Fractured aquifers: a review of geological constraints and methods of investigation

Abstract

Detailed investigation of the geological conditions that constrain the circulation of groundwater in fractured aquifers allows the elaboration of conceptual models which are necessary for the suitable design of hydraulic tests and sampling, whose results, in turn, improve the conceptual models. This procedure leads to the prediction of the preferential flow pathways and, therefore, saves time and diminishes costs. In Brazil there is still little experience in the study of fractured aquifers. The majority of the investigations have dealt with the indication of criteria for the location of wells and are based on the relation between specific capacity or discharge and a number of factors, such as lineaments, lithology, topographic setting, etc., that traditionally are thought to be influential in well production. It is herein suggested that well production data be presented in the form of cumulative distributions, as average and median values alone, as presented in the majority of studies, are not enough to separate data populations of distinct behaviour and limit assessment of the results. This type of study should be conducted as a preliminary evaluation, as it suggests the general outline of the behaviour of the fractured aquifer at regional and semi-detailed scales. More recently, and mainly outside of the country, studies have also been conducted that relate neotectonics and/or current stress fields with the orientation of transmissive fracture zones, that is, with preferential flow pathways,. These have concluded that recent or current tectonics are the predominant control on the circulation of groundwater. However, the origin of the stresses that determine the attitude of the more transmissive fractures is still a matter of debate and may be influenced by local factors. The present work provides a synthesis of theoretical fundamentals with regard to groundwater flow in fractured aquifers, brittle deformation and generation of extensional fractures, geometric characteristics of fractures as a function of scale and lithology, and direct and indirect methods of fracture survey. Furthermore, this work also presents an analysis of well data regarding the factors that control flow and indicates methods for identifying preferential flowpaths in fractured media.