You may need to hire an environmental engineer or hydrogeologist to find the problem. Lists of water system consultants and water treatment options are available from the Department of Health. Before you begin, disconnect or remove any water treatment devices, such as activated carbon filters, water softeners or reverse osmosis units. Highly chlorinated water can make them less effective or even damage them. For a dug or drilled well, add one gallon of household laundry bleach for every 525 gallons of water.
Pellets are available at pool supply or hardware stores.
Whenever possible, run the chlorinated water through a garden hose back into the well for an hour so the chlorinated water washes down the inside of the casing. Use rubber gloves and eye protection. After you recap the well, open one faucet at a time throughout the house. Run the water until you smell a strong chlorine odor, then turn tap off. Strong chlorine solutions may damage rubber and polybutylene gaskets and fittings.