Los yacimientos de hierro, alunita y jarosita de la zona de Riaza como posible origen del arsénico en las aguas subterráneas

Arsenic in concentrations above the legal limit of 10 µg/l has been detected in groundwaters of the Duero Cenozoic Basin. The origin of arsenic is related to sedimentary units with arsenic content above the background value of 28 .5 mg/kg. Thus, iron-rich deposits located at the base of Cenozoic succession may constitute a potential source of arsenic in the groundwaters . Three outcrops of iron-rich conglom - erates in the Riaza's area of Segovia province (one in El Negredo and two in Madriguera) have been studied to determine the significance of these materials as a potential source of arsenic in groundwater. These outcrops occur above an unconformity separating them from strongly altered Paleozoic slates, rich in alunite and jarosite. The work is based in geochemical (trace elements detection by INAA) and mineralogical analyses (through XRD-EDAX and ESEM) of 18 samples of altered slates and materials of ferriferous deposits. Besides, 3 water samples from springs have subjected to hydrochemical analysis to establish major ionic species and trace elements. Although min - eralogical study reveals that arsenic occurs in iron oxides and high arsenic concentrations have been identified in rocks of El Negredo (up to 361 mg/kg, average 143 .3 mg/kg), the arsenic concentrations in Ca-Mg-HCO 3 - water type are always lower than 10 µg/l. Therefore, it does not seem to be probable that these deposits act as arsenic source, at least at the present physic-chemical conditions.