Shock chlorination procedures

For shock chlorination, an initial chlorine concentration of 50 to 100 parts per million (ppm) with a contact time of at least 6 hours is recommended. To obtain a concentration of 100 ppm you need to know (or estimate) the diameter of your well and the depth of water in the well. Note: Do not use the total depth of the well. The depth of water is the distance from the water surface to the bottom of the well. Your well driller may have this information or you can determine it yourself by lowering a weighted string to the bottom of the well. Table 1 shows the amounts of chlorine bleach to add for an initial concentration of about 100 ppm.

Table 1. Chlorine Bleach Additive Quantities
Well Diameter	Depth of Water in Well (feet)
  (inches)    less than 50    100     150     200
		     Quarts of Laundry Bleach
    2		  1/4	      1/4     1/4     1/4
    4		  1/4	      1/2     3/4     1
    6		  1/2	      1	      1.5     2
    8		  1	      2	      2	      3
   10		  1.5	      3	      3	      3
   12		  2	      3	      4	      4

Any type of laundry bleach containing 5.25% hypochlorite solution will work. Because bleach is corrosive to metals it should be diluted in the ratio of 12 parts of water to 1 part bleach prior to adding to the well water. These corrosive properties also mean that care should be taken in handling to avoid contact with skin and especially eyes. Rubber gloves and goggles are recommended when handling chlorine solutions. If you are exposed, flush repeatedly with clean water. This should be done immediately after exposure.

Follow the steps below to properly shock chlorinate your well:

Step 1. Determine the amount of bleach and dilution water to use (see Table 1). For example if you have a 4 inch diameter well with 40 feet of water, Table 1 recommends using 1/4 quart of bleach. To dilute, use 3 quarts of water (12 times 1/4 quart).

Step 2. Remove the cap from your well. There are many types of well caps. If you have questions or need instructions to remove the cap, contact your well driller.

Step 3. Pour the mixture of chlorine and water down the well. Try to coat the casing (sides of the well) as you pour. To get good distribution in the well, attach a hose to a nearby hydrant or faucet and put the discharge end of the hose in the well. Then start the pump to recirculate chlorinated water back into the well. The recirculating water should have a strong odor of chlorine if the chlorine demand has been met.

Step 4. Run water through the service lines in the house until you detect the odor of chlorine at each tap. Make sure you run the chlorinated water through every line in the system. You may also want to flush toilets. Note: If you have an activated carbon filter in the system, temporarily remove the cartridge or bypass the filter completely.

Step 5. Let the chlorinated water stand in the system for at least 6 hours, preferably 8 to 12 hours.

Step 6. Flush out the system beginning with the well. Use a hose connected to an outside hydrant and discharge the water to an appropriate waste system. Note: This large volume of chlorinated water should not be put into a septic system or onto delicate plants or lawns. After water from the well is free from chlorine odor, flush the rest of the piping system. Draining this volume of water into your septic system should be acceptable.

Depending on the age and condition of your well, you may want to shock chlorinate the well up to 10 times before abandoning the well. If you must use the present contaminated water supply until the new supply can be developed, be sure to follow a process of continuous chlorination.

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