Rising Nitrate in a Municipal Well Near a CAFO
A community water system has two wells in a designated drinking water source water assessment area. Well 4 produces up to 1,200 gpm and historically tested at 6.5 to 7.9 mg/L nitrate as nitrogen. The latest sample is 8.8 mg/L. Well 2 produces up to 1,500 gpm and tests at 3.2 mg/L nitrate. Peak system demand is 1,300 gpm. A concentrated animal feeding operation expanded about 1,200 feet up-gradient of Well 4 two years ago, and the well has a gravel pack and a shallow grout seal. The system has no nitrate treatment. The operator wants to run Well 4 at full capacity because it has lower iron and manganese than Well 2. The nitrate MCL is 10 mg/L as nitrogen. The utility has not updated its source water protection plan since the CAFO expansion, and nitrate monitoring has been annual.
1. Is the system currently in compliance with the nitrate MCL? What does the trend imply?
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The latest result of 8.8 mg/L is below the 10 mg/L nitrate MCL as nitrogen, so this sample alone is not a violation. The trend is the concern: 6.5 to 7.9 to 8.8 mg/L over time, with a CAFO expansion up-gradient and a well construction that may not provide a strong annular seal. Compliance is determined by the required monitoring results and averaging rules in the system permit, so the operator should verify whether the result is a single sample or part of a running average and confirm the reporting basis with KDHE. The system should not wait for a 10 mg/L result to act. A result above 10 mg/L would be an MCL violation with acute health significance for infants, pregnant women, and others, and would trigger immediate public notice and alternative water or treatment. The practical conclusion is that the well is approaching the MCL and the source water protection plan is out of date.
2. At peak demand of 1,300 gpm, what is the maximum Well 4 flow that keeps the blended nitrate at or below 8.0 mg/L? Show the calculation.
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Let x be the fraction of total flow from Well 4. The blend concentration is 8.8x + 3.2(1 - x). Set that equal to the target of 8.0 mg/L: 8.8x + 3.2 - 3.2x = 8.0; 5.6x = 4.8; x = 0.857, or 85.7%. At 1,300 gpm total, the maximum Well 4 flow is 0.857 x 1,300 = about 1,114 gpm. The remaining 186 gpm would come from Well 2. This calculation shows that Well 4 can supply most of the peak demand while holding the blend at 8.0 mg/L, but the margin is small because the blend is sensitive to any further rise in Well 4 nitrate. If Well 4 reached 10 mg/L, the same target would require much more Well 2 water or treatment. The operator should blend deliberately and record the ratio, not simply run Well 4 at full capacity because of lower iron and manganese.
3. What source water protection and monitoring actions should the utility take?
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Update the source water protection plan and inspect the Well 4 area, including the casing, grout seal, and gravel pack, because a shallow seal can allow contaminated shallow groundwater to enter. Increase nitrate monitoring from annual to quarterly until the trend is understood, and add at least one raw water sample from Well 4 before blending. Review the CAFO nutrient management plan, manure storage, and application rates with the appropriate state agency, and document any evidence of runoff or leaching. Evaluate blending, treatment such as nitrate-selective ion exchange or reverse osmosis, and a backup supply. Communicate with customers if results approach or exceed the MCL. Source water protection is preventive: once nitrate reaches the well, blending buys time but does not remove the source of contamination.