Wells that have been submerged beneath floodwater or high groundwater tables should be disinfected and tested for safety before using water from them for drinking or food preparation, warns Tom Glanville in the department of Agricultural and Biosystems Engineering at Iowa State University.

Most wells do not have watertight caps, so bacteria, silt and other pollutants are likely to enter them if they are submerged, Glanville said. Wells located near streams or drainage ditches are particularly vulnerable to flooding following rapid snowmelt or heavy rainfall, but wells located far from drainage ways also can become submerged if they are located inside leaky subsurface frost pits that become flooded as shallow water tables rise during wet seasons.

To reduce the risks of well contamination caused by submergence, wells should be constructed with watertight casing that extends at least one to two feet above ground or above the highest known flood level in low lying areas, Glanville said. In addition, earth should be mounded around the casing to prevent ponding of contaminated water around the well. For more information on proper well construction or repair, obtain a copy of ISU Extension publication PM 840, Good Wells for Safe Water, from the ISU Extension Online Store or from your county ISU Extension office.

After a well has been flooded, it will be necessary to sanitize it using a procedure called shock chlorination, Glanville continued. Experienced and properly equipped do-it-yourselfers may be able to shock chlorinate their own wells, but the procedure requires removal of the well cap, and dealing with electrical wiring and piping that may obstruct the interior of narrow diameter wells. For these reasons it is recommended to contact a well driller or pump installer to handle the task, as they have the professional training and equipment to perform the job thoroughly and safely. For more information on shock chlorination, see the ISU Agricultural and Biosystems Engineering Web site.

After the well is disinfected, a water sample must be tested for coliform bacteria to ensure that the disinfection procedure was effective and that the water is again safe to drink.  Until test results indicate that water from the well is safe for human consumption, use bottled water from a safe source for drinking and food preparation. If bottled water is not available, Glanville said, small batches of clear (not cloudy) well water can be disinfected by vigorously boiling the water for at least five minutes.

For more information on water sampling, see ISU Extension publication PM 1335, Sampling Your Drinking Water.

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3/08
Contact:
Tom Glanville, Extension Agricultural Engineer, (515) 294-0463, tglanvil@iastate.edu