Seasonal variations of the isotopic ratios for oxygen and deuterium in the groundwater of the Serra Geral Aquifer System in the northeast region of the state of Rio Grande do Sul, Brazil

Abstract

This paper presents the results of the monitoring of oxygen (18O) and deuterium (2H) isotopes in groundwater of the Serra Geral Aquifer System (SGAS) in the region of Carlos Barbosa, Rio Grande do Sul. Rainwater, and water from 12 tubular wells of the SGAS was collected monthly (from April to November). The results indicated that groundwater originates from meteoric water. For rainwater, values were found to be between -11.73 and -3.25‰V-SMOW for δ18O and -61.56 and -8.63‰V-SMOW for δ2H. The air masses simulated by the HYSPLIT model revealed that retro-trajectories for depleted rainfall (d18O <—7.5‰) are mostly influenced by moisture transported from Antarctica, through the Atlantic Polar mass (April, October, and November months). For enriched rainfall, the Atlantic Ocean appears as a more important moisture source, with most of the trajectories coming from this region. This moisture transport pattern occurred mainly during winter, when the South Atlantic Subtropical Anticyclone is closer to the continent. For groundwater, the monthly average values of all wells ranged from -9.02 to -5.06‰V-SMOW for δ18O and from -43.32 to -27.27‰V-SMOW for δ2H and indicated a similar trend in rainwater. Thus, the results show the different climatic conditions that occur over the months, influence of recharge processes, and isotopic ratios of groundwater.