.
  2. Students will explain how galvanic corrosion and pipe disturbance can raise lead after a partial replacement.
  3. Students will evaluate full and partial replacement against cost, health, and reliability criteria.

Vocabulary

Service line
A service line is the small pipe that connects the water main under the street to the plumbing inside one home or building. It is the last leg of the trip water makes to your tap.
Orthophosphate
A chemical that utilities add to water in very small amounts. It coats the inside of pipes so lead cannot easily get into the water.

Also: galvanic corrosion, passivation, sequential sampling

Teacher overview

A lead service line runs from the water main to the building, and utilities have often replaced only the portion they own, leaving the homeowner's half in the ground. Removing half of the lead pipe would seem to cut the lead at the tap roughly in half, yet sampling frequently shows higher lead for weeks or months after a partial job, because cutting the pipe knocks loose the protective scale and because joining new copper to old lead sets up a galvanic cell that drives the lead to corrode faster. This lesson uses sequential sampling data from two homes in the fictional city of Rilbeck to make that pattern visible, then asks students to evaluate the two replacement policies against stated criteria. The federal rule now in effect requires nearly all lead service lines to be replaced within ten years, which makes the choice between partial and full work a practical question for utilities rather than a hypothetical one.

Procedure

Framing the problem (5 min): Project a simple cross-section of a service line running from the main to a house, with the utility-owned half in one color and the customer-owned half in another. Tell students that the fictional city of Rilbeck replaced its half on some streets and the whole line on others, and ask them to predict which homes will show lower lead afterward. Record the class prediction on the board so it can be checked at the end.

Scale and passivation (12 min): Explain that lead pipe in service for decades develops an interior scale of lead carbonate and, where the utility feeds orthophosphate, lead phosphate. These solids are only slightly soluble, so the scale, not the metal, is what the water actually touches. Emphasize two consequences: the scale is protective as long as it stays intact, and anything that changes water chemistry or physically disturbs the pipe can release both dissolved lead and scale particles. Draw the pipe wall with the scale layer labeled, then draw a fragment breaking loose during a cut.

The galvanic cell (8 min): Sketch a lead pipe joined to a new copper pipe with water inside as the electrolyte. Identify lead as the more active metal, which becomes the anode and dissolves, while copper serves as the cathode. Point out that the connection is exactly what a partial replacement creates, and that the effect is strongest in the first months and near the joint. Ask one check question before moving on: which metal loses mass in this pair, and why does that matter for a homeowner's tap. Expect students to name lead as the anode.

Handout analysis (20 min): Distribute the student handout and have students work in pairs through items 1 through 9. Circulate and watch for two errors: averaging only the first-draw sample instead of all five, and reporting the Home B change as a decrease because the arithmetic was reversed. Ask early finishers to mark on the data table which sample number represents water that sat in the service line itself.

Discussion of the trade-off (7 min): Bring the class together and compare the two homes' results against the prediction recorded at the start. Push students to separate the health criterion from the cost criterion, and to state plainly that a partial job costs less per home while leaving lead in the ground and raising exposure in the short term. Ask whether a filter provided for six months changes their recommendation, and require students to say what the filter does and does not solve.

Exit ticket (3 min): Have every student complete handout item 10 and hand it in on the way out. The class is ready to move on if at least four out of five students name both the disturbance of the scale and the galvanic couple in their answer; if fewer do, reopen the galvanic sketch at the start of the next session.

Rilbeck's Partial Lead Line Replacements: A Corrosion Lesson — Grade 10 Science
Name: ____________________________ Date: ______________

Student Handout

Suppose the fictional city of Rilbeck sampled two homes before and after service line work. Each sample is one liter, taken in sequence from the same tap after the water sat unused for six hours. Sample 1 is the water standing in the household plumbing, samples 2 and 3 are the water that sat in the service line, and samples 4 and 5 come from the main. Results are in parts per billion of lead. Home A received a full replacement of both halves of the line. Home B received a partial replacement, in which the utility replaced its half with copper and left the customer's lead half in place. The federal action level for lead falls to 10 ppb in 2027.

Sample Home A before Home A after 3 months Home B before Home B after 3 months
1 21 4 18 16
2 34 5 30 41
3 28 3 26 33
4 12 2 14 12
5 7 2 8 6
  1. Using Home A's five before values of 21, 34, 28, 12, and 7 ppb, calculate the average lead concentration before the replacement.
  2. Using Home A's five after values of 4, 5, 3, 2, and 2 ppb, calculate the average lead concentration three months after the replacement.
  3. Using your averages of 20.4 ppb before and 3.2 ppb after, calculate the percent reduction in average lead at Home A. Round to the nearest tenth.
  4. Using Home B's five before values of 18, 30, 26, 14, and 8 ppb, calculate the average lead concentration before the work.
  5. Using Home B's five after values of 16, 41, 33, 12, and 6 ppb, calculate the average lead concentration three months after the work.
  6. Using your averages of 19.2 ppb before and 21.6 ppb after, calculate the percent change in average lead at Home B, and state whether it is an increase or a decrease.
  7. Home B's highest result after the work was 41 ppb. Calculate how many times the 2027 action level of 10 ppb that single sample represents.
  8. Identify which sample number carried the highest lead at Home B after the work, and explain what that sample number tells you about where in the plumbing the lead is coming from.
  9. Rilbeck must choose between replacing whole lines on fewer streets and replacing utility-owned halves on many more streets for the same budget. Using the Home A and Home B results, recommend one policy and defend it using at least two of these criteria: health risk to residents, cost per home, and how long the fix lasts.
  10. In two or three sentences, explain why cutting out half of a lead service line can leave a home with more lead at the tap than it had before. Name both the physical effect on the pipe wall and the electrical effect of joining two different metals.

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: 20.4 ppb.
    Add the five values and divide by five: 21 + 34 + 28 + 12 + 7 = 102, and 102 / 5 = 20.4 parts per billion.
  2. Answer: 3.2 ppb.
    Add and divide the same way: 4 + 5 + 3 + 2 + 2 = 16, and 16 / 5 = 3.2 parts per billion.
  3. Answer: 84.3 percent.
    Subtract, divide by the original, and multiply by 100: 20.4 minus 3.2 = 17.2, and 17.2 / 20.4 = 0.8431, so lead fell by 84.3 percent. Accept 84 percent.
  4. Answer: 19.2 ppb.
    Add and divide: 18 + 30 + 26 + 14 + 8 = 96, and 96 / 5 = 19.2 parts per billion.
  5. Answer: 21.6 ppb.
    Add and divide: 16 + 41 + 33 + 12 + 6 = 108, and 108 / 5 = 21.6 parts per billion.
  6. Answer: 12.5 percent increase.
    Subtract in the order after minus before: 21.6 minus 19.2 = 2.4, and 2.4 / 19.2 = 0.125, so average lead rose by 12.5 percent. Students who report a decrease have reversed the subtraction and should compare the two averages directly.
  7. Answer: 4.1 times.
    Divide the sample result by the action level: 41 / 10 = 4.1, so that one sample carried just over four times the level that takes effect in 2027.
  8. Answer: Sample 2, the water standing in the service line.
    Sample 2 was the highest at 41 ppb, and samples 2 and 3 represent the volume that sat inside the service line itself rather than in household plumbing or the main. The pattern points at the remaining lead half of the line, not at fixtures inside the house.
  9. Answer: Full replacement on fewer streets.
    A defensible recommendation cites health risk first: the Home B data show that a partial job can raise average lead by 12.5 percent and produce single samples above four times the coming action level, so spreading partial work across more homes spreads a short-term hazard. Students should also address durability, since a partial job leaves lead in the ground that must be dug up again later, which makes the cheaper option a second cost rather than a saving. Accept an argument for partial replacement only if it names a specific mitigation, such as filters and follow-up sampling for every affected home, and acknowledges the higher short-term exposure.
  10. Answer: The cut disturbs the protective scale, and the copper-to-lead joint drives corrosion.
    Decades of service build a scale of lead carbonate and lead phosphate on the pipe wall, and the water contacts that scale rather than bare metal. Cutting and pulling the pipe shakes fragments of scale loose, so particles of lead travel to the tap. Joining new copper to the remaining lead also creates a galvanic cell in which lead acts as the anode and dissolves faster than it did before. Both effects are strongest in the first months, which is why a three-month sample can look worse than the original.

Differentiation

Support

Give students a version of the handout table with the sums already written under each column so the averaging step in items 1, 2, 4, and 5 becomes a single division. Work item 3 aloud with the class using the Home A numbers before releasing pairs to work, and provide a percent change formula card that shows the subtraction order explicitly.

Extension

Have students calculate what Home B's average would need to fall to for the city of Rilbeck to claim a 50 percent reduction from its 19.2 ppb baseline, then design a follow-up sampling schedule that would confirm or refute the claim. Ask them to state how many months of data they would require before drawing a conclusion.

ELL supports

Provide a labeled diagram of the service line with the five sample volumes marked in order, so students can see what each sample number represents without reading the description first. Pre-teach "anode," "scale," and "disturbance" with images, and offer sentence frames for items 9 and 10, such as "I recommend ___ because ___ and because ___."

Extensions

Have students write a short notice that the fictional Rilbeck water department could mail to a household scheduled for service line work. The notice must explain in plain language what the crew will do, why lead may rise for a period afterward, and what the household should do in the meantime, and it must state the action level of 10 ppb correctly rather than promising that the water will be safe immediately.