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Lesson plan Grade 11 Science

Rendon's Lead Service Lines: Corrosion Control and the Action Level

A grade 11 science lesson on why corrosive water leaches lead, how corrosion control coats pipes, and how the 90th-percentile action level drives replacement.

Objectives

  1. Students will explain the chemistry by which corrosive water strips corrosion-control coatings and leaches lead.
  2. Students will apply the 90th-percentile action level to sample data and interpret the replacement requirement.

Vocabulary

Corrosion control
Treatment that keeps water from slowly eating away at the inside of pipes. It helps stop metals like lead from getting into drinking water.
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.
Action level
A trigger number in lead and copper testing. If too many homes test above this level, the utility must take steps like improving treatment and telling the public.
Leaching
When water slowly pulls a substance out of the material it touches. Corrosive water can leach lead from old pipes into drinking water.

Also: 90th percentile

Teacher overview

Lead service lines remain a source of lead as long as they stay in the ground, which is why federal rules now require replacing them; until then, water utilities reduce lead release by adding corrosion-control chemicals that coat the inside of old pipes with a thin protective layer. This lesson uses the fictional town of Rendon to connect that chemistry to the federal rule that governs it: the lead action level, currently 15 ppb and dropping to 10 ppb in 2027, and the requirement to replace nearly all lead service lines. EPA estimates that about 4 million lead service lines remain nationwide (a November 2025 figure built from utilities' service line inventories, replacing an earlier estimate of about 9 million), so this rule governs current practice rather than a historical case. A common misconception the lesson corrects is that the action level is a health limit for an individual tap; it is instead a 90th-percentile trigger across many samples that tells a utility when to act. EPA's health-based goal for lead is zero, so the action level is not a safe level.

Procedure

Hook (5 min): Remind students of the Flint case, in which the city switched to more corrosive river water in 2014 without corrosion-control treatment, and lead leached from the pipes. Ask: if the pipes were already lead before the switch, why did the problem start only after the source changed? Take a few predictions without confirming them.

Direct instruction, corrosion chemistry (12 min): Explain that lead service lines connect a water main to a house. Corrosion-control treatment adds chemicals such as orthophosphate that build a protective mineral scale on the pipe wall, so water does not touch bare lead. Water that is more corrosive, because it is more acidic, lower in alkalinity, or higher in chloride, dissolves that scale, and lead then leaches into the standing water. This is the chemistry answer to the hook: the source switch changed the water's corrosivity, not the pipe.

Direct instruction, the rule and the government (8 min): Introduce the action level as a regulatory trigger set by the Environmental Protection Agency under the Lead and Copper Rule. Utilities collect first-draw tap samples from high-risk homes. If the 90th-percentile result is above the action level, the utility must expand treatment and public education and accelerate replacement. Note that the action level is 15 ppb now and drops to 10 ppb in 2027, and that federal rules now require replacing nearly all lead service lines within a decade. Emphasize that this is a policy decision made through federal rulemaking, an example of how government regulation shapes local water chemistry.

Guided practice, worked example (13 min): Work this demonstration on the board for the town of Rendon. Rendon collects 10 first-draw samples, in ppb: 2, 4, 5, 7, 8, 10, 13, 15, 19, 24. With 10 samples, the 90th-percentile value is the 9th-ranked reading, which is 19 ppb. Compare to the current 15 ppb action level: 19 is above 15, so Rendon exceeds the action level and must act. Ask students what happens to this conclusion once the level drops to 10 ppb.

Independent practice (10 min): Students complete the Student Handout, which uses a different sample set and asks them to find the 90th percentile, compare it to both the current and future action levels, and compute a replacement schedule.

Closure and exit ticket (2 min): On an index card, students answer: "In one sentence, why can water that has always flowed through lead pipes suddenly show high lead?" Collect and confirm that at least 80 percent name a change in the water's corrosivity or loss of the protective coating before moving on.

Rendon's Lead Service Lines: Corrosion Control and the Action Level — Grade 11 Science
Name: ____________________________ Date: ______________

Student Handout

Use the fictional town of Rendon for every question. With 10 samples, the 90th-percentile value is the 9th-ranked reading.

  1. Rendon collects 10 first-draw lead samples from homes with lead service lines, in ppb: 3, 5, 6, 8, 9, 11, 12, 14, 16, 21. Rank them and identify the 90th-percentile lead level.
  2. Compare your item 1 result to the current action level of 15 ppb. Does Rendon exceed the action level, and by how much?
  3. In 2027 the action level drops to 10 ppb. Would this same sample set exceed the future action level, and what does that tell Rendon about planning now?
  4. Rendon has 4,000 lead service lines and must replace nearly all of them within 10 years. What average number of lines per year must it replace to finish on schedule?

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: 16 ppb.
    Ranked from least to greatest: 3, 5, 6, 8, 9, 11, 12, 14, 16, 21. The 9th-ranked value is 16 ppb, which is the 90th percentile for a set of 10 samples.
  2. Answer: yes, by 1 ppb.
    16 ppb is above the current 15 ppb action level, so Rendon exceeds it by 16 - 15 = 1 ppb and must take action.
  3. Answer: yes, by 6 ppb.
    16 ppb is above the future 10 ppb action level by 16 - 10 = 6 ppb. Because the sample set already exceeds the stricter 2027 level, Rendon should plan corrosion-control improvements and replacements now rather than wait.
  4. Answer: 400 lines per year.
    4,000 lead service lines / 10 years = 400 lines per year on average to finish within the required decade.

Differentiation

Support

Give students the item 1 sample values already ranked from least to greatest, so they can locate the 90th-percentile position without re-sorting. Provide a one-line reminder that "9th of 10" marks the 90th percentile.

Extension

Have students research, using only the lesson's facts, why the rule targets the 90th percentile rather than the average, and write a short argument for why a percentile-based trigger better protects the homes at highest risk than a simple mean would.

ELL supports

Pre-teach corrosion control, service line, leaching, and action level with a labeled diagram of a coated versus a bare lead pipe. Provide a sentence frame for the exit ticket: "Lead can rise suddenly because the water became more ___, which removed the ___ inside the pipe."