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Interpreting Drinking Water Test Results and Correcting Unsatisfactory Water

Finding here: Explanation of common tests for bacteria in water samples; Description of common corrective measures when water test results are unsatisfactory; Warnings for homeowners and especially for homebuyers when certain conditions are encountered, with advice about what to do.

This document combines information from various source 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 20 years experience in the field.

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

Common Tests for Bacteria Level


This standard water test required by many banks involves pouring use of a chemical Defined Substrate Technology (DST) reagent which produces a colour change (or another signal, ie. fluorescence), both indicating and confirming the presence of total coliforms 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 coliforms and E. coli at 1 CFU/100mL of the 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.


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.)

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 a followup.

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 the water supply.

The need to go to an MPN test often explains the additional delay of up to a week beyond the usual time for obtaining water test results.


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 a sample in liquid nutrients which favour 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.

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.)

Correcting Unsatisfactory Water


If a result is not satisfactory the following questions must be addressed:

  • Was the sample properly taken?

(We remove the faucet, aerator which can harbour bacteria, run at least 20 minutes of water, 24 hours if the house is unoccupied, and fill the container without touching its interior.)

  • Has work been done on the water system that was not followed by adequate disinfection when the work was completed? (New pump or pipes installed recently?)
  • Is it a new well that has not been in daily use and/or properly disinfected
  • Has the house been vacant and the water system not in use for an extended period of time
  • Is the well properly sealed
  • Are there cartridge type or R.O. filters in use on the water system? (These can be a source of bacterial contamination.)
  • If the source of the contamination is not due to an ongoing situation, for example, bad groundwater source, then disinfection of the water system will solve the problem.


This is a general description of a common procedure used to sterilize well water and water equipment.

This information is from the Dutchess County Health Department’s environmental laboratory.

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 is constructed so that no new contamination may enter the well following completion of the shock disinfection.

In order to achieve 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.


When working with chlorine, people should be in a well-ventilated place.

The powder or strong liquid should not come in contact with skin or clothing.

Solutions are best handled in wood or crockery containers because metals are corroded by strong chlorine solutions.


If drinking water has been tested and has not passed standards for safe drinking, or any time the building water supply system has been opened for repairs, the well should be disinfected following these procedures and should be re-tested as described herein.

Pour CloroxÔ or hypochlorite granules down into the well.

Some people use swimming pool chlorine tablets which have the advantage that they sink to and sterilize water at the well bottom, and the disadvantage that it takes longer to flush out the chlorine.

Health department officials can give more precise guidance about the amount of disinfectant needed based on the depth of the well.

Common guidelines:

  • Well depth 100′ – 3 cups Clorox or 2 oz. of granules.
  • Well depth 200′ – 6 cups Clorox or 4 oz. of granules.
  • Well depth 300′ – 9 cups Clorox or 6 oz. of granules.
  • Well depth 400′ – 12 cups Clorox or 9 oz. of granules.
  • Well depth 500′ – 1 gallon Clorox or 12 oz. of granules.

Introduce the chlorine solution into the top of the well. Remove the cap at the upper terminal of the well casing and pour the chlorine solution down the inside of the casing.

If the well casing terminates through the floor of a pump house, then the casing is required to have a well seal at the upper terminal [ie. at the top of the casing].

This well seal can be loosened and the chlorine solution introduced into the well at that point.

In a large diameter well [such as a public supply company’s well], the chlorine solution should be poured or splashed around the wall of the well so that all inside surfaces of the well are brought into contact with the strong chlorine solution.

Using a garden hose, spray water down into the well pipe to wash the chlorine solution down to the bottom of the well.

Ten gallons of water should be enough. [More won’t hurt nor risk running the well dry since you’re recycling the well water through the plumbing and back to the well.]

Turn on all cold water household taps until you can smell the Clorox coming out of the faucet farthest from the well.

Turn off the water and do not use it for 8 to 24 hours. Seal the top of the well.

Do not run laundry with this chlorinated water or it may bleach clothing unexpectedly.

At the end of the standing period, operate the pump (run the water) water until you can no longer smell the Clorox.

Do not run Clorox into the septic system – run water outside through an outside faucet or hose.

There should be a hose connection at the bottom of the water tank.

When you no longer smell chlorine at the hose draining the water tank, close off the drain and open all faucets in the house to flush out house piping for fifteen minutes or until you no longer smell or taste chlorine [whichever is longer].

Retest the water after all the Clorox or chlorine is out of the system and the water has been used for 5-7 days (typical health department guideline) or 7-10 days (my suggestion) after the disinfection. The longer you wait until the retest the more valid will be the results.

Acting On Water Test Results

If water passes after disinfection

  • If the contamination was a relatively low bacteria count and was caused by an unsanitary growth in plumbing equipment this treatment can “correct” the problem.
  • If the contamination was a relatively low to modest count and is caused by a persistent source of contamination, this treatment can appear to correct the problem by killing off bacteria. But the problem can recur.
  • If the contamination is from a major source, with a high bacteria count, repeated sterilization and testing of the well may yield inconsistent results, sometimes passing, sometimes not.
  • “Shocking” a well can temporarily make unacceptable water look fine. If no other corrective measures were taken than to “shock” the well, it is appropriate to follow the first acceptable water test with a second and perhaps even a third, spaced a week or more apart.
  • If in a real estate sale transaction the schedule does not permit these tests before closing, a buyer should consider requiring an escrow fund set aside during purchase of the property. This fund will guarantee acceptable water while giving the buyer a reasonable time, perhaps a month, to perform additional testing.

If water fails after disinfection

If the test still is not satisfactory, it is likely that there is a persistent source of contamination.

Common sources of contamination include loose or damaged cap on buried well casings, loose or damaged pitless adapter (where buried water line enters the well casing), a bad plumbing connection in piping between the well and the building, spiders living in the wellhead and possibly insects they’ve dropped into the well, a dead animal in a well which was not properly capped, and, less common, a cracked or damaged well casing.

Damaged well casings can involve a significant expense to repair or replace.

Since water systems can change, drinking water from private wells should be analyzed on a regular basis.

The frequency of testing should be determined by the history of the water quality of the well.

Usually, the property owner/seller is responsible for correcting unacceptable water. Check with your attorney and your contract regarding this matter.

Often sellers have the well “shocked” using the sterilization treatment described above.

In addition to sterilizing the well, an owner/seller might:

  • have an experienced plumber or well service company look for, find, and correct a source of contamination such as a bad plumbing connection at the well (pitless adapter or well casing cap), bad plumbing between well and house (broken or leaky pipe joint), or bad or soiled plumbing components inside the house.
  • install water treatment/sterilization equipment such as a chlorinator/charcoal filter system or an ultra-violet light system. These systems work but require maintenance.

Correcting the source of contamination is naturally preferred.

But if a home is being sold, often schedule pressures do not permit longer investigation by a plumber to find a problem if it is not obvious.

Therefore to provide safe potable (drinkable) water immediately, water treatment may be installed.

If that course of action is followed, we still recommend that the new owner/occupants attempt to find and correct the source rather than having to maintain equipment.

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