Groundwater and surface water connectivity within the recharge area of Guarani aquifer system during El Niño 2014–2016

Abstract

Recharge areas of the Guarani Aquifer System (GAS) are particularly sensitive and vulnerable to climate variability; therefore, the understanding of infiltration mechanisms for aquifer recharge and surface run‐off generation represent a relevant issue for water resources management in the southeastern portion of the Brazilian territory, particularly in the Jacaré‐Pepira River watershed. The main purpose of this study is to understand the interactions between precipitation, surface water, and groundwater using stable isotopes during the strong 2014–2016 El Niño Southern Oscillation event. The large variation in the isotopic composition of precipitation (from −9.26‰ to +0.02‰ for δ18O and from −63.3‰ to +17.6‰ for δ2H), mainly associated with regional climatic features, was not reflected in the isotopic composition of surface water (from −7.84‰ to −5.83‰ for δ18O and from −49.7‰ to +33.6‰ for δ2H), mainly due to the monthly sampling frequency, and groundwater (from −7.04‰ to −7.76‰ for δ18O and from −49.5‰ to −44.7‰ for δ2H), which exhibited less variation throughout the year. However, variations in deuterium excess (d‐excess) in groundwater and surface water suggest the occurrence of strong secondary evaporation during the infiltration process, corresponding with groundwater level recovery. Similar isotopic composition in groundwater and surface water, as well as the same temporal variations in d‐excess and line‐conditioned excess denote the strong connectivity between these two reservoirs during baseflow recession periods. Isotopic mass balance modelling and hydrograph separation estimate that the groundwater contribution varied between 70% and 80%, however, during peak flows, the isotopic mass balance tends to overestimate the groundwater contribution when compared with the other hydrograph separation methods. Our findings indicate that the application of isotopic mass balance methods for ungauged rivers draining large groundwater reservoirs, such as the GAS outcrop, could provide a powerful tool for hydrological studies in the future, helping in the identification of flow contributions to river discharge draining these areas.

The use of isotopes in evolving groundwater circulation models of regional continental aquifers: The case of the Guarani Aquifer System

“…In the northern part of the GAS, a wide variation of isotopic δ 18 O and δ 2 H values was observed, suggesting that no evaporation processes had taken place prior to recharge (Aravena, ). Later, Batista et al (), in a minor scale study in a GAS recharge area, revealed isotopes data showing secondary evaporation processes during GAS recharge events. In the southern part, isotopic composition was slightly different, being correlated more directly to the air masses responsible for precipitation in that region.…”

Tracing Water Sources and Fluxes in a Dynamic Tropical Environment: From Observations to Modeling

“…The distinct isotopic pulses or input functions provided by rainfall in the tropics are commonly damped or modified by different ecological and hydrogeological processes across the landscape. Connectivity between groundwater and surface water compartments have been widely studied in humid tropical watersheds (Gordon et al, 2015;Lagomasino et al, 2015;Batista et al, 2018;Oiro et al, 2018;Villegas et al, 2018;Welsh et al, 2018;Tsuchihara et al, 2019); however, large scale or regional approaches are uncommon across tropical biomes. Figure 9 shows locations of groundwater (N 1,156) and surface water (N 603) sampling efforts across Costa Rica (2016-2019).…”

Preface to stable isotopes in hydrological studies in the tropics: Ecohydrological perspectives in a changing climate

“…Kirchheim, Gastmans, and Chang () reviewed how stable isotope and noble gas methods have influenced the evolution of knowledge on the Guaraní aquifer (Brazil) groundwater circulation models in the past decades. Batista et al () studied groundwater and surface water connectivity within the recharge area of the same aquifer. Their isotopic mass balance and hydrograph separation estimated variations between 70% and 80% in the groundwater contribution during the last strong ENSO event coupled with a notable correlation of d ‐excess between surface and groundwater reservoirs.…”