Two Wells, One Nitrate Problem
Your system serves 4,200 connections from two wells that blend into a single entry point. Well A (1.2 MGD firm capacity) has climbed from 5.8 to 9.6 mg/L nitrate as N over 18 months; a 180-home subdivision on septic systems was built upgradient. Well B (0.8 MGD) runs 2.4 mg/L. For years the wells ran roughly 50/50 and the blend sat near 6.0 mg/L. Last week Well B's motor failed and it is out for 10 days, so you are running Well A alone at 1.1 MGD. Your internal operating target is 8.0 mg/L as N, and the state requires quarterly nitrate samples as long as results stay above 5 mg/L. Peak-day demand is 1.6 MGD.
1. What nitrate concentration are customers receiving right now? Once Well B is back, what is the highest percentage of Well A water you can blend and still hold the 8.0 mg/L internal target - and can the two wells meet a 1.6 MGD peak day inside that limit?
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Right now (Well A only, 1.1 MGD): customers receive about 9.6 mg/L as N. That is under the 10 mg/L MCL, but above your 8.0 target - only 0.4 mg/L of margin remains.
Maximum blend. Let x = fraction from Well A:
9.6x + 2.4(1 − x) = 8.0 9.6x + 2.4 − 2.4x = 8.0 7.2x = 5.6 x = 0.78 → 78% Well A / 22% Well B
Capacity check. If Well A may supply no more than 78% of the total, the maximum total production is 1.2 ÷ 0.78 = 1.54 MGD. Peak demand is 1.6 MGD, so you are about 0.06 MGD (≈40 gpm) short. The internal target cannot be met at peak with both wells inside firm capacity.
So the answer is not "blend harder" - it is a capacity problem. Realistic responses: treat a slipstream of Well A (IX or RO), accept operation between the 8.0 target and the 10 mg/L MCL with an explicit written justification, or add storage to shave the peak and let the wells run at a steady low nitrate fraction.
2. A council member asks why you cannot simply keep running Well A alone. Explain the health basis, the monitoring consequence of the current number, and the units trap in the lab reports.
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Health basis. Nitrate is an acute contaminant. Infants under about six months convert nitrate to nitrite, which binds hemoglobin (methemoglobinemia, "blue baby"). Because the effect is acute rather than cumulative, the MCL is 10 mg/L as N and a single exceedance matters.
Monitoring consequence. Once a result is at or above 5 mg/L (50% of the MCL), quarterly nitrate monitoring applies and cannot be reduced to annual. You are at 9.6, so quarterly is locked in, and a rising trend means you should be sampling more often than required on your own initiative.
Units trap. The MCL is expressed as N. Labs sometimes report as NO3. Convert with the factor 4.43: 9.6 mg/L as N = 42.5 mg/L as NO3. A report showing "42.5 mg/L nitrate" is not a violation; a report showing "10.5 mg/L nitrate as N" is. Check the reporting basis before anyone reacts to a number.
Acute response. A compliance sample at or above 10 mg/L must be confirmed by a second sample within 24 hours. If confirmed, that is a Tier 1 public notice within 24 hours plus immediate action - not something that waits for the quarterly cycle.
3. The 10-day outage passes with no violation. What do you put in front of the utility board as the long-term plan, and what are the two things you must not defer?
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The board plan has four parts:
Source control (slowest, cheapest). Work with the county on septic inspection and pumping, wellhead protection zoning, and a casing/grout integrity check on Well A - a failing annular seal can short-circuit surface contamination. Nitrate from a subdivision does not reverse quickly, so do not plan on it.
Blending and storage. Formalize the 78% blend limit in the operating plan and SCADA, and evaluate storage to keep Well A at a steady fraction instead of chasing peaks.
Treatment. Pilot or design nitrate removal (ion exchange with brine disposal, or RO with concentrate handling). Budget a year for permitting and piloting; the trend line says you will need it.
Monitoring and notification. Quarterly at minimum, an entry-point sample split confirmed by a second method (ion chromatography versus cadmium reduction), and a written procedure for the 24-hour confirmation and Tier 1 notice.
Do not defer: (a) the trend - 5.8 to 9.6 in 18 months means the MCL is roughly a year away at this slope, so treatment planning cannot wait for a violation; (b) the acute-response procedure - the 24-hour confirm-and-notify must be written and practiced before the first 10 mg/L sample, not after.