Multi-isotope (δ 2H, δ 18O, δ 13C-TDIC, δ 18O-TDIC, 87Sr/86Sr) and hydrochemical study on fractured-karstic and detritic shallow aquifers in the Pampean region, Argentina

Abstract

Fluxes between fractured-karstified and detritic aquifers are commonly poorly understood in many environments. These two types of aquifers are in contact in the southeastern Pampean region in the Argentine Buenos Aires province, and the aim of this work is to analyze their relationship contributing to improve the hydrological model. A joint application of hydrochemical and multi-isotope (δ 2H, δ 18O, δ 13C-TDIC, δ 18O-TDIC, 87Sr/86Sr) tools was used. TDIC, δ 2H, δ 18O and δ 13C-TDIC allowed differentiating two main end members. Water in the Pampeano aquifer (PA) which is transferred from the fractured-karstic aquifer (F-KA) is characterised by high TDIC around 500–700 mg/L, isotopically depleted in 18O (about −5.5 ‰) and high δ 13C-TDIC (around −10.0 ‰). The other end member is direct recharge water infiltrated into the PA with TDIC ranging from 400 to 500 mg/L, slightly enriched in 18O (δ 18O = −4.8 ‰), and δ 13C-TDIC in the range of soil CO2 as a result of reactions with calcrete concretions (from −20.0 to −9.0 ‰). Dolomite dissolution is the main process controlling the chemistry of the low-mineralized (Mg–Ca-HCO3) waters, whereas high-mineralized (Na-HCO3) waters are strongly influenced by ion-exchange reactions with adsorbed Ca2+ and Mg2+ and by evaporation.