Source Change and Lead Corrosion Control
A 2,200-connection system with 280 known lead goosenecks switches from local groundwater to purchased surface water. Old water: pH 7.3, alkalinity 180 mg/L as CaCO3, hardness 220 mg/L, chloride 20 mg/L, sulfate 40 mg/L. New water: pH 7.8, alkalinity 45 mg/L, hardness 60 mg/L, chloride 35 mg/L, sulfate 25 mg/L. Lead and copper tap samples show a 90th-percentile lead of 0.008 mg/L, below the action level, but one site at 0.052 mg/L and another at 0.031 mg/L; both are lead service lines. The utility has not done a corrosion-control study, and the new water is corrosive by calculation. Customers are asking whether the water is safe. The operator must decide on immediate public communication, sampling, and treatment before the next LCR monitoring period.
1. Interpret the lead results in light of the source change. Is the system in compliance, and is the water safe?
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The 90th percentile is below the lead action level of 0.015 mg/L, so the system is not over the LCR trigger, but the individual results of 0.052 and 0.031 mg/L at lead service lines are a warning. The source change matters: the new water has much lower alkalinity, 45 versus 180 mg/L, and a higher chloride-to-sulfate mass ratio, 35/25 = 1.4 versus 20/40 = 0.5. Those changes increase lead solubility and galvanic corrosion. A 90th percentile can hide a small number of high sites, especially when sampling is limited. The utility should not say the water is safe in all homes based only on the 90th percentile. It should resample those sites, check sampling protocol, and start a corrosion-control study. The switch is a plausible cause, but plumbing and stagnation also matter.
2. What immediate actions and treatment options should the utility pursue?
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Immediate actions: resample the high sites with proper pre-stir stagnation, test pH, alkalinity, chloride, sulfate, and lead at entry and at taps, and tell affected customers how to flush and use cold water for formula. Do not issue a blanket all-clear. For treatment, optimize alkalinity and pH first, then evaluate orthophosphate; the new water is low in alkalinity, so pH adjustment alone may not be enough. A corrosion-control study should test the actual blended water and include lead and copper coupons or pipe loops. Replace lead goosenecks on a schedule and prioritize the high sites. Any treatment change needs Oklahoma DEQ review and a revised monitoring plan. The goal is to lower the highest sites, not to keep the 90th percentile just below the action level.
3. How should the utility communicate with customers and regulators while the investigation is underway?
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Communication should be honest and specific: the system is below the action level overall, but some lead service lines have elevated results, and the source change may increase corrosion. Provide flushing, filters, and replacement information, and coordinate with DEQ on language. Regulatory path: continue LCR monitoring, do the corrosion-control study, and if optimization fails, consider treatment or source blending. Document decisions and sampling. The utility should avoid promising that the new water is safe for everyone; it should explain the uncertainty and the actions. This is a judgement call because the LCR is a statistical trigger, while individual exposure is what customers care about. The defensible answer is to act on the high sites while the study is underway.