Nitrate Trend in a Small Groundwater System
A community groundwater system serves 1,150 people from two wells. Well 2, completed at 62 feet in sandy soil, is surrounded by irrigated corn fields and several septic systems. The most recent annual nitrate result is 9.6 mg/L as N; the two previous results were 8.1 and 8.8 mg/L. Well 1 is 4.2 mg/L. Arsenic at Well 2 is 0.006 mg/L and manganese is 0.18 mg/L. The system chlorinates with sodium hypochlorite but has no other treatment and no blending facility; both wells pump to a common 150,000-gallon tank. The system has never been determined to be groundwater under the direct influence of surface water. The owner asks whether the nitrate result is a violation and whether a new well is needed immediately.
1. Is the nitrate result a violation, and what monitoring change is triggered?
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Nitrate MCL is 10 mg/L as N, so 9.6 mg/L is not an MCL violation, but it is a trigger for more monitoring. Under the nitrate rule, a result above 5 mg/L requires quarterly monitoring instead of annual. The system should confirm the result with a repeat sample and check nitrite as well. It should also review whether the sample was properly preserved and analyzed. Because Well 2 is shallow and surrounded by corn fields and septic systems, the trend from 8.1 to 8.8 to 9.6 mg/L is more important than a single result. The owner should not wait for an exceedance to plan. If nitrate reaches 10 mg/L, it is a Tier 1 violation with immediate public notice, especially for infants under six months.
2. What immediate treatment or blending options should be evaluated?
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Immediate treatment options: blend Well 2 with Well 1 to keep finished nitrate below 10 mg/L. With Well 1 at 4.2 mg/L and Well 2 at 9.6 mg/L, a blend of about 60% Well 1 and 40% Well 2 gives roughly 6.4 mg/L, which is a reasonable operating target. That requires a blending facility or flow control, not just a common tank. Other options are ion exchange, reverse osmosis, or a new well in a lower-nitrate aquifer. Arsenic at 0.006 mg/L and manganese at 0.18 mg/L are below their MCLs but should be monitored. Before selecting treatment, run pilot testing and check waste disposal for ion exchange brine.
3. What source water protection and communication steps should accompany the technical response?
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Source water protection should be part of the answer: inventory septic systems, fertilizer application, and irrigation practices near Well 2; work with the farm owner on nutrient management; and inspect the well casing and grout for pathways. The system should update its source water assessment and emergency plan. Because the system is groundwater and not determined GWUDI, the Ground Water Rule applies, but the immediate issue is nitrate. Notify MDE of the trend and the monitoring change. Public education can explain nitrate and infant risks before any violation. If a new well is proposed, test water quality before construction and permit it properly.