Nitrate at 10.8 mg/L and a Board That Wants to Wait
A 3,400-connection community system serves an agricultural valley with two wells and no treatment. Well A produces 13.5 mg/L nitrate as N; Well B produces 4.2 mg/L. Historically the system blended about 55% Well A with 45% Well B and stayed near 9.3 mg/L. In July, peak demand forced the operator to run 70% Well A for 11 days, and a routine entry-point sample collected Friday at 15:00 came back Monday morning at 10.8 mg/L. The board wants to keep the blend as-is until September because Well B's pump needs a $40,000 repair. The system has no bottled-water contract, one elementary school, and about 60 residences on private septic.
1. Is this a violation? What must happen and on what clock?
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Yes. This is a primary MCL violation.
Why: the nitrate MCL is 10 mg/L measured as nitrogen. Nitrate compliance is based on the sample result, not on a running annual average - a single entry-point result of 10.8 mg/L is an exceedance, and there is no averaging provision to lean on. The historical 9.3 mg/L blend does not offset it.
Clock: nitrate is a Tier 1 violation because of the acute health risk to infants. That means:
- Notify the primacy agency as soon as possible and no later than 24 hours after learning of the result - here, Monday.
- Public notice within 24 hours, by the fastest feasible method (hand delivery, door hangers, radio, social media, posted notices), using state-approved acute health-effects language.
- Content: the notice must state the level and the MCL, explain methemoglobinemia - "blue baby syndrome" - in infants under six months, and tell parents not to use the tap water to mix infant formula or to give to infants, and to use bottled or an alternate source. It must also note that boiling does not remove nitrate - boiling concentrates it, which is the most common and most dangerous misunderstanding in a nitrate notice.
- The elementary school gets a direct hand delivery, not a posted notice.
The board's preference is not a legal option. The operator's duty runs to the agency and the public, not to the schedule the board prefers, and the notice must go out even while the fix is being arranged.
2. Calculate the blending limit that keeps the entry point under the MCL, and explain why that number is a starting point rather than a guarantee.
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Mass balance on the blend. Let f = fraction of total flow from Well A:
10 = 13.5f + 4.2(1 − f) 10 = 4.2 + 9.3f 5.8 = 9.3f f = 0.624 → 62% Well A maximum to land exactly at 10 mg/L.
Do not operate at 62%. That is the line, not a target. Build in a margin - targeting 8 mg/L gives (8 − 4.2) ÷ 9.3 = 0.41, or about 41% Well A. The gap between 62% and 41% is the room you need for the things the simple calculation ignores:
- Well B's own nitrate drifts seasonally. 4.2 mg/L is one number on one day in an agricultural basin; if it rises to 6 mg/L, the 62% blend no longer holds.
- Blending is not perfectly mixed. Short-circuiting at the entry point and dead ends in the distribution system can produce local concentrations above the blend average.
- Flow is not constant. The July event happened precisely because peak demand changed the ratio. A blend ratio set at the plant is only as good as the flow control that maintains it, and it must be verified with a flow meter, not by valve feel.
Therefore: set the ratio conservatively, verify with entry-point sampling after any change in the ratio or in seasonal demand, and post the calculated limit at the control point so the next operator does not have to re-derive it at 2 a.m.
3. The board wants to wait until September. Lay out the options, their tradeoffs, and the compliance consequences of proceeding as they propose.
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The status quo is a continuing violation, not a holding pattern. Each entry-point result above 10 mg/L while the violation continues means ongoing Tier 1 public notice, re-notification on the state's schedule (typically every three months while it continues), and exposure to a state compliance order and penalties. The notices also have to be paid for and delivered, twice - once for the violation and once to rescind it.
Options, roughly in order of speed:
- Cut Well A to about 40% immediately. Costs nothing but capacity. Verify with flow meters that the ratio is actually being held, and resample the entry point. This is available today and should be done now, before any other decision.
- Bottled water for the school and for households with infants. Cheap relative to a violation. Get a standing order or an agreement with a vendor - the system currently has none, which is the real gap the July event exposed.
- Repair Well B's pump. $40,000 to restore the second source is very likely the cheapest durable fix on the table; frame it against the cost of the notices, the state order, and the risk.
- Buy or wheel water from a neighboring system if an interconnection exists or can be made quickly.
- Treatment or a new source - reverse osmosis, ion exchange, or a deeper well - is the long-term answer in a basin where nitrate is rising, but it is a year-plus and should be started now in parallel.
What to tell the board: the technical decision is not "wait or fix." It is "reduce the blend today at no cost, buy bottled water for the school this week, and fund the pump repair now." The only question genuinely open is how to finance the repair, and that is a board question - the violation is not.