Interpreting Your Laboratory Results


Bacteriological | Nitrate | Fluoride


Old Sample


If your laboratory report indicates Old Sample, your water sample was over 30 hours old for compliance samples or 48 hours old for private samples when received in the laboratory. These samples are not tested because they may give inaccurate results. Check delivery/mail schedules from your community to Madison and resubmit a fresh sample.


Bacteriological Interpretation


Total Coliform Absent (SAFE)


If your laboratory results indicate Total Coliform Absent (SAFE), no coliform bacteria were found in your water sample. If you collected the sample according to directions supplied with the kit, you can be reasonably sure your water is safe for drinking or general domestic use, from a bacteriological standpoint. Retest a “safe” well annually—or at any time it has been repaired or modified. Also test whenever there is a change in appearance, taste, odor or flow.


Total Coliform Present (UNSAFE)


If your laboratory results indicate Total Coliform Present (UNSAFE), coliform bacteria were found in the water sample. Total Coliform bacteria are found in human and animal feces, as well as in surface water. Their presence in wells indicates unfiltered or poorly filtered surface 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. This water is a potential health hazard.


How Wells Become Contaminated


Wells of insufficient depth or substandard construction are more susceptible to bacteriological contamination. This is particularly true of dug wells that are walled up with boards, brick, stone or tile sections. These linings let unfiltered surface water and near-surface water seep in through cracks. Properly constructed wells are usually free from bacterological contamination because they tend to seal off near-surface and surface waters from the well. However, if they are contaminated, one of the following reasons is likely the cause:

  • The casing is not properly sealed into the rock formation.
  • The casing is not terminated far enough above the ground.
  • If the well is equipped with a hand pump, the pump has not been mounted watertight on the casing, permitting surface water to enter the well at the top.
  • The well terminates in a nonconforming pit, which may be subject to flooding or seepage of groundwater.
  • In old wells, the casing may have rusted through, leaving holes near the ground surface through which polluted surface water can enter.
  • Rock outcroppings, sinkholes, quarries or abandoned wells in the production well area may allow surface water to contaminate the groundwater aquifer supplying the well.
  • New wells often show contamination because the drill hole becomes contaminated through dirty tools, pipe and drilling water.
  • New piping, pump or pressure system components may also contaminate a well if they are dirty and not disinfected prior to use, assembly or installation. Therefore, new wells, pumping equipment and water systems should be disinfected prior to use. The state code requires such disinfection.


Locating the Contamination Source


Before attempting to locate the contamination source of an unsafe well, first be certain that you closely followed the instructions for collecting water samples. If not, collect another sample following instructions closely. When sampling error has been ruled out, the well and the surrounding area should be inspected for possible pollution sources. These include:

  • Openings at the top of the well
  • Old, rusty, or damaged casings
  • Improper casing installation
  • Faulty pump installation
  • Close proximity of the well to septic tanks, tile fields, sewers, sink drains, privies, barnyards, feedlots, abandoned wells, rock outcroppings, sinkholes and quarries

If any of the above are found to cause contamination problems, you must make changes or repairs to ensure safe drinking water. Licensed well drillers, pump installers, DNR officials or county sanitarians can assist you in making observations and recommend improvements.


Disinfecting the Well and Water System


Once an inspection has determined that your water system is free from any sources of apparent contamination, you should disinfect the well. Although there are many ways to properly disinfect a well, the following technique has proven to be effective in most cases:

  • Mix one gallon of household laundry bleach with 100 gallons of water. If your well is more than 150 feet deep, mix two gallons of bleach with 200 gallons of water. If you do not have a container for mixing the solution all at once, you can mix 25 gallons at a time in a clean plastic garbage can.
  • Remove the cap from the well and pour the entire bleach and water solution into the well.
  • Rinse down the sides of well casing with a garden hose for 5 to 10 minutes. The rinse water should be from a hose bib on the water system being disinfected. This procedure circulates the bleach through the water system to insure better disinfection.
  • If you wish to disinfect your plumbing system, turn on all cold water taps until you smell the bleach. Then turn the taps off. Hot water lines don’t need disinfection.
  • Let the bleach remain in the system for at least eight hours and preferably 24 hours.
  • Pump all of the bleach out of the water system by running the water through a garden hose to an area where the bleach will not damage lawns, shrubs or septic systems. Pump until you can no longer smell the bleach.
  • Two or three days after disinfection, a sample from the well should be submitted for bacteriological analysis.

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Nitrate Interpretation


Nitrate levels in water can be represented as Nitrate + Nitrite (as N) or as Nitrate (as N).

If your laboratory report shows your water’s nitrate level is less than 10 mg/L* as N (the symbol < indicates “less than”), the water is below the federal standard for nitrate in public drinking water supplies.

If your nitrate level is 10 mg/L* as N or greater, your water exceeds the federal standard for nitrate in public drinking water supplies and the following actions are recommended:

  • No infant or any female who is or may become pregnant should consume any water that exceeds this standard (either by drinking or by eating foods prepared with the water such as soups, juices, and coffee).
  • The Wisconsin Division of Public Health recommends that people of all ages avoid long-term consumption of water that has a nitrate level greater than 10 ppm.
  • Do not boil the water to reduce the nitrate level—boiling increases the nitrate concentration due to evaporation of the water.

* mg/L is equal to parts per million or ppm.


Learn More

WI Department of Health Services – Nitrate in private wells

WI Department of Natural Resources – Nitrate in drinking water


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Fluoride Interpretation


Because fluoride in drinking water may reduce cavities by 20 to 40%, fluoride supplements are recommended for children when the drinking water concentration is low. However, fluoride levels that are too high can also be problematic. Your physician or dentist can prescribe the correct dosage of supplements based on the child’s age and your water test results.

Use the following chart to determine if the children in your home should receive fluoride supplements:


Drinking Water Fluoride Concentrations
Concentration Interpretation
0.7 mg/L Optimal
>2.0 mg/L Children under 8 should not consume
>4.0 mg/L Children and adults should not consume

*Mg/L is equal to parts per million or ppm.


WI Department of Health Services Oral Health Program, includes information on Fluoride


What are fluoride’s health effects?

Exposure to excessive consumption of fluoride over a lifetime may lead to increased likelihood of bone fractures in adults, and may result in effects on bone leading to pain and tenderness. Children aged 8 years and younger exposed to excessive amounts of fluoride have an increased chance of developing pits in the tooth enamel, along with a range of cosmetic effects to teeth.


How does fluoride get into my drinking water?

Some fluoride compounds, such as sodium fluoride and fluorosilicates, dissolve easily into ground water as it moves through gaps and pore spaces between rocks. Most water supplies contain some naturally occurring fluoride. Fluoride also enters drinking water in discharge from fertilizer or aluminum factories. Also, many communities add fluoride to their drinking water to promote dental health.


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For More Information


If you need more information about bacteriological interpretation, contact your local Department of Natural Resources office or the Wisconsin State Laboratory of Hygiene (WSLH) Water Microbiology Unit, 2601 Agriculture Drive, Madison, WI 53718 (608-224-6262).

For more information on nitrates found in ground water, contact your local Department of Natural Resources office or the WSLH Inorganic Chemistry Unit, 2601 Agriculture Drive, Madison, WI 53718 (608-224-6280).

For additional information about fluoride in your water, contact your dentist; the Wisconsin Division of Public Health’s Chief Dental Officer (608-266-5152); or the WSLH Inorganic Chemistry Unit (608-224-6280).