Hydrogeologic Assessment and Simulation of Stream-Aquifer Relations in the Lower Apalachicola-Chattahoochee-Flint River Basin

Project Chief: Lynn Torak
Cooperators: Georgia Department of Natural Resources, Environmental Protection Division
Year started: 2000

THIS PROJECT HAS BEEN COMPLETED AND IS BEING ARCHIVED IN ITS FINAL CONFIGURATION

Problem

Current hydrologic information and groundwater-flow modeling in the lower Apalachicola - Chattahoochee - Flint (ACF) River Basin (inset map) are insufficient to describe effects of time-variant irrigation pumping on streamflow. Therefore, existing models cannot accurately predict ground-water or streamflow conditions during a growing season. The Georgia Department of Natural Resources, Environmental Protection Division (GaEPD) has implemented a hydrologic assessment of the Upper Floridan aquifer in southwestern Georgia to obtain new information and to further understanding of stream-aquifer relations and the effects of groundwater pumping on streamflow in a karst hydrologic setting. The U.S. Geological Survey has engaged in a cooperative effort with GaEPD to develop a groundwater-flow model that can account for stream-aquifer interaction and streamflow reduction caused by agricultural pumping. Information obtained from the model is vital to the State’s management of ground-water resources and for providing early indication of low-streamflow conditions that would affect delivery of water to downstream, out-of-state users.

Objectives

• Develop new data for the stream-lake-aquifer system by evaluating well-drilling and aquifer-test information.
• Obtain accurate locations of pumped wells for municipal, industrial, and irrigation purposes.
• Collect and compile ground-water-level, stream-seepage, and off-stream spring-discharge data.
• Synthesize newly collected and existing hydrologic data into a transient finite-element model of ground-water flow that can simulate seasonal ground-water levels, stream-aquifer interaction, and pumpage-induced streamflow reduction, and assess the sensitivity of streamflow to ground-water pumping.

Publications

Torak, L.J., and Painter, J.A., 2006, Geohydrology of the Lower Apalachicola– Chattahoochee –Flint River Basin, southwestern Georgia, northwestern Florida, and southeastern Alabama: U.S. Geological Survey Scientific Investigations Report 2006-5070, 67 p. and interactive map, Web-only publication.

Fact Sheets (PDF)

• 2004—2005
• 2002—2003
• 2001

Progress and Significant Results, 2004—2005

Development of hydrogeologic framework revealed that:

• Recharge to the Upper Floridan aquifer occurs by vertical leakage through a thin veneer of residuum or surficial deposits located in the outcrop areas of the Upper Floridan aquifer along the northwestern basin boundary and in the northern and central parts of the Dougherty Plain (recharge map, facing page).
• Sparse data defining the lithology, hydraulic properties, and ground-water level of the upper semiconfining unit limits the description of recharge to the Upper Floridan aquifer to the delineation of hydrogeologic zones and a general temporal distribution of saturated proportions of total thickness (hydrogeologic zone map, facing page).
• Large variations in hydraulic conductivity of the Upper Floridan aquifer exist at regional scales in the lower ACF River Basin; equally large variations at the local scale are inferred from descriptions of lithologic heterogeneity of limestone penetrated by closely spaced wells.
• Overpumping the Upper Floridan aquifer in specific areas of the lower ACF River Basin can have negative hydrologic effects on the Upper Floridan aquifer basinwide, such as ground-water-level decline, aquifer dewatering, reduced regional (intrabasin) flow and reduced interbasin flow to the east and south (hydrograph, facing page).
• Negative effects of increased pumping can occur where ground-water resources are limited or inadequate to sustain pumpage increases, such as in outcrop areas of the aquifer and downdip, along the Solution Escarpment, where diminished recharge from the outcrop area reduces intrabasin flow.

Model of ground-water flow with stream-aquifer interaction in the Upper Floridan aquifer included:

• Simulated pumpage at 3,280 irrigation, municipal, and industrial wells tapping the Upper Floridan aquifer; ground-water and surface-water exchange along 36 streams; recharge by direct infiltration and vertical leakage, and discharge to the upper semiconfining unit; and regional flow across model and basin boundaries.
• Calibration to October 1999, steady-state drought conditions using 275 measured ground-water levels and streamflow gains and losses along 53 reaches.
• Simulated transient conditions of March 2001– February 2002 containing time-varying irrigation pumpage, stream and lake stage, upper-confining unit water level, and recharge by direct infiltration to the Upper Floridan aquifer.

Progress and Significant Results, 2002—2003

• Collected new hydrogeologic data defining aquifer and semiconfining-unit thickness and extent, and evaluated results of aquifer-performance tests; incorporated new information into Ground-Water-Site-Inventory database.
• Compiled recent (post-1986) hydrogeologic information on aquifer and semiconfining-unit thickness and extent, hydraulic properties, and pumpage, from GaEPD records.
• Incorporated well coordinates from agricultural wells, obtained by GaEPD using global-positioning-system technology, into local database used for developing model inputs.
• Analyzed agricultural withdrawal data for spatial and temporal relations.
• Evaluated ground-water-level measurements, stream-discharge data, hydrograph-separation methods, and off-stream springflow for October 1999, April 2000, and August 2000 conditions to define ground-water flow to streams.
• Installed five real-time streamgaging stations and upgraded one station for water-quality and acoustic velocity metering.
• Added 12 sites to monitor-well network of hourly groundwater- level recorders and one real-time satellite station.
• Initiated application of USGS transient finite-element model, MODFE, and development of automated input/output graphical user interface.

Progress and Significant Results, 2001

• Located 25 sites for test boring, geophysical logging, well drilling and installation, and aquifer testing; 23 sites were found suitable for drilling and aquifer testing (map, facing page).
• Compiled existing hydrogeologic information into digital form and assembled structure-contour maps of Upper Floridan aquifer and surficial units. Maps enabled development of hydrogeologic framework.
• Prepared well data for entry to Ground-Water Site Inventory (GWSI) database.
• Initiated development of graphical user interface (GUI) for processing data into model inputs and for depicting model results.
• Estimated ground-water contribution to streamflow for selected reaches during three periods of low flow during 1999–2000 using hydrograph separation techniques.