The location and depth of a well and the type of thickness of soil around it will help determine the level of risk of contamination. Natural soil conditions and depth to the groundwater source can't be controlled, but there are a number of protective measures that well owners can take to reduce the risk of well contamination. These include proper well construction, regular inspection and maintenance.
Unused or abandoned wells that have not been properly sealed can provide a direct pathway for contaminants to enter the groundwater. Contaminants from surface water, runoff, or sources, such as leaking sewers or septic systems, can enter the well through casings that have deteriorated.
Regardless of the reason for well abandonment, it is imperative that unused wells are sealed by professional contractors. It is also necessary to seal a well when water contamination is discovered in an area where there are multiple-aquifer systems because the contaminated water can migrate between the aquifers.