Hydrochemical and isotopic evidence of groundwater contribution to wetlands in a coastal plain aquifer, southern Brazil

The Coxilha das Lombas Aquifer System is located in the coastal plain of the Pelotas Basin, southern Brazil. Associated with Pleistocene eolian deposits, it is considered the most productive aquifer system in the Porto Alegre Metropolitan Region, capital of the state of Rio Grande do Sul. Despite its strategic importance and the pressure related to its use, the system still lacks a more comprehensive understanding of its groundwater flow and hydrochemical characteristics. This aquifer system constitutes a groundwater recharge area extending approximately 70 km in length (NE–SW direction) and about 10 km in width. Its topographic highs coincide with a regional groundwater divide, characterized by divergent flow paths. According to several authors, groundwater discharge contributes to lagoons, wetlands, and watercourses that drain toward the Lagoa dos Patos to the southeast—the world’s largest choked coastal lagoon—and toward the Gravataí River basin to the northwest. In this context, a hydrochemical study of the Coxilha das Lombas Aquifer System was carried out, integrating pre-existing data with new field data obtained from the collection and analysis of 85 groundwater samples along the aquifer extent, encompassing different depths and well types. The analyzed parameters included pH, electrical conductivity, total dissolved solids, and the major anions and cations. The results indicate predominantly sodium–chloride hydrochemical facies, with a median pH of 5.3, characterizing acidic waters, likely of meteoric origin. Low values of total dissolved solids, electrical conductivity, and alkalinity were observed (TDS = 36 mg/L, EC = 46 μS/cm, and alkalinity = 4.83 mg/L), reflecting the low degree of mineralization of the aquifer waters, probably related to a short groundwater residence time. Additionally, 21 groundwater and surface water samples were collected for oxygen and hydrogen isotope analyses. These results will be evaluated using precipitation data from the Global Network of Isotopes in Precipitation (GNIP-POA) station in Porto Alegre, operated by the Geological Survey of Brazil, and are expected to provide insights into water origin and groundwater contribution to wetlands and surface watercourses in the region through groundwater-surface water mixing relationships. This work is part of the project “Geological evolution and hydrostratigraphy of Coxilha das Lombas, northwestern coastal plain of Rio Grande do Sul” (no. 407572/2023-6), supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).