Hydrochemistry of hot springs from Caldas Novas Thermal Complex, Brazil

Abstract

Central Brazil is home to the world's greatest geothermal water complex that is unrelated to magmatism. Hot springs arise along a metamorphic terrain in two main locations 35 km apart, i.e. Caldas Novas and Rio Quente cities. A large-scale dome of 20 × 12 km oval feature with a long axis going NNW-SSE, primarily made of quartzite from the Paranoá Group and surrounded by schists of the younger Araxá Group, works as the aquifer's main recharge area for rainwater. This geological structure separates both cities by a few kilometers and grants its waters a few chemical differences. Three groups of samples from the area, consisting of rainwater, surface water, and groundwater have been analyzed for major and minor constituents. Two major types of groundwater composition have been identified. The first occurs at Rio Quente city, featuring a moderate temperature (37.5 °C), slightly acidic pH (mean = 6.3 ± 0.3), lower total dissolved solids (TDS, mean = 59.6 ± 9.8 mg/L), and lower dissolved concentrations of Ca²⁺ (mean = 5.7 ± 3.1 mg/L), Mg²⁺ (mean = 2.0 ± 1.1 mg/L), and HCO₃⁻ (mean = 20.0 ± 2.2 mg/L). The second arises at Caldas Novas city, exhibiting relatively higher temperature (mean = 41.9 °C), pH (mean = 7.3 ± 0.6), TDS (mean = 147.1 ± 28.6 mg/L), and dissolved concentrations of Ca²⁺ (mean = 16.4 ± 8.4 mg/L), Mg²⁺ (mean = 7.4 ± 2.6 mg/L), and HCO₃⁻ (mean = 99.8 ± 23.5 mg/L). The crystalline rocks terrain, while extensively weathered, also provide a wide range of minor elements to those waters, which are chiefly found on the suspended solids fraction rather than in its dissolved form. Dissolved Pb and Fe exceeded the maximum contaminant levels for drinking water in some surface and groundwater samples, whilst some calculations indicated that P, K, Mn, Fe, Ni, Cu, Zn, Rb, Pb, Br and Ba tend to remain adsorbed in the particulate matter. However, the use of the USSL salinity diagram pointed out that all analyzed water samples offer no hazard to irrigation in most soil types. Subsurface reservoir temperatures were also estimated by means of different solute geothermometers and, among them, the quartz geothermometer yielded the most reasonable values.