Multi-isotopic approach to interpret groundwater storage and movement in Northeast Brazilian Aquifers

The application of techniques using multi-isotopic tracers has been widely used in groundwater studies with variable purposes e.g. estimates of recharge rates, changes caused by overexploitation, and paleoclimatic interpretations. The intense exploitation of groundwater for public and agroindustrial supply in semi-arid regions represents a combination of the limited availability of surface water resources associated to the lack of management control, that led to a scenario of constant reduction in water stored, sometimes intensified due to the consequences of climate changes. The presence of important groundwater reservoirs in the states of Ceará and Rio Grande do Norte, encompassed in the Araripe and Potiguar basins, led to a pronounced economic development in recent years, however, overexploitation and contamination of groundwater have been observed. In this sense, the use of a set of radioactive tracers (3H and 14C) associated with stable isotopes of 2H and 18O, aiming the better understanding of the circulation and storage processes of groundwater in the aquifers is required. The preliminary results represent part of a three-year monitoring program for wells located in Ceará State encompassing monthly data from water isotopes (2H and 18O), in addition to radioactive isotopes (3H and 14C), as well as hydrochemical data. In the Araripe Basin we are sampling 29 wells located in Cariri Valley and one well in Araripe Plateau, in addition to 10 springs that emerging in Araripe Plateau. Meanwhile in the Potiguar Basin, 8 wells are being monitored from the Açu Aquifer and 2 wells from the Jandaíra Aquifer. The results show three main groups based on isotopic ratios, in the Araripe Basin, the first group corresponds to the more depleted values (δ2H-23.2±3.44‰ and δ18O-4.39±0.44‰) the second the most enriched values of isotopic signatures (δ2H-10.50±-1.25‰ and δ18O-2.76±0.37‰), while the third group corresponds to intermediate values. The springs has an isotopic fingerprint of δ2H-14.13±1.02‰ and δ18O-3.47±0.17‰. Residence times were estimated using 14C and 3H data. The 3H concentrations of the springs are below the detection limit (0.3 UT), while C14 data indicates residence times varying from very old waters (<5pcm), old waters (~50pcm), and recent waters (>100pcm) in the Cariri Valley. After the first year of monitoring, it is possible to interpret that in the Araripe Basin, the isotopic variations were not in agreement with the previous hydrostratigraphic framework, and our conceptual model proposes mixing conditions, resulting from flows facilitated by fault systems. As expected, the longest residence time was observed in more depleted isotopic fingerprints. In the Potiguar Sedimentary Basin, the Açu Aquifer presented isotopic ratios in the confined portion (δ2H-21.92±2.79 and δ18O-3.99±0.52) are slightly more depleted when compared to the isotopic signatures of the outcropping portion (δ2H-22.48±2.91 and δ18O -4.03±0.56). The Jandaíra Aquifer presented enriched isotopic ratios (δ2H-7.46±2.66 and δ18O-1.81±0.47) indicating strong evaporation processes. These variabilities confirm isolation between the units. The 14C data for the confined Açu Aquifer, indicated very old groundwater (14C<1.87Pmc). The first year of monitoring brought perspectives that allowed progress in investigations using isotopes. In the case of Araripe Basin, other regions will be dated to test the mixing model between the aquifers units in the Cariri Valley, in addition to the use of 39Ar in spring samples to understand the Araripe Plateau flow system. The possible residence time observed in the Potiguar Basin is older than the observed in previous studies carried out in Parnaíba Basin (Stute, 1995) and the Paraíba-Pernambuco Basin (Bertrand et al 2017), can contribute to the advancement of Kr dating, and enables new paleoclimatic interpretations with the use of noble gases.