Arsenic Compliance When Demand Outgrows the Blend
A city blends two wells to meet demand. Well 1 produces up to 400 gpm with arsenic at 0.014 mg/L; Well 2 produces up to 250 gpm with arsenic at 0.003 mg/L. The arsenic MCL is 0.010 mg/L. Peak-day demand is 550 gpm and the system has no treatment other than chlorination. Well 1's arsenic is about 60% As(III); Well 2 carries 0.6 mg/L iron and 0.15 mg/L manganese. Compliance samples are collected at the entry point after blending, quarterly, per the state's schedule. A new school is scheduled to connect next summer, adding 60 gpm to peak demand.
1. Find the maximum fraction of Well 1 water that can be blended and still meet 0.010 mg/L. Can the system meet today's 550 gpm peak, and what happens when the school connects?
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Blend calculation
Let x = fraction from Well 1:
0.014x + 0.003(1 − x) = 0.010
0.014x + 0.003 − 0.003x = 0.010
0.011x = 0.007
x = 0.636
So Well 1 can be no more than about 63% of the blend. Round down for safety - 63% gives 0.014(0.63) + 0.003(0.37) = 0.0099 mg/L, which is compliant but with almost no margin on a quarterly sample. Aim for a practical operating target below the MCL, not at it.
Today's 550 gpm peak
- Well 1 at 63%:
550 × 0.63 = 346 gpm - Well 2 at 37%:
550 × 0.37 = 204 gpm- under its 250 gpm limit. ✅ Feasible.
When the school connects (610 gpm peak)
- Well 1 at 63%:
610 × 0.63 = 384 gpm- under 400 gpm. ✅ - Well 2 needed:
610 × 0.37 = 226 gpm- under 250 gpm. ✅ Still feasible on paper.
The system works - but watch the margins. Well 2 is close to its capacity ceiling, Well 1 has only 16 gpm of headroom, and there is essentially zero arsenic margin. Any loss of Well 2 (maintenance, pump failure, declining yield) forces a higher Well 1 fraction and an exceedance. That is a plan failure waiting for a bad week.
2. The As(III) fraction and the iron/manganese in Well 2 matter. How do they change your options, and what would you test before choosing a treatment approach?
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Why speciation matters
- As(III) is much harder to remove than As(V) by coagulation, adsorption, or ion exchange. About 60% of Well 1's arsenic is arsenite. If you ever need treatment, you will oxidize first - chlorine, permanganate, or ozone - to convert As(III) to As(V) and then remove it.
- The system already chlorinates, which oxidizes some As(III), but the contact time, dose, and pH determine how much. That is a testable question, not an assumption.
- Well 2's iron (0.6 mg/L) and manganese (0.15 mg/L) are useful for arsenic: iron oxides adsorb As(V) well. It also means Well 2 water will form rust and manganese deposits in the distribution system and stain fixtures - a separate aesthetic and potentially a secondary-standard issue.
Tests to run before selecting treatment
- Arsenic speciation on both wells (field or preserved, properly handled) and on the blended, chlorinated entry point.
- Full water quality: pH, alkalinity, sulfate, phosphate, silica, vanadium - all compete with arsenic for adsorption sites and change the answer.
- Chlorine demand and oxidation test: how much As(III) is converted at your actual dose and contact time?
- Iron/manganese oxidation test and whether the chlorinated blend will form particulate that must be filtered.
- Pilot testing on the actual blend ratio - arsenic treatment sizing off jar tests alone is unreliable.
Practical options, roughly in cost order: better blending control and a third low-arsenic source; oxidation plus iron coagulation/filtration; adsorptive media (which must be regenerated or replaced and whose capacity depends on pH, silica, and phosphate); ion exchange; reverse osmosis (expensive, brine to manage). In New Mexico, arsenic treatment often comes with a concentrate or media disposal problem - price that in.
3. What operating and monitoring controls do you put in place for the next 12 months to keep the system in compliance through the school connection?
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Controls
- Flow-based blending. Install and calibrate flow meters on both well discharges and interlock or alarm the blend ratio. Set a high-arsenic-fraction alarm below the theoretical 63% - e.g., an alarm at 60% - so operators act before an exceedance. Manual valve positioning is not a control.
- Chlorine dose and contact time documented. If oxidation is part of your arsenic strategy, the dose and contact time are treatment parameters now.
- Well 2 is a critical asset. Protect its capacity: pump maintenance, backup power, and monitoring of static and pumping levels. If yield declines, the blend falls apart. Have a written contingency for Well 2 outage - likely reduced peak demand through storage management.
- Iron/manganese. Watch for distribution discoloration and complaints once well 2 is pushed to its ceiling; flushing may be needed.
Monitoring
| Sample | Location | Frequency |
|---|---|---|
| Arsenic (total) | Entry point, blended | Quarterly per rule, plus monthly internally |
| Arsenic speciation | Each well | Semiannual |
| Iron, manganese | Blend and distribution | Quarterly |
| Blend ratio | SCADA log | Continuous |
| Chlorine residual | Entry point and distribution | Daily |
Planning
- Run the school connection as a hydraulic and water-quality stress test before summer: simulate the new peak and confirm the blend and the storage can carry it.
- If Well 2 cannot sustain 226+ gpm at peak, the answer is either additional storage to shave the peak, a third source, or treatment - decide that now, not in July when the school opens.
- Keep a running arsenic trend. Quarterly compliance samples with near-zero margin can hide a drift for months; internal monthly data is what tells you the truth.