Compositional data analysis and hydrogeochemistry of the Continental Intercalaire aquifer (northwestern Africa): A regional synthesis

The Continental Intercalaire (CI) aquifer, spanning Algeria, Tunisia, and Libya, is analyzed using piezometry, major ions, and stable isotopes (δ2H, δ18O) to track flow paths from recharge to discharge areas. A new piezometric map was generated for the CI from the interpolation of extensive datasets from the three countries (measured in 2008). A hydrochemical study was conducted on data compiled from previous investigations (n = 248 samples) and compared to new samples from this study (n = 10 samples). Hierarchical cluster analysis (HCA) and principal component analysis (PCA) were applied to hydrochemical data transformed using the centered log ratio method. The piezometric map, created using a nested model (spherical and gaussian variograms), achieved high accuracy (MAE = 0.09 m; RMSE = 26.62 m). Variography revealed the lateral continuity of the CI aquifer over ~600 km, in agreement with previous studies on flow paths and recharge characteristics. Depth to groundwater is shallowest in coastal areas (~20 m) and deepest in the Sahara (440 m). The hydrochemistry data shows that there is a progression towards the NaCl-SO4 water type along flow paths. Compositional data analysis (CoDA) successfully differentiates similar water types, providing additional insights into CI aquifer geochemistry. Isotopes proved essential in interpreting both paleo-recharge and modern-recharge compositions, considering depletion and enrichment in δ2H and δ18O compared to the global meteoric water line (GMWL) and the palaeo water line (PWL). These results are vital for sustainable water management and environmental planning.