Nitrate Climbing in a Two-Well Blended Supply

Average-day blend:

• W-1 share = 0.55 / 0.90 = 61.1%
• W-2 share = 0.35 / 0.90 = 38.9%
• Blended nitrate = (0.611 × 9.4) + (0.389 × 4.1) = 5.74 + 1.59 = 7.3 mg/L as N, against a 10 mg/L MCL. Comfortable-looking margin of 2.7.

But the margin that matters is the peak-day margin:

• W-1 alone at 9.4 mg/L leaves 0.6 mg/L of headroom.
• The trend is +0.35 mg/L per year, and field-versus-lab agreement is only ±0.2 mg/L.
• 0.6 / 0.35 ≈ 1.7 years before W-1 alone reaches the MCL - and considerably less before you are operating with no defensible margin at all.

The honest answer to how much time you have is one summer, maybe two, and that assumes the trend is linear, which rising nitrate from a septic plume rarely is.

The deeper point: blending is a flow-control strategy, not a treatment strategy. The moment W-2 trips, is taken offline for maintenance, or fails to keep up on a peak afternoon, the blend becomes W-1 alone. The system is one pump failure away from an exceedance right now.

Operating (this quarter):

• Stop running W-1 alone. Enforce a minimum W-2 run rate or install a ratio control that hard-limits W-1's share. If that is not possible, run W-1 at a reduced pumping rate alongside W-2.
• Install an online nitrate analyzer on W-1 and at the finished-water entry point, keep the weekly field checks, and keep quarterly certified lab confirmation. You need to know when the peak-day blend crosses 8 mg/L, not when the quarterly sample does.
• Watch the source water for other agricultural markers - the same plume that carries nitrate carries pesticides and pathogens; confirm the well construction and grouting are sound and that the well is not drawing shallow water.

Capital and planning (this year):

• Get a time-of-travel map for W-1's contributing area and use it in the wellhead protection plan.
• Intervene in the septic development review. Submit comments through the wellhead protection plan and the municipal planning process, and ask NJDEP to evaluate the proposed septic density against groundwater quality standards in the recharge area. Once those 240 units are built, the nitrate load is permanent.
• Evaluate real treatment options with costs per 1,000 gallons: nitrate-selective anion exchange (with a brine disposal plan), reverse osmosis, a third lower-nitrate well, or purchased water. Treat a blending-only plan as a bridge, not a solution.

The failure mode here is a utility that watches a trend line for five years and then needs a treatment plant in eighteen months.

Treat it as an MCL violation from the moment the result is reported.

• Nitrate is acute. The health endpoint is methemoglobinemia in infants under six months - blue baby syndrome. There is no waiting for a reanalysis to decide whether to act.
• Public notice: Tier 1, within 24 hours, using the mandatory health effects language, with explicit advice not to use tap water for infant formula or food preparation for infants under six months. Confirm tier classification and the exact language with NJDEP, but assume the shortest clock until told otherwise.
• Notify NJDEP immediately.
• Get the water below the MCL now. Take W-1 out of service or blend it down hard against W-2 until the blended result is well under 5 mg/L as N, and verify with a field check plus a rush lab sample.
• Protect the sensitive customers directly - bottled water to households with infants, to the pediatric office, and to any day care in the service area.
• Check nitrite as well, and pull the W-2 sample from the same period to confirm the blend you thought you were running.
• Verify the result in parallel, not in lieu of action: duplicate sample, lab QC blanks, and a check that the sample was not contaminated by handling. Do not let the reanalysis deadline set the response time.

After the acute response, the follow-up is a source water protection and treatment decision made under a compliance schedule, which is a much worse position than making it voluntarily a year earlier.