Everyone thinks holes drilled in the ground are just holes...
But, there is a lot that goes into the borings, much more than you'd expect. There are grouted in casings (2"+/- concrete) with multiple pipes and tubes that extend tbrough the boring, through confining layers, through the water table, through bedrock, etc. that aid in the protection of the water supply. There are well defined and viable methods to abandon wells that preserve the integrity of aquifers and the water table, etc.
Think about a well dug below a confining layer in the soil to give you water at your house but expand that idea to much greater depths and some more details.
Frankly, I'd be more concerned about contaminating a water supplyby improperly installed water wells or surface runoff (oils/chemicals, etc) that could enter an existing well.