Hydrogeological framework and numerical model of the Bajos Submeridionales System, Chaco Argentino

THALMEIER, M.B; RODRIGUEZ, L.; HEREDIA, J.; VEIZAGA, E.

Brasil

Congreso; 47th IAH Congress; 2021

The Bajos Submeridionales (BBSS) is located in north-central Argentina, South America. It covers 54,280 km 2 in the distal part of the Salado-Juramento fluvial megafan, Gran Chaco. The BBSS is affected by extreme events of droughts and floods that cause serious socio-productive problems. Geomorphological and stratigraphic antecedent data, with the support of hydrochemistry and environmental isotopes of surface waters and groundwaters data, were integrated to identify hydrogeological units (HU) to contribute to the conceptual understanding of the groundwater flow system of the BBSS. The sedimentary column simplification in different HU summarizes the fundamental characteristics of the dynamics of underground flows and their interactions with surface water bodies and contributions from the atmosphere. The hydrogeological system is defined by four HUs, associated with the main geological formations from the Late Cenozoic to the Holocene. HU4 consists of sands from the middle and lower members of the Chaco Fm that determines a confined aquifer. HU3 is composed by sandy clays of the Paraná Fm that behaves as an aquitard. HU2 consists of fluvial sands of the Ituzaingó Fm and Chaco Fm upper member and constitutes a semi-confined aquifer. HU1 contains silty clay of the lower member of the Fortín Tres Pozos Fm that confers local semi-confinement conditions to the underlying units (HU1b); and by silt-clay of the Fortín Tres Pozos Fm upper member and Urundel Fm that constitute the phreatic aquifer (HU1a). The regional flow patterns of the HUs have a NW-SE direction, associated with the Chaco topography in HU1, with hydraulic gradients between 1 m/3 km and 1 m/8 km. Other HUs circulation have similar patterns. The conceptual model suggests the presence of regional and local flows, differing from Toth's scheme in which intermediate flows are also present. The circulation pattern of the BBSS is usual for large plains systems. At the local level, there are particularities reflected on the chemical and isotopic composition of groundwater. Local flows are conditioned by the vertical circulation of water, influenced by hydroclimatic pulses and evaporation, which contribute to the hydrochemical heterogeneity of waters in HU1. Regional recharge occurs in the Sub-Andean System and local recharge from atmospheric water and flows from underlying units. Both local and regional flows discharge into the Golondrinas-Calchaquí creek and the Salado-Juramento River and ultimately to the Paraná River. The conceptual model was numerically tested under a steady state flow regime. The numerical model was instrumental for evaluating the water balance components for the entire aquifer, highlighting the occurrence of flow exchanges between HUs. Transient simulations will follow to investigate the system evolution after historical land use changes, and to explore interventions on it with environmental sustainability criteria. This study was funded by projects nº2559-IGME (Spain), nº2010-019-14-ASaCTeI (Argentina), and PICT-2016-1334-ANPCyT (Argentina).