Chloroform in Water Test: Not Coliform

A chloroform in water test is a chemical analysis, not a bacteria result. Coliforms are bacteria; chloroform is a chemical in the trihalomethane group. If someone tells you a well has “chloroform bacteria,” the wording needs clarification. The exact name on the laboratory report determines which result you need to address. CDC testing guidance and EPA contaminant definitions distinguish these categories.

Take immediate precautions when contamination is suspected

If harmful contamination is known or suspected, switch to bottled water or another safe source for drinking and food preparation before arranging more testing or treatment.

• Bacteria concern: A positive E. coli result likely indicates fecal contamination. Use an alternative safe water source and contact your health department promptly. For a total-coliform-positive result, use an alternative source while seeking guidance about possible harmful germs and follow-up testing. If the concern is germs alone and safe alternative water is unavailable, CDC recommends boiling under boil-water advisory guidance.
• Chemical concern: Boiling does not remove chemical contamination. For known or suspected harmful chemical contamination, CDC advises stopping water use pending treatment and guidance. Contact your health department for advice specific to the chemical, concentration, and household uses. A chloroform detection alone needs interpretation; it does not establish the same restrictions for every household.

Do not assume boiling is appropriate when the contamination could include chemicals. CDC explains these different emergency responses.

Identify the exact report entry

Report entry	What it means	What to clarify
Total coliform	A group of bacteria found in the environment and digestive systems; an indicator of possible contamination	Was E. coli also tested, and what was its result?
E. coli	A bacterium within the fecal coliform group; a positive indicator result likely means fecal contamination	What immediate precautions and follow-up does the health department recommend?
Chloroform	An individual chemical in the trihalomethane group	What concentration and unit were reported?
Total trihalomethanes, or TTHMs	The combined concentration of four chemicals, including chloroform	Are all four individual results and their total available?

An E. coli indicator test does not identify whether a dangerous strain is present. Likewise, a bacteria result does not answer a chloroform question. Read each requested analyte separately; a limited panel cannot establish that water is safe from every contaminant. CDC explains bacterial indicators; EPA identifies the trihalomethane group.

For the fuller bacterial follow-up process, see our coliform in well water guide.

Understand the 80 µg/L benchmark

EPA’s public-water-system maximum contaminant level for total trihalomethanes is 0.080 mg/L, equivalent to 80 µg/L. It covers the combined total of chloroform, bromodichloromethane, dibromochloromethane, and bromoform.

That is a group standard, not a chloroform-alone limit. For a private well, it provides comparison context rather than a federal legal requirement. A chloroform-only result below 80 µg/L does not establish the four-compound total or the water’s overall suitability for drinking.

Also distinguish an MCL, an enforceable public-system standard, from an MCLG, a non-enforceable health goal. Do not read a goal or another contaminant’s “zero” entry as a chloroform legal limit. EPA’s standards table and definitions explain the distinction.

How to test well water for chloroform

A chloroform in water test has to name chloroform, or the four-compound TTHM group, on the laboratory order. Have the complete report ready and ask:

1. What exact analyte was measured, and what are its result and unit?
2. Does the panel include chloroform specifically, all four TTHMs, bacteria, or some combination?
3. What does any “not detected” notation mean at this test’s reporting limit?
4. Does the laboratory’s state certification cover the requested analysis and, if relevant, the property transaction’s requirements?
5. What collection instructions and follow-up timing apply, given recent chlorination, repairs, flooding, or nearby spills?

Use EPA’s state laboratory certification contacts to find an appropriate laboratory. Our well water testing guide helps organize the broader testing plan.

Avoid an automatic chlorine response

Chlorination can form disinfection byproducts when disinfectants react with naturally occurring material in water. A chloroform finding therefore calls for chemical interpretation, not automatic shock chlorination. Have the health department or a well contractor assess treatment and follow-up testing without independently changing disinfectant dosing. EPA explains byproduct formation.

Keep routine testing separate from this investigation: CDC recommends annual total coliform, nitrate, pH, and total dissolved solids testing, plus locally relevant contaminants and testing after events such as repairs or flooding. Use our annual well water check to track that baseline. CDC testing recommendations.

Chloroform vs. Coliform Bacteria: Side-by-Side Comparison

Homeowners and voice-search tools frequently confuse chloroform (a volatile organic chemical and disinfection byproduct) with coliform (a group of indicator bacteria). Use this quick-reference chart to verify which test you ordered and what your lab report actually shows:

Feature	Chloroform (Chemical / THM)	Total Coliform & E. coli (Bacteria)
What it is	A volatile organic compound (CHCl3), one of the four regulated trihalomethanes (TTHMs)	A family of bacteria used to indicate sanitary pathway entry into a well
How it enters well water	Usually formed when chlorine bleach from shock chlorination reacts with natural organic matter, or from industrial/solvent contamination	Surface runoff, cracked well casing, loose well cap, or nearby septic drain field failure
EPA Drinking Water Benchmark	Regulated under Total Trihalomethanes (TTHM) MCL of 0.080 mg/L (80 ppb)	Public water MCLG is 0; any positive detection in a private well warrants retesting and inspection
Standard Certified Lab Method	VOC GC/MS analysis (e.g., EPA Method 524.2)	Microbiological presence/absence or MPN count (e.g., SM 9223B Colilert)
Does boiling remove it?	No — prolonged boiling can volatilize chemicals into indoor air; use a certified activated-carbon or VOC filter	Yes — a rolling boil for 1 minute kills bacteria for drinking and cooking while you inspect the well
Where to go next	Compare VOC panels in our well water testing guide	Follow our step-by-step coliform bacteria in well water guide

If you are testing both systems during a rural home purchase, review our complete well and septic testing checklist for buyers and check whether iron or manganese in well water could also increase chlorine demand during disinfection.