This is especially true in farming areas. The water table can drop at rates up to 1 meter per year. As water is withdrawn from unconsolidated sediment, the water pressure drops, and the weight of the overburden is transferred to the sediment. Several meters of subsidence is not uncommon - e.g., San Joaquin Valley and Mexico City. Occurs along coastlines where saltwater replaces freshwater which has been withdrawn.
Fresh water floats on top of salt water.
A cone of depression in the top of the zone of freshwater will produce an inverted cone in the interface between freshwater and saltwater. Sewage, liquid wastes such as solvents, highway salt, fertilizers, and pesticides are some of the worst offenders. These enter the groundwater through septic tanks, holding ponds, landfills, leaking underground tanks and pipelines, and infiltration. By chemical oxidation or other bio-chemical processes. The Geologic Work of Groundwater Limestone caverns.