Nitrates in groundwater of the state of São Paulo

Abstract

As groundwater consumption increases, its quality and consequently its potability are proportionally affected. Nitrates stand out among the agents that contribute to the loss of groundwater potability, as they are the most common groundwater contaminants, notably in urban regions. Furthermore, because of their high solubility and chemical stability, nitrate contamination can extend over large areas. The main source of nitrate contamination is domestic sewage, particularly where basic sanitation is incipient, or maintenance of the sewerage system is inadequate to prevent significant sewage leakage to the subsurface. A total of 840 groundwater samples collected from the main aquifer systems distributed throughout the State of São Paulo (Cristalino, Tubarão, Guarani, Serra Geral, Bauru and Taubaté) were analyzed for this study, which aimed at a detailed diagnosis of the nitrate contamination in the State. The results were compared with the potability standards established by the Brazilian Ministry of Health in the Consolidation Ordinance nº5/2017 and the Prevention Value established by CETESB in the document “Groundwater Quality in the State of São Paulo - 2016-2018”. From the samples analyzed, 70 (8.4%) exceeded the maximum value allowed by Ordinance nº5, and 159 samples (18.9%) exceeded the Prevention Value. The Bauru Aquifer System is the most affected by nitrate contamination, due to its size, its characteristics typical of free aquifers, and the large number of wells that extract water from this system. Conversely, the Guarani Aquifer System is the least impacted, because it is predominantly a confined aquifer system, preventing it from being affected by surficial contamination.