Geological factors controlling occurrence and distribution of arsenic in groundwaters from the southern margin of the Duero Basin, Spain

Abstract

Groundwater from springs and boreholes on the southern edge of the Cenozoic Duero Basin (DB) of Spain has concentrations of arsenic (As) which are commonly above the EC drinking-water limit of 10 µg/L, and reach observed values up to 241 µg/L. Groundwater compositions within the sedimentary aquifer vary from Ca-HCO 3 type, variably affected by evaporation and agricultural pollution at shallow levels, to Na-HCO 3 compositions in deeper boreholes of the basin. Groundwater conditions are mainly oxidising but reducing groundwaters exist in sub-basins within the aquifer, localised flow paths likely being influenced by basement structure. Arsenic concentrations are spatially variable, reaching up to 38 µg/L in springs of the Spanish Central System (SCS) basement aquifer and up to 62 µg/L in springs from the DB. Highest As concentrations are associated with the Na-HCO 3 compositions in deep boreholes (200-450 m depth) within the DB. These have high pH values (up to 9.6) can give rise to associated elevated concentrations of V and U (up to 64 µg/L and 30 µg/L respectively). In the deep borehole waters of the DB, oxidising flows derived from the mineralised igneous-metamorphic basement and discharging via major faults, are considered the origin of the higher concentrations. Compositions are consistent with desorption of As and other anionic species from metal oxyhydroxides in an oxic environment. Under locally reducing conditions prevalent in some low-flow parts of the DB, an absence of detectable dissolved As is coincident with low or undetectable SO 4 concentrations, and consistent with loss via formation of authigenic sulphide minerals. Mitigation measures are needed urgently in this semi-arid region where provision of alternative sources of safe drinking water is logistically difficult and expensive.

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

“…Towards the north, in the centre of the study area, a crossshaped distribution can be identified following these main fracture orientations. This distribution corroborates earlier theories about the control exercised by the structure of the basement on the distribution of As (Giménez-Forcada and Smedley, 2014;Rey-Moral et al, 2016;Giménez-Forcada et al, 2017).…”

“…Analyses of sediment samples undertaken as part of the various studies cited indicate that the clayey-silty materials contain the highest concentrations of As. Furthermore, and without disregarding water-sediment interactions, recent studies of the distribution of As within the area allude to the distribution of As in the southern CDB being strongly controlled by the structure of the Basin (Giménez-Forcada and Smedley, 2014;Rey-Moral et al, 2016;Giménez-Forcada et al, 2017).…”

Métodos geoestadísticos para la elaboración de mapas de probabilidad de riesgo hidrogeotóxico (HGT) por altas concentraciones de As en las aguas subterráneas. Aplicación a la distribución de HGT en la provincia de Ávila (España)

“…A esta estructura se asocian las cuencas pull-apart terciarias del Valle Amblés y Corneja, que forman parte también de la zona de estudio. Se han detectado anomalías significativas de As en la prolongación de esta estructura bajo el relleno terciario de la cuenca del Duero (Giménez-Forcada y Smedley, 2014;Rey-Moral et al, 2016). Aproximadamente la cuarta parte de la provincia de Ávila corresponde a los depósitos terciarios de la Cuenca del Duero, mientras que el resto del territorio se encuentra sobre el Sistema Central (Figura 2).…”

“…Este último método utiliza la geología para el cálculo de la media local variable, dando como resultado un mapa más detallado que los métodos que utilizan sólo las concentraciones de As (Figura 4(c)). Por otro lado todos los mapas que han tenido en cuenta la litología de la zona estudiada (Figuras 4 (c), (e) y (f)) reflejan un mayor contraste entre las áreas sedimentarias (con altas concentraciones de As en las aguas subterráneas) y las del Sistema Central, donde generalmente los valores de concentración de As son más bajos, a excepción de las 2017, IWA Publishing, Editorial UPV, FFIA fracturas donde se aprecia un HGT As mayor, especialmente en torno al trazado del dique Messejana-Plasencia, corroborando de este modo la hipótesis sobre el control estructural de la distribución de As (Giménez-Forcada y Smedley, 2014;Rey-Moral et al, 2016).…”

“…Una de las zonas más afectadas es el sur de la Cuenca del Duero donde un buen número de abastecimientos con agua subterránea superan el nuevo valor umbral. Aunque generalmente estos valores están solo ligeramente por encima de 10 µg/L, puntualmente se pueden alcanzar concentraciones de varias centenas de µg/L (Giménez-Forcada y Smedley, 2014). La provincia de Ávila es un buen ejemplo de lo que sucede en la zona.…”