LOWER ACF BASIN

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

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.