Spatial and Temporal Variability of Nitrate and Chloride in Aquifer Bauru System, São Paulo State, Brazil

Abstract

The Bauru Aquifer System (BAS) is a source of essential importance for the State of São Paulo. This fact raises the attention with its state of conservation before the possibility of contamination by pollutants. Monitoring in the SAB has provided important but timely information, not reflecting a spatial picture of every area of the aquifer. In this sense, geostatistics can be an important analytical tool since water quality parameters can vary in an orderly way in time and space. This study used data from 75 monitoring wells from CETESB, between 2010-2012, and aimed to evaluate the variability and spatial/temporal correlations of nitrate (N-NO₃) and chloride (Cl^-) concentrations in BAS. Descriptive statistical analysis, correlation, and semivariographic analysis (simple and cross) were performed. The results indicate a high linear correlation between N-NO₃ and Cl- (R² = 0.69**). From a geostatistical point of view, all the parameters present spatial dependence, independent of the period, denoting good semivariographic adjustments and well-delineated maps of spatial variability estimates. The Nitrate mapping corroborated previous studies performed in the SAB BAS area, indicating the presence of N-NO₃ above the limit value established by the Ministry of Health. Finally, there are significant spatial correlations between Nitrate and Chloride, allowing spatial estimations of N-NO₃ from the co-variable Cl-.