Environmental isotopes contribution to the characterization of the flow pattern of a large plain hydrogeological system

Abstract

[EN] 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. It is located at the distal portion of the Salado-Juramento fluvial mega-fan and presents a smooth NW-SE slope (gradient < 1 0/00). The north-western and eastern surface watersheds are defined by the Otumpa Hills and the Vera block, respectively. Both are raised blocks originated by regional neotectonic movements. However, the regional bound of the hydrogeologic system is unknown. It could extend from a recharge zone on the Subandean 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 regional flows exist. Intermediate scale flows are not relevant due to low topographic gradients. 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. A new geological interpretation of the regional system allowed defining consistent hydrogeologic regional units with the 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 fluvial mega-fan of the Salado-Juramento River, with recharge originating on the Sub Andean 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 Golondrina-Calchaquí stream which is a tributary of the Salado-Juramento 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. 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.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.Peer reviewe