The Madrid Tertiary Detrital Aquifer is one of the largest and most important aquifers of Spain. This aquifer was declared a strategic resource for Madrid´s drinking water supply in the Water Plan of the Tagus Basin. High arsenic concentrations (up to 91 µg/L) have been measured in groundwaters from the aquifer. Around 20% of the samples analyzed show arsenic concentrations that exceed the maximum level established by the current Spanish law for drinking water (50 µg/L). Furthermore, in December 2003 the European Directive 98/93/CE related to the water quality for human consumption will come into effect in Spain. This Directive will establish a maximum contaminant level for arsenic in drinking water of 10 µg/L; consequently, the number of water supply wells affected by excessive concentrations of arsenic will be increased substantially.

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.