El problema del arsénico en Castilla y León/The arsenic problem in Castilla y León (Spain)

Resumen de la Comunicación presentada al "2nd International Congress ARSENIC IN THE ENVIRONMENT" , celebrado en Valencia, España, Mayo 2008.

Hydrochemistry of groundwater from the Tierra de Pinares region (Douro basin, Spain) affected by high levels of arsenic
Marisol Vega, Concepción Carretero, Luis Fernández, Rafael Pardo, Enrique Barrado, Luis Debán
Department of Analytical Chemistry, Faculty of Sciences, University of Valladolid. Ps. Prado de la Magdalena, s/n. 47005-Valladolid, Spain. E-mail: solvega@qa.uva.es

The occurrence of high arsenic levels in groundwater from the Tierra de Pinares region (Castilla y León, Spain) , has prompted an investigation into the hydrogeochemistry of the aquifer in order to understand the sources and mobilization pathways of this element.

A confined area of the Tierra de Pinares region covering the municipalities of Mata de Cuellar, Vallelado and San Cristóbal de Cuellar (Segovia province, Spain) was selected to investigate the groundwater hydrochemistry. More than 80 sampling points, including shallow dug wells, deep drilled wells and springs were located and sampled twice (August 2002 and June 2003). Twenty hydrochemical variables were determined, including pH, conductivity, temperature, dissolved oxygen, arsenic, vanadium, major cations and anions, iron and manganese.

Two shallow aquifers, alluvial (Ca-Mg-SO4-HCO3) and carbonated (Ca-Mg-HCO3), exist in the area hydraulically interconnected, and below them there is a deep detritic aquifer (Na-HCO3). The distribution of As in groundwater varies widely: 16% of the water points sampled contained less than 10 μg/L As, all of them belonging to the carbonated aquifer; 10-50 μg/L As was found in 25% of the sampling points; As levels of 50-200 g/L occurred in 47% of the sampling points; and 12% of the wells analysed contained As levels higher than 200 μg/L (all of them in the alluvial aquifer). The highest arsenic contents were found in the alluvial aquifer (maximum concentration, 385 μg/l), but maximum mean values correspond to the detritic aquifer (90 &mug/l).

The correlations of As with other hydrochemical variables were investigated by principal component analysis and cluster analysis and allowed to elucidate the mechanisms that likely contribute to the mobilization and dispersion of arsenic in the aquifer. Within the distinct mechanisms that can lead to the release of arsenic from the parent rock to the groundwater the one dominating here seems to be the development of high pH (>8.5) conditions as a combination of mineral weathering and high evaporation rates. This pH change leads to the release of adsorbed As species (mainly AsV) and other anion-forming elements from clays and, in less extent, from iron and manganese oxides. The large concentrations of bicarbonate and sulphate found in groundwater can enhance the desorption of As because of competitive effects.

The intense exploitation of both the deep and shallow aquifers and the recirculation of irrigation excess water cause an important mixing of groundwater from different aquifers and hence increase the mobility and dispersion of arsenic in the environment.

Acknowledgements
The authors wish to thank the economical support provided by Consejería de Educación y Cultura, Junta de Castilla y León (Project VA077A05) and the Spanish MEC (Project CTM2006-02249/TECNO).