The causes of bacterial contamination in a well are usually due to:

• Lack of, or improper, disinfection of a well following repair or construction.
• Failure to seal the space between the drill hole and the outside of the casing.
• Failure to provide a tight sanitary seal at the place where the pump line(s) passes through the casing.
• Wastewater pollution caused by contaminated water percolating through surrounding soil and rocks into the well.

At the time when a new well is constructed or repairs are made to a well, pump or piping, contamination from the work is possible. Therefore, it is important that the well, pump, piping and associated structures should be regularly disinfected using chlorine solution.

Tracing the source of contamination

There are different methods which help to identify a possible source of groundwater contamination. One method is sodium or potassium fluorescein. This is a brightly-coloured, fluorescent, water-soluble dye and can be used as a tracer when a sewage disposal system is suspected of contaminating groundwater. A solution flushed into the disposal system or suspected source may appear in the well water within 12–24 hours. It can be detected by sight, taste or analysis.