Lead Action Level Exceedance With a Bad Sample in the Set
Elmwood Water Department has 4,200 connections and has just completed its 60-sample lead and copper tap monitoring. Treatment is free chlorine at 0.7 mg/L, pH 7.5, alkalinity 40 mg/L as CaCO3, and no corrosion inhibitor. Sorted results: 51 of the 60 samples are below 0.015 mg/L; nine are above, ranging from 0.016 to 0.052 mg/L. The 54th value in ascending order is 0.017 mg/L. Two of the nine high results came from homes with lead service lines. One high result, 0.052 mg/L, came from a home where the resident ran the tap for five minutes at 06:00 and filled the bottle at 08:00, and the sample form is annotated that way. The department has no lead service line inventory. All copper results are below the action level.
1. Is this an action level exceedance? Show the calculation and explain how the 0.052 mg/L result affects it.
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The calculation: the lead action level is 0.015 mg/L, evaluated at the 90th percentile of the tap samples.
- n = 60; the 90th percentile is the value at rank 0.90 × 60 = 54 in ascending order.
- The 54th value is 0.017 mg/L, which exceeds 0.015 mg/L.
- This is an action level exceedance.
The 0.052 mg/L result: that sample is not a valid first-draw sample. The resident ran the tap for five minutes and then filled the bottle two hours later - that is flushed water, not the six-hour-stagnation sample the rule requires. It should be invalidated and replaced.
Does invalidating it change the answer? No. Recompute with 59 results: rank 0.90 × 59 = 53.1, so use the 54th value of the reduced set. Removing one of the nine high values still leaves eight above the action level, which means the 54th value of the new set is the sixth-highest result - still above 0.015 mg/L. The exceedance survives the invalidation.
Two cautions: you do not get to unilaterally void a sample that hurts you, so coordinate the invalidation and the recomputation with NJDEP in writing; and do not let the argument over one sample delay the public education clock. The exceedance stands either way.
2. What must the department do, and by when?
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Immediately: notify NJDEP of the exceedance and open the record.
Within 60 days of the end of the monitoring period: public education. This is the deadline that matters and the one that gets missed.
- Deliver the required educational material to all bill-paying customers - bill inserts, and posters or brochures at public locations.
- Deliver it directly to high-risk customers: households with pregnant women or children under six, and homes known or suspected to have lead service lines.
- Content is prescribed: health effects, sources of lead, and specific steps - flush before drinking, use only cold water for formula and cooking, clean aerators, consider a certified filter, and get children's blood lead tested.
Also on the compliance track:
- Water quality parameter monitoring for pH, alkalinity, calcium, conductivity, and temperature at the required sites.
- A corrosion control treatment recommendation and study, delivered on the schedule NJDEP sets. You currently have no inhibitor and no formal corrosion control, which is the underlying finding here.
- Begin the lead service line inventory and the replacement planning that goes with it. Inventory deadlines have been revised more than once - get the current NJDEP schedule in writing rather than working from memory.
Message discipline: do not issue a boil water notice and do not tell people the water is unsafe to drink in general terms. Lead is not an acute contaminant. The message is flushing, cold water, filters, and blood testing - a confused message erodes confidence as fast as no message.
3. What corrosion control change would you recommend, and what is the trap?
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The starting point: pH 7.5, alkalinity 40 mg/L, no inhibitor, lead service lines present, and nine high taps. This is under-stabilized water. The two standard levers are raising pH toward 7.8–8.0 and feeding orthophosphate in the low mg/L range.
The traps:
- You do not know which lead is the problem. Nine high results, two from known lead service lines - what about the other seven? Leaded brass, old solder, or interior plumbing produce a different response to pH adjustment than a lead service line does. Corrosion control and service line replacement are not substitutes for each other.
- Raising pH is not free. In a low-alkalinity water, pushing pH up can reduce lead solubility but can increase copper solubility and pitting risk. Your copper results are all below the action level - do not trade a lead problem for a copper problem without data.
- Chlorine and DBP interactions. Changing pH shifts the chlorine/organic chemistry. Check what it does to TOC removal and disinfection byproduct formation before you commit.
- Blind dosing. Do not pick a phosphate dose out of a vendor brochure.
What to actually do: run a corrosion control study - coupon or pipe-loop testing on the department's own water, at the candidate pH and phosphate levels - and submit the resulting optimal corrosion control treatment recommendation to NJDEP for approval before full-scale implementation. In the interim, do the public education thoroughly, and start the service line inventory so the treatment decision and the replacement program can be designed together. Guessing at the chemistry and then defending the guess in a compliance review is the expensive path.