Bajos Sumeridionales (BBSS) is an extensive region (54 280 km2) on the Chaco Austral in Argentina. Mean annual precipitation is 950 mm, ranging from 400 mm/y to 1600 mm/y due to the occurrence of hydroclimatic extremes. Potential evapotranspiration reaches 1500 mm/y, and mean annual temperature is 21oC. From a geomorphological perspective, it is located at the distal portion of the Salado del Norte river alluvial mega-fan. It presents a smooth NW-SE slope. Land depressions are occupied by permanent or semi-permanent lagoons. Northwestern surface water divide is located on the Otumpa Hills, an uplift block emerged as a result of regional neotectonics movements. The regional bound of the hydrogeologic system is unknown. It could extend from a recharge zone on Subandinas mountain range on the West end of the region, up to a regional discharge level imposed by the Paraná River on the East end. As is typical on large plain hydrogeological systems, local recharge/discharge vertical flows condition the flow pattern, though slow, regional, chemically evolved flows exist. Intermediate scale flows do not play a relevant role due to low topographic gradients, less than 1 0/00. All flow types discharge locally in numerous surface water bodies and crypto-wetlands, and their interaction is dependent on hydroclimatic pulses. These flow features make difficult to contrast Toth´s hydrogeological conceptual model proposed for sedimentary basins. Nonetheless, some previous works postulated the validity of this model, although with limited field contrast, while others indicated an hydrochemical evolution that can be considered consistent with this conceptualization. A new geological interpretation of the regional system allowed defining consistent hydrogeologic regional units that assisted a new hydrochemical study. Preliminary environmental isotopes results (18O y 2H) presented in this work enabled stablishing a complex and sectorized hydrogeologic flow system for the BBSS. In the Southern portion, water circulation would correspond to the current alluvial mega-fan of the Salado del Norte river, with recharge originating on the Subandinas mountain range. In the northern portion, it would correspond to local flows, with a likely contribution from deep thermal flows. Both sectors would discharge into the Salado del Norte river, and finally, into the Paraná river. This sectorization would highlight that the uplift of the Otumpa Hills, which occurred after the formation of the mega-fan, conditioned its primitive flow pattern. Besides, results have shown the strong influence of climatic conditions on water chemistry.
This study is developed under project nº 2559 from Instituto Geológico y Minero de España, project nº 2010-214-16 from ASaCTeI (Santa Fe, Argentina), and project PICT-2016-1334 from Agencia Nacional de Promoción Científica y Tecnológica, Argentina.