Isotopic and chemical assessment of surface water and groundwater in the Bajos Submeridionales Basin, Chaco Austral

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

Basil

Congreso; 47th IAH Congress; 2021

Bajos Sumeridionales (BBSS) Basin occupies 54 280 km2 of the distal region of the Salado-Juramento River megafan in the Chaco Austral. The area presents a very gentle NW-SE slope. Annual precipitation is 950 mm, ranging from 400 mm/y to 1600 mm/y. Potential evapotranspiration reaches 1500 mm/y. A 29,000 km2 wetland covers the southeasterrn portion of the basin. The wetland and numerous permanent and semipermanent lakes located on deflation hollows discharge into a single river outlet (Golondrina-Calchaquí River) discharging into the Salado-Juramento River. The stratigraphic column is represented by: silty sand and sandy silt, with abundant gypsum and calcareous concretions (Chaco Formation); fine quartz sands, with intercalations of grey-greenish clays with calcareous fossils (marine Paraná Formation); fluvial deposits of the Paraná River composed of yellowish and reddish quartz sands, interstratified with illitic and kaolinitic silt and clays (Ituzaingó Formation), and interbedded clayey silts covered by loessic deposits (Urundel Formation and Fortin Tres Pozos Formation). A network of sandy palaeochannels is identified. Previous works postulated the validity of Tóth ́s model, and hydrochemical evolution of groundwater consistent with this conceptualization, although with limited field contrast. Three sampling surveys (2016/2017/2018) of surface-SW and groundwaters-GW plus background data, allowed the chemical and isotopic characterization of four hydrogeologic units, from HU1 the shallowest, to HU4 the deepest, to support the understanding of the groundwater flow system. Studied GW have low to high mineralization (EC= 458-65,000 μS/cm). An EC= 132,000 μS/cm was measured at an evapoconcentrated lake at the headwaters. Na-Cl and Na-Cl/SO4 types were present in all waters. Na-HCO3/Cl waters were found in SW, HU1 and HU2, but not in HU4. Na-Cl/HCO3 was found in SW, in HU1 and HU2, though with lower representation percentages. Na, Cl and SO4 were the dominant ions. Na is mainly contributed by dissolution of halite, cation exchange, evapoconcentration combined with saline soil whashing, specially in HU1. Cl abundance is related to halite dissolution while SO4 originates from gypsum dissolution. Paleochannel waters have lower ion concentrations than surrounding waters, and mineralization increases with depth, tough the spatial distribution of chemical types along longitudinal flow lines is not conclusive. A strong influence of climatic conditions on water chemistry was noticeable. 18O and 2H results enabled stablishing a sectorized hydrogeologic flow system: in the Southern portion, water circulation would correspond to the current fluvial megafan, with regional recharge originating on the Subandinas mountain range. In the northern portion, local flows would prevail, with a likely contribution from deep thermal flows favored by vertical hydraulic gradients. Both sectors would discharge into the Salado-Juramento River, and finally, into the Paraná River. This study was funded by projects nº2559- IGME (Spain), nº2010-214-16- ASaCTeI (Santa Fe, Argentina), and PICT-2016-1334- ANPCyT (Argentina).