Use of a multi-tracer approach for the hydrogeochemical characterization of fractured crystalline aquifers in semi-arid region

In crystalline aquifers, groundwater resources are generally scarce and may be subject to salinization, notably in semi-arid areas. In semi-arid Northeastern Brazil (NEB), the processes at the origin of the high and heterogeneous salinity observed in the crystalline basement are still controversial. This field study demonstrates how the use of stable and radioactive isotopes and dissolved gases, combined with hydrodynamic and hydrochemical methods, are powerful tools to understand the hydrogeological functioning and the salinization processes of fractured aquifers in such a region.

The study area is located in the state of Ceará (NEB). Monthly and hourly piezometric data were collected respectively from March 2018 to December 2019 over a network of 56 boreholes, and from November 2016 to December 2019 in four of these boreholes. Chemical sampling (major ions and/or minor and trace elements) was carried out in 27 boreholes in December 2017, in 38 in June 2018 and 20 in June 2019. Stable (¹⁸O, ²H) and radioactive (¹⁴C, ³H) isotopes, as well as CFC and SF₆ concentrations were also measured on a total of 137, 10 and 10 samples, respectively.

The strong heterogeneity of the crystalline aquifer was highlighted by Electrical Conductivity (EC) ranging between 1000 and 15000 μS/cm and contrasted temporal variations of EC or piezometric levels. Stable isotope data showed that groundwater recharge occurs through both direct and indirect infiltration of evaporated surface (45-65% on average for the latter). Multi-tracer dating confirms that groundwater is relatively recent (from decades to hundreds of years, with a large contribution of post-1950 water) and that circulation processes are fast. The chemical study suggests that the groundwater, originally bicarbonated, becomes progressively enriched in chloride and sodium, due to the strong evapotranspiration in the quite endorheic context. The NaCl salinity is interpreted as originating from rainfall. However, water and chloride budgets modeling shows that the highest salinities can only be explained by direct withdrawal of groundwater by the vegetation roots of the native bush, with an evapotranspiration rate equivalent to the annual recharge.