| Summary |
The yield of water wells drilled in crystalline rock aquifers is determined by the occurrence and interaction of open, saturated fractures which decrease with increased depth, outside of the influence of large-scale features such as fault zones. These trends suggest that in the shallow subsurface, (i) fluid flow is controlled by specific fracture types, and (ii) there is a bounding depth below which groundwater flow is significantly reduced. These hypotheses are tested in this paper. The analysis of fracture properties identified in boreholes offers an approach to investigate the depth evolution of groundwater systems in crystalline rocks. Optical televiewer, caliper and heat pulse flow meter logs are utilized to investigate the attributes, distributions, orientations, and the contribution to flow of 570 fractures that intersect 26 bedrock wells drilled in crystalline rocks of North Carolina. Results indicate that the dominant fracture types are foliation parallel fractures (FPFs) (42%), other fractures (32%) and sheet joints (26%). The boreholes are drilled into five lithologic terranes and intersect seven major lithologic and rock fabric types, consisting of: (1) felsic gneiss, (2) andesitic to basaltic flows, (3) diorite, (4) mafic gneiss, (5) gneiss and amphibolite, (6) gneiss and mylonite and (7) schist. The dominant fracture type in the upper 40 m is sheet joints. Rocks with sub-horizontal planar fabric prefer to develop FPFs instead of sheet joints. Fractures with the largest apertures are mostly restricted to shallow depths, where sheet joints dominate. However, some sheet joints and FPFs with large apertures are observed at depths greater than 40 m. Flow intervals identified from borehole logs reveal that sheet joints and FPFs are the dominant fracture type where the majority of flow is observed and are therefore likely to control most of the flow in the shallow subsurface. In contrast, FPFs and other fractures are the dominant conduits for channeling flow at greater depths. Fracture and flow analyses indicate that the majority of flow (> 80%) occurs at depths shallower than 75 m, suggesting the existence of a bounding depth in the shallow subsurface below which permeabilities of fractured crystalline rocks are significantly reduced. |
