6 Wrap-up

Fractured aquifers occupy large areas of all continents and, although they are relevant or essential for water supply and other applications, there are many areas of study still to be developed and challenges to be overcome, as presented in this section.

Fractured aquifers pose scientific and methodological challenges because fracture porosity is heterogeneously distributed, and its characterization requires detailed studies. To understand the factors that control fracture heterogeneity, theoretical fundamentals of rock mechanics and structural geology must be applied so that the data collected in the field can be properly organized, allowing the mechanisms controlling the heterogeneity to be revealed.

There has been a significant increase in the application of structural geology studies to the investigation of fractured aquifers. Nevertheless, detailed structural surveys need to be carried out more frequently to build an increasingly sound knowledge about these aquifers. For this to be accomplished, the training of professionals and/or the migration of structural geologists to this area of study is needed.

Another fundamental issue is that geologists and hydrogeologists must interact and carry out a better coordination of their respective work, which will provide a mutual advancement of both specializations. This will certainly help to overcome some of the major difficulties in structural and hydrogeological characterization of fractured media.

Fracture surveys need to be carried out on large outcrops to obtain a representative description of field conditions. Quarries present good opportunities for study because they provide long horizontal scanlines with different orientations, as well as vertical scanlines. The development of technologies for data collection in places of difficult access, such as high vertical exposures with risk of collapse, is progressing. The use of drones will allow for more representative surveys, as they offer the possibility to observe otherwise inaccessible exposures. This facilitates the gathering of more complete data including fracture trace length (i.e., persistence) which is a parameter that faces a restrictive limitation in most traditional surveys due to the usually limited height of vertical exposures that can be directly and safely accessed. Fracture trace length is one of the fundamental geometric parameters used for defining the fracture network configuration, especially connectivity. This, together with the fracture aperture, determines the potential for flow in these media.

The difficulty in estimating the aperture of fractures imposes a significant limitation to the development of mathematical models. For this reason, a single value of aperture is assigned to all fractures in a model. This limitation can lead to unrealistic results, particularly when considering that flow is proportional to the cube of the fracture aperture. As emphasized in Section 4.4, the systematic collection of evidence indicating flow along fractures may lead to a ranking of fracture sets with regard to aperture. Determination of in-situ stresses is another relevant research topic because such stresses affect fracture aperture. It is important to conduct studies that assess how the in-situ stresses vary from place to place and with depth, and to what extent they affect the variation of aperture. This tool could prove to be useful for inferring the orientation of the main flow paths in areas where structural data are scarce such as, for instance, places with wells but without rock exposures.

The extrapolation of fractured aquifer characterization to locations beyond the area where fracture and hydraulic data were collected, is an imposing challenge; to address this issue, it is necessary to make a distinction between the terms “surveying” and “mapping.” Mapping assumes that the mapped elements are in fact where they are represented on the map. Lithological units and some fault zones are large enough to be mapped. On the other hand, a fracture survey is the systematic collection of fracture data but it does not result in a fracture map, as this would demand an exorbitant amount of work due to the required map scale. In addition, such a survey would be applicable only to the area where the data were collected.

Thus, the only way to extrapolate the fracture network characteristics from one place to another is to use statistical analyses, although this approach is often insufficient for some types of applications such as the migration of a contamination plume. Plume migration is a local phenomenon and, therefore, in some cases the accurate location of the fracture or fracture zone along which contaminants are transported needs to be known. Geological contacts and fault zones represented in detailed geological maps need to be considered in the extrapolations because they constrain specific properties of the fracture networks (Sections 3.4 and 4). Fault zones may be addressed in a deterministic manner because many of them can be mapped. Thus, an effort needs to be made to produce detailed geological maps in regions where only regional-scale maps are available. This is the case for nearly all of Brazil and for many other regions throughout the world.

We conclude that, although much has already been done, much remains to be accomplished. Thus, we wrote this book to motivate students and professionals to dedicate themselves to the study of fractured aquifers.