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Introduction 1 Purpose and scope 2 Location of study area 2 Previous investigations 2 Hydrogeologic units 2 Acknowledgments 5 Well inventory 5 Well-numbering system 5 Well-site data 5 Natural-gamma geophysical logs 10 Ground-water levels 10 Chloride concentration in ground water Aquifer-test results 16

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For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit https://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.Suggested citation: Jones, L.E., Painter, Jaime, LaFontaine, Jacob, Sepulveda, Nicasio, and Sifuentes, D.F., 2017, Groundwater-flow budget for the lower Apalachicola-Chattahoochee-Flint River Basin in southwestern Georgia and parts of Florida and Alabama, 2008-12 By L. Elliott Jones, Jaime Painter, Jacob LaFontaine, Nicasio Sepulveda, and Dorothy F. Sifuentes AbstractAs part of the National Water Census program in the Apalachicola-Chattahoochee-Flint (ACF) River Basin, the U.S. Geological Survey evaluated the groundwater budget of the lower ACF, with particular emphasis on recharge, characterizing the spatial and temporal relation between surface water and groundwater, and groundwater pumping. To evaluate the hydrologic budget of the lower ACF River Basin, a groundwater-flow model, constructed using MODFLOW-2005, was developed for the Upper Floridan aquifer and overlying semiconfining unit for 2008-12. Model input included temporally and spatially variable specified recharge, estimated using a Precipitation-Runoff Modeling System (PRMS) model for the ACF River Basin, and pumping, partly estimated on the basis of measured agricultural pumping rates in Georgia. The model was calibrated to measured groundwater levels and base flows, which were estimated using hydrograph separation.The simulated groundwater-flow budget resulted in a small net cumulative loss of groundwater in storage during the study period. The model simulated a net loss in groundwater storage for all the subbasins as conditions became substantially drier from the beginning to the end of the study period. The model is limited by its conceptualization, the data used to represent and calibrate the model, and the mathematical representation of the system; therefore, any interpretations should be considered in light of these limitations. In spite of these limitations, the model provides insight regarding water availability in the lower ACF River Basin.

Groundwater conditions during 1994 and period of record in Georgia were evaluated using data from precipitation, groundwater level , and groundwater quality monitoring networks. Data for 1994 included in this report are from precipitation records from 10 National Weather Service (NWS) stations, continuous water-level records from 72 wells, and chloride analyses from 13 wells. Cumulative departure from normal precipitation in 1994 was above normal at 9 of the 10 NWS sites. Cumulative departure from normal precipitation for the 10-year period (1985-94) was above normal at 6 of the 10 stations. Annual mean groundwater levels in Georgia in 1994 ranged from about 2.3 feet (ft) lower to about 18.3 ft higher than in 1993. Of the 72 wells summarized in this report, 51 wells had annual mean water levels that were higher, 20 wells had annual mean water levels that were lower and one well had an annual mean water level that was the same in 1994 as in 1993. Record-high daily mean water levels were recorded in two wells tapping the surficial aquifer, one well tapping the Upper Floridan aquifer, one well tapping the Lower Floridan aquifer, two wells tapping the Claiborne aquifer, and one well tapping the Midville aquifer system. These record highs were from about 0.3 foot (ft) to about 1.8 ft higher than previous record highs. Record-low daily mean water levels were recorded in one well tapping the Clayton aquifer, and one well tapping the Dublin-Midville aquifer system. These record lows were from about 0.3 to 1.3 ft lower than previous record lows. Chloride concentration in water from the Upper Floridan aquifer in most of coastal Georgia was below drinking-water standards established by the Georgia Department of Natural Resources and the U.S. Environmental Protection Agency and has not changed appreciably with time. However, chloride concentration in water from some wells that tap the Floridan aquifer system in the Brunswick area exceeds the drinking-water standards. Well-Numbering System Wells described in this report are numbered according to a system based on the USGS index of topographic maps of Georgia. Each 71/2 minute topographic quadrangle in the State has been assigned a six-digit number and letter designation beginning at the southwestern corner of the State. Numbers increase sequentially eastward and letters advance alphabetically northward. Quadrangles in the northern part of the State are designated by double letters; AA follows Z, and so forth. The letters "I", "O", "II", and "OO" are not used. Wells inventoried in each quadrangle are numbered consecutively, beginning with 01. Thus, the fourth well scheduled in the 11AA quadrangle is designated 11AA04.