Coliform Bacteria water testing in Iowa.

By the Closer Look Inspectors team · Facts checked October 2026 against Iowa DNR, Iowa HHS, State Hygienic Laboratory and EPA sources · About our inspectors

A total coliform test shows whether bacteria found in soil, sewage and surface water are present in your well water. In a 2006 to 2008 University of Iowa survey of rural wells for Iowa DNR, 43% of samples had total coliform bacteria.

Health standard: Iowa Department of Natural Resources (DNR): private wells have no mandated state or national water quality standards, so use EPA standards for public water supplies. State Hygienic Laboratory: water is bacterially safe when total coliform and E. coli are absent.

Where coliform bacteria come from in Iowa wells

Total coliform bacteria live in soil, sewage, surface water and very shallow groundwater influenced by surface water. Soil normally filters them out as water moves down, unless a pathway bypasses it. The State Hygienic Laboratory (SHL) says the pathway in older wells can be pits, cracks in the casing, inadequate grouting, brick or concrete wells, or cisterns. In new wells the problem is more often the plumbing, such as cross-connections, faulty pressure tanks, or repairs done without proper disinfection. Iowa DNR says unused or improperly plugged wells can cause issues with a working well. It adds that old wells may give surface water and very shallow groundwater a direct connection to the aquifer. In the University of Iowa survey for DNR, bacteria turned up less often in deeper wells: total coliform was found in 54 of 82 wells shallower than 50 feet, but in 80 of 266 wells deeper than 100 feet. Bacteria were more common in northwest, south-central and southwest Iowa, where wells are generally shallow.

Health effects and who is most at risk

Total coliforms are mostly harmless. SHL says finding them points to a possible sanitary defect that could give contamination a way into the well or the plumbing in the home. Iowa DNR says their presence can indicate that more dangerous bacteria, particularly fecal coliforms, have a pathway into the well. SHL rates water with total coliform present as unsafe, with potential health concerns. If harmful microbes are in a water supply, DNR says symptoms such as diarrhea, cramps, nausea and headaches are possible. It adds that microbes can pose a much greater health risk for infants, young children, senior citizens and people with severely compromised immune systems. SHL calls water bacterially safe when total coliform and E. coli are absent.

Signs and when to test

You cannot tell by look, taste or smell whether well water is safe; Iowa DNR says only lab analysis can show it. Iowa DNR recommends testing for bacteria at least once a year. It also says to test when a well is brand new and after any service on the well, pump or water system. Other times are after a power outage that drops water pressure, after flooding near the well, and when the water looks, smells or tastes different. A home that has sat empty for more than a few weeks is another reason. DNR says older wells, especially those built before 1982, may or may not have adequate protections to draw from deeper, protected aquifers. An older well, or one with unknown depth and construction, should be sampled at least yearly. Iowa DNR says coliform is most likely in wet weather, when runoff and excess soil moisture carry contaminants into shallow groundwater or through well defects. In general those are the wet periods of late spring, early summer and fall. Iowa DNR says the well owner is responsible for testing. A home purchase is a sensible time for a baseline result.

If your result is positive: what to do next

First, stop drinking the water as it is. SHL says to boil it for one minute, or use a known safe source such as bottled water or water from a public supply. It says not to brush teeth with the water unless it is disinfected. Next, find how the bacteria entered the water system. SHL says to start by reviewing how the sample was collected. It notes that improper collection can easily contaminate a sample. If it was collected correctly, SHL says to have the whole water system, the well and the area around it checked for defects, and to correct any that are found. Iowa DNR says a certified well contractor can help diagnose water issues and advise on repairs or a new well. After flooding, DNR's guidance for a well that tests positive is to purge and shock chlorinate the well and water system, retest 7 to 10 days after, and repeat until there is no coliform or fecal coliform.

Facts checked October 2026 against: shl.uiowa.edu, iowadnr.gov, iowadnr.gov, iowadnr.gov, iowadnr.gov, iowadnr.gov.

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