Volumetric Groundwater Depletion in the Atlantic Coastal Plain During the Twentieth Century

Substantial declines in groundwater levels have been observed since predevelopment times in many areas of the Atlantic Coastal Plain aquifer system.  This system underlies a large part of the east coast of the U.S. and includes a series of mostly unconsolidated sediments forming unconfined and confined aquifers separated by low hydraulic conductivity confining layers.  Corresponding with water-level declines is a long-term reduction in the volume of groundwater in storage in the system.  Estimates indicate that between 15 and 20 km³ of depletion has occurred from New York to Georgia during the 20^th century.  Most of the depletion is attributable to leakage out of low-permeability clayey confining layers and the coincident reduction in the volume of water in storage (though generally without desaturation).  Calculation of depletion volumes requires estimating the specific storage of the confining layers, for which few measurements are available.  This uncertainty represents the principal source of error in the estimated depletion volume.  The total depletion in the Atlantic Coastal Plain system represents only about 2 to 3 percent of the total estimated volumetric depletion of groundwater in the United States during the 20^th century.