Modeling of hydrochemistry evolution in carbonatic–siliciclastic aquifer system in coastal environment

Abstract

Coastal regions are quite populous, causing coastal aquifers to be overexploited, due to which contamination of these aquifers is observed. The municipality of Belém, located on the eastern edge of the Amazon Forest, is severely deficient with respect to sanitation services, resulting in contamination by domestic wastewater becoming a widespread problem. Furthermore, groundwater overexploitation induces the migration of the saline wedge into the continent. To evaluate the natural and anthropic processes controlling the water quality, we conducted a large-scale (152 samples) hydrochemical analysis, stable isotope analysis, 14C dating of the Belém region. We also performed geochemical simulations to assess the mass balance of the identified process governing the water hydrochemistry. We found that the groundwater of the studied area may be classified into eight groups (based on hydrochemistry and sampled aquifer) or five hydrochemical clusters (solely as per hydrochemical affinity). The natural composition of siliciclastic aquifers Post-Barreiras and Barreiras (Cluster 1) is less mineralized, closely resembling rainwater, composed by recent groundwater recharge. In urban areas of Belém, nitrate contamination is quite significant (Cluster 2), while some samples present noticeable salinization induced by excessive groundwater pumping (Cluster 3). The TDS of deep samples of Barreiras and Pirabas (Clusters 4 and 5) increase as depth and groundwater age increases by the dissolution of calcite, dolomite, and pyrite. Our results allowed us to characterize the natural composition of the water and to measure the intense process deterioration of the water quality of the shallow aquifers.