Hydrogeotoxicity from arsenic and uranium in the southern Duero Basin, Spain

Abstract

In the southern part of the Cenozoic Duero Basin (Spain), naturally occurring arsenic and uranium in groundwater sometimes occur in concentrations exceeding the limits for potable water. Usually, concentrations of these elements are only slightly above the limit, but this is sufficient to invalidate its use as drinking water.Hydrogeotoxic risk or hydrogeotoxicity (HGT) is defined as the quotient between the observed concentration of an element and the limit set by health regulations (such as by the World Health Organization, WHO). Areas at risk of HGT from a given Potentially Toxic Geogenic Trace Element (PTGTE) are defined a quotient of >1.Hydrogeotoxicity of arsenic and uranium (HGTAs and HGTU) were calculated for water samples and 34% returned HGTAs above 1. For uranium, different HGT indexes were considered against a variety of thresholds for drinking water established by different countries and organizations (10μg/L, Germany; 15μg/L, WHO, 2004; 30μg/L, WHO, 2011/US-EPA, 2011): 48% samples returned HGTU10>1; 29% samples, HGTU15>1 and 7% had HGTU30>1. In the water samples analyzed, the highest arsenic content were usually associated with the highest uranium concentrations. However sometimes these two hydrogeotoxicities do not coincide and the HGTAs and HGTU10 distributions show different patterns.Hydrogeotoxicity due to more than one element (HGTAs and HGTU10) was also considered; 17% samples showed a potentially hydrogeotoxic risk linked to both PTGTEs, leading to the delimitation of these zones as especially vulnerable to a combined hydrogeotoxic hazard.The linear distribution of HGTAs seems to be related to the structure of the Cenozoic Duero Basin and to deep water inflows that are enriched in As and U, which are associated with its fissured basement. The HGTU can be partly linked to these deep flows with a significant hydrothermal component, but they are also linked to secondary processes occurring in the sedimentary basin, which could explain the scattered distribution of HGTU in some areas.As well as the HGT values, which are based on total concentrations, bioavailability of arsenic and uranium species was also considered in order to provide a comprehensive assessment of hydrogeotoxic risk over the study area. In all samples with HGTAs>1, arsenic was present as As(V) inorganic form. Likewise, in samples with HGTU10>1, uranium appears as U(VI). This hydrogeotoxic anomaly must be related to both the chemical and radiological hazards.