(USGS)

Influence of Geologic Setting on Ground-Water Availability in the Lawrenceville Area, Gwinnett County, Georgia

Lester J. Williams1, Randy L. Kath2, Thomas J. Crawford3, and Melinda J. Chapman4

U.S. Geological Survey Scientific Investigations Report 2005-5136, 43 pages (Published September 2005) ONLINE ONLY

ABSTRACT

Cover photograph: Top left: Differential weathering in an amphibolite-biotite gneiss outcrop. Bottom right: Steeply-dipping joints in a granite gneiss. Notebook (4.5 inches by 7 inches) for scale.Obtaining large quantities of ground water needed for municipal and industrial supply in the Piedmont and Blue Ridge physiographic provinces can be challenging because of the complex geology and the typically low primary permeability of igneous and metamorphic rocks. Areas of enhanced secondary permeability in the bedrock do occur, however, and “high-yield” wells are not uncommon, particularly where careful site-selection techniques are used prior to test drilling. The U.S. Geological Survey — in cooperation with the City of Lawrenceville, Georgia — conducted this study from 2000 to 2002 to learn more about how different geologic settings influence the availability of ground water in igneous and metamorphic bedrock with the expectation that this knowledge could be used to help identify additional water resources in the area.

In compositionally layered-rock settings, wells derive water almost exclusively from lithologically and structurally controlled water-bearing zones formed parallel to foliation and compositional layering. These high-permeability, water-bearing zones — termed foliation-parallel parting systems —combined with high-angle joint systems, are the primary control for the high-yield wells drilled in the Lawrenceville area; yields range from 100 to several hundred gallons per minute (gal/min). Near Lawrenceville, areas with high ground-water yield are present in sequences of amphibolite, biotite gneiss, and button schist where the structural attitude of the rocks is gently dipping, in areas characterized by abundant jointing, and in topographic settings with a continuous source of recharge along these structures.

In massive-rock settings, wells derive water mostly from joint systems, although foliation-parallel parting systems also may be important. Wells deriving water primarily from steeply-dipping joint systems typically have low yields ranging from 1 to 5 gal/min. Joint systems in massive-rock settings can be identified and characterized by using many of the methods described in this report. Geologic mapping was the primary method used to determine the distribution, variability, and relative concentrations (intensity) of joint systems. In the subsurface, joints were characterized by taking orientation measurements in the open boreholes of wells using acoustic and/or optical televiewers.

In this investigation, the only practical approach found for locating areas of high ground-water potential was first through detailed geologic mapping followed by test drilling, borehole geophysical logging, and aquifer testing. Geologic methods help characterize both large- and small-scale structures and other lithologic and stratigraphic features that influence development of increased secondary permeability in the bedrock. The rock types, discontinuities, depth of weathering, topographic position, and recharge potential — which were the principal factors assessed through detailed geologic mapping — must be evaluated carefully, in relation to one another, to assess the ground-water potential in a given area.


TABLE OF CONTENTS

Abstract

Introduction

Purpose and Scope

Methods of Study

Previous Studies

Acknowledgments

Description of the Study Area

Water Use

Geologic Setting

Hydrogeologic Setting

Recharge and Regolith/Bedrock Storage Capability

Water-Bearing Zones

Influence of Geologic Setting on Ground-Water Availability

Geologic Factors

Compositionally Layered-Rock Settings

Example of a Productive Well (14FF59) in a Compositionally Layered-Rock Setting

Examples of Other Wells in Compositionally Layered-Rock Settings

Aquifer Tests of Productive Wells in Compositionally Layered-Rock Settings

Influence of Joint Systems

Conceptual Model of Ground-Water Availability

Methods for Identifying and Characterizing Foliation-Parallel Parting Systems

Massive-Rock Settings

Example of a Well (14FF58) in a Massive-Rock Setting

Other Wells in Massive-Rock Settings

Conceptual Model of Ground-Water Availability

Methods for Identifying and Characterizing Joint Systems

Potential for Future Ground-Water Development

Summary and Conclusions

References Cited

 

1 U.S. Geological Survey, Atlanta, Ga.
2 Associate Professor, University of West Georgia, Carrollton, Ga.
3 Professor Emeritus, University of West Georgia, Carrollton, Ga.
4 U.S. Geological Survey, Raleigh, N.C.


REPORT AVAILABILITY

This report is available online in pdf format: Scientific Investigations Report 2005-5136 (5.9 MB)
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