Abstract: The U.S. Geological Survey continuously monitors groundwater levels in a network of wells completed in major aquifers in Georgia. This network includes 18 wells in the surficial aquifer, 15 wells in the upper and lower Brunswick aquifers, 64 wells in the Upper Floridan aquifer, 13 wells in the Lower Floridan aquifer and underlying units, 12 wells in the Claiborne aquifer, 1 well in the Gordon aquifer, 11 wells in the Clayton aquifer, 12 wells in the Cretaceous aquifer system, 2 wells in Paleozoicrock aquifers, and 7 wells in crystalline-rock aquifers. Data from these 155 wells were evaluated to determine whether mean-annual groundwater levels were either within, below, or above the normal range during 2003, based on summary statistics for the period of record. Information from these summaries indicates that water levels in 120 of the 155 wells (77 percent) monitored during 2003 ranged from predominantly normal to above normal. INTRODUCTION Monitoring groundwater levels is important for management of water resources. The U.S. Geological Survey (USGS)—in cooperation with State, Federal, and local agencies—collects and disseminates groundwater-level data from a network of wells completed in major aquifers in Georgia (Fig. 1). This paper presents an overview of groundwater levels in selected aquifers in Georgia during 2003 based on continuous water-level measurements obtained from 155 wells. All wells are equipped with electronic data recorders that register at 60-minute intervals and are retrieved monthly. Eighteen of the wells are equipped with real-time satellite telemetry that transmits data every 4 hours. Telemetered data are displayed on the USGS Georgia District Web site at http://water.usgs.gov/ ga/nwis/current?type=gwx Method of Study Monthly median water levels for 2003 were compared to period-of-record monthly normal water levels to determine if water levels were either above normal, below normal, or normal. In this paper, the normal range is defined as those monthly water-level observations that lie between the 25th and 75th percentiles (first and third quartiles), also known as the interquartile range, for the period of record (Leeth and others, 2003). This can be shown by examining a graphical representation of these values known as a boxplot (Fig. 2.). The results of comparing monthly median water levels for 2003 with period-ofrecord monthly normal water levels are graphically depicted on maps (Fig. 1) by either an up arrow—2003 monthly mean water levels above period-of-record normal range; a down arrow—2003 monthly mean water levels below period-of-record normal range; or a circle—2003 monthly mean water levels within the period-of-record normal range. Occurrence of Groundwater Contrasting geologic features and landforms of the physiographic provinces in Georgia affect the occurrence of groundwater in the State. Surficial aquifers are present in each physiographic province and generally are under water-table (unconfined) conditions. The most productive water-bearing units are in the Coastal Plain in the southern half of the State and include, in order of increasing depth, the surficial and Brunswick aquifer systems (Clarke, 2003); Upper and Lower Floridan aquifers; Claiborne, Gordon, and Clayton aquifers; and Cretaceous aquifer system. In the Piedmont and Blue Ridge Provinces in northern Georgia, groundwater occurs in the regolith and in fractures in crystalline bedrock (referred to as “crystalline-rock aquifers”). In the Valley and Ridge and Appalachian Plateau Provinces, groundwater occurs largely in secondary openings in folded and faulted sedimentary and metasedimentary rocks (referred to as “Paleozoic-rock aquifers”). Changes in groundwater levels are caused by changes in aquifer storage—when recharge exceeds discharge, groundwater levels rise, and when discharge exceeds recharge, groundwater levels decline. Recharge varies in response to precipitation and surface-water infiltration to an aquifer. Discharge occurs as natural flow from an aquifer to streams and springs, as evapotranspiration, and as withdrawal from wells. Water levels typically show a cyclic pattern of seasonal fluctuation, with higher water levels in the winter and spring due to greater recharge, and lower water levels in the summer and fall due to less recharge, greater evapotranspiration, and increased pumpage. Groundwater pumpage is the most significant human activity that affects the amount of groundwater in storage and rate of discharge from an aquifer (Taylor and Alley, 2001).
Publication Year: 2005
Publication Date: 2005-04-01
Language: en
Type: article
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