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

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Water Potability Eests: Bacteria Levels in Water

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.

The illustration of different common bacteria tests or counts and their relationships shown above is adaptged from Coliform Bacteria in Drinking Water, Washington State Department of Health, cited in detail near the end of this article.

[Click to enlarge any image]

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.

This article explains how to interpret the results of water tests for bacteria.

At COLIFORM STANDARDS for DRINKING WATER we explain that virtually all standards, world wide, for water potability require that the level of bacterial contamination be at or below the standards we list here:

Watch out: the presence of bacteria in drinking water from a well has more significance than just the possibility of a bacterial health hazard itself.

Because the most-common source of bacteria in well water is the entry of surface-runoff into the well water supply, any other contaminants (besides bacteria) that are on or near the ground surface may also be contaminating the well water.

That means that bacteria testing is a check for a well-contamination indicator. Before installing a well-water disinfection system such as a UV light or chlorination system it is essential to perform further tests to screen for other contaminants in the bacteria-contamintaed well.

More numerical result details for water tests can be read at

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.

A MFT/MPN test (millipore filter test), instead of giving a present or absent result, actually gives a quantitative measure of bacterial level found in water.

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 3) or if the water is stunningly contaminated with bacteria (say a count of 200 cfus or higher, or maybe "TNTC" which is jargon for "too numerous to count").

Why do we care about the actual count of bacteria in water?

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.

(Synonyms / similar procedures: Membrane Filtration Test or Millipore Filter Test)

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,

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.

More about the Millipore Filtration Methods in Coliform Bacteria Testing

TVC Water Test & Aerobic Colony Count Test Interpretation

TVC tests or Total Viable Count watyer tests estimate the total number of microorganisms in a water sample. THis test counts not just bacteria, but also yeast spores and other mold spores or particles found in the sample.

TVC tests are also called ACCs or Aerobic Colony Count tests because the test detects aerobic microorganisms - a group of bacteria or other orgsanisms that require oxygen to survive.

A TVC or ACC test is incomplete in that it is not attempting to count anaerobic bacteria - species that do not require the presence of air or oxygen, but the test is considered a general measure of water quality.

Details about TVC testing are gven in this nice UK publication:

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 WELL CHLORINATION SHOCKING PROCEDURE (live link is given above and again below)

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)

What if My Water "Fails" the Bacteria Test?

What to do after a seller/owner has "done something" to the water supply and re-testing has been done is discussed

at FAILED WATER TESTS - WHEN to RE-TEST

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.

Bacteria Tests in Water Research


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