Stream base flow and potentiometric surface of the Upper Floridan aquifer in south-Central and southwestern Georgia, November 2008

Abstract

An investigation to document groundwater levels and stream base flow in the lower Chattahoochee-Flint and western and central Aucilla-Suwanee-Ochlockonee River basins during low-flow conditions was conducted by the U.S. Geological Survey in November 2008. During most of 2008, moderate to severe drought conditions prevailed throughout southwestern Georgia. Groundwater levels were below median daily levels throughout most of 2008; however, in some wells, groundwater levels rose to median daily levels by November. Discharge in most of the streams in the study area also had risen to median levels by November. The potentiometric surface of the Upper Floridan aquifer was constructed from water-level measurements collected in 21 counties from 376 wells during November 1-10, 2008. The potentiometric surface indicates that groundwater in the study area generally flows to the south and toward streams except in reaches discharging to the Upper Floridan aquifer. The degree of connection between the Upper Floridan aquifer and streams decreases east of the Flint River where the overburden is thicker. Decreased connectivity between ground and surface water is evident from the stream-stage altitudes measured in November 2008 east of the Flint River, which are not similar to water-level altitudes measured in the Upper Floridan aquifer. Stream-stage measurements were collected at 111 sites-26 U.S. Geological Survey streamgaging sites and 85 additional synoptic sites without gages. Streamflow measurements were made at 87 of the sites during November 2008 and were used to estimate base flow. The measurements indicate that stream reaches range from losing up to 10 cubic feet per second to gaining up to 4,559 cubic feet per second; five stream reaches were determined to be losing stream reaches. Of the 11 stream reaches in the Alapaha River subbasin, 7 were dry when measured in November 2008.

Groundwater-flow budget for the lower Apalachicola-Chattahoochee-Flint River Basin in southwestern Georgia and parts of Florida and Alabama, 2008–12

“…Regionally, groundwater flow in the Upper Floridan aquifer is from the northwest toward the southeast as indicated from synoptic potentiometric maps (Gordon and Peck, 2010;Kinnaman and Dixon, 2011), The simulated direction of local flow from recharge areas toward streams in the study area is superimposed on the direction of regional flow (Jones and Torak, 2006). Simulated flow rates in the Upper Floridan aquifer in the study area are highest where the aquifer discharges along major streams.…”

“…5). Losing reaches of streams may also provide inflow to the groundwater system (Gordon and Peck, 2010) as well as karstic sinks and lakes, but these sources are likely much smaller components of inflow than direct or indirect recharge (Jones and Torak, 2006). Groundwater may also flow into the study area from adjacent or updip equivalent aquifer units.…”

“…Groundwater pumping also removes some water from the groundwater system, although some excess irrigation may recharge the shallow groundwater system. Some groundwater flows out of the study area, likely toward the south and southeast following the dip of the Upper Floridan aquifer and where indicated by potentiometric surfaces (Gordon and Peck, 2010;Kinnaman and Dixon, 2011;Williams and Kuniansky, 2015). Some groundwater may also leave the system through evapotranspiration from the shallow part of the groundwater system.…”

“…In the model, the general-head boundary cells along lateral boundaries of layer 3 were simulated as spatially variable, but constant in time, although measured values indicate the cells are temporally variable (Gordon and Peck, 2010;Kinnaman and Dixon, 2011;Gordon and others, 2012). This assumption may result in larger or smaller flows across the model boundaries and may result in an inability to accurately simulate groundwater levels near the boundaries.…”

“…Additional groundwater pumping data were taken or derived from Marella and Dixon (2015) and the USGS National Water Information System (NWIS) database (U.S. Geological Survey, 2017). In addition, groundwater-level data are available for three synoptic surveys conducted during the study period (Gordon and Peck, 2010;Kinnaman and Dixon, 2011;Gordon and others, 2012), as well as continuous groundwater-level data at 31 sites and continuous stream stage and streamflow data at 27 sites (appendix 1).…”