Natural processes and human impacts on groundwater: example of a Cretaceous volcanic aquifer in southern Brazil

The Paraná 3 Hydrographic Basin (BP3) drains its waters into the reservoir of Itaipu Binacional, the largest electricity generator in the world. It is located in southern Brazil, on basic volcanic rocks (basalts ands volcanoclastic) from the Cretaceous age. This paper presents the results of 12 analytical campaigns of the Hidrosfera project, a partnership between the Hydrogeological Research Laboratory of the Federal University of Paraná (LPH-UFPR), Itaipu Binacional and Itaipu Parquetec, where water samples are collected quarterly from 42 tubular wells that supply the 29 municipalities of BP3, among the main municipalities being Foz do Iguaçu, Cascavel, Toledo and Guaíra. Electrical conductivity, temperature, pH, STD, dissolved oxygen, ORP, alkalinity, nitrate, nitrite, ammoniacal nitrogen, phosphate, and acidity are analyzed in the field. At the LPH (Hydrogeological Research Laboratory), complementary analyses are carried out using titrators, spectrophotometry and ICP-OES. BP3 has an area of ​​8,000 km² and has different chemical signatures depending on the flow depth. The calcium bicarbonate chemical type predominates in flows up to 59 meters deep. In these wells, the oxygen and deuterium results are superimposed on the local meteoric line, indicating a short residence time. In this shallower flow, alteration of ferromagnesian minerals predominates. Concentrations of calcium, magnesium, nitrate, chloride, potassium, phosphate, silica, strontium, and dissolved CO2 are higher when compared to flow depths greater than 119 meters. Micropollutants such as atrazine, DEA, and nicotine also occur more frequently in shallow flows (<59 m). At this depth, the statistical correlation between nitrate (NO₃^-), phosphate (PO₄^2-), and potassium (K), highlighting the possible source of “NPK fertilizers” as the origin of this contamination. The results of nitrogen isotope analyses suggest sources related to fertilizers and/or manure/sewage. The source from organic soil is also a hypothesis for some sampled wells. These sources are consistent with the land use at BP3, where agricultural use predominates, followed by pasture and urban areas. Wells with deeper water flows (>119 meters deep) have sodium bicarbonate and sodium carbonate waters. There is a tendency for pH, electrical conductivity, STD, temperature, alkalinity, sodium, arsenic, sulfate, fluorine, and vanadium to increase with increasing depth of the groundwater flow. In these deeper flows, processes related to cation exchange in the aquifer predominate, and C14 ages reach 17,000 years. Considering that BP3 has wells with excellent production (above 220 m³h^-1), used for public supply, contaminants in the groundwater, even if below the guideline values, are a warning sign for local water security. Wells with contamination evidence should attempt to manage land use and occupation, seeking to delimit capture zones and regulate the land use in these areas. In areas where the aquifer has deeper circulation with more mineralized waters, strategic actions such as artificial recharge will prevent the total exploitation of the resource (unsustainable mining of the aquifer).