Lead Action Level Exceedance After a Free Chlorine Conversion
Your 1,100-connection groundwater system has three wells, free chlorination with gas, pH 7.3, alkalinity 40 mg/L as CaCO3, hardness 85 mg/L, and no corrosion inhibitor feed. The distribution system includes roughly 90 lead goosenecks and lead service lines on the older streets. In May, to control coliform hits in a storage tank, you converted from chloramines to free chlorine and kept it for six weeks before returning to chloramines. Lead monitoring completed in September (20 sites, standard volumes) gave a highest result of 0.034 mg/L, a second-highest of 0.026 mg/L, and a fourth-highest of 0.014 mg/L; the previous round's 90th percentile was 0.009 mg/L. The copper 90th percentile is 0.9 mg/L against a 1.3 mg/L action level. Finished-water TTHM is 62 micrograms/L against an 80 micrograms/L MCL. Operators report red water complaints on two dead-end streets, and the utility board wants to know whether to add phosphate, raise pH, or just add more chlorine. The state has not yet reviewed a corrosion control recommendation for this system.
1. Calculate the 90th percentile lead result and determine whether the action level is exceeded. What does the exceedance trigger?
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With 20 samples, the 90th percentile is the third-highest value: 0.9 x 20 = 18, so you take the 18th value in ascending order, which is the third-highest. That is 0.021 mg/L.
The action level is 0.015 mg/L, so the system exceeds it, roughly 1.4 times over, and the 90th percentile has more than doubled since the last round. Copper at 0.9 mg/L is under the 1.3 mg/L action level, which tells you the problem is lead-specific rather than a general metal release - consistent with a lead scale that was destabilized.
An action level exceedance is not an MCL violation. It triggers a defined set of duties: public education for consumers at risk within the required timeframe, delivery of the message in a specific form, corrosion control treatment and re-optimization, water quality parameter monitoring, and source water treatment evaluation. Confirm the exact deadlines with the state primacy agency - the rule has been revised and the phase-in dates have changed, so an outdated timeline is worse than no timeline.
The likely cause is the six-week free chlorine conversion. Free chlorine is a stronger oxidant than chloramine and can alter lead scales (particularly lead(IV) dioxide and lead carbonate scales) and mobilize lead into the water. The red water complaints on dead ends support a scale disturbance that has not yet re-stabilized.
Note also that 90 lead goosenecks means the exposure is real and not a sampling artifact. Do not resample to try to get a lower number - sample to find the worst sites.
2. Evaluate the board's three options - add orthophosphate, raise pH, or add more chlorine - on chemistry and on secondary consequences.
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Adding more chlorine is not a lead control strategy. It increases the oxidant load that destabilized the scale in the first place, raises DBP formation (TTHM is already at 62 micrograms/L), and does nothing for pH or mineral scaling. Rule it out as a corrosion control measure; keep chlorination at whatever level is needed for disinfection only.
Raising pH is the most direct lever for this water. At pH 7.3 with only 40 mg/L alkalinity the water is aggressive and lead solubility is relatively high. Moving to about 7.8-8.2 with caustic or lime reduces lead solubility substantially and also lowers TTHM formation. The limits: the higher pH weakens free chlorine efficacy, so disinfection must be re-checked; the feed system has to be reliable and continuously monitored, because a pH excursion in either direction will move lead again; and the target must be verified by a corrosion control study, not by assumption.
Orthophosphate works by forming lead phosphate scales, typically 1-3 mg/L as PO4, and is often the more effective option for high-lead waters where pH alone will not get there. Its costs: added phosphorus load to the receiving stream, a nutrient source in the distribution system, a new feed and monitoring system, and a scale that must be maintained continuously. Do not substitute blended or polyphosphate products - polyphosphates can increase lead solubility.
Most likely answer: a combination, with pH and alkalinity adjustment as the backbone and orthophosphate added if pH alone does not bring the 90th percentile under 0.015. The state will want the recommendation supported by a study with water quality parameter monitoring - pH, alkalinity, calcium, orthophosphate if used, silica, and temperature - at the plant and in the distribution system.
3. What do you do in the next 30 days to protect consumers while the treatment decision is worked out?
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Communicate first. The public education requirement is not optional and it is time-bound. Notify the state of the exceedance as required, tell affected customers what the results mean and what to do (run cold water before use, use cold water for formula, flush after periods of stagnation), and target the message to the streets with known lead service lines and to pediatric and prenatal care providers.
Control the scale disturbance without making it worse. Use unidirectional flushing at controlled velocity on the dead ends with red water - enough to clear the sediment and the disturbed scale, not so much that you scour the pipe. Flush to waste with dechlorination, and sample after flushing to confirm the water has stabilized. Avoid abrupt changes in disinfectant or pH while the system is unsettled.
Sample deliberately: expand beyond the routine sites, target the highest-risk homes with lead service lines, and collect both first-draw and flushed samples on a subset so you can see whether the lead is from the service line or the interior plumbing.
Operational checks: confirm the pH is stable and the chlorine residual is adequate but not excessive; inspect the storage tank for sediment; and look for electrical grounding or stray current on the lead services, which can accelerate corrosion independently of water chemistry.
Finally, get the corrosion control study started now, with a written schedule. The board's question about phosphate versus pH should be answered by data from this water, not by preference, and the study is what the state will ask for.