Relation of Well Yields to Fracture Patterns and Lineaments in the Rolesville Batholith, Wake County, North Carolina

Abstract

In Wake County over 140,000 county residents (about 25% of the population) rely on groundwater resources for drinking and other everyday uses. The increasing population of the county is causing an increase in demand for water resources. As surface water supplies for the county reach maximum capacity, groundwater will become an increasingly important source of water.

Most drinking water supply wells in the county are completed in the crystalline bedrock. There is little matrix permeability in these rocks, therefore groundwater storage and flow is primarily through secondary fractures. The three goals of this study are 1) to determine via field study the primary surface fracture directions present at pavement outcrops within the Rolesville granite area of Wake County, 2) to perform a larger scale land-surface lineament analysis using aerial photographs and remote sensing data to compare with the field data results, and 3) to use available well yield data to correlate with the observed primary surface fracture directions to identify possible areas within the Rolesville granite of Wake County where the subsurface fracturing will allow for the construction of wells with higher than average yields.

A total of 151 individual joints and fractures were identified by field study and were recorded. A rose diagram of these fractures finds the primary fracture azimuth to be 90° ± 5°, with 30.67 % of all measured fractures falling in this range. The secondary fracture azimuth in the field is approximately 0° ± 5° (18 % of all fractures). The third most common fracture azimuth is roughly 280°-290° (14 %), and the fourth most common azimuth is 10°-15° (8.67 %).

The computer program ENVI was used to identify a total of 2362 lineaments on aerial photographs of the same geographic area where the fractures were located in the field study. The rose diagram of these land-surface lineaments also supports the E-W trend (90° ± 5°) found in the fracture data measured in the field. Lineaments in this range account for 15.66 % of those found. The second most common direction amongst the computer lineaments is 0° ± 5°. The number of fractures in this direction is roughly half that found in the 90° trend, accounting for 7.28 % of all lineaments found.

The consistency between the results of the computer and field data confirm the validity of using the computer program ENVI for identifying lineaments in the Rolesville batholith portion of Wake County. This suggests that the directional filter of the program ENVI may be a useful tool in helping locate optimal sites for new high yielding wells.