This article explains how to make sense out of a water test report or bacteria test: is the water safe to drink? What do bacteria levels mean? What do we do about bacteria contamination in drinking water?

The author has worked as a professional home inspector, environmental test consultant, has been a member of AHIA, the American Industrial Hygiene Association, and is a member of ASHRAE, American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

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.

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.

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

As 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 our inspection client demonstrating 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:

• T coli & E Coli:
  
  Both total coliform and E-coli in drinking water must be at 0 per 100 ml of water or in some standards, < 1 coliform bacteria per 100 ml.
  
  Your water test report might include reporting of both Tcoli or total coliform count and Escherichia coli / E. coli. E. coli present in water usually indicates sewage contamination while other coliform bacteria in water can be from other surfaces, even simple groundwater runoff.
• HPC counts:
  
  Heterotrophic plat counts (HPC tests) in drinking water (potable water) must be < 500 CFU/ml, that's less than 500 colony forming units per milliter of water.

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

• WATER CONTAMINANT LEVELS & LIMITS - maximum allowable contaminants in drinking water

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,

• 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.

More about the Millipore Filtration Methods in Coliform Bacteria Testing

• SC DEHC, COUNTING RULES FOR MEMBRANE FILTRATION METHODS FOR COLIFORM BACTERIA [PDF] (2010) South Caroliina Department of Health & Environmental Control, Office of Environmental Laboratory Certification at 803-896-0970. retrieved 2023/10/04, original source: scdhec.gov/sites/default/files/docs/Environment/docs/MF Counting Rules 1-15-2010.pdf
  

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:

• Greaves, Jon, What is TVC TOTAL VIABLE COUNT in water testing and why is it important? [PDF] (2002), WCS Group, Unit 17 Wheatstone Court, Davy Way, Quedgeley, Gloucester, GL2 2AQ UK, Email: info@wcs-group.co.uk Tel: +44 (0)1454 299 310 - retrieved 2023/10/04, original source: https://www.wcs-group.co.uk/wcs-blog/total-viable-count-in-water-testing

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

• AB, Understanding Your Drinking Water Bacteria Testing Results, MyHealth Alberta, Canada, retrieved 2023/10/04, original source: myhealth.alberta.ca/Alberta/Pages/Understanding-drinking-water-bacteria-testing-results.aspx
• Handbook of Disinfectants and Antiseptics, Joseph M. Ascenzi (Editor), CRC, 1995, ISBN-10: 0824795245 ISBN-13: 978-0824795245 "The evaluation of chemical germicides predates the golden age of microbiology..." -
  
• Potable Aqua® emergency drinking water germicidal tablets are produced by the Wisconsin Pharmacal Co., Jackson WI 53037. 800-558-6614 pharmacalway.com
• Principles and Practice of Disinfection, Preservation and Sterilization (Hardcover)
  by A. D. Russell (Editor), W. B. Hugo (Editor), G. A. J. Ayliffe (Editor), Blackwell Science, 2004. ISBN-10: 1405101997, ISBN-13: 978-1405101998.
• Stein, Matthew, When Technology Fails, Chelsea Green Publisher, 2008,493 pages. ISBN-10: 1933392452 ISBN-13: 978-1933392455, "... how to find and sterilize water in the face of utility failure, as well as practical information for dealing with water-quality issues even when the public tap water is still flowing". Mr. Stein's website is www.whentechfails.com/
  
• U.S. Army Field Manual 21-10, Field Hygiene and Sanitation, 1988, web search 07/02/2010, original source: http://www.enlisted.info/field-manuals/fm-21-10-field-hygiene-and-sanitation.shtml
  
  The purpose of this manual is to assist individual soldiers, unit commanders, leaders and field sanitation teams in preventing disease and environmental injuries. The manual provides information on preventive medicine measures (PMM) to the individual soldier as well as essential information for the unit commander, unit leaders, and the unit field sanitation team on applying unit level PMM.
• U.S. CDC, Guidelines for Environmental Infection Control in Health-Care Facilities [PDF] (2003 - retrieved 2023/10/04, original source: https://www.cdc.gov/infectioncontrol/guidelines/environmental/appendix/water.html#:
  ~:text=Heterotrophic plate count evels in,general decrease in ater quality.
• US EPA, HETEROTROPHIC PLATE COUNT (HPC) WATER TESTING (STANDARD PLATE COUNT) [PDF] U.S. Environmental Protection Agency, retrieved 2023/10/04, original source: downloads.regulations.gov/EPA-HQ-OW-2014-0408-0021/content.pdf
  
  Excerpts:
  The heterotrophic plate count (HPC), formerly known as the standard plate count, is a procedure for estimating the number of live culturable heterotrophic bacteria in water and measuring changes during water treatment and distribution or in swimming pools.
  
  Colonies may arise from pairs, chains, clusters, or single cells, all of which are included in the term “colony-forming units” (CFU). The final count also depends on interaction among the developing colonies.
  
  Bacteria testing methods described in detail n this document include the pour plate method, the spred plate methods, the membrane filter method, and the enzyme substrate method for hetertrophic bacteria.
• US EPA, REVISED TOTAL COLIFORM RULE: A QUICK REFERENCE GUIDE [PDF] (2013) U.S. Environmental Protection Agency - retrieved 2023/10/04, original source: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100K9MP.txt
• USGS, BACTERIA & E. Coli IN WATER [PDF] (2018) USGS Water Science School, - retrieved 2023/10/04, original source: https://www.usgs.gov/special-topics/water-science-school/science/bacteria-and-e-coli-water
  Excerpts:
  
  Total coliform
  Total coliforms are gram-negative, aerobic or faculative anaerobic, nonspore forming rods.
  
  These bacteria were originally believed to indicate the presence of fecal contamination, however total coliforms have been found to be widely distributed in nature and not always associated with the gastrointestinal tract of warm blooded animals.
  
  The number of total coliform bacteria in the environment is still widely used as an indicator for potable water in the U.S.
  Fecal coliform
  Fecal coliform bacteria are a subgroup of coliform bacteria that were used to establish the first microbial water quality criteria.
  
  The ability to grow at an elevated temperature (44.5 degrees Celsius) separate this bacteria from the total coliforms and make it a more accurate indicator of fecal contamination by warm-blooded animals.
  
  Fecal- coliform bacteria are detected by counting the dark-blue to blue-grey colonies that grow on a 0.65 micron filters placed on mFC agar incubated in a 44.5º C oven for 22-24 hours.
  
  The presence of fecal coliforms in water indicates that fecal contamination of the water by a warm-blooded animal has occurred, however, recent studies have found no statistical relationship between fecal coliform concentrations and swimmer-associated sickness. E. coli
  Escherichia coli (E. coli) is a rod-shaped bacteria commonly found in the gastrointestinal tract and feces of warm-blooded animals. It is a member of the fecal coliform group of bacteria and is distinguished by its inability to break down urease. 
  
  E. coli numbers in freshwater are determined by counting the number of yellow and yellow brown colonies growing on a 0.45 micron filter placed on m-TEC media and incubated at 35.0º C for 22-24 hours. The addition of urea substrate confirms that colonies are E. coli.
  
  This bacteria is a preferred indicator for freshwater recreation and its presence provides direct evidence of fecal contamination from warm-blooded animals.
  
  Although usually harmless, E. coli can cause illnesses such as meningitis, septicemia, urinary tract, and intestinal infections. A recently discovered strain of E. coli (E. coli 0157:H7) can cause severe disease and may be fatal in small children and the elderly.

• WA, COLIFORM BACTERIA in DRINKING WATER [PDF] Washington State Department of Health, retrieved 2023/10/04, original source: doh.wa.gov/community-and-environment/drinking-water/contaminants/coliform

...

Continue reading at WATER CONTAMINANT LEVELS & LIMITS - allowable levels of contaminants in drinking water, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CHEATING ON WATER TESTS - how people accidentally or on purpose can obtain incorrect water test results.
• COLIFORM STANDARDS for DRINKING WATER
• CONTAMINANT CLASSES, in WATER
• DUG WELL SANITATION PROTECTION
• FAILED WATER TESTS - WHEN to RE-TEST
• LEAD in WATER, ACTION LEVELS - Lead Pipe Problems/Advice, Lead Testing & Correcting Contamination from Lead Water Supply Lines/Entry Mains
• WATER CONTAMINANT LEVELS & LIMITS - maximum allowable contaminants in drinking water
• WATER TEST PROCEDURAL ERRORS
• WATER TESTS for CONTAMINANTS - home
• WATER TEST INTERPRETATION - Interpreting Drinking Water Test Results and Correcting Unsatisfactory Wat
• WATER TESTING ADVICE - Drinking Water Testing Advice for home buyers home owners home inspectors
• WATER QUALITY TEST CHOICES & WATER TEST FEES - home
• WATER TESTING ADVICE - for home buyers and building owners:
• WELL CHLORINATION & SHOCKING - Procedure for Shocking a Well to (temporarily or maybe longer) "Correct" Bacterial Contamination

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BACTERIA LEVELS in WATER, INTERPRETATION at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to WATER TREATMENT SYSTEMS

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