Nitrate Exceedance, Blending and a Late Confirmation at a Two-Well System
A community system is supplied by two Type I wells in a sand-and-gravel aquifer beneath row crops. Well 1: nitrate 11.4 mg/L as N, 700 gpm capacity, static level 38 ft, pumping level 74 ft, pump set at 110 ft. Well 2: nitrate 3.6 mg/L as N, 450 gpm capacity, static level 51 ft, pumping level 96 ft, pump set at 130 ft. Raw arsenic is 0.004 mg/L at both. July demand averages 0.85 MGD with a peak of 1.15 MGD. Well 1 nitrate has risen for three years (7.1 to 11.4 mg/L) and its discharge now carries sand. The system has no treatment; both wells feed a common clearwell where chlorine is added. The 11.4 mg/L sample was collected six days ago; nothing has been reported and no confirmation sample has been taken.
1. Can Well 1 stay in service? Compute the blend and state the operating constraints you would impose.
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Mass balance with the 10 mg/L MCL: (Q1 x 11.4) + (Q2 x 3.6) is less than or equal to 10 x (Q1 + Q2), which reduces to 1.4 Q1 is less than or equal to 6.4 Q2, or Q1 is less than or equal to 4.57 Q2. With Well 2 at its 450 gpm maximum, Well 1 could theoretically run about 2,050 gpm - so total capacity is not the binding constraint.
The binding constraint is turndown. At the 1.15 MGD peak (about 800 gpm total), running both wells wide open gives (350 x 11.4) + (450 x 3.6), all divided by 800 = 7.0 mg/L, comfortably compliant. But at the 0.85 MGD average (about 590 gpm), Well 1 alone would deliver 11.4 mg/L. You must run Well 2 at its full 450 gpm and trim Well 1 to about 140 gpm: (140 x 11.4) + (450 x 3.6), divided by 590 = 5.5 mg/L.
Constraints to impose: flow-paced blending with a nitrate interlock; Well 1 never the sole source; if Well 2 goes down for maintenance, Well 1 comes off or you buy water; sample the blended finished water, not just each well; and blend against an internal target of 8-9 mg/L, not 10, to leave margin for analytical variability.
2. What are your confirmation, notification and public notice obligations for the 11.4 mg/L result?
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Nitrate above the 10 mg/L MCL is an acute-risk contaminant, so the rule requires a check sample at the same sampling point within 24 hours. Six days have passed with none taken - you now have a possible MCL exceedance and a missed confirmation.
Actions now, in order:
- Collect the confirmation sample immediately, split with EGLE, and request expedited analysis.
- Notify EGLE today and state plainly that the confirmation is late and why. Do not wait for a report cycle.
- If confirmed above 10 mg/L, a Tier 1 public notice goes out within 24 hours of confirmation, with the required acute health-effects language (infants under six months are the critical population - methemoglobinemia), delivered to all persons served by methods that actually reach them.
- Take Well 1 out of service or blend it below the MCL immediately - the notice obligation does not wait on the confirmation if you cannot show the water is safe, and it is a poor trade to keep supplying water you have reason to believe exceeds an acute MCL.
- Do not treat the Consumer Confidence Report as the vehicle for this. The CCR is annual and cannot substitute for Tier 1 notice; the violation is also disclosed in the next CCR.
3. Sand in the discharge and a rising nitrate trend together - what do you investigate, and what does it suggest about the well itself?
Read the worked answer
Specific capacity right now is 700 gpm divided by (74 - 38) ft of drawdown = 19.4 gpm/ft. Get the original number from the well log: a material drop means the well, not the pump curve, has changed.
The pairing of sand with a rising nitrate trend is the important clue. Nitrate comes from the shallow, agricultural part of the aquifer. If the well is increasingly drawing shallow young water, the causes are mechanical: a failing annular seal, a breached casing, collapsed gravel pack, or a pump misaligned or running with insufficient submergence (only 36 ft above the pump here - marginal, with vortexing and air entrainment risk).
Investigate: pull and inspect the pump and column, check impeller wear and bowl clearance, camera the casing and annular seal, run a step-drawdown test against baseline, and review pumping water levels continuously rather than by spot reading. If the seal is breached, re-seal and redevelop; sand production will otherwise destroy the pump and the water quality record together.
Longer term: quarterly nitrate at both wells, an updated wellhead protection area delineation, work with the farm operators on nutrient management in the recharge area, and begin costing treatment or a replacement well. A rising three-year trend rarely reverses on its own.