5.4 Corrosion Control & the Lead and Copper Rule
Key Takeaways
- The lead action level is 0.015 mg/L and the copper action level is 1.3 mg/L, both evaluated at the 90th percentile of first-draw tap samples.
- Action levels are not MCLs; exceeding one triggers treatment, monitoring, public education, and possible service line replacement rather than an automatic violation.
- Lead and copper compliance samples are first-draw, one-liter samples taken after at least six hours of stagnation from cold-water taps.
- The Langelier Saturation Index indicates whether water tends to deposit or dissolve calcium carbonate, with negative values indicating corrosive water.
- Orthophosphate is the most widely used corrosion inhibitor because it forms a low-solubility protective scale on lead and copper surfaces.
5.4 Corrosion Control & the Lead and Copper Rule
Lead almost never comes out of a river or an aquifer. It comes out of lead service lines, lead solder, and brass fixtures between the water main and the consumer's tap. That single fact reframes the operator's job: you are not removing a contaminant at the plant, you are conditioning the water so it stops dissolving the pipe. In South Carolina this also drives facility classification — adding corrosion control treatment places a plant in Group II, requiring at least a Class D operator-in-charge.
1. What Corrosion Actually Is
Corrosion is an electrochemical process. A galvanic cell forms with four parts:
- Anode — metal oxidizes and dissolves into the water (this is where pipe is lost)
- Cathode — reduction occurs, commonly of dissolved oxygen
- Electrolyte — the water itself, carrying ions
- Metallic path — the pipe wall connecting anode and cathode
Remove any one and corrosion stops. Practical control usually attacks the electrolyte — the water chemistry.
Factors that accelerate corrosion
| Factor | Effect |
|---|---|
| Low pH | Strongly increases metal solubility; the dominant lever |
| Low alkalinity | Little buffering, so pH swings and scale will not form |
| High dissolved oxygen and free chlorine | Support the cathodic reaction |
| High temperature | Roughly doubles reaction rate per 10 °C |
| High velocity | Erosion corrosion, especially at elbows in copper |
| Dissimilar metals joined | Galvanic corrosion; the less noble metal sacrifices |
| High chloride or sulfate relative to alkalinity | Aggressive to lead solder and brass |
2. Saturation Indices
The Langelier Saturation Index (LSI) compares actual pH to the pH at which the water would be saturated with calcium carbonate:
| LSI | Tendency |
|---|---|
| Positive | Supersaturated — tends to deposit calcium carbonate scale |
| Zero | At equilibrium |
| Negative | Undersaturated — corrosive, tends to dissolve scale and pipe |
A slightly positive LSI is the classic target. But be careful with the exam framing: a calcium carbonate film is a blunt tool and modern practice relies more on orthophosphate passivation than on deliberate scaling, because heavy scale reduces carrying capacity and does not protect lead well on its own.
3. Corrosion Control Treatment
| Approach | Chemicals | Notes |
|---|---|---|
| pH and alkalinity adjustment | Lime, soda ash, caustic soda, sodium bicarbonate | The first and most powerful lever. Raising pH toward 7.5–9 sharply lowers lead solubility |
| Orthophosphate inhibitor | Phosphoric acid, zinc orthophosphate, blended ortho/polyphosphate | Forms an insoluble lead- and copper-phosphate film. The most common modern approach. Typical dose ~1–3 mg/L as PO₄ |
| Silicate inhibitor | Sodium silicate | Less common; forms a protective silicate film |
| Calcium carbonate saturation | Lime addition to a positive LSI | Older approach, still used where alkalinity is very low |
Critical operating rule: never make an abrupt change in source water, pH, or disinfectant without evaluating the corrosion consequences. Changing water chemistry can destabilize an existing protective scale and release accumulated lead. This is the mechanism behind the best-known lead crises in the United States, and it is heavily tested.
4. The Lead and Copper Rule
Action levels — not MCLs
| Metal | Action level | Basis |
|---|---|---|
| Lead | 0.015 mg/L (15 ppb) | 90th percentile of first-draw tap samples |
| Copper | 1.3 mg/L | 90th percentile of first-draw tap samples |
An action level is not a maximum contaminant level. Exceeding it is not automatically a violation; it triggers required actions. Failing to perform those actions is the violation.
How the 90th percentile works
Rank all sample results from lowest to highest. With 20 samples, the 90th percentile is the 18th value (20 × 0.9 = 18). If that value exceeds 0.015 mg/L, the system has a lead action level exceedance — even if 17 homes were perfectly clean.
Sampling protocol — the details are testable
- First-draw sample, collected after the water has stood motionless in the plumbing for at least 6 hours.
- One liter, from a cold-water kitchen or bathroom tap normally used for consumption.
- Taken from Tier 1 sites — the highest-risk locations: homes with lead service lines, lead solder, or lead interior plumbing.
- Do not remove or clean the aerator before sampling, and do not pre-flush.
What an exceedance triggers
- Water quality parameter monitoring and installation or optimization of corrosion control treatment.
- Public education delivered on a required schedule, and individual notification of sampled residents.
- Source water monitoring and treatment where source contribution is significant.
- Lead service line replacement obligations where a lead action level exceedance persists.
- Increased tap sampling frequency until the system returns to compliance.
Service line inventory and records
Systems must develop and maintain a service line material inventory identifying lead, galvanized-requiring-replacement, non-lead, and unknown materials on both the utility and customer sides. As noted in Section 1.3, lead and copper tap monitoring results and service line inventory records are retained for 12 years — the longest retention in the drinking-water rules.
5. Copper-Specific Behavior
Copper leaching is worst in new copper plumbing before a protective oxide film develops, and is aggravated by low pH, high free chlorine, and high velocity. Blue-green staining of fixtures and a metallic taste are the field indicators. Pinhole leaks in copper often trace to erosion corrosion at elbows where velocity is excessive, or to aggressive water with low pH and low alkalinity.
A system collects 20 lead tap samples. Ranked lowest to highest, the 18th value is 0.018 mg/L. What is the correct conclusion?
Which sampling protocol is correct for Lead and Copper Rule compliance samples?
A water with a Langelier Saturation Index of −1.8 is best described as:
Which corrosion control chemical is most commonly used specifically to form a protective film on lead and copper surfaces?