The most common odour problem is a "rotten egg" smell. This is caused by hydrogen sulphide in the water. Hydrogen sulphide may be inherent in the water supply but may also be introduced in the well itself through the actions of biofouling bacteria.
Hydrogen sulphide can be treated through the use of an activated carbon filter. If the hydrogen sulphide is caused by biofouling then treating the biofouling through the use of Wellrific shocking or a down well injection system may be more appropriate.