Influence of Foliation Fracture Systems on Water Availability in the Lawrenceville, Georgia, Area
Williams, Lester J.
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The city of Lawrenceville, Georgia, pumps ground water mainly from wells completed in a system of productive fractures formed parallel to foliation and compositional layering in layered crystalline rocks. Much less production is obtained from planar, more steeply dipping joints intersecting boreholes. The combination of subhorizontal sets of foliation fractures and a well-developed network of steeply dipping joints that connect these fractures to regolith source water creates a fracture system that can sustain large well yields ranging from 100 to 400 gallons per minute (gal/min). Joints and joint sets generally are low yielding and produce water ranging from 1 to 5 gal/min, and in few cases as high as 30 to 35 gal/min where the joint opening is created from dissolution of a pre-existing mineral infilling. Fractures formed from weathering along foliation planes yield water ranging from 1 to 15 gal/min for individual foliation partings and ranging from 50 to 100 gal/min for larger openings. The model presented in this paper indicates that a “weathering wedge” or a zone of weathered rock either originates from the outcrop area or develops from steeply dipping joint systems transmitting water into the bedrock, or a combination of these two. Once a ground-water flow path is established, progressive differential weathering along foliation planes and layering forms zones of increased bedrock permeability. Where foliation fracture systems dominate the flow system, this model could be used to better understand ground-water availability and recharge to bedrock.