QUESTION AND ANSWER :
COLIFORM BACTERIA AND WATER CONTAMINATION
WELL DISINFECTION PROCEDURES
What are coliform? Coliform bacteria are “indicator organisms” used in water microbiological
analysis. Coliform is a group of bacteria readily found in soil, decaying vegetation, animal feces,
and untreated surface water. They are not normally present in deep groundwater or treated
surface water. These indicator organisms may be accompanied by pathogens (i.e., diseasecausing organisms). Coliform bacteria do not normally cause disease in healthy individuals.
However, pregnant women, the elderly or persons with compromised immune systems are
considered to be at risk. Coliform bacteria, rather than the actual pathogens, are used to
assess water quality because their detection is more reliable and cost efficient. Pathogens are
less viable and appear in smaller numbers than coliforms, so are less likely to be isolated.
Drinking water found to contain coliform bacteria is considered contaminated and unsafe, and
should not be consumed. We recommend that a well or spring source be sampled for total and
fecal coliform at least once a year during the rainy, winter season.
How does the laboratory detect coliform bacteria in a water sample? Since visual
examination of water under the microscope is unreliable, most laboratories use the Colilert™
presence/absence test method. In brief, this method combines two separate examinations.
First, a 100-milliliter water sample is combined with growth media, and then incubated for 24
hours at 35.0 degrees Centigrade. After 24 hours, only coliform bacteria will have used the
growth media for food, resulting in a change in the water sample’s appearance.
Simultaneously, a second analysis is performed to determine if fecal coliform organisms are
present. The test is completed 24 hours after it was begun.
A TOTAL COLIFORM ABSENT report means that there was no coliform bacteria present at the
time of sampling and the water can be considered bacteriologically safe to drink.
A TOTAL COLIFORM PRESENT report means that coliform bacteria were present at the time
of sampling and the water should not be considered safe to drink. In most cases, we would
advise you to super-chlorinate the water system components after inspecting for system
deficiencies. This may need to be done several times to eliminate the contamination.
A FECAL COLIFORM (E.COLI) PRESENT report means that fecal coliform bacteria were
present at the time of sampling and the water is not considered safe to drink. We would advise
you to super-chlorinate and/or make modifications to the water system after inspecting for
system deficiencies.
The California Department of Health Services set drinking water standards and has determined
that the presence of any coliform bacteria in drinking water presents a health concern. Water
must not have any coliform bacteria in order to be considered safe for consumption.
How did the water become contaminated? Surface waters, such as streams, creeks, and
lakes, will almost always contain some degree of contamination due to exposure to animals,
humans, aquatic life, etc. “Raw” surface water should always be disinfected and filtered prior to
domestic use. Filtration is necessary to remove pathogens that are resistant to simple
disinfection. Wells and springs can become contaminated due to faulty construction, poor
protection from surface drainage including rain and flooding, a shallow water table, entrance of
surface water via rock fractures, close proximity to a sewage disposal systems or water
impoundments, and/or contamination during repair or reconstruction. Make certain that the
wellhead is adequately sealed against contaminants. All openings must be sealed, the casing
intact, and the well cap (sanitary seal) tightly bolted to the casing. Also, seal openings at the
back and bottom of the electrical box. If the location is approved by Planning and building, we
recommend installing a concrete pad around and contiguous with the wellhead after seal setup
and a well house to protect against the elements. A well or spring should be super-chlorinated
prior to the coliform test, especially if a repair has been made or if the water system plumbing
has never been chlorinated.
A “false” positive coliform sample result from poor sampling technique can occur by accidentally
contaminating the water sample (e.g., touching the inside of the sterile water sample bottle or lid
or by using a non-approved water sample bottle). We recommend sampling from an
unthreaded tap or bib as close to the water source as possible and from one that points
downward and has a minimum six-inch clearance space below it to allow for sampling access.
Why can’t I just provide treatment to get rid of the bacteria? Disinfection eliminates
coliform bacteria but may not eliminate all pathogens. Viruses, parasites, cysts, protozoans,
amoebas, Giardia, etc. are serious human disease-causing organisms that are resistant to
simple chlorination or UV disinfection. In addition to disinfection, expensive ultra-filtration (nanofiltration), water treatment additives, continuous online monitoring equipment with alarms, and
water treatment operators will also be needed to adequately treat the water to meet drinking
water standards. Installing a disinfection treatment system may give a negative bacteria test,
but as explained above, will only give a false sense of security.
Then how do I “cure” my contamination problem? Locate and permanently eliminate the
source of the contamination. Examine the well, spring, storage tank, distribution system, etc. to
determine if each is adequately sealed to prevent surface water entry. If the well/spring is
properly constructed, the contamination can usually be eliminated by chlorination. This involves
pouring chlorine bleach down the well shaft (or into the spring containment box), and allowing it
to remain in contact with the well casing (or box). Recirculating the chlorinated water back
down the well casing will help disinfect the interior of the casing. If a storage tank is used, pour
additional chlorine in the tank and ensure the storage tank is adequately protected from
contamination. Next, each water tap should be opened one at a time to bring the chlorine
solution in contact with all of the plumbing. Once the chlorine odor is noticeable, shut off the tap
and proceed through the remainder of the system. Make sure every tap is operated, both inside
and outside the house, including the hot water. When done, let the chlorine stand in the pipes
and well overnight. The next day, open each tap and run water until the chlorine odor is
undetectable. Once the chlorine is completely flushed from the entire system, the water should
be retested for coliform bacteria. (We recommend checking the water using a swimming
pool/spa test kit to verify no chlorine residual is present). Chlorine in your water will prevent the
laboratory from processing your sample. Normally, the contamination will be eliminated by this
temporary disinfection method, although it may be necessary to repeat the process with
stronger chlorine dose and/or longer contact time. If the contamination persists, at some point,
a qualified professional should be hired to adequately evaluate the situation, source
construction and location, and past chlorination procedures. If the contamination cannot be
eliminated, the well/spring may require reconstruction or a new well drilled in a different location.
New construction or modifications to wells or water systems require permits from the County
building and environmental health departments issued to licensed professionals.
[To estimate the amount of 5.25% chlorine (household type bleach) to use: for a 6-inch
diameter well, figure the depth of water in the well and use 1 quart of chlorine for every 66 feet
of water (100 gallons) =~ 25 parts per million chlorine].
What are some other forms of water contamination? Water contaminants commonly found
in this area include: hydrogen sulfide (rotten egg odor), excess iron (reddish brown stains), iron
bacteria (foul taste and odor), manganese (blackish stains, metallic taste), hardness (white
deposits, high soap usage), salinity, corrosiveness (evidenced by the dissolving of copper
plumbing leading to blue-green stains and bitter taste or lead leaching from lead solder from
copper pipes), turbidity (cloudy and/or dirty water), and sediment. With the exception of lead,
these may be unpleasant but are not generally considered hazardous to health. There are water
treatment systems available for the removal/reduction of all of the above. Contact a qualified
water treatment company and surf the web for more information. State certified water treatment
devices can be found at:
http://www.dhs.ca.gov/ps/ddwem/technical/certification/Devices.html
(The state certification program regulates only those water treatment devices for which health
benefit claims are made.)
A water source can also be susceptible to land use activities that may have occurred in the past.
Mining processing or extracting constituents and naturally occurring metals (arsenic, etc.) can
be present in your water. Contact a State-approved water laboratory if you want to test for
certain inorganic chemical constituents. Man-made chemicals, such as pesticides, herbicides,
solvents, etc., can easily leach into a subsurface water supply through careless handling and
dumping, or even normal application. Fortunately, these forms of water contamination are
rarely found in this area, but chemicals should never be applied or dumped near any water
source.
Printed by Nevada County Community Development Agency
Department of Environmental Health
950 Maidu Avenue
Nevada City, CA 95959
(530) 265-1787
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