SUGGESTED PROCEDURES FOR DISINFECTED WELLS
Disinfection of all wells is recommended to eliminate pathogenic organisms as
well as organisms that can grow in wells and thereby cause clogging and
effect the quality of water produced. Disinfection of the well is the final
act of well construction or repair before it is placed in service. Wells
should also be disinfected following repair or replacement of the pump and/or
well maintenance. The procedures described in this appendix are recommended
for disinfecting wells; however, other methods may be used provided it can be
demonstrated that they will yield comparable results. For new wells,
disinfection should take place following development (this will assure that
the well is purged of drilling mud, dirt, and other debris that reduces the
effectiveness of the disinfection), testing for yield, and installation of
the pump. When there is a delay in pump installation, interim or partial
disinfection should be undertaken.
Disinfection involves seven steps:
- A chlorine solution containing at least 50 mg/l (or parts per million)
available chlorine, is added to the well. Table 6 lists quantities of
various chloride compounds required to dose 100 feet of water-filled casing at 50 mg/l for diameters ranging from 2 to 24 inches.
For wells that have been repaired or when a pump has been
repaired or replaced and, bringing the well back into service quickly is
desired, the solution should contain at least 100 mg/l available chlorine.
To obtain this concentration, double the amounts shown in Table 6.
- The pump column or drop pipe shall be washed with the chlorine solution
as it is lowered into the well.
- After it has been placed into position, the pump shall be turned on and
off several times (i.e., “surged”) so as to thoroughly mix the disinfectant
with the water in the well. Pump until the water discharged has the odor of
chlorine. Repeat this procedure several times at one-hour intervals.
- The well shall be allowed to stand without pumping for 24 hours.
- The water shall then be pumped to waste until the presence of chlorine is
no longer detectable. The absence of chlorine is best determined by testing
for available chlorine residual using a test kit designed for this purpose.
Disposal of the waste should be away from trees, shrubs, or lawns and into
storm sewers, drainage ditches, etc. Note that heavily chlorinated water
should not be wasted into the plumbing system of homes that utilize
individual sewage disposal systems (septic tanks). Such strong disinfectants
could neutralize the bacteria needed to stabilize the sewage and also could
damage the soil adsorption system.
- A bacteriological sample shall be taken and submitted to a laboratory for
examination (see Appendix D).
- If the laboratory analysis shows the water is not free of bacterial
contamination, the disinfection procedure should be repeated. Depending on
the level of contamination, it may be necessary to use a higher concentration
chlorine solution (several times that shown in Table 6). The water should
then be retested. If repeated attempts to disinfect the well are
unsuccessful, a detailed investigation to determine the cause of the
contamination should be undertaken.
Where small individual domestic wells to be treated are of unknown depth or
volume, at least one pound (0.45 kilograms) of calcium hypochlorite (70
percent available chlorine) or two gallons (7.5 litres) of household bleach
(sodium hypochlorite), such as Clorox or Purex, may be used in lieu of the
chemicals shown in Table 6.
CHLORINE COMPOUND REQIRES TO DOSE 100 FEET
OF WATER-FILLED CASING AT 50 MILLIGRAMS PER LITER1
1/4 oz (7 g)
1 oz (28 g)
2 oz (57 g)
3 oz (85 g)
4 oz (113 g)
6 oz (0.2 kg)
10 oz (0.3 kg)
1 lb (0.45 kg)
1-1/2 lb (0.7 kg)
1/2 oz (14 g)
2 oz (57 g)
4 oz (113 g)
7 oz (0.2 kg)
11 oz (0.3 kg)
1 lb (0.45 kg)
2 lb (0.9 kg)
3 lb (1.4 kg)
4 lb (1.8 kg)
2 oz (59 mL)
9 oz (266 mL)
20 oz (0.6 L)
2-1/8 pts (1.0 L)
3-1/2 pts (1.7 L)
5 pts (2.4 L)
1 gal (3.8 L)
1-2/3 gal (6.3 L)
2-1/3 gal (8.8 L)
1. Some authorities recommend a minimum concentration of 100 mg/L.
To obtain this concentration, double the amounts shown.
2. HTH, Perchloron, Pittchlor, etc.
3. Where dry chlorine is used, it should be mixed with water to form a chlorine solution prior to placing it into the well.
Note that dry chlorine should always be added to water, not vice versa.
Further, the chemical should be added slowly.
These precautions are necessary to lessen the possibility of a violent chemical reaction.
4. Household bleaches such as Chlorox, Purex, etc.