Assessments of Groundwater Circulation and Recharge Paleotemperature in the Açu Aquifer, Northeastern Brazil

The Potiguar Basin is a passive margin sedimentary basin formed during the breakup of Africa and South America. It encompasses important onshore oil reservoirs which have been exploited since the 1970s. It is located in the Atlantic Septentrional margin in the semi-arid region of Northeastern Brazil. The hydrogeological framework of the Potiguar Basin includes two main reservoirs: the karst Jandaíra Aquifer and the porous Açu Aquifer. The latter is the most significant aquifer in the region, responsible for supplying water to the population and for fruit production irrigation. Nowadays exhaustive drilling and groundwater extraction have led to a decrease in the water table posing problems for water resources sustainability in this semi-arid region. Despite the increasing demand of groundwater in the last decades, the Açu Aquifer has not been properly studied regarding groundwater flow, isotopic, and hydrochemical groundwater evolution. The semi-confined Açu Aquifer is bounded by the Potiguar basin, at the contact with the overlying carbonate platform, the Jandaíra Aquifer, which confines the Açu Aquifer across the entire platform toward the offshore areas. From the outcrop area, groundwater flows preferentially in the SW to NE direction influenced by the geological and structural framework of the basin. Aiming at a better understanding of groundwater flow and paleorecharge temperatures in the Açu Aquifer, environmental isotopes (H, O, and C) and noble gases were measured. Groundwater isotopic signatures values were found to be more depleted than those of precipitation (<-3.5‰ for δ¹⁸O and -10‰ for δ²H in groundwater, and up to -2.0‰ for δ¹⁸O and close to 0 ‰ for δ²H in precipitation). The age tracer C-14 indicates that young groundwater (close to 100 pMC) is present in the outcrop area and that very old groundwater is present at the deepest zones of the basin, where the C-14 concentration was near the detection limit of the method (0.3 pMC). In addition, the noble gas concentrations also suggest colder climate conditions during recharge. Our first results yield temperatures up to 8ºC below the modern mean annual temperature, in agreement with previously paleoclimate temperature studies in the region. The high excess air values ​​from older deep waters indicate a larger fluctuation of the water table level. These preliminary results suggest that the recharge of the Açu Aquifer took place under colder climate conditions, in possible association with the Last Glacial Maximum (LGM). Even though the groundwater residence time was not precisely determined, it is expected to be over 40k years. Our results are consistent with previous studies that point to changes in temperature since the LGM for the Northeastern Brazil climate. Although further sampling and more precise data analysis are needed, these initial findings highlight the complexity of groundwater flow in the Açu Aquifer, and the importance of effective groundwater management to ensure the sustainability of the resource.