Rising Nitrate and Manganese in a Three-Well Sand and Gravel Supply
A community well system has three wells in a sand and gravel aquifer beneath farmland and a lakeside subdivision of 60 homes on septic systems. Well 1 (400 gpm) tests nitrate 3.1 mg/L as N and manganese 0.02 mg/L; Well 2 (350 gpm) tests nitrate 9.4 mg/L as N and manganese 0.06 mg/L; Well 3 (150 gpm) tests nitrate 6.8 mg/L as N, manganese 0.42 mg/L, and iron 0.9 mg/L. Treatment is 0.6 mg/L free chlorine only. The system has never exceeded a nitrate MCL and the state requires quarterly nitrate samples. Historical data show Well 2 rising from 5.0 to 9.4 mg/L as N over 18 months while Well 1 has stayed flat. Summer peak-day demand requires all three wells for about six hours; the rest of the year Well 1 alone can meet demand. A customer near Well 3 has complained of black staining, and a tap sample measured 0.31 mg/L manganese.
1. Calculate the blended nitrate under normal operation and under the loss of Well 1, and explain how the nitrate MCL is actually applied.
Read the worked answer
Normal blend at 900 gpm total: Well 1 is 44.4%, Well 2 is 38.9%, Well 3 is 16.7%.
(0.444 x 3.1) + (0.389 x 9.4) + (0.167 x 6.8) = 1.38 + 3.66 + 1.13 = about 6.2 mg/L as N.
With Well 1 out, the remaining 500 gpm is 70% Well 2 and 30% Well 3:
(0.70 x 9.4) + (0.30 x 6.8) = 6.58 + 2.04 = about 8.6 mg/L as N.
That is under the 10 mg/L MCL but the margin is thin, and if Well 2 continues its 18-month trend to 10.5 mg/L the blend becomes roughly 9.4 mg/L as N with Well 1 down. Compliance is based on the running annual average of quarterly samples at each entry point, not on a single grab, so one low quarter can mask a climbing trend; the operator should project the annual average forward, not just report the current quarter.
Units matter: the MCL is 10 mg/L as nitrogen, equal to 44.3 mg/L as nitrate. A lab report in mg/L NO3 is not directly comparable and must be converted before anyone declares compliance.
2. The manganese complaint comes in. What sampling and analysis would you require before choosing a treatment, and why does the 0.9 mg/L iron matter?
Read the worked answer
First establish whether the manganese is dissolved or particulate, because the treatment is different.
Collect a fresh sample at the wellhead and at the tap. Split each into an unfiltered aliquot preserved with nitric acid to pH below 2 (total recoverable) and a field-filtered aliquot through a 0.45 micron filter, then acidified (dissolved). Hold both for the metals analysis within the method holding time, typically six months for acidified metals. Compare the pairs: if dissolved manganese is most of the total, the metal is in the reduced Mn(II) form and needs oxidation, sequestration, or media treatment rather than filtration alone.
Also re-sample after flushing the service line, because 0.31 mg/L at the tap in a system with only chlorine may reflect scale sloughing rather than raw water.
The 0.9 mg/L iron is a strong hint: reduced iron and manganese travel together in anoxic groundwater and both consume chlorine. Any oxidation-based treatment (chlorine, permanganate, ozone) must be sized for the combined iron plus manganese load, and the chlorine demand will change the residual the system can carry.
3. The state asks for a plan. What are your next steps on source water protection and treatment, and what would you do first?
Read the worked answer
First, protect the compliance margin. Cut Well 2 back or rotate it out where demand allows, and formalize a well-blending procedure with a written maximum Well 2 fraction so nobody has to do the arithmetic under pressure during a summer peak.
Second, characterize the source. Well 2's rise from 5.0 to 9.4 over 18 months while Well 1 stayed flat points to a local source, likely the septic systems or farm inputs near that wellhead rather than a regional trend. Sample all three wells quarterly for nitrate and nitrite, and add ammonia and chloride, which help distinguish septic influence from fertilizer. Update the source water assessment and check whether the wellhead protection area accounts for the subdivision.
Third, plan treatment if the trend continues. Blending is the cheapest lever but is already limited; anion exchange or reverse osmosis removes nitrate, while oxidation plus filtration handles the manganese and iron. Get a pilot or at least a design basis started now, because a nitrate exceedance would require immediate action and treatment takes years to permit and build.