Final answer:
Excessive groundwater extraction near coastlines mainly leads to saltwater intrusion into freshwater aquifers, causing land subsidence, sinkholes, and the potential reversal of groundwater flow which can introduce pollutants.
Step-by-step explanation:
The primary problem with drawing excessive amounts of groundwater from near ocean coastlines is saltwater intrusion into freshwater aquifers. When large amounts of groundwater are extracted, the nearby ocean water, which is saline, can move into the spaces formerly occupied by fresh groundwater. Another major concern is land subsidence, which is the gradual sinking of the land surface, and sinkholes, which are sudden collapses of the ground.
Both are direct results of the reduction in pressure within the aquifer system due to groundwater extraction. Continuous pumping can reduce the water table to such an extent that it becomes necessary to drill deeper wells, which are more expensive and likely to yield saline water. Additionally, overpumped aquifers near coastlines lead to the reversal of the natural groundwater flow, potentially pulling pollutants toward the pumping wells.