Bacteria Contamination In Well Water

Most private wells provide a safe and uncontaminated source of drinking water. Some wells do however become contaminated with bacteria. Fortunately certified labs can easily test water for coliform bacteria, a common indicator of bacterial contamination in wells. To ensure your well is not contaminated, it is a good idea to regularly test your water. You should have your water tested at least annually and whenever you notice a change in the taste, odor or color of the water.

Most bacteria entering the ground surface along with rainwater or snowmelt are filtered out as the water seeps through the soil. Several strains of bacteria can survive a long time and find their way into the groundwater by moving through coarse soils, shallow fractured bedrock, quarries, sinkholes, inadequately grouted wells or cracks in the well casing. Insects or small rodents can also carry bacteria into wells with inadequate caps or seals. Coliform bacteria are naturally occurring in soil and are found on vegetation and in surface waters. Water from a well properly located and constructed should be free of coliform bacteria.

While coliform bacteria do not cause illness in healthy individuals, their presence in well water indicates the water system is at risk to more serious forms of contamination. The presence of another type of bacteria, Escherichia coli (E. coli), indicates fecal contamination of the water. Fecal coliform bacteria inhabit the intestines of warm-blooded animals and are typically found in their fecal matter. Pathogenic bacteria, viruses and parasites often present in fecal matter can cause illnesses, some having flu-like symptoms such as nausea, vomiting, fever and diarrhea. In some cases, symptoms can be more severe. Many labs now routinely test for E. coli bacteria along with total coliform. The presence of E. coli bacteria in water represents a serious problem. If your water sample is positive for E. coli, it is important you stop consuming your water and deal with this problem immediately.  


How Can I Tell If My Well Is Contaminated?

Your water supply may become bacteriologically contaminated because of one or more of the following reasons:

1. There is a source of contamination too close to the well and the casing does not extend deep enough to assure bacteria have been adequately filtered out of recharge water to the aquifer.

2. The well may have been constructed using poor sanitary practices. Wells can become contaminated during the drilling process by the improper use of contaminated drill tools, casing pipe or drilling water. The installation of the pump, its discharge piping, or any other pump or pressure system component can also be the cause of contamination if they are not assembled and installed in a sanitary manner prior to their use. The State Private Well Code (NR 812) requires disinfection of any new well, the pump, pump discharge piping and the pressure tank, prior to being placed in service.

3. Contaminated surface or near-surface water can enter a substandard or improperly constructed well in any of the following ways:

• Dug wells lined with poorly sealed brick, stone or tile curbing, or having unsealed covers, can allow unfiltered water to get into the well.*

• Casing improperly sealed through a shallow unconsolidated or bedrock geological formation may allow contaminated water to migrate downward into the aquifer.*

• Surface water can enter the top of the well casing if the casing does not extend far enough above the ground surface.

• The well casing may terminate in a basement, pit or alcove subject to fl ooding or seepage of water.

• An old well casing may become badly corroded and allow water to seep into the well through holes in the casing.*

• A well with a noncomplying casing depth setting can allow contaminated near-surface water to enter the well.*

• A well having old, substandard ‘stove-pipe’ casing can allow near surface water to enter the well.* 

* Note: A well with the defects indicated with an asterisk cannot be easily repaired and typically need to be replaced with a new code-complying well.

4. The aquifer may be a highly fractured bedrock formation or a coarse gravel deposit that does not adequately fi lter recharge water percolating down from the ground surface into the aquifer.

5. The well cap may be loose or poorly installed allowing insects, spiders or small animals to enter the well.

6. There may be a ‘cross-connection’ between the well or plumbing system and the septic or sewerage system.

Contact the Winneabago County Health Department and we can assist in investigating well contamination issues (920-232-3000)



When should I test my well for bacteriological contamination?

The State Well Code requires all new wells to be tested for bacteriological quality. Wells must also be tested following the installation or reinstallation of a pump, or anytime a well is entered for repairing or reinstalling equipment within the well. Existing wells should be tested annually, after modifying the well in any way, or whenever there is any change in the taste, odor or appearance of the water.The best times of the year to test your well water are when it is most likely to be unsafe. Statistically these times occur following a period of heavy snowmelt in early spring or during the hot stagnant time of late summer and early fall. A test kit (including sampling instructions) may be obtained from any laboratory certified to test water for bacteriological contamination. For private well owners, certified bacteria labs can be found online at Wisconsin DNR Drinking Water Data or use the Certified Water Labs. 

Where can I obtain a water-sampling kit for bacteriological testing?

A test kit (including sampling instructions) may be obtained from any laboratory certified to test water for bacteriological contamination. Some certified labs in our area are below: 

  • Badger Labs and Engineering - 501 West Bell St Neenah WI 54956 (920) 729-1100 (Lab #205) (800) 776-7196  
  • Clean Water Testing - 1990 Prospect Ct Appleton WI 54914 (920) 733-7590 (Lab #291)
  • Fox Valley Quality Control Lab - 2224 American Dr Neenah WI 54956 (920) 731-5484 (Lab #239)
  • Oshkosh Health Dept - 215 Church Ave Oshkosh WI 54901 (920) 236-5030 (Lab #028)
  • TG Analytical Labs - PO Box 170 N1022 Quality Dr Greenville WI 54942 (920) 757-1355 (Lab #452)
  • University of Wisconsin Oshkosh - Aquatics Research Laboratory - 783 Pearl Ave, Oshkosh, WI 54901 (920) 424-3148


What do the test results tell me? 

Bacteriological analyses of water samples are completed to determine the safety of the water for drinking and preparation of food. If a sample was collected according to directions included with the kit and the lab subsequently reports the sample as bacteriologically “safe,” then “total coliform bacteria” were not found in the water. You can then be reasonably sure the water is bacteriologically safe to drink. On the other hand, when the lab reports the sample as either bacteriologically “at risk” or “unsafe,” then total coliform and/or E. coli bacteria were found in the sample and you should not drink the water. Total coliform bacteria are only an indicator bacteria and are not, by themselves, usually a health concern for healthy individuals. But their presence in well water indicates an increased risk that pathogenic (disease-causing) bacteria are also present in the water. Well water reported by the lab as being “at risk” or “unsafe” should not be consumed or used for preparation of food unless it is fi rst boiled for at least one minute, at a rolling boil. If you need additional help in interpreting the results of your water analysis, contact your laboratory. (Note: Boiling water for a long time reduces the volume of water and can increase the concentration of any nitrate that may be present in the water. This can make the water more hazardous for infants.) 

What should I do if my well water is bacteriologically unsafe?  

1. First resample your well. Collect another water sample and have it analyzed to confirm your fi rst “at risk” or“unsafe” result. Be sure to use the proper sampling procedure when you collect the sample. This will help you determine if your original sample result could have simply been a result of an improper sampling technique. 

2. If the second sample result is also reported as being “at risk” or “unsafe”, do not consume the water unless you boil it, at a rolling boil, for at least 1 minute.

 3. If you find no obvious sources of contamination of your well or water system, you should have your entire system inspected and disinfected by a reputable Licensed Well Driller or Pump Installer. You can disinfect your well yourself if you follow the precautions and directions of this brouchure - Well Chlorination In Arsenic Sensitive Areas.

4. If your well does not have a Department-approved vermin-proof well cap or seal, have one installed by your Licensed Well Driller or Pump Installer. To find a Licensed Well Driller or Pump Installer, look in the back of your phone book under “Water Well Drilling & Service,” “Pump Service & Repair” or “Water Supply Systems.

Contact the Winneabago County Health Department and we can assist in investigating well contamination issues.


WI Department of Natural Resources - PUB-DG-003-2009