how do I disinfect a well with plain bleach
1)             Use common household bleach as the chlorine source for
  disinfection. 
  
  [Bleach products usually contain 5 to 6 percent chlorine.  For the
  disinfection procedure to be effective, the pH (acidity) of the well
  water should be between 6 to 7.5.  If it is not in this range, then a
  well treatment professional experienced in disinfection of high pH
  water should be consulted to adjust the pH and maintain it in the
  correct range during the disinfection process. 
  
  2)             Find the total depth of water in the well. 
  
  [For example - If the well is drilled to 200 feet and the water level
  is 50 feet down from the top, then there is a 150 feet depth of water
  in the well]
  
  3)             Determine how many gallons of water are stored in the
  vertical column of the well.  Once you know depth you can work out the
  gallons.  (See Table 1 for specific  Water Volumes)
  
  Well diameter gallons per foot of well gallons in 100 feet
  
  4-inch 0.65   65
  
  6-inch 1.47 147
  
  8-inch 2.61 261
  
  [NOTE - disinfection is not feasible for large-diameter dug wells]
  
  4)             Work out how much bleach will be needed:  For every 50
  gallons of water in the well use one quart of laundry bleach - (4
  quarts in a gallon).  Do not use excessive amounts of bleach - more is
  not more effective.
  
  [In most cases, one gallon of bleach will disinfect 200 gallons of
  well water at a chlorine concentration between 200 to 300 ppm (parts
  per million)]
  
  [NOTE - Bleach loses strength in its container over time, Check the
  “sell by date” and use bleach that is less than three months old.  Use
  unscented bleach to avoid adding unneeded chemicals to the water (It
  is usually less expensive too)]
  
  5)             For best results the bleach should be combined with
  water before adding it to the well.
  
  [By reducing the concentration there is less chance of corrosion of
  the cables and pipes in the well.  The greater volume of water helps
  get the chlorine mixed in the well column.]
  
  [NOTE: - Do not mix chlorine solutions with other cleaning products,
  including ammonia, because toxic gases will be created.
  
  6)             Make the Chlorine-Water Mixture by filling a five
  gallon bucket with water and adding about one quart of bleach.  Pour
  the mixture into the well and repeat until the right amount of bleach
  has been added to the well (see #4 above).
  
  7)             Remove the well cap and pour the mixture directly down
  inside the well casing.
  
  [NOTE - Wear rubber gloves and safety glasses to protect skin and eyes
  from splashes.  Spills contacting the body should be rinsed
  immediately and thoroughly with fresh water.]
  
  8)             Recirculate the water in the well by running water with
  a hose back into the well for 30 minutes.
  
  [This will help draw the Chlorine-Water Mixture down the well and will
  wash the “dry” portion of the well casing or drilled hole above the
  water table more thoroughly.] 
  
  9)             To disinfect the household plumbing first turn off the
  electric or gas supply to the hot water heater.  Turn on all the
  faucets, shower heads, clothes washers, dish washers and outside
  faucets, etc. until there is a chlorine smell at each location.
  
  [NOTE - If you have water conditioning equipment check the owners
  manual to see if it is sensitive to chlorination. All equipment in the
  water system should be disinfected because it may serve as a safe
  haven from which the bacteria may reinfect the system.]
  
  10)         Leave the Chlorine-Water Mixture in the plumbing system
  and well for 12 to 24 hours before removing the chlorinated water.
  
  [NOTE - During the disinfection process, the well water should not be
  used for consumption, so make plans for essential water needs before
  you begin the disinfection process!]
  
  11)         Remove the chlorinated water from the well by running the
  pump and leading a hose from outside faucets to a safe area. 
  
  [Don’t put the chlorine solution into a septic system or in a creek or
  pond where it may kill fish. Flowers and vegetables may be affected by
  chlorinated water.]
  
  12)         When the chlorine odor has gone the well flushing is
  complete.
  
  [If you have difficulty smelling the chlorine odor a swimming pool
  chlorine test kit can indicate whether or not there is chlorine
  remaining in the well water.]
  
  13)         Once the chlorine has been removed from the well water,
  the indoor plumbing may be flushed out to the septic or sewer system
  to remove the chlorinated water.
  
  [Re-energize the hot water heater. The water supply should now be
  bacteria free.]
  
  14)         Wait about 5 to 10 days before resampling the water supply
  and testing for bacteria.
  
  [If bacteria are still present, repeat the process.  It may take
  several treatments to remove long-established bacteria colonies.  If
  the problem persists, contact a professional familiar with
  microbiological contamination in wells.]
  
  15)         Repeat testing in three months to confirm treatment
  success, and then test annually.


https://agwt.org/content/water-well-disinfection-procedure