Photograph of  this antiquated laundry sink with several unsanitary plumbing violations in view.What Does the Water Test Bacteria Count Mean?

  • BACTERIA LEVELS in WATER - CONTENTS: how to interpret and what to do about bacteria in drinking water. What to do when a well fails a drinking water test - step by step action guide
  • POST a QUESTION or READ FAQs about the meaning of different levels of bacteria found in well water after a well test
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Water bacteria tests or potability tests:

How to Interpret the Level of Bacteria Reported in a Well Water Potability Test. This article explains the significance & diagnostic implications of different levels of bacteria found in water during a well test.

If we determine that the source of well water contamination is persistent (for example we've recently disinfected the well and that did not cure the problem) then water treatment may be the best and fastest "cure" for this problem.

Understanding the level of bacterial contamination in a well can help us interpret the meaning of a well water bacteria or potability test. So what was the level of well water contamination detected? This question is explored here.

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Interpreting and Acting On the Significance of the Level of Bacteria Found in a Water Test

Dirty well top (C) Daniel FriedmanAs we introduced at Water Test Procedure Errors if the source of the contamination is not due to an ongoing situation or a persistent source of well contamination, for example bad ground water source, and assuming that the well has not already been recently disinfected or "shocked" then a disinfection of the water system will solve the problem.

See WATER TEST PROCEDURAL ERRORS if you missed that article.

Our photo (left) shows how easily a well might be contaminated by a simple error such as a loose well casing top or the placement of well piping on the ground during well repairs.

Comparing Well Bacteria Test Types & Understanding What the Results Mean

P/A or "Present/Absent" Water Bacteria Tests vs MFT / MPN Bacteria Test

A P/A test for bacteria in well water gives a "yes" or "no" or "OK" or "Not OK" test result. If the P/A result is "unacceptable" that means that there is bacteria in the drinking water above the acceptable level for where you live. Depending on where you live, a P/A test that says your water has an acceptable level of bacteria means that the bacteria level, if cultured on a suitable culture plate, will produce less than one colony per 100 ml of water for both coliform bacteria and for the total bacteria count.

But we have no idea if the actual bacteria level, expressed for example in colony forming units (CFUs) per some measure of quantity of water (say per milliliter or ml) is just barely unacceptable (say a count of 11 when a count of 10 would have been OK) or if the water is stunningly contaminated with bacteria (say a count of 200 cfus, or maybe "TNTC" which is jargon for "too numerous to count").

Why do we care? Because the implications of these numbers are different. For example shocking a well is unlikely to cure a very high bacteria count problem. And conversely, shocking a well and then testing as soon as permitted afterwards may give a "pass" result on water that is actually contaminated by a persistent source that will show up again a few days or weeks later.

We prefer the MFT/MPN test to the P/A test because getting an actual count or "number" of CFU/100ML is diagnostic (as we describe here) whereas a P/A test simply says OK or NOT OK with no indication of the level of contamination present.

For example, consider two wells with a persistent source of bacterial contamination. If a well that fails at 15 CFU/100mL is "shocked" the low starting count means it's easier to cover up the persistent problem source and more time may be needed for the bacteria to reappear. This would affect your follow-up water testing strategy.

By contrast, if a second well with a persistent source of bacterial contamination fails at >10,000 CFU/ml, first it is very unlikely that any "well shock" treatment will be effective, and second, the level of contamination is so great that if a seller came up with a subsequent "pass" on a bacteria test without installing purification equipment or without telling me what problem was found and fixed, we would be very skeptical about the reliability of the follow-up test.

So while a property seller/owner may prefer to "shock" a well and re-test, that procedure, used alone, might or might not be acceptable. What to do after a seller/owner has "done something" to the water supply and re-testing has been done is discussed at WATER RE TESTING

Watch out: a bacteria test on well water is also a great indicator that other contaminants can be present. Often the source of bacteria is surface water that leaks into the well. That means any other contaminants on the ground surface may also be in the well water. Discuss this with your lab - further testing may be in order.

See WATER CONTAMINANT LEVELS & LIMITS for examples of contaminant limits permitted in drinking water.

When and How to Shock or Chlorinate a Well - Procedure for Shocking a Well to (temporarily or maybe longer) "Correct" Bacterial Contamination

At WELL CHLORINATION SHOCKING PROCEDURE we provide a description of a common procedure used to sterilize well water and water equipment. The purpose of shock disinfection of a well system is to destroy bacterial contamination present in the well system at the time of disinfection and is not intended to kill bacteria that might be introduced at a later time.

Therefore it is vital that the well be constructed so that no new contamination may enter the well following completion of the shock disinfection. In order to achieve a satisfactory disinfection of the system, the bacteria must be brought in contact with a chlorine solution of sufficient strength and remain in contact with that solution for a sufficient time to achieve a complete kill of all bacteria and other microorganisms.

If you are going to have a well disinfected after a failed bacteria or water potability test, follow the well shocking procedures and warnings that we provide at

This series of articles explains many common water contamination tests for bacteria and other contaminants in water samples. We describe what to do about contaminated water, listing common corrective measures when water test results are unsatisfactory.

We include water testing and water correction measures warnings for home owners and especially for home buyers when certain conditions are encountered, with advice about what to do when these circumstances are encountered. If a well potability test "fails" and the well is considered contaminated with bacteria, be sure you have also reviewed WATER TEST PROCEDURAL ERRORS (live link is below)


Continue reading at WELL CHLORINATION SHOCKING PROCEDURE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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