Nitrate in the Adamantina Aquifer: the case of the Monte Azul Paulista Municipality, SP

Abstract

The municipality of Monte Azul Paulista is supplied exclusively by groundwater, mainly from the Adamantina Aquifer, captured by more than one hundred wells. In recent years high nitrate concentrations have prohibited the use of some of these wells for public water supply. In the central urban core a strategically deep tubular well drilled to supply the water deficit has allowed a conceptual hydrogeologic model based on the interpretation and integration of hydrochemical, geophysical and hydrogeological results to be tested. This model indicated two distinct potentiometric levels: one in a shallow aquifer from the surface to 40 m and the other in a deep aquifer between 80 and 152 m, separated by a clayey layer 40 m thick. Groundwater may be classified in two types: 62% is bicarbonate calcic (Ca-Na-CO₃, Ca-HCO₃) water, and 38% is chlorinated calcic (Cl-Ca-HCO₃) water, as per the Piper diagram. The class of the dominant calcium bicarbonate hydrochemical type is compatible with and strengthens regional findings by other researchers over more than two decades. On the other hand, the chlorinated calcic waters are related to waters that have experienced human intervention with the presence of the dopant nitrate. Although nitrate comes from surface sources, it is already distributed in the deep aquifer. This observation is also supported by hydrochemical modelling showing the occurrence of mixed polluted and natural waters in different proportions with chlorinated calcic characteristics. The presence of interdigitated clay layers decreases the possibility of contamination with nitrate by vertical flow into the deeper portions. On the other hand, an excess of wells and their poor construction comprise a potential source of contamination.