As already written by ...

As already written by Francisco, I think the most common impact of water wells is the connection between shallow and deep aquifers, with the possibility to prejudice the water quality of the deep one. Is very common in water well drilling to install filters at every coarse level to improve the well productivity. This malpractice cause a flux of  water from layers with greater hydraulic potential, through the well casing, to the layers with lower potential. This problem can be fixed with proper well design and proper building operations. On the contrary, it is rather impossible to fix the problem on existing wells.