Nitrate in the groundwater of the Bauru Aquifer System, Marília Municipality (SP)

Abstract

Anomalous nitrate concentrations have been detected in the Bauru Aquifer System by groundwater monitoring wells located at the urban areas of many cities throughout the State of São Paulo. Focusing on this problem, the Geological Institute, together with other institutions, is developing a project to evaluate the trends of nitrate concentration in groundwater over time and space related to urban development. One of the study areas comprises the Marília municipality, where the Bauru Aquifer System is composed of the Marília and Adamantina aquifer units. The activities conducted include: record of drilling wells, statistical treatment of previous hydrochemical data and evaluation of urban expansion over the last decades. The results indicate that the highest nitrate concentrations (up to 16.9 mg/L N-NO3-) occur in older urban areas and in drilling wells up to 150 m in depth. This indicates that the contamination is probably related to the sewage system (old septic tanks and sewer network leakage) and that it occurs mainly in the Marília Aquifer. The deepest parts of the aquifer are less permeable due to carbonate cementation, which restricts the percolation of this contaminant into the Adamantina Aquifer. It is evidenced by the groundwater quality of the deepest wells, which showed low nitrate concentrations, in general, lower than 3.0 mg/L N-NO3-.