Well Water Coliform Bacteria Testing Guide

Volume I  ·  May 2026  ·  494 words

Coliform bacteria are a group of microorganisms found in soil, vegetation, and the intestinal tracts of warm-blooded animals. Their presence in well water signals a breach in the sanitary barrier between surface contamination and the groundwater source — a cracked well casing, a failed grout seal, surface water intrusion at the wellhead, or a compromised aquifer. Total coliform bacteria themselves are not typically pathogenic, but their presence indicates that fecal coliforms (including E. coli) — which are pathogenic — may also have a pathway into the well. The EPA Total Coliform Rule requires public water systems to test routinely; private well owners must test on their own initiative, and the CDC recommends testing at least annually.

Test types. Home coliform test kits fall into two categories. Presence/absence (P/A) tests use a growth medium that changes color when coliform bacteria metabolize the nutrients — results appear in 24–48 hours of incubation at body temperature. The Safe Home DIY bacteria test and Health Metric water test are P/A kits suitable for annual screening. A positive P/A result should be confirmed by a quantitative laboratory test (membrane filtration or most probable number method) that reports colony-forming units per 100 mL (CFU/100 mL). The EPA action level is 0 CFU/100 mL for total coliforms and E. coli — any detectable presence requires corrective action.

Sampling technique. The most common cause of a positive coliform test in a well with historically clean water is sampling error — bacteria introduced from the sampler's hands, the faucet aerator, or a contaminated sample bottle. The correct procedure: remove the faucet aerator or strainer, disinfect the faucet opening with a 10% bleach solution or alcohol swab, run the cold water for 5 minutes to flush the plumbing, then collect the sample without touching the inside of the bottle or cap. The sample must be kept cold (below 10°C / 50°F) during transport to the lab and delivered within 24 hours of collection — incubated bacteria in a warm sample bottle will multiply and produce a false positive.

After a confirmed positive. A laboratory-confirmed positive total coliform result requires shock chlorination of the well — introducing a chlorine solution to achieve 50–200 mg/L free chlorine concentration throughout the well column and plumbing, with a contact time of 12–24 hours. The chlorine must reach every tap and fixture, including outdoor spigots and washing machine connections. After shock chlorination, the system is flushed until chlorine is undetectable, and a follow-up test is performed 7–14 days later. If coliforms return, the contamination source is persistent and requires a well inspection for structural defects or, as a permanent solution, a point-of-entry ultraviolet sterilization system — an in-line UV sterilizer rated for the well's flow rate destroys bacteria without chemical residuals.