U.S. Geological Survey Scientific Investigations Report 2005-5047, 77 pages (Published July 2005) ONLINE ONLY
During 1997, the Dougherty County Health Department sampled more than 700 wells completed in the Upper Floridan aquifer in Dougherty County, Georgia, and determined that nitrate as nitrogen (hereinafter called nitrate) concentrations were above 10 milligrams per liter (mg/L) in 12 percent of the wells. Ten mg/L is the Georgia primary drinking-water standard. The ground-water flow system is complex and poorly understood in this predominantly agricultural area. Therefore, the U.S. Geological Survey (USGS) — in cooperation with Albany Water, Gas and Light Commission — conducted a study to better define ground-water flow and water quality in the Upper Florida aquifer in the southwestern Albany area, Georgia.
Ground-water levels were measured in the southwestern Albany area, Georgia, during May 1998 and March 1999 (spring), and October 1998 and September 1999 (fall). Groundwater levels measured in 75 wells open only to the Upper Floridan aquifer were used to construct potentiometric-surface maps for those four time periods. These maps show that ground water generally flows from northwest to southeast at gradients ranging from about 2 to greater than 10 feet per mile. During spring and fall 1998, ground-water levels were high and mounding of the potentiometric surface occurred in the central part of the study area, indicating a local recharge area. Water levels declined from December through February, and by March 1999 the mound in the potentiometric surface had dissipated.
Of the 75 wells in the potentiometric network, 24 were selected for a water-quality network. These 24 wells and 1 spring were sampled during fall 1998 and spring 1999. Samples were analyzed for major chemical constituents, selected minor constituents, selected nutrients, and chlorofluorocarbons (CFC). Water-quality field measurements — such as water temperature, pH, specific conductance (SC), and dissolved oxygen (DO) — were taken at each well. During August 2000, a ground-water sample was collected and analyzed for selected sewage tracers. During March 2001, water samples from selected wells were analyzed for nitrogen and oxygen isotopes. Age-dating analysis using CFCs yield apparent groundwater ages that range from modern to greater than 50 years.
The chemistry of ground water in the Upper Floridan aquifer varies widely throughout the southwestern Albany area, Georgia, and in general represents the chemistry commonly found in recharge areas. From fall 1998 through spring 1999, median values of pH, SC, and DO concentration were 7.6 standard units, 266 microsiemens per centimeter at 25 degrees Celsius (μS/cm), and 5.6 mg/L, respectively. The SC is highest (350 – 400 μS/cm) where mounding of the potentiometric surface exists. Specific DO concentrations indicate an area of anoxic ground water in the north-central part of the study area.
Water samples indicate that ground water in the study area is dominated by calcium and bicarbonate ions, which is consistent with the limestone lithology of the aquifer. About 25 percent of the samples contained sodium and chloride at ratios similar to those in rainfall, indicating a close proximity to recharge areas. The remaining water samples, however, had sodiumchloride ratios less than 0.90, the ratio in Tift County, Georgia, rainfall samples. These low sodium-chloride ratios are consistent with chloride enrichment. Minor constituent and nutrient concentrations typically are below laboratory reporting limits; however, the maximum nitrate concentration measured during the study period was 12.2 mg/L, and the median concentration for the study period was 3.0 mg/L. Samples collected during 1999 had a higher median nitrate concentration than the 1998 samples. Regression analysis indicated that nitrate concentrations are related exponentially to chloride concentrations.
Four distinct groups of ground-water-quality samples, plus four unique samples, were identified using cluster analysis. Water-quality groups I and II occur in the north-central part of the study area and generally are chemically similar. These groups represent an area of anoxic ground water where reducing conditions prevail. Water-quality group III occurs in the central part of the study area and represents a mixing zone where three chemically different ground waters merge. Water-quality group IV occurs in the south-central part of the study area, has nitrate concentrations less than 2.5 mg/L, and probably represents background conditions of the Upper Floridan aquifer in this area. The four unique samples collected from two wells represent ground water with elevated nitrate concentrations. Ground water drawn from one well possibly is contaminated by the application of biosolids in fields upgradient from the study area. Ground water from the other well apparently is influenced by agricultural fertilizer.
Abstract
Introduction
Purpose and Scope
Description of Study Area
Previous Investigations
Acknowledgments
Methods of Investigation
Well and Spring Naming System
Well Inventory and Measurement of Ground-Water Levels
Collection and Chemical Analysis of Water Samples
Age-Dating Ground Water Using Chlorofluorocarbons
Statistical Analysis of Water-Quality Data
Hydrogeology of the Upper Floridan Aquifer
Undifferentiated Overburden
Upper Floridan Aquifer
Lisbon Confining Unit
Ground-Water Flow in the Upper Floridan Aquifer
Water Levels
Flow Directions and Gradients
Recharge and Ground-Water Ages
Ground-Water Chemistry and Water Quality in the Upper Floridan Aquifer
Description of Ground-Water Chemistry and Water Quality
pH, Specific Conductance, and Dissolved Oxygen
Major Chemical Constituents
Minor Chemical Constituents
Dissolved Nutrient Concentrations
Nitrate
Nitrogen and Oxygen Isotopes
Emerging Contaminants
Spatial Patterns in Ground-Water Chemistry and Water Quality
Spatial Distribution of Specific Conductance
Spatial Distribution of Nitrate as Nitrogen
Identification and Spatial Distribution of Water-Quality Groups
Unique Samples
Water-Quality Groups I and II
Water-Quality Group III
Water-Quality Group IV
Summary and Conclusions
References Cited
Appendix A
Appendix B
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