Coliform-Positive New Well With Elevated Nitrate
A new 4-inch well is completed at 120 feet with 40 feet of casing grouted to 40 feet. The driller shock-chlorinates with about 100 ppm chlorine, waits 24 hours, flushes until the chlorine odor is gone, and takes a coliform sample the next morning. The sample is total-coliform positive and E. coli negative; nitrate is 14 mg/L as N. The well is 80 feet from a horse paddock, 60 feet from a septic tank, and the casing terminates 6 inches above grade with a loose well cap. The homeowner took the sample at an outdoor faucet downstream of a water softener and a 50-foot garden hose. The local health department requires a repeat sample and an inspection before the well is placed in service. The driller wants to shock-chlorinate again and resample immediately.
1. Was the coliform sample properly collected and interpreted? Identify every defect and state whether the positive result can be dismissed.
Read the worked answer
The sample has serious collection defects, but the positive result cannot simply be dismissed.
Sampling defects:
- The sample came from an outdoor faucet downstream of a water softener and a 50-foot garden hose. Softeners, hoses, and faucet aerators can harbor biofilm and produce a false positive. The correct sample point is a cold, unsoftened, frequently used tap supplied directly by the well, with the aerator removed, the tap disinfected, and water run long enough to clear the line.
- Sampling the morning after chlorination and flushing is too soon. Many programs recommend waiting about 7 to 10 days after disinfection so that chlorine residual does not mask or alter the result. A positive result despite recent chlorination is therefore more suspicious, not less.
Construction defects that could make the result real:
- The casing ends only 6 inches above grade and the cap is loose. Surface water can enter the well at the top.
- The grout extends to 40 feet, but the driller must verify that it seals into a low-permeability layer. If the upper formation is sandy, a 40-foot seal may not be enough.
- The well is 80 feet from a horse paddock and 60 feet from a septic tank. Both are plausible sources of bacteria and nitrate.
Interpretation: The E. coli negative result is reassuring but does not prove the water is safe. Total coliform can indicate a pathway or regrowth. The nitrate result of 14 mg/L, above the 10 mg/L maximum contaminant level, supports a real contamination concern. The driller should not dismiss the sample; the well should be repaired, disinfected, and resampled properly.
2. Design the corrective action and resampling sequence. Include physical repairs, disinfection, and regulatory reporting.
Read the worked answer
The response should fix the well, not just add chlorine.
Immediate protection:
- Tell the homeowner not to drink the water until the repeat samples are satisfactory. Provide bottled water or an alternate source.
Physical repairs:
- Extend the casing to at least 18 to 24 inches above grade, install a sanitary well cap, and build a grout pad that slopes away from the casing.
- Inspect the annular seal. If the grout is incomplete or the formation around the casing is sandy, pressure-grout the annulus or extend the seal.
- Remove the hose and softener from the sampling path. Check for cross-connections and fix any that are found.
Disinfection:
- Shock-chlorinate with about 100 to 200 ppm chlorine, circulate to contact the casing and drop pipe, and hold for 24 hours.
- Flush until the chlorine residual is gone, then wait about 7 to 10 days before sampling. Sampling immediately after disinfection can give a misleading result.
Sampling and reporting:
- Collect total coliform and E. coli from a proper cold, unsoftened tap. Also retest nitrate and any required chemical parameters.
- If the repeat is positive, do not place the well in service. Investigate with a video inspection, pressure test, or additional grouting, and consult the local health department.
- Report the repairs and sample results as required by Part 127 and the local health department permit. Keep the well record current.
3. What health advice would you give for nitrate at 14 mg/L, and what are the likely sources? What treatment is effective?
Read the worked answer
Nitrate at 14 mg/L as N exceeds the 10 mg/L maximum contaminant level. It is not an acute emergency for most adults, but it is a serious concern for infants under six months because of methemoglobinemia, sometimes called blue-baby syndrome.
Health advice:
- Do not use the water for infant formula or for mixing infant foods. Use bottled water or an alternate safe source.
- Breastfeeding mothers should consult a health care provider if they are considering using the water.
- Adults can use the water for bathing and general household use, but drinking-water advice should come from the local health department.
Likely sources:
- The horse paddock 80 feet away and the septic tank 60 feet away are both plausible. Nitrate is mobile in groundwater, so separation distances reduce risk but do not guarantee protection.
- Fertilizer, manure storage, and older unsealed wells nearby can also contribute.
Treatment:
- Reverse osmosis or anion exchange are effective point-of-use treatments for nitrate. Distillation can also work but is energy-intensive.
- Boiling does not remove nitrate; it concentrates it. Standard carbon filters and water softeners do not remove nitrate.
- Treat the water and then retest the treated water to confirm performance. Also retest the raw well water and investigate the source so that the problem is not simply moved downstream.