COLIFORM BACTERIA AND YOUR DRILLED WELL
A properly constructed and adequately cased drilled water well
usually obtains its water at a depth where coliform bacteria
are not present. All water in ponds, streams, and rainwater
running over the ground surface contains harmful bacteria, viruses,
and other organisms. These organisms die or are filtered out
as water moves through the subsurface ground water environment.
However, groundwater can be contaminated when there is insufficient
filtration or travel time between the land surface and the ground
water, such as inadequate separation distances from the well
to sewage systems and other pollution sources. Some of the common
ways in which bacteria can get into a well are as follows:
- The pitless adapter can leak or the well casing can become
cracked. Visually inspect the interior of the casing on a
regular basis to make sure there are no cracks or leaks present.
- Over time well casings may rust through, leaving holes near
the ground surface where water can seep in and contaminate
deeper ground water.
- Surface water could enter the top of the well if the casing
does not extend far enough above the ground. Consider extending
the casing to at least 24” above the ground surface.
- Bacteria can be introduced into a well when it is drilled,
or when a pump is installed or serviced. Water wells should
be sanitized after any service or installation work. Refer
to the TCHD (Tompkins County Health Department) handout for
directions on shock disinfecting wells.
- Unsealed abandoned water wells can directly channel contaminated
surface water into groundwater. Refer to the TCHD handout
for more information regarding well abandonment.
- Backflow can occur if water systems are not installed with
properly functioning backflow prevention devices. Backflow
prevention devices are essential to prevent any risk of bacteria
being siphoned back into the well. For example, installation
of an atmospheric vacuum breaker on each outside faucet will
provide backflow protection if the end of a hose is accidentally
submerged in material that could contaminate a well, such
as lawn fertilizers, herbicides, swimming pools, or puddles.
- Surface water and insects can easily enter the well through
a poorly constructed or unsecured well cap. Several types
of common well covers are not vermin proof. Look to make sure
that the electrical conduit (where the wires enter the well)
is sealed and the gasket around the well is tight and intact,
so that when the cover is bolted down the gasket will be pushing
into the well’s casing. The well cover should include
a downward facing screened vent that serves to prevent a vacuum
forming within the well. Also make sure that bolts or screws
are not missing from the well cover.
Homeowners should test their well water annually for coliform
bacteria. Although most coliforms are harmless, they are an
indication that disease-causing bacteria, viruses, or protozoa
may be present in the drinking water. The TCHD maintains listings
of certified testing laboratories.
If coliform bacteria are detected in your drinking water, the
TCHD recommends making sure that the well is in good condition
and shock disinfecting the well before collecting another water
sample. Contact the TCHD for assistance and advice.
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