Georgia Water Science Center
This report is available online in pdf format (2 MB): USGS WRIR 98-4233 (
)
Melinda J. Chapman, Thomas J. Crawford, and W. Todd Tharpe
U.S. Geological Survey Water-Resources Investigations Report 98-4233, 46 pages and 2 plates (Published 1999)
The population of the Atlanta Metropolitan area continues to grow at a rapid pace and the demand for water supplies steadily increases. Exploration for ground-water resources, as a supplement to surface-water supplies, is being undertaken by many city and county governments. The application of effective investigative methods to characterization of the complex igneous and metamorphic fractured bedrock aquifers of the Piedmont physiographic province is essential to the success of these ground-water exploration programs. The U.S. Geological Survey, in cooperation with the City of Lawrenceville, Ga., began a study in December 1994 to apply various investigative techniques for field characterization of fractured crystalline-bedrock aquifers near Lawrenceville.
Five major lithologic units were mapped in the Lawrenceville, Ga., area as part of an ongoing study of ground-water resources-amphibolite, biotite gneiss, button schist, granite gneiss, and quartzite/aluminous schist. These units generally are thin in outcrop width, have low angles of dip (nearly 0 to 20 degrees, dip reversals occur over short distances), and exhibit some shearing characteristics. The most productive unit for ground-water resources, on the basis of subsurface data collected through 1997, is the amphibolite. Historically, two wells drilled into this unit are recognized as having possibly the highest yields in the Piedmont region of northern Georgia. The City of Lawrenceville refurbished one well at the Rhodes Jordan Wellfield in 1990, and has pumped this well at an average rate of about 230 gallons per minute since 1995. In general, the composition of water collected from the bedrock wells, regolith wells, and City Lake is similar; calcium and bicarbonate are the dominant cation and anion, respectively. Water from the regolith wells and the lake have lower concentrations of major ions than does water from the bedrock wells. Many of the ground-water samples collected from the Rhodes Jordan Wellfield during October-November 1995, and from the wellfield and three additional observation well sites during August 1996, contain volatile organic compounds. Volatile organic compounds were detected in ground-water samples collected from several bedrock and regolith wells located in urban areas. Trace concentrations of tetrachloroethylene, trichloroethylene, 1,1-dichloroethane, trichlorofluoromethane, 1,1,1-trichloroethane, and cis-1,2-dichloroethene were detected. Methyl-tert-butyl ether (MTBE)-a compound used to increase the octane level in gasoline-was detected at concentrations above expected urban background levels in bedrock wells in the Rhodes Jordan Wellfield. Concentrations of MTBE ranged from 0.6 to 12 micrograms per liter in October-November 1995, and from 0.6 to 26 micrograms per liter in August 1996.
Continuous ground-water-level data suggest that the fractured crystalline-bedrock aquifer (amphibolite unit) at the Rhodes Jordan Wellfield, generally is dewatered to a depth near a productive fracture during the regular pumping cycle of 18 hours per day, 5 days on and 2 days off per week. However, when the stress on the aquifer is increased by extending the pumping period up to as much as 18 days, or by pumping longer that 18 hours per day, the aquifer exhibits an unusual condition of recovery. Areal effects of pumping have been observed at distances of as much as one mile, extending across surface-water drainage divides.
Abstract
Introduction
Background
Purpose and scope
Description of the study area
Methods of investigation
Previous studies
Well and surface-water station numbering systems
Acknowledgments
Regional geology
Geologic mappingÑinterpretations and limitations
Lithologic units
Amphibolite
Biotite gneiss
Button schist
Granite gneiss
Magnetite quartzite
Quartzite and aluminous schist
Diabase dikes
Hydrogeologic sections
Ground-water resources
Well inventory and data compilation
City of Lawrenceville historical well records
Well yields
Relation of well yield to lithology
Amphibolite
Biotite gneiss
Button schist
Granite gneiss
Quartzite and aluminous schist
Interpretation of borehole geophysical logs
Rhodes Jordan Wellfield
Maltbie Street well
Pike Street well
Gwinnett County Airport well
Analyses of ground-water levels
Response of local ground-water levels to pumpage
Areal effects of ground-water withdrawals
Ground-water quality
Summary and conclusions
References cited
Plate 1. Lithologic map showing the distribution of major rock types, fault contacts, and well locations in the Lawrenceville area, Georgia
Plate 2. Hydrogeologic sections showing general dip of lithologic units inferred from surface geologic mapping and subsurface lithologic and fracture data collected from wells in the Lawrenceville area, Georgia
This report is available online in pdf format (2 MB): USGS WRIR 98-4233 ()
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