2011 Ground Water Summit and 2011 Ground Water Protection Council Spring Meeting

The Rate and Magnitude of Regional Groundwater-Level Decline in the Floridan Aquifer System and Its Relation to Ground-Water Pumpage and Relative Confinement of the System to Recharge

Monday, May 2, 2011: 11:45 a.m.
Constellation C (Hyatt Regency Baltimore on the Inner Harbor)
Lester J. Williams, U.S. Geological Survey;
Alyssa M. Dausman, U.S. Geological Survey;
Jason C. Bellino, U. S. Geological Survey;

The rate and magnitude of long-term groundwater-level decline in the Floridan aquifer system was evaluated to gain a better understanding of hydrologic responses to groundwater pumpage and the effects of confinement on recharge rates in the aquifer system.  Because of the large geographic area and widely varying time periods covered, a simple screening approach was taken.  To conduct the analysis, data from monitoring wells with at least twenty years of record were analyzed using linear regression to determine the slope of water-level change – a positive slope indicates a decreasing water-level trend and negative slope indicates an increasing water-level trend.  The slopes were then converted to a 10-year rate-of-decline coefficient which was used for mapping decline patterns in the aquifer system.  The results of the mapping indicate a strong correlation between aquifer confinement and long-term groundwater-level decline.  Areas of greatest decline correspond to thickest confinement with low recharge rates while areas of least decline correspond to thin or absent confinement with high recharge rates.   One of the surprising findings from the evaluation was the identification of an exceptionally large area of decline in the confined portions of the aquifer system extending from south central and southeastern Georgia into the Florida Panhandle and the northeastern Florida.  This pattern suggests that the gross vertical leakage through the upper confining unit (or upward leakage from deeper aquifers) in these areas is not able to fully satisfy current pumping conditions.  It is theorized that as water levels have declined over the past several decades, broad cones of depression have developed and have been correspondingly balanced by induced recharge contributed from nearby adjoining recharge (karst) areas.  Numerical modeling is being conducted to provide insight into recharge-discharge relations on a regional scale.