Well-Water Bacterial Testing Protocol

Why this matters

Private wells serve roughly 23 million U.S. households and sit outside the federal Safe Drinking Water Act monitoring regime defined in 40 CFR 141. The well owner is the de facto utility, and the contractor running the sample is the person whose technique decides whether the lab result is meaningful. A sample collected with poor aseptic technique, the wrong bottle, or held too long produces a false positive that triggers an unnecessary shock chlorination, or a false negative that leaves a contaminated well in service. The procedure below mirrors the sampling protocol U.S. EPA recommends for compliance monitoring of public ground-water systems (EPA 815-R-09-007) adapted for residential field collection.

When to sample

CDC and EPA private-well guidance: at minimum once per calendar year for total coliform and E. coli. Additional triggers for an out-of-cycle sample:

• After any well repair, pump replacement, pressure-tank replacement, or well-cap work
• After any flooding event that submerged the wellhead
• After a known septic-system failure within 100 feet
• After any unexplained gastrointestinal illness in the household
• When a new infant, immunocompromised resident, or pregnant resident joins the household
• After purchase or transfer of the property (lender or buyer requirement in most states)
• After shock chlorination, to confirm clearance

Equipment

• Lab-supplied sterile 100 mL or 120 mL polypropylene bottle with sodium thiosulfate preservative (neutralizes residual chlorine for chlorinated wells)
• 70 percent isopropyl alcohol wipes
• Disposable nitrile gloves
• Bic lighter or propane micro-torch for flame sterilization of metallic faucets
• Cooler with reusable ice packs (NOT loose ice; dilution risk)
• Pre-printed chain-of-custody form from the certified lab
• Sharpie marker for bottle labeling

Selecting the sampling tap

Pick a tap that draws directly from the pressure system without going through any treatment device. The goal is to characterize the well water, not the post-treatment water.

Acceptable taps:

• Outside hose bib upstream of any treatment (rare; verify with plumbing)
• Indoor utility-sink cold tap with the aerator removable
• Pre-treatment sampling port installed by the contractor (best practice)

Unacceptable taps:

• Any tap downstream of a softener, UV unit, filter, or RO
• Taps with non-removable swivel aerators, screens, or carbon attachments
• Refrigerator water dispensers
• Outside hose bibs that have a hose, splitter, or vacuum breaker permanently attached
• Threaded laundry faucets with anti-siphon devices

Procedure

1. Schedule the sample so it arrives at the lab within the regulatory holding time. EPA holding time for total coliform by Method 9223B or Colilert is 30 hours from collection to analysis start. Plan around lab cutoffs and shipping.
2. Don nitrile gloves. Do not touch the inside of the bottle, the inside of the cap, or the bottle threads.
3. Remove the aerator, screen, and any attachments from the faucet. Set aside.
4. If the faucet is metal, flame-sterilize the outside of the spout for 5 to 10 seconds with the lighter or torch. Skip for plastic faucets (will melt); wipe with alcohol instead.
5. Open the cold tap to full flow. Let the line run for at least 5 minutes to clear standing water from the service line and pressure tank. For wells with long lateral runs, extend to 10 minutes or until water temperature stabilizes.
6. Reduce flow to a steady pencil-diameter stream (no splashing, no aeration).
7. Open the sample bottle. Do not set the cap down. Hold the cap by its outside, threads down.
8. Fill to the 100 mL line marked on the bottle. Leave the small headspace; the lab requires it for mixing. Do not overfill chlorinated-well bottles (overfills sometimes flush the thiosulfate preservative).
9. Cap immediately and tighten firmly.
10. Label with sample ID, address, tap location, date, time of collection, collector name, and free-chlorine residual if chlorinated.
11. Place upright in the cooler with ice packs. Target storage temperature is 1 to 10 degrees C; freezing invalidates the sample.
12. Complete the chain-of-custody form and ship same day. If shipping by overnight carrier, include sufficient ice packs to maintain temperature for 30 hours.

Reading the results

Two field-relevant analytes:

• Total coliform: a hygiene-indicator group, not pathogens in themselves. A positive result means surface water, soil, or biofilm has reached the sample. Either a wellhead-integrity issue (cracked casing, missing well cap, surface intrusion) or contamination during sampling. EPA action: investigate and re-sample.
• E. coli: a fecal-contamination indicator. A positive result is a presumptive public-health emergency. EPA recommends boil-water advisory until the well is shock-chlorinated and re-tested clean two consecutive times spaced 1 to 2 weeks apart.

Corrective action sequence after a positive

1. Re-sample within 24 hours from a different tap to confirm the result is not sampling-technique contamination.
2. If the confirming sample is also positive, inspect the wellhead: cap integrity, casing condition, grouting, surface drainage away from the well, separation distance from septic absorption fields (50 feet minimum for sealed casings, 100 feet for unsealed; check state code).
3. Shock chlorinate per Manuall's well-chlorination SOP using NSF/ANSI 60 certified hypochlorite.
4. Flush the system until free-chlorine residual is below 1 mg/L at every fixture.
5. Re-sample after a 7 to 14 day waiting period.
6. Two consecutive clean samples are required to clear a boil-water advisory.
7. If positives recur within 6 months, recommend continuous disinfection: NSF/ANSI 55 Class A UV (40 mJ per cm-squared validated dose) sized to peak demand, or chlorine pellet/hypochlorite injection with downstream contact tank sized for 30 minutes at peak flow.

References

• EPA Safe Drinking Water Act, 40 CFR 141.21 (Total Coliform Rule sampling requirements)
• EPA 815-R-09-007, Quick Reference Guide for Total Coliform Rule
• EPA Method 9223B / IDEXX Colilert-18 (most-used residential coliform method)
• CDC, Well Water Testing recommendations (national.gov private-well guidance)
• NSF/ANSI 55, Ultraviolet Microbiological Water Treatment Systems
• NSF/ANSI 60, Drinking Water Treatment Chemicals: Health Effects
• ANSI/AWWA C654, Disinfection of Wells