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Monday, March 24, 2008
10:27 AM

 

What should I do if my water is contaminated with bacteria?

 

The Shock Chlorination of the Well


 

AnswerFirst, don't panic. Bacterial contamination is very common. Studies have found that more than 40 percent of private water supplies are contaminated with coliform bacteria. Spring water supplies are the most frequently contaminated, with more than 70 percent containing coliform bacteria.

Improving protection of a well or spring from the inflow of surface water is an important option to consider if the supply is contaminated with bacteria. It is important to remember that the groundwater is not necessarily contaminated in these cases; rather, the well is acting to funnel contaminants down into the groundwater.

Although well pits were the common method of construction several years ago, they are no longer considered sanitary construction and are not approved. Pitless well adapters are approved, but the casing must extend at least one foot above ground and be provided with an approved cap.

A properly protected well is evidenced by the well casing extending above the surface of the ground and the ground sloping away from the well to prevent water from collecting around the casing.

A properly protected spring is developed underground and the water is channeled to a sealed spring box. At no time should the water be exposed to the ground surface.

Keeping the plumbing system clean is an important part of maintaining a sanitary water supply. Any time work is performed on the plumbing or pump, the entire water system should be disinfected with chlorine. Simply pulling the pump out of the well, setting it on the grass to work on it, and returning it to the well is enough to contaminate the well with bacteria.

The procedure for cleaning and sanitizing a well or spring with chlorine is called shock chlorination. Concentrations of chlorine ranging from 50 to 200 milligrams per liter are used in the shock chlorination process. This is 100 to 400 times the amount of chlorine found in city water. The highly chlorinated water is held in the pipes for 12 to 24 hours before it is flushed out and the system is ready to be used again.

Periodic shock chlorination also may be effective in reducing an iron bacteria problem. The amount of chlorine needed to shock chlorinate a water system is determined by the amount of water standing in the well.

Table 3 lists the amount of chlorine laundry bleach or powdered high-test hypochlorite (HTH) that is needed for wells. If in doubt, it is better to use more chlorine than less.

Table 3
Amount of chlorine needed for shock chlorination

Liquid laundry bleach (about 5.25 percent Hypochlorite)

Depth of water in well

Casing diameter

4-inch

6-inch

8-inch

10-inch

12-inch

10 feet

1/2 cup

1 cup

1-1/2 cups

1 pint

2 pints

25 feet

1 cup

1 pint

2 pints

3 pints

4-1/2 pints

50 feet

1 pint

1 quart

2 quarts

3 quarts

1 gallon

100 feet

1 quart

2 quarts

1 gallon

1-1/2 gallons

2 gallons

150 feet

3 pints

3 quarts

1-1/2 gallons

2 gallons

3 gallons

High-test Hypochlorite (HTH 65 to 75 percent Hypochlorite)

Depth of water in well

Casing diameter

4-inch

6-inch

8-inch

10-inch

12-inch

10 feet

 

 

 

 

 

25 feet

 

 

 

1/4 pound

1/2 pound

50 feet

 

 

1/3 pound

1/2 pound

3/4 pound

100 feet

 

1/3 pound

3/4 pound

1 pound

1-1/2 pounds

150 feet

1/4 pound

1/2 pound

1 pound

1-1/2 pounds

4 pounds

Shock chlorination process

Most water treatment equipment, such as water softeners, iron filters and sand filters, should also be shock chlorinated. Check manufacturers' literature before chlorinating treatment equipment and pressure tank to prevent damage from strong chlorine solutions. Do not chlorinate carbon or charcoal filters or reverse osmosis units because it will use up their capacity.

Be careful when handling concentrated chlorine solutions. Wear rubber gloves, goggles and a protective apron when handling chlorine solutions. If it accidentally gets on your skin, flush immediately with clean water.

Never mix chlorine solutions with other cleaning agents or ammonia because toxic gases are formed.

Do not use fresh scent bleach or other special laundry products to disinfect wells. The plain and usually least expensive laundry bleach should be used for disinfecting water.

 

 

 


University Outreach and Extension David Reinbott, reinbottd@missouri.edu
Agriculture Business Specialist
Last modified: March 24, 2008