Isotopic signature of precipitation in the semi-arid region of Ceará, Northeastern Brazil

The study of the stable isotopic composition of precipitation is an essential element to characterize the groundwater input signal. Indeed, the content analysis of stable isotopes (¹⁸O, ²H) in the different compartments of the water cycle constitutes a powerful integrative approach to understand the origin and organization of groundwater flows within aquifers, and to identify potential relationships between surface water and groundwater. To date, the isotopic characterization of rainwater of the semiarid regions of Northeastern Brazil was only addressed by a few studies. This study aims, therefore, to improve the characterization of the isotopic signal of rainfall in the semiarid region of Ceará, and more specifically in the municipality of Quixeramobim, located in the center of the state.

The stable isotopes of rainwater were measured in monthly rainfall at three different stations between 2011 and 2019 (172 samples), and in daily rainfall at two stations between 2018 and 2019 (212 samples). The results showed that the frequency of rainfall sampling may induce an isotopic bias, due to the partial evaporation from the water collectors during monthly storage causing an isotopic fractionation. This bias may lead to a wrong definition of the Local Meteoric Water Line (LMWL) in the absence of adequate data processing. The processing of the monthly data (elimination of the evaporated data) resulted in the preliminary LMWL of Quixeramobim characterized by the equation δ²H(‰)=8.06×δ¹⁸O(‰)+12.6 (r²=97%). The isotopic monitoring of daily rainfall highlighted the high d-excess values observed  in the rainwater (between +10.1‰ and +22.1‰) and allowed to confirm the LMWL derived from the monthly data. The y-intercept value of the LMWL of Quixeramobim, greater than +12‰ in both datasets, indicates that rainwater is composed of atmospheric vapors from marine origin (from the Atlantic, considering the proximity of the study zone to the coast) but also  from continental recycling. As a consequence, d-excess value < +12‰ in groundwater evidences the presence of water that has undergone evaporation, revealing an indirect infiltration process. This observation has major consequences in the understanding of aquifer recharge processes.