They are present in dust, carried by insects, and can be introduced to your water supply through routine well and well pump maintenance. In the majority of circumstances, contamination is an isolated occurrence due to one of several easily remedied situations. Serious contamination occurs relatively infrequently. Occasionally a new well will require several consecutive disinfections before all the residual bacteria are removed. Normal dirt and debris carried on the equipment and hands of well drillers or repairmen will contain coliform bacteria.
Bacteria can enter the system any time there is a break.
Dirt and insects that carry bacteria can easily enter a well or cistern through damaged seals, or when the covers are removed. The well vent should be covered with a small mesh screen to prevent the entrance of insects. If there are problems with any of these, contact your well driller for repair or replacement. Shallow wells typically draw their water from surface water supplies and are often contaminated. Wells may also become contaminated during periods of heavy rain or flooding.