Stable isotopes (H-2, O-18 and C-13) in groundwaters from the northwestern portion of the Guarani Aquifer System (Brazil)

Abstract

The groundwater flow pattern of the western part of the Guarani Aquifer System (GAS), Brazil, is characterized by three regional recharge areas in the north, and a potentiometric divide in the south, which trends north-south approximately. Groundwater flow is radial from these regional recharge areas toward the center of Parana Sedimentary Basin and toward the western outcrop areas at the border of the Pantanal Matogrossense, because of the potentiometric divide. The isotopic composition of GAS groundwater leads to understanding the paleoclimatic conditions in the regional recharge areas. The delta O-18 and delta H-2 isotopic ratios of GAS groundwaters vary, respectively, from -9.1 to -4.8aEuro degrees V-SMOW and -58.4 to -21.7aEuro degrees V-SMOW. In the recharge zones, enriched delta O-18 values are observed, while in the confined zone lighter delta O-18 values are observed. These suggest that climatic conditions were 10A degrees C cooler than the present during the recharge of these waters. The delta C-13 ratios in groundwater of GAS, in the study area, vary from -19.5 to -6.5aEuro degrees VPDB, increasing along the regional flow lines toward the confined zone. This variation is related to dissolution of carbonate cement in the sandstones.