NATURALLY OCCURRING ARSENIC IN GROUNDWATERS OF THE MADRID TERTIARY DETRITAL AQUIFER (SPAIN)
The arsenic contamination phenomena in groundwaters of the Madrid Tertiary Detrital Aquifer respond to natural causes. High arsenic levels appear in groundwaters with high residence time, that is, they occur in groundwaters characterized by a) high pH values, between 8.3 and 9, and the resulting occurrence of carbonate anion, b) high sodium concentrations, and c) low calcium and magnesium concentrations. Furthermore, the groundwaters which display high concentrations of arsenic show high levels of vanadium, fluoride and boron. It is very interesting to note that the stable forms of these four constituents in aqueous systems are anionic complexes. Consequently, it is really possible that the concentration and mobility of As could be controlled by anion exchange processes frequently related to evolved groundwaters characterized by high pH values and tendency to softening. Although arsenic contamination is naturally occurring, arsenic release could be also affected by fluctuating water levels related to high exploitation of water wells.