Arsénico en aguas subterráneas e interacción agua-roca: un ejemplo en la Cuenca terciaria del Duero (Castilla y León, España)

Abstract

The presence of arsenic in groundwaters in noxious amounts for health constitutes a problem of primary importance at an international scale. Multi-element analysis carried out in the southeastern portion of the Duero Basin have detected arsenic contents above potability limits in the supplies of several towns. To study the distribution and origin of this element was carried out an hydrochemical study on 514 samples, a field sedimentological study on 69 stratigraphic sections and 7 diamond drill holes, a lithogeochemical study of 553 samples, and a mineralogical study of the samples with the highest arsenic contents. The natural origin of arsenic and its correlation with the lithogeochemical anomaly recorded in the Zaratan Facies (Middle Miocene, palustrine facies) have been recognized. It is assumed that most of the arsenic present in the groundwaters is derived from this unit. There is also a correlation with the anomalies recorded in other stratigraphic intervals. In addition, the rocks of the Upper Cretaceous-Middle Miocene succession constitute an arsenic “storage” potentially removable to the groundwaters of the northwestern zone and to the deep aquifers of the central sector. Arsenic has been identified in iron oxides, authigenic pyrite, manganese oxides, inherited titanium-iron oxides, phyllosilicates and organomineral colloids. Arsenic mobilization to groundwater corresponds to arsenic desorption from iron and manganese oxides and arsenic desorption by organic matter transformation.