Photograph of  this antiquated laundry sink with several unsanitary plumbing violations in view.How to Interpret & Act On Drinking Water Test Results
How to remove water contaminants indicated by a water test

  • WATER TEST INTERPRETATION - CONTENTS: How to interpret water test results - types of water quality & purity tests & what they mean; What to do if your water fails a water test, when to re-test, detecting cheating on water tests; When and how to shock or chlorinate or sterilize a water well; How to get rid of odors in drinking water; How to treat water for hardness, minerals, odors; How to treat water that is contaminated: choices of types of water treatment equipment
  • POST a QUESTION or READ FAQs about how to understand the meaning of water potability test reports & what actions to take based on water test results
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Failed well test:

What to do when the well fails a potability or bacteria test.

Here we explain how to interpret and act on the results of drinking water or well water tests for various types of water contamination. 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.

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Common Water Contamination Tests for Bacteria Levels - Identifying Unsanitary Drinking Water

EPA sketch of common sources of water contaminants

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.

Various treatment methods for contaminated water are reviewed and the pros and cons of each are discussed.

Emergency drinking water needed? Basic water purification procedures that can be used in an emergency such as after a hurricane, flood, or earthquake are described

emergency sources of drinking water are described at  DRINKING WATER EMERGENCY SOURCES.

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If community or private wells are back in operating and providing water, do not assume that the water is sanitary and ok to drink until responsible authorities have said so. Even then, local water pipes in a building may be unsanitary and additional cleaning or disinfection may be needed.


See WATER TESTS for CONTAMINANTS for advice on using a private well for drinking water.

This document combines information from various sources including from the Dutchess County NY health department, from Smith Laboratory in Hyde Park NY (914-229-6536), water test developer/suppliers, and other independent sources. Pending review corrections by these sources, the author is responsible for the content which includes opinion and advice based on more than 40 years experience in the field.

Because water quality can have major effect on personal health, home owners and home buyers, & ASHI home inspectors should consult with their local health department before performing tests or taking corrective actions regarding water quality.

PA - Coliform Bacteria Test for Contamination in Water: Presence/Absence Test

This standard water test required by many banks, involves pouring use of a chemical Defined Substrate Technology (DST) reagent which produces a color change (or another signal, i.e. fluorescence), both indicating and confirming the presence of total coliform and E. Coli in a sample of drinking water. The test indicates either the presence or absence of this bacteria.

This test, used by most water test companies for real estate transactions does not produce a bacteria colony count. We order this procedure, or the lab will elect this procedure when the water sample is turbid (contains sediment or other material which precludes alternative test procedures).

The test is sensitive and specific for the detection of total coliform and E. coli at 1 CFU/100mL of sample in water samples with as many as 20,000 heterotrophic bacteria present per ml. This means that this test for coliform bacteria is not obscured by the presence of other bacteria in the water.

P/A Test results: if the test did not find an indication of a coliform bacteria problem the result will say "Less than 1" or if the lab reports carelessly, "Zero".

Accurate lab test reporting would be to say "below the limits of detection" of a given test, and then to specify the limits of detection used. The acceptable level of bacteria in water varies by jurisdiction. For example in some Canadian provinces a higher level of 10 CFU/100mL is permitted. These very slightly higher numbers are allowed because the opinion of health experts is that there is no measurable risk at those levels.

See TOTAL COLIFORM TESTING for details about coliform bacteria test procedures and standards.

See WATER TEST CHOICES & WATER TEST FEES for choices of water tests and our advice about choosing among them.

M.F.T. - Coliform Bacteria Test for Contamination in Water: Membrane Filter Technique

This standard water "test" required by many banks, involves pouring 100 ml of water through a membrane filter. The bacteria present in the water are trapped on the surface of the filter. After an incubation period of 24 hours the coliform colonies present are counted. The count may not exceed 0 per 100 ml according to recently updated New York State Drinking Water Regulations. (MFT=Multiple tube Fermentation Test.)

MFT Test results: If the lab finds apparent unacceptable results with this general test for the presence of any bacteria, the revised NY State procedure requires the lab to measure for E-coli, by performing the MPN procedure as follow up.

This is because the presence of non-coliform bacteria present in the water can obscure the test and prevent counting E-coli. The NY State Sanitary Code has no standard for total bacteria count in water supply. The need to go to an MPN test often explains additional delay of up to a week beyond usual time for obtaining water test results.

MPN - Coliform Bacteria Test for Contamination in Water: MPN testing

This test is used when the water cannot be filtered due to turbidity, high iron, large amounts of sediment, or high non-coliform bacteria count. This test involves incubation of measured volumes of sample in liquid nutrients which favor the growth of any coliform bacteria present. This is a statistical method of testing based on the number of positive tubes of media after 48 hours of incubation and 48 additional hours of confirmation.

MPN Test results: As of modified NY State procedures starting in 1991 a count of 0 indicates no coliform and water of satisfactory quality. (MPN=MilliPore Nutrient test.)

Interpreting Other Water Test Results

Examples of allowable limits (subject to change) for other tests are listed at Drinking Water Contaminant Levels - New York State Maximum Allowed or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

A Quick EPA Guide to Understanding Your Water Test Results

Photograph of  a loose, unsanitary well plumbing system exposed to surface water runoffr  © DJ FriedmanHave your well water tested for any possible contaminants in your area. Use a state-approved testing lab. (See below for sources of approved laboratories.) Do not be surprised if a lot of substances are found and reported to you.

The amount of risk from a drinking water contaminant depends on the specific substance and the amount in the water. The health of the person also matters.

Some contaminant cause immediate and severe effects. It may take only one bacterium or virus to make a weak person sick.

Another person may not be affected. For very young children, taking in high levels of nitrate over a relatively short period of time can be very dangerous.

Many other contaminants pose a long-term or chronic threat to your health - a little bit consumed regularly over a long time could cause health problems such as trouble having children and other effects.

EPA drinking water rules for public water systems aim to protect people from both short and long term health hazards. The amounts of contaminants allowed are based on protecting people over a lifetime of drinking water. Public water systems are required to test their water regularly before delivery.

They also treat it so that it meets drinking water standards, notify customers if water does not meet standards and provide annual water quality reports.

Compare your well's test results to federal and state drinking water standards. (You can find these standards at or call the Safe Drinking Water Hotline 800-426-4791.) In some cases, the laboratory will give a very helpful explanation. But you may have to rely on other experts to aid you in understanding the results.

also see

This article weries on water contaminants describes types of activities in your area that can create threats to your water supply. It also describes problems to look for and offers maintenance suggestions. Sources for more information and help are also listed. [Editing for clarity by DF are marked by brackets or italics] Initial Source of a portion of the above text: EPA 816-K-02-003 January 2002

Article series contents


Continue reading at WATER QUALITY TEST CHOICES & WATER TEST FEES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see WATER CONTAMINANT LEVELS & LIMITS for allowable water contaminant limits

Or see FAILED WATER TESTS - WHAT TO DO - water testing procedural errors can foul up water test results


Or see WATER TREATMENT EQUIPMENT CHOICES for treating contaminated water

Or see WELL CONSTRUCTION & MAINTENANCE for methods of building a safe water well


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