4.5 Highlights on Groundwater Flow along Fracture Networks with Opportunities to Exercise Knowledge Gained by Reading Sections 1 through 4‌

Highlights on groundwater flow along fracture networks dealt with in sections 3.5 and 4.3 can be summarized as follows.

• Reactivation of previously existing planar structures (e.g., foliation and older fractures) through shear or opening can increase fracture transmissivity. This is a common phenomenon because fractured rock masses usually have several fracture sets, some of them with orientations that facilitate reactivation during later tectonic events. Fractures that are reactivated will likely have an increased aperture, particularly if this occurs under the current tectonic stress field.
• Because of reactivation, rocks showing anisotropy (e.g., foliation in gneisses) and pre-existing discontinuities (e.g., bedding surfaces and frequent intercalation of different lithologies) will tend to have a denser and more connected fracture network when compared to a massive and homogeneous rock body. These characteristics increase the potential for transmitting water.

• Fracture persistence is key for the connectivity of a fracture network having important implications for flow over larger distances, i.e., hundreds of meters or kilometers.

• The current stress field may control the final stage of the brittle deformation history and can influence the aperture and transmissivity anisotropy of a fracture network. This occurs because the critically stressed fractures, i.e., the ones parallel to or at an angle smaller than 30° with the current maximum principal stress have favorable orientations for slip or dilation. Consequently, they are potential groundwater flow pathways.
• High dip fractures, parallel and close to a steep topographic slope can be more transmissive due to the influence of the laterally free slope face on the in-situ stress.This is called effect of topography and locally influences the in-situ stress field within distances of about a 100 m from steep slope faces.
• Orientation of the more transmissive fractures can vary substantially over tens of kilometers, thus, the simple extrapolation of results from one region to another is not recommended. Local data should always be collected and lineament interpretation can be an auxiliary method for the identification of the main directions of high-dip fractures.
• Fracture interactions, along with transmissivity values of fracture sets, bear a strong influence on the configuration of the fracture network and its flow anisotropy. These interactions, such as crosscuttings and abutments, are to a great extent controlled by the brittle deformation history. Thus, a better characterization of the succession of tectonic events is important for constructing conceptual models as well as for finding out similarities between different regions.

Exercises 15 through 17 offer opportunities to learn more on some of the aspects listed above by analyzing data collected in field work (as did Exercise 12 that was provided in Section 3.7). The exercises provide photographs, stereograms, drawings of outcrop walls and scanlines, as well as information on: orientation of fractures belonging to different sets; features present on fracture faces; fracture persistence and patterns (parallel or conjugate); evidence of groundwater flow along specific fractures; interaction between fracture sets. The solutions of the exercises explore the analysis and determination of preferential groundwater pathways, brittle deformation history, connectivity, and flow anisotropy. Links to the exercises 15 through 17 are provided here: Exercise 15; Exercise 16; and Exercise 17.