Lead Action Level Exceedance in an Older Distribution Area
A utility serving 48,000 people runs a lead and copper sampling program. The 90th percentile lead result from the most recent round is 0.019 mg/L, above the 0.015 mg/L action level. The affected samples cluster in a 1950s neighborhood with lead service lines and copper plumbing with lead solder. The utility currently doses zinc orthophosphate at 0.5 mg/L as PO4 and maintains finished water pH at 7.4 with alkalinity of 55 mg/L as CaCO3. The corrosion control treatment was originally optimized years ago, but a recent switch to a different coagulant lowered finished pH slightly. The state requires a public education program within 60 days of the exceedance and a corrosion control treatment re-evaluation. The utility has budget for either increasing phosphate dose, raising pH, or both, but the distribution system has a nitrification concern in a chloraminated area and the operators worry that raising pH will worsen it. The utility must also decide whether to replace lead service lines or treat the water.
1. What is the first technical step in responding to the action level exceedance, and how should the utility frame the choice between treatment changes and line replacement?
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The first step is not to pick a chemical fix but to establish the cause. The 90th percentile at 0.019 mg/L is above the action level, and the clustering in a lead-service-line neighborhood with lead solder points to both source water corrosivity and premise plumbing. The utility should review the corrosion control re-evaluation data: pH, alkalinity, phosphate residual, and whether the coagulant change shifted the water's scaling tendency. A slight pH drop from the coagulant change can destabilize the protective phosphate film, which would explain a rise in lead.
On treatment versus replacement: the regulatory framework generally expects corrosion control optimization first, because it protects the whole system, while lead service line replacement addresses the highest-risk homes. The utility should not treat them as either-or. The defensible sequence is to restore and verify optimal corrosion control, then plan replacement for the lead lines. Replacement without stable water chemistry can still leave lead solder and brass fixtures as sources. Document the re-evaluation, the pH and phosphate history, and the sampling plan.
2. How should the utility evaluate raising pH versus increasing phosphate dose, given the chloramination nitrification concern?
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The operator should think in terms of the corrosion control objective, which is to form and maintain a stable passivating film, not simply to hit a pH number. Zinc orthophosphate works by forming a lead phosphate film; its effectiveness depends on adequate phosphate residual, pH in the right range, and sufficient contact. Raising pH can reduce lead solubility but may narrow the chloramine stability window and, in a nitrifying distribution system, higher pH can favor nitrification in some circumstances by affecting chloramine decay and ammonia availability. The utility should not assume raising pH is safe or unsafe; it should evaluate with distribution monitoring.
The practical approach is to run a bench or pilot study and a distribution monitoring plan: measure lead at the tap, phosphate residual, pH, alkalinity, and chloramine residual and nitrification indicators before and after a change. Increasing phosphate dose may be lower risk for nitrification and can be trialed first if the film is phosphate-based. If pH is raised, do it in small increments with nitrification monitoring. The judgement is to make one change at a time, verify with data, and keep the state informed.
3. What public education and reporting obligations follow from the exceedance, and what should the operator ensure is documented?
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The utility must deliver public education within the required timeframe, with content covering the health effects of lead, sources of lead in drinking water, steps to reduce exposure (flushing, using cold water for formula, cleaning aerators), and the utility's actions. The education must reach the affected customers, and the utility should not limit it to the sampled homes. The state also expects the corrosion control re-evaluation and a monitoring plan.
The operator's documentation should include the 90th percentile calculation and the sample site list, the treatment change history (including the coagulant switch and pH effect), the corrosion control re-evaluation results, the public education distribution record, and the distribution monitoring data. The reasoning is that an action level exceedance is a data-and-documentation event as much as a treatment event; regulators and the public will judge the response on whether the utility can show it understood the cause and acted. The operator should also confirm the sampling was valid, since improper sampling can produce misleading results.