FRACTURE ZONES AND LITHOLOGIC CHARACTERISTICS ASSOCIATED WITH HIGH-YIELD WELLS IN LAWRENCEVILLE, GEORGIA

A crystalline bedrock ground-water-resource study by the U.S. Geological Survey, in cooperation with the City of Lawrenceville, Ga., a suburb of Atlanta, began in 1994. The study is focused on improving the understanding of hydrogeologic controls affecting well yields in a fractured-bedrock aquifer system. Subsurface geophysical data collected from 21 bedrock wells in Lawrenceville indicate that high-yield fracture zones occur within amphibolite and biotite gneiss units and near lithologic contacts. Productive wells have initial estimated air-lift yields of as much as 200 gallons per minute (gal/min) and sustained yields of as much as 350 gal/min. Borehole geophysical logs, in conjunction with drill cuttings or cores, were used to describe hydrogeologic conditions at the bedrock wells. Caliper and electrical resistivity logs were used to delineate depths to fractures within the wells; and natural gamma logs were used in conjunction with cuttings or cores, to characterize rock type and lithologic units. Fracture orientations were interpreted from digital acoustic televiewer logs, directional borehole radar, and digital-oriented video logs. Fracture orientations in production zones were compared in 11 high-yield wells. At the Rhodes Jordan well field, an area of about 2 acres, dominant strike orientations are east-west, N40-60W, and N40-60E. A second planned Maltbie Street well field is located about 0.9 mile west of the currently active Rhodes Jordan wellfield. At this location, ground-water levels in five high-yield wells, located at distances of as much as 1.25 miles from each other, respond as a connected fracture network during well drilling activities and short-term aquifer tests. Fracture orientations measured at the second well field indicate that dominant strike orientations are east-west and N30-60W. The comparable strike orientations for both production well centers and the observed hydraulic response to pumpage at the Rhodes Jordan well field, suggest a possible regional fracture system.