ENVIRONMENTAL ISOTOPES CONTRIBUTION TO THE CHARACTERIZATION OF THE FLOW PATTERN OF A LARGE PLAIN HYDROGEOLOGICAL SYSTEM: BAJOS SUBMERIDIONALES (SOUTH AMERICA)

HEREDIA, J.; RODRIGUEZ, L.; CASTRO, S.; THALMEIER, M.B; VEIZAGA, E.; MORENO MERINO, L.

Congreso; 46th International Association Of Hydrogeologists Congress; 2019

Bajos Sumeridionales (BBSS) is an extensive region (54 280 km2) on the Chaco Austral in Argentina. Meanannual precipitation is 950 mm, ranging from 400 mm/y to 1600 mm/y due to the occurrence of hydroclimaticextremes. Potential evapotranspiration reaches 1500 mm/y, and mean annual temperature is 21oC. From a ge-omorphological 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 ofregional neotectonics movements. The regional bound of the hydrogeologic system is unknown. It couldextend from a recharge zone on Subandinas mountain range on the West end of the region, up to a regionaldischarge level imposed by the Paraná River on the East end. As is typical on large plain hydrogeologicalsystems, local recharge/discharge vertical flows condition the flow pattern, though slow, regional, chemicallyevolved flows exist. Intermediate scale flows do not play a relevant role due to low topographic gradients, lessthan 1 0/00. All flow types discharge locally in numerous surface water bodies and crypto-wetlands, and theirinteraction is dependent on hydroclimatic pulses. These flow features make difficult to contrast Toth ́s hydro-geological conceptual model proposed for sedimentary basins. Nonetheless, some previous works postulatedthe validity of this model, although with limited field contrast, while others indicated an hydrochemical evo-lution that can be considered consistent with this conceptualization. A new geological interpretation of theregional system allowed defining consistent hydrogeologic regional units that assisted a new hydrochemicalstudy. Preliminary environmental isotopes results (18O y 2H) presented in this work enabled stablishing acomplex and sectorized hydrogeologic flow system for the BBSS. In the Southern portion, water circulationwould correspond to the current alluvial mega-fan of the Salado del Norte river, with recharge originating onthe Subandinas mountain range. In the northern portion, it would correspond to local flows, with a likely con-tribution 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 occurredafter the formation of the mega-fan, conditioned its primitive flow pattern. Besides, results have shown thestrong 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 PromociónCientífica y Tecnológica, Argentina.