Bacteriological Contamination of Drinking Water

Wisconsin's groundwater is normally free from bacteria, which are filtered out of surface water as it percolates downward through the soil. In some localities groundwater can become bacteriologically contaminated because of insufficient filtration between the land surface and the groundwater. Surface water can also enter the ground through rock outcroppings, sink holes, quarries and abandoned wells.

For a free brochure containing the information on this page, contact the nearest DNR office and request publication number WS-003.

Table of contents

1. How can my well become contaminated with disease causing bacteria?
2. When should I test my well for bacteriological contamination?
3. Where can I obtain a sampling kit for bacteriological testing?
4. What do the results of this test tell me?
5. What should I do if my sample is analysed unsafe?
6. How can I locate possible sources of pollution?
7. How can I disinfect my water system?

How can my well become contaminated?

Your well may become bacteriologically contaminated in one or more of the following ways:

1. The well may not have a vermin-proof cap, allowing insects that carry bacteria to enter the well.
2. There is a source of contamination, such as a septic system, too close to the well or the well casing isn't deep enough to assure that recharge water receives sufficient filtration to remove bacteria.
3. The well may be constructed using poor sanitary practices. New wells often show contamination because:
• The drillhole was contaminated by dirty tools, pipe or drilling water
• New piping, pump or pressure system components were not disinfected prior to use, assembly or installation

Note The state well code requires disinfection of new wells, pumping equipment and water systems prior to use.

4. Contaminated surface water or groundwater can enter an improperly constructed well in the following ways:
• Dug wells walled with boards, brick, stone or tile sections permit unfiltered surface water and near-surface waters to seep into the well through cracks in the wall.
• Casing improperly sealed into the rock and/or unconsolidated geological formation may permit surface water or contaminated groundwater to move vertically downward, contaminating good aquifers.
• The well casing doesn't extend far enough above the ground allowing surface water to enter the well or because a hand pump base doesn't have a watertight seal where it attaches to the casing.
• The well top ends in a nonconforming well pit subject to flooding or seepage of contaminated groundwater.
• Old well casings may rust through, leaving holes near the ground surface where surface water or near-surface waters can seep in and contaminate deeper groundwater.
• The aquifer supplying the well is fractured rock, which has poor water-filtering properties.

When should I test my well for bacteriological contamination?

Construction of new wells and pump work involving entry into a well, requires testing the well for bacteriological quality. It is advisable to recheck wells annually or after modifying them in any way. Wells should also be tested when any change in taste, odor or appearance is noticed.

Where can I obtain a sampling kit for bacteriological testing?

A test kit (including sampling instructions) may be obtained for a fee from the Microbiology Unit, State Laboratory of Hygiene at 1.800.442.4618 or from any private laboratories certified for bacteriological testing of water. Make sure to follow all instructions carefully.

What do the results of this test tell me?

Bacteriological examinations are made to determine the suitability of water for drinking and food preparation uses. When a sample is reported "safe bacteriologically," it means that coliform bacteria (a group of indicator bacteria) were not found in the sample. If the sample was taken according to directions enclosed with the water sampling kit, you can be reasonably sure that the water is suitable for drinking and general domestic use.

When a sample is reported "unsafe bacteriologically," it means that coliform bacteria were found in your sample. Coliform bacteria are found in the feces of humans and other animals as well as in surface water. Their presence in groundwater (wells) shows that unfiltered or poorly-filtered surface water or near-surface waters have found their way into the groundwater or entered through an opening in, around or at the top of the well casing.

Presence of coliform bacteria indicates that the water is potentially dangerous and should not be consumed unless boiled.

If additional help is needed in interpreting the lab results, contact the water Microbiology Unit, State Laboratory of Hygiene at 1.800.442.4618 or your local DNR statewide office.

What should I do if a sample results in an unsafe test?

1. Resample. A second sample must be taken to confirm your first unsafe result. Be sure to use the proper sampling procedure in taking the sample because it will help you determine if your original unsafe sample was due to human error in sampling.
2. If the second sample results are unsafe, do not consume the water unless it is boiled at a rolling boil for at least 5 minutes.
3. Replace any old, poorly-sealed well caps with a properly-fitting vermin-proof cap.
4. If you can find no obvious sources of well contamination, your water system should be disinfected by a qualified individual (well driller, pump installer or owner).
5. If disinfection is unsuccessful or there is no obvious cause of the well contamination, contact your local well driller, pump installer or the nearest DNR statewide office for assistance.

For assistance with well or pump work or chlorination, contact a licensed well driller or pump installer (see telephone yellow pages under Well Drilling, Pumps or Water Supply Systems). For further assistance, you may call DNR Regional Staff, County Sanitarians or Health Departments.

How can I locate possible sources of pollution?

Before you attempt to locate the source of contamination for an unsafe well, be certain that the instructions for collecting water samples were followed closely. If they were not or you're uncertain, another sample should be collected following instructions closely.

If sampling error can be ruled out, the surrounding area should be inspected for possible pollution sources. These would include;

• Openings at the top of a well
• Old, rusty or damaged well casings
• Improper well casing installation
• Faulty pump installation
• Close proximity of a well to septic tanks, tile fields, sewers, kitchen sinks, drains, privies, barnyards, animal feed lots, abandoned wells, rock outcroppings, sink holes and quarries.

If any of the above are found to cause contamination problems, the proper changes or repairs must be made. Qualified well drillers, pump installers, DNR drinking water specialists or county sanitarians can assist you in making these observations and recommending improvements.

How can I disinfect my water system?

New wells and wells that are bacteriologically contaminated should be disinfected according to the following steps:

1. Determine the amount of chlorine solution (prepared in step 2) needed to displace the entire volume of water standing in the well according to the following:
• 2 " casing diameter: prepare two gallons of chlorine solution per ten feet of well depth
• 4" casing diameter: prepare seven gallons of chlorine solution per ten feet of well depth
• 6" casing diameter: prepare 15 gallons of chlorine solution per ten feet of well depth
• 8" casing diameter: prepare 26 gallons of chlorine solution per ten feet of well depth
2. To prepare the chlorine solution, mix one unit volume of household laundry bleach with 100 units of water. Be sure to use pure bleach without additives, like "fresh scent". For example, mix one gallon of bleach with 100 gallons of water. Prepare enough solution to meet or exceed the total volume of your well. Mixing can be done 25 gallons at a time in a new garbage can. Note: Never use (even new) garbage cans to store drinking water.
3. Remove the cap from the well and pour the entire bleach and water mixture into the well in one continuous, fast pour.
4. Rinse down the sides of the well casing with a garden hose for 5-10 minutes. Make sure the hose is connected to the system being chlorinated. This procedure circulates the chlorine solution throughout the water system to insure total disinfection.
5. To disinfect your plumbing system, you can turn on each of your water taps until the bleach smell is just detected and then turn them off. You should turn off the heating element in your water heater to save energy during this process. The water softener should be bypassed after allowing a low concentration of chlorine to pass through it.
6. Let the chlorine solution remain in the system for at least 12 hours, but preferably 24.
7. Pump all of the chlorine solution out of the well by attaching a garden hose and running the water to an area where the chlorine will do no damage. Remember that chlorine can kill grass and fish. Do not dump the spent chlorine solution into your private septic system and check with your municipality before dumping into any public sewer system. Pump until you can no longer detect the chlorine smell. If necessary, follow this procedure for your plumbing system by running each of the cold water taps.
8. The well should be resampled only after all traces of chlorine have been flushed from the system.