Structural and Lithologic Controls on Ground-Water Availability in a Granite and Biotite Gneiss in the Conyers, Georgia, Area

Proceedings of the 2003 Georgia Water Resources Conference, held April 23-24, 2003, at the University of Georgia.Historically, the lack of primary permeability in crystalline rocks of the Piedmont physiographic province of Georgia was believed to result in low ground-water yields. However, focused study of lithology, foliations, and fractures in crystalline rocks in the vicinity of the city of Conyers, Rockdale County, Georgia, shows that geologic controls can contribute to high well yields in these rocks. Although surface geologic mapping indicates that the Conyers area primarily is underlain by granite gneiss, borehole video images show that subsurface lithology is an interlayered granite gneiss and biotite gneiss. Preferential weathering of the biotite gneiss is a significant factor controlling well yield. Most of the yield in each of the two wells studied is from openings along lithologic contacts between the granite gneiss and biotite gneiss, or from openings along compositional layering within the biotite gneiss. The biotite gneiss is thin at land surface, but distinctive enough to be mapped. This geologic unit could be useful for identifying high-yielding areas within the granite gneiss