An atypical model of groundwater storage in hard rock in semi-arid regions: a weathered compartment

Abstract

Well siting in hard rock terrains within semi-arid regions typically follows two conceptual models of groundwater accumulation. The first, traditionally termed the fracture-controlled stream model, associates structurally controlled drainage networks with brittle structural features. The second, known as the eluvium-alluvium trough model, relates structurally controlled drainages to ductile deformation zones. Both models have demonstrated practical success in groundwater exploration; however, the resulting wells generally exhibit modest yields, typically ranging between 2 and 3 m³/h. In the Lagoa do Bom Pasto area, located in the rural zone of the municipality of Serrinha, Rio Grande do Norte (NE Brazil), within a semi-arid climate and hard rock basement setting, historical records since the 1980s have documented anomalously high well yields, commonly between 9 and 12 m³/h, and an exceptional case reaching 51.4 m³/h. These wells were sited along a structure interpreted in the field as a fracture-controlled stream model. Given the discrepancy between the observed high yields and the expected hydraulic performance in such geological and climatic settings, the area was selected for the development of an alternative conceptual model of groundwater storage in hard rock media under semi-arid conditions. An integrated approach combining geoelectrical surveys, structural geological analysis, and geomorphological mapping enabled the identification of a high-conductivity zone along ductile structural discontinuity. This zone, herein referred to as a weathering compartment, is hydraulically connected in the subsurface to the fracture-controlled stream model. This conceptual model, likely not considered during the original siting efforts, is characterized at depth by the development of pseudo-secondary permo-porosity resulting from weathering processes acting along anisotropic planes associated with gneissic banding. These processes significantly enhance the effective storage capacity of the aquifer, providing an alternative explanation for the anomalously high well yields recorded in the area.