Laboratory > Bacteria > PRESENCE/ABSENCE & MPN

Presence/Absence & MPN

HEALTH SIGNIFICANCE OF BACTERIOLOGICAL TESTS OF POTABLE WATER SUPPLIES

Most of the coliform bacteria are not themselves hazardous to human health. They are used as "indicator bacteria" to evaluate the disease producing potential of a water source. That is, if coliform bacteria can find their way into a water supply, then disease bacteria could find their way into the supply by the same route.

"Total Coliform Bacteria" refers to the entire coliform group. These bacteria are abundant in soil, decaying vegetation, animal fecal matter, and raw surface water. They are not normally present in deep groundwater and properly treated surface water. "Fecal Coliform Bacteria" refers to a sub-group of coliform bacteria many of which are found in the intestines of warm-blooded animals, including humans. E. coli is the primary fecal coliform of interest.  The presence of these bacteria in a water supply should be taken very seriously as they may well be accompanied by disease bacteria that are transmitted in animal feces.

The absence of all coliform bacteria is a clear indication that a water supply is free from bacterial contamination at the point in time when the sample was collected.  This is the only result that is considered acceptable by health departments, lending institutions, building departments, etc.  as demonstrating the biological safety of a water source.

The presence of coliform bacteria does two things to the evaluation of a water supply:

    * It removes the certainty that the supply is completely free of health hazard.
    * It shows that some outside bacterial contamination is getting into the water. This contamination may or may not constitute a health hazard depending on its sources. However, caution dictates that the presence of coliform bacteria be considered indicative of potential health hazard.

Some of the sources of coliform bacteria contamination are:

    * Surface water runoff or groundwater from near the surface (especially common when the well is located near a stream or pond) which enters a well, spring, or storage tank through cracks and unsealed openings, around casings (especially where there is a breach in the integrity of the clay, concrete, or cement seal), etc.
    * Vegetation that accumulates and decays in the well, spring, or storage tank.
    * Droppings from any warm-blooded animals (birds, mice, squirrels, etc.) that gain access to the water supply.
    * Raw sewage that makes its way into the water.
    * New or recently serviced water systems usually harbor coliform bacteria until they have been pumped out or killed by sterilization.
    * Unsterilized or improperly sterilized sampling containers.
    * The sampling point itself might harbor coliform bacteria. This is especially true of hoses, aerator screens, long-arm faucets, and infrequently used taps.

If your result indicates contamination, steps should be taken to determine the cause.

    * Determine that all septic tanks, leach fields, or other sources of sewage contamination are more than 100 feet away from the water source.
    * Examine the well, spring, storage tank, etc. to determine if it is adequately sealed to prevent entry of surface water, leaves, rodents, insects, etc.  Do not allow surface water to form puddles in the vicinity of the water source.

Correct any deficiencies you found. Disinfect the system according to the accompanying instructions and resample for coliform bacteria analysis to ascertain if the corrections were successful. Standard health department practice requires that two successive repeat samples showing no coliform bacteria present are necessary to show that the problem has been corrected. This is because it can take some time for bacteria to make their way back into a water system after they have been eliminated by disinfections.  It is best to wait at least several days before taking the repeat samples

HOW OFTEN SHOULD A WELL BE TESTED FOR BACTERIA?

As a safety measure, water systems serving the public are required to test regularly at frequencies ranging from quarterly for small systems to dozens of samples weekly for large systems. If you do not wish to check your water on a routine basis, we recommend that it be checked at least once during the rainy season, when contamination is most likely to occur. Also check your water if one of the following occurs:

    * A change in the quality of the water (color, odor, dirt, etc.).
    * Construction or other disturbance near the well.
    * Repairs or maintenance of the water system (disinfect before sampling).
    * Repeated gastrointestinal illness noted in users of the well.

 

DISINFECTION OF WATER DISTRIBUTION SYSTEMS

WHAT IS DISINFECTION?

            Disinfection is the process by which chlorine is added to a water system in large amounts to kill bacteria and to burn up organic materials which serve as food for bacteria.

WHEN DOES A WATER SYSTEM NEED DISINFECTION?

    * After construction of a new water supply.
    * After making repairs to an existing water supply.
    * After a water system has been unused for several months.
    * When the results of a coliform bacteria test indicate contamination.
    * Sometimes disinfection will give relief from iron bacteria and from sulfide odors caused by sulfate-reducing bacteria often associated with iron bacteria.

HOW IS A WATER SYSTEM DISINFECTED?

    * Pour household bleach (Clorox, Purex, etc.) into the well, spring, or storage tank, and mix thoroughly if possible.

        For most water systems, a gallon of bleach is more than adequate. For systems with large storage tanks, use an extra gallon for each 1,000 gallons of storage capacity. Use bleach with no additives such as scents. Chlorinated well water can be circulated by pumping it through a hose and discharging it back into the well.  This serves to clean parts of the well above the water line, as well as to aid in mixing.

    * Pump the chlorinated water through the entire system.

        Do this by opening each tap until you can smell the chlorine coming through. Shut off the tap and go on to the next one.  Leaving a line un-chlorinated can cause recontamination of a system, so be sure to open every tap, including hot water and irrigation taps.

    * Allow the chlorinated water to stand at least overnight.
    * Flush out the system until the chlorine is gone.

        This can take several hours and it can be difficult to determine if all of the chlorine has been removed. The best results are obtained with a chlorine test kit, like a swimming pool test kit (chlorine testing materials are also available at the lab for a nominal charge). As far as possible, do not flush highly chlorinated water into a septic system, as the chlorine may have an adverse effect on the bacteria that make septic systems work.  Do not use highly chlorinated water for irrigation purposes or for drinking.  Once the odor of chlorine has become very faint there is no harm in using the water as usual.

    * Take a sample in a laboratory-sterilized bottle for coliform bacteria analysis.

        Do this only after all chlorine has been flushed from the system or the sample will be invalid. It is highly recommended to wait a few days after all the chlorine has been pumped out of the system before you collect a sample.  Health department guidelines require that two successive repeat samples be coliform-free before the water source can be considered safe. This is because it often takes some time for bacterial contamination to reappear in a disinfected water system. Sampling again during the rainy season, when contamination is most likely to occur, is also recommended.

    * If test results continue to show contamination, you have several choices:
          o Look again for possible sources of contamination, fix them, and re-sterilize the system as above. Occasionally repeated sterilizations do remedy the problem if bacteria are protected from the chlorine by large amounts of organic matter, like wood, dead animals, bacterial slimes, algae, etc.
          o Install an approved chlorination system to continually disinfect the water. These require meticulous maintenance and may not be approved by your local health department (check with them for current regulations).
          o Use the contaminated supply for irrigation only and hook the domestic system up to a coliform-free water supply, such as a new well or a public water system.
          o Sometimes surface water contamination can be eliminated in a well by lining the casing and resealing it.  Check with a well drilling or service company for further information.

SAMPLING INSTRUCTIONS FOR COLIFORM BACTERIA TESTING

ABOUT COLIFORM TESTING:

Many human pathogens (disease-causing organisms) can be transmitted by water, especially when it is contaminated by human sewage or other animal fecal matter.  However, even when they are present in a water supply, most pathogens occur in such small numbers that testing for them is difficult and often inconclusive.  Instead, the Coliform Bacteria test is used to ascertain if a water supply is adequately protected from potential contamination by pathogens.  This is accomplished by checking for the presence of bacteria from the coliform group.  These "indicator bacteria" are abundant in soil, surface water, and animal feces (including human sewage), but are not normally found in ground water.  This analysis can also be used to check the effectiveness of disinfection systems (chlorinators, Ultra-violet, etc).

NEW WELLS, REPAIRS, POWER OUTAGES:

Coliform bacteria are often temporarily introduced into a well during the well and water system construction process, any time the well or water system is opened for repairs, or when a power outage results in loss of pressure to the water system.  For this reason it is a good practice to disinfect the well and water system with chlorine bleach.  Disinfection instructions are available at the lab upon request.

SPRINGS:

Interpretation of test results for springs, especially unprotected springs, is difficult, since they often contain coliform bacteria naturally.  The MPN test can, however, give you some idea of the extent of contamination from vegetation and animal sources.

OPTIONS:

       1. Standard Presence-Absence.  For routine monitoring of drinking water
       2. Most Probable Number (MPN): Gives an estimate of the number of bacteria in the range often found in contaminated wells (1.0 - 200.5 per 100 mL).
       3. Swim Water MPN:Gives an estimate of the number of bacteria in the range often found in surface water (or very contaminated wells).  (2 - 1,600 per 100 mL -- higher ranges are available; check with lab for prices).
       4. Suspected Sewage: For assessing whether a water source (spring, creek, etc.) is contaminated with sewage.  Fecal coliform analysis, up to 160,000 per 100 mL).

IMPORTANT POINTS: (See below for complete instructions.)

       1. All chlorine must be gone from samples. It is highly recommended to wait a few days after chlorine is pumped out of a well system before collecting a sample.
       2. Maintain sterility of the bottle and sample at all times.
       3. Fill bottle to above 100 mL fill line. Fill out form.
       4. Drinking water testing must begin within 30 hours of collection (surface water within 8 hours, wastewater within 6 hours) to be legally valid. Keep samples cool.
       5. If possible, please deliver samples before 2:00 PM Monday - Thursday, and before 12:00 noon on Friday. Payment is expected when the sample is submitted.

HOW TO TEST YOUR WATER FOR BACTERIA

1. CHLORINE:

      a)       Well systems are often disinfected with chlorine after installation or repair work, or to clear up contamination.  Unless your water system is normally chlorinated (see 1b below), be certain that all traces of chlorine are gone before you sample.  Use a chlorine test kit, such as a pool test kit.  (Chlorine testing materials are also available from the lab at a nominal charge.)  If chlorine remains in the water, the test results will be invalid. The sample will be screened at the laboratory and rejected if chlorine is present. It is highly recommended that you wait a few days after pumping out the chlorine before you collect a sample to avoid false negative results.

      b)       If the water system is normally chlorinated (city water, chlorinator, etc.), the sample MUST be collected in a special sampling bottle that contains a chemical to deactivate the chlorine.  These bottles have white labels (bottles without the chemical have orange labels).

2. STERILITY: The sample bottle is sterile and will stay that way until opened.  Be careful not to touch inside the cap or bottle with anything but the water being sampled.  Do not rinse out the bottle and do not open it until you are ready to fill it.  It is normal for there to be a small amount of liquid or white crystals in the bottle and for the bottle to be slightly discolored.

3. SAMPLING POINT:  The cold water faucet you choose should be clean and free of leaks or drips, should face downwards, and should be at least 18” above the ground.  Do not use a "swing faucet" (like most kitchen faucets). Remove any attachments such as hoses or faucet aerators.  Spraying the tap with a 10% solution of bleach can help prevent false results due to contamination from the tap itself.

4. SAMPLE COLLECTION:

    a)       Run the water at full force for at least two minutes.  (If the well has not been used for more than a day, run the water longer so that you will be sampling fresh water.)  Reduce the flow to one-third of full force and run again for about two minutes.

    b)         Open the bottle and fill it to ABOVE the "fill line" or 100 mL mark, leaving about one half inch of head space.  Be sure the bottle is held so that no water touching your hands enters the bottle and nothing other than the water being sampled touches the inside of the bottle or cap.  Put the cap on immediately.

5. SAMPLING SLIP: Fill out the sampling slip as completely as you can.  If the sample is for real estate or legal purposes, be certain that the street address and/or assessor's parcel number are recorded on the form ("Sampling Point").  Write the bottle number (found in the upper right corner of the sampling slip) on the bottle label.

6. Deliver the sample as soon as possible to the laboratory.  A DRINKING WATER SAMPLE MORE THAN 30 HOURS OLD IS LEGALLY INVALID (8 hr. for surface water, 6 hr. for wastewater). If you cannot deliver the sample to the lab within one hour, keep it in a refrigerator or ice chest.  Do not expose the sample to direct sunlight for extended periods of time or allow it to get hot.

7. Since the test takes 24-28 hours, we ask that you do not submit samples after 2:00 PM Monday - Thursday, or after 12:00 noon on Fridays if at all possible.  Samples received after 12:00 on Fridays will be run by the classic 2 – 4 day method.  Results may not be available until Tuesday.  If results MUST be ready by Monday, please request the expedited test ($10 extra).  Thank you.