Structural and neotectonic criteria for location of water wells in semi-arid crystalline terrains: a preliminary approach in the eastern domain of Rio Grande do Norte state, northeast Brazil

Abstract

The traditional methods to locate water wells in crystalline terranes are being re-evaluated aiming to improve the success rate of the procedures. We describe the preliminary approach being used in the eastern crystalline domain of Rio Grande do Norte (NE Brazil),encompassing areas around Santo Antonio and Santa Cruz cities. Emphasis is given to re-evaluate the classical structural criteria in the light of modern concepts on rock deformation and the regional tectonic framework, especially in the context of the neotectonic stress regime. At a first step, the structural analysis is applied to better understand the formation and/or reactivation of brittle structures, their chronology, and past and present kinematics. Then, these informations are used to understand water flow and accumulation in fracture aquifers, and the reasons for success or failure in the location of water wells already drilled in the region, in the hope to establish a more consistent and practical routine methods. Lineament maps of the studied areas were generated to determine the main fracture sets for well location. Structural data were collected near selected water wells, locally combined with'geophysical data. Neotectonic reactivation is evaluated both from regional and local information, as available. In some cases, as in Tararaca Farm near Santo Antonio, a correlation was found between a fracture zone and a photointerpreted lineament following a drainage; the favorable orientation of the fractures, at a high angle to the present (neotectonic) extension direction, appears to convey an important control in water accumulation and well production. In other cases, as in the Santa Rita Farm near Santa Cruz, the photolineament and the drainage do not correspond to a fracture zone; and the accumulation of water is related to the opening of foliation surfaces by weathering, its intersection with different fracture sets and favorable recharge conditions provided by locally thicker, alluvial covers, forming a through-like structure.