Hydrogeochemical evolution of mesozoic aquifers in the southern Paraná basin in western Rio Grande do Sul State, Brazil

Abstract

The western region of Rio Grande do Sul is marked by successive water supply crisis during the dry season. Hydrogeological, hydrogeochemical and structural studies were done to understand the dynamics of groundwater flow in the aquifers of the region. The aquifers are the Guarani Aquifer System (SAG) and Serra Geral Aquifer System (SASG). Approximately 90 % of the wells capture groundwater exclusively from the SASG, while 10% capture from SAG. The SAG consists of four sedimentary hydrostratigraphic units: Botucatu, Guará and Pirambóia Formations and Rosário do Sul Group, while the SGAS consists of one hydrostratigraphic unit, the Serra Geral Formation. The main objective of this study is the hydrogeochemical characterization of Juro-Cretaceous aquifers from the region. The hydrochemical data are from the SIAGAS database, belonging to Brazilian Geological Survey (CPRM). Statistical analysis by principal components and clustering identified three groundwater groups,  confirmed by dispersion diagrams. Group 1 groundwater percolates only the SGAS while Groups 2 and 3 show sodium enrichment indicating mixture with groundwater from deeper, confined sedimentary aquifers of the Paraná Basin. The enrichment in sodium results from cation exchange of calcium from clay minerals. In addition to sodium, ions like fluoride, sulfate and chloride appear with significant concentrations in many wells. They could be originated in the Irati Formation, which is stratigraphically below the SASG  and SAG.