Estimate of water reserves in the São Joaquim Fault Thermal Aquifer System, Chapada dos Veadeiros, Goiás

Abstract

The Chapada dos Veadeiros region, northeast of Goiás State, Central Brazil, shows occurrences of thermal waters associated with fractured structures in Aurumina Suite granitic rocks and metasedimentary rocks of the Araí, Traíras and Paranoá groups. The aim of this study was to estimate the volume of water available in the thermal aquifer system associated with the São Joaquim Fault, through the analysis of hydrogeological parameters such as fracturing index, saturated thickness and storage coefficient. The methodology was based on the hydrothermal flow conceptual model proposed for the region by Junqueira et al. (2022), in which the main controlling factors for the occurrence of thermal waters are regional groundwater flow, the geothermal gradient, and the presence of open fractures at minimum depths of about 1 km. The exploitable reserve or availability was obtained as a percentage of the permanent reserve, which was defined, considering conservative criteria regarding the input parameters in the equations. The aquifer system associated with the São Joaquim Fault is conditioned by the presence of open fractures at depths of up to 1 km, which enable the deep hydrothermal flow. The effective recharge areas of the thermal waters correspond to the highest topographic portions of the Rio Preto and Santana ranges, located in the northeastern sector of Chapada dos Veadeiros, characterized by elevations above 1.100 m and slopes below 20%. In these sectors, infiltration occurs mainly on exposed rock surfaces or within shallow soil profiles. The higher elevation establishes a regional hydraulic gradient that directs infiltrated water to deeper levels, where the natural geothermal gradient raises the temperature of the flow, enabling its ascent and thermal exudation along structural conduits (faults/fractures). For the reserve estimates, the following parameters were adopted: (i) an interconnected fracturing index ranging from 2.0 to 2.5% in the confirm areas of naturally warm water occurrence; (ii) saturated thicknesses of the aquifer varying according to the hydrostratigraphic units considered; and (iii) 5% of the permanent reserve (comprising the saturation and pressure components), defined as the exploitable fraction in the availability estimates. Studies carried out in the Caldas Novas region, Southeast Goiás State, and in the Federal District show similarities in the structural complexity and anisotropic behavior of the fractured aquifers, but with different aquifer potential, due to differences in relief, soil types, geological context and degree of water stagnation in the aquifer. The results obtained should support the actions for the operation, management of exploitation systems and protection of groundwater resources, fundamental to maintain the sustainability of thermal waters in the face of growing demands. Among the good management practices that must be applied to the region aquifer, the following stand out: active monitoring of the potentiometric levels of wells and spring flows; and monitoring of water quality.