Hydrodynamic, geochemical and isotopic analysis for the assessment of low temperature hydrothermal systems in the Argentine plain

Abstract

A hydrogeological model of the low temperature aquifer system in the South of the pampean plain is presented, with emphasis on dynamic, geochemical and isotopic groundwater studies. Isotopic analysis (¹⁸O, ²H and ³H) and geothermometers (SiO₂, Na/K, Na/K/Ca, Na/Li and Mg/Li) were used. The area is part of the lower megablock associated to a geological regional fault system (N-S). The deep aquifers are related to a multilayer sedimentary system associated with different stages of a Tertiary fluvial network. Three deep multilayer aquifers were defined taking into account hydraulic behaviors, depths and groundwater quality. The two shallower and less confined aquifers had higher salinities and similar isotopic composition to the unconfined aquifer (-4.8 ‰≈δ¹⁸O; δ²H≈-30 ‰) suggesting hydraulic connection between them. The deepest and more confined level, showed water more fresh and a more impoverished isotopic composition (-6.6 ‰≈δ¹⁸O; δ²H≈-43 ‰), suggesting allochthonous recharge. In this deepest level, the sulphate water type and hydraulic estimations suggest aging from recharge areas, which is supported by very low ³H values (≈ 1.5±0.4 UT). The groundwater has temperatures (≤ 35.8 °C) exceeding up to 10 °C those expected by the laying depths. The geothermal anomaly, according to the regional tectonic model, is due to a possible cortical thinning, the probable presence of deep granitic rocks (U, Th, K) which would have capacity to produce high internal radiogenic heat and the geological faults that facilitate the circulation of water at different depths.