How do I disinfect a drilled well?
Calculate the amount of water in
the well by multiplying the gallons per foot by the depth of the well in feet.
For example, a well with a 6inch diameter contains 1.5 gallons of water per
foot.
If the well is 120 feet deep, and the water level is 20 feet, then you have
100 ft of water in the well,
multiply 1.5 gallons per foot by 100 feet to get 150 gallons of water in the
well.
CHLORINE
BLEACH TO DISINFECT A DRILLED WELL 

Diameter of well (in inches)  Gallons (per foot) 
3  0.37 
4  0.65 
5  1.0 
6  1.5 
8  2.6 
10  4.1 
12  6.0 
* For each 100 gallons of water in the well, use the amount of chlorine bleach (liquid or granules) indicated in the table below.
AMOUNT
OF DISINFECTANT REQUIRED FOR EACH 100 GALLONS OF WATER 

Chemical  Amount 
Laundry bleach (5.25% Chlorine)  3 cups (*) 
Hypochloride Granules (70% Chlorine)  2 ounces (**) 
(*)
1 cup = 8 ounce measuring cup (**) 1 ounce = 2 heaping tablespoons of granules 
*Mix the total amount of liquid
or granules with about 10 gallons of water.
*Remove the access nut on the well casing top.
*Use a funnel and pour the solution into the top of the well.
*Connect a hose to a faucet on the discharge side of the pressure tank and insert
it into the well casing top.
*Start the pump. Spray the water back into the well and wash the sides of the
casing for at least 15 minutes.
*Open every faucet in the system and let the water run until the smell of chlorine
can be detected.
*Close all the faucets and seal the top of the well.
*Let the chlorinated water stand
in the system several hours, preferably overnight.
*After letting the water stand, turn on the pump and open all faucets. Let the
water run until all odor of chlorine disappears.
Be sure to adjust the flow of water from faucets of fixtures that discharge
into septic tank systems to a low flow to avoid overloading the drain field.
*After 5 days of normal usage, test
water for coliform.
***If you have a large bored or hand dug well, contact our office for information on the suggested treatment method.
All information is general
in nature and is not intended to be used as a substitute for appropriate professional
advice.
Source: http://www.ashiinspection.com/welldisinfection.htm