Blending Two Wells With Nitrate and Manganese
A community water system uses two wells and chlorination only. Well A produces 0.9 MGD with nitrate 8.9 mg/L as N and manganese 0.32 mg/L; Well B produces 0.6 MGD with nitrate 3.4 mg/L and manganese 0.06 mg/L. Current demand is 1.05 MGD, and the state nitrate MCL is 10 mg/L; manganese has a secondary limit of 0.05 mg/L. The operator blends both wells at equal flow, sending 0.75 MGD from each, and the finished nitrate is 6.2 mg/L. A new 120-unit subdivision is projected to raise demand to 1.3 MGD. The system has no nitrate removal or manganese treatment, only 250,000 gallons of storage. Well A's static water level has dropped 6 feet since last year, and a nearby farm applies anhydrous ammonia in spring. The state has asked for a source-water protection plan update.
1. Calculate whether the current 50/50 blend meets the nitrate MCL at 1.05 MGD, and determine the maximum Well A flow if total demand is 1.3 MGD and Well B runs at 0.6 MGD. What about manganese?
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At equal flow, nitrate is (0.75 x 8.9 + 0.75 x 3.4) / 1.5 = 6.15 mg/L, so the current blend meets the 10 mg/L MCL. Manganese is (0.75 x 0.32 + 0.75 x 0.06) / 1.5 = 0.19 mg/L, well above the 0.05 mg/L secondary limit. At 1.3 MGD with Well B at 0.6 MGD, Well A must supply 0.7 MGD. Nitrate becomes (0.7 x 8.9 + 0.6 x 3.4) / 1.3 = 6.36 mg/L, still below 10. Manganese becomes (0.7 x 0.32 + 0.6 x 0.06) / 1.3 = 0.20 mg/L, still above 0.05. Nitrate is not the limiting factor here; manganese is. The system could meet nitrate by blending, but it cannot meet the manganese secondary limit with these two wells and chlorination alone. The calculation should be checked with actual annual averages, because a single sample can mislead.
2. What are your options to meet demand and the manganese limit, and how would you sequence them?
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Immediately, I would maximize Well B and reduce Well A to lower manganese as far as nitrate and demand allow, while monitoring manganese at the entry point and in the distribution system for discoloration complaints. That is only a partial fix because Well B is limited to 0.6 MGD and demand will reach 1.3 MGD. Short term, I would evaluate treatment: oxidation followed by filtration, such as chlorine or permanganate with greensand or manganese-coated media, because chlorination alone does not remove manganese. I would pilot or jar-test before design. I would also evaluate a new lower-manganese well or interconnection with a neighboring system. Sequencing matters: secure additional supply or treatment before approving the subdivision, and do not rely on sequestration without checking state approval and aesthetic results. Long term, I would add manganese monitoring and treatment capacity to the capital plan.
3. What source-water protection actions would you recommend given the farm and water-level decline?
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I would inspect the wellhead area for proper casing, grout, and sanitary seal, and confirm that the well is not drawing shallow agricultural recharge. I would add nitrate and manganese to a routine source-water monitoring plan, with more frequent sampling in spring after anhydrous ammonia application. I would meet with the farm to encourage nutrient management, proper application timing, and setbacks from the wellhead, and I would check whether the state offers agricultural best-management-practice cost share. The 6-foot static water-level decline needs investigation: compare seasonal trends, pumping rates, and nearby high-capacity wells, and consider a dedicated monitoring well. I would update the source-water protection plan with a contaminant source inventory, wellhead protection area map, and contingency plan for losing Well A. If nitrate trends upward, I would plan treatment or a new source before the MCL is exceeded.