If the local geology can handle you may be able to completely evacuate a cavern. Over the course of a riverbed you may find faults and fissures that can quickly regenerate "ground water" much better then soil absorption. The surface area of a riverbed facilitates soil absorption also. Water seeks it own level. Vertical relief of a well is always bad. Contaminants spread horizontally for the most part (In a high volume aquifer) much in the same way a aquifier regenerates it self.The 4 causes listed here
A large amount of rainwater is typically absorbed by the ground ending up as ground water. Large paved areas cause that rainwater to typically be drained directly into streams or rivers where there is little chance to be absorbed into the ground.
- Overwithdrawal of groundwater
- Diverting surface water from a large area and concentrating it in a single point
- Artificially creating ponds of surface water
- Drilling new water wells