Multivariate statistical interpretation of the distribution of arsenic and other geogenic trace elements in groundwater from a semiarid basin (Central Spain)

Abstract

The occurrence of high levels of arsenic and other geogenic trace elements in groundwater from the Southern bank of the Duero River basin (Central Spain) has prompted an investigation into groundwater hydrogeochemistry to understand the sources and mobilization pathways of these toxic elements. More than 70 groundwater samples were collected (2015-2016) from springs and deep wells (>50 m) from an area of 45x25 square kilometers. Hydrochemical parameters determined in the samples included pH, electrical conductivity, E.C., redox potential, Eh, temperature, major ions and trace elements (As, B, Ba, F, Fe, Li, Mn, Sr, U, V and W). The distribution of As in groundwater varies widely: 32% of the samples contained less than 10 µg/L, maximum allowable level in drinking water according to WHO and the Council Directive 98/83/EC, whereas As levels above 100 µg/L occurred in 18% of the samples. Spatial variability was also observed for the rest of trace elements. The interpretation of the results by multivariate statistical tools (principal component analysis, PCA, and hierarchical agglomerative cluster analysis, HCA) revealed significant correlations amongst groups of variables: As-V-F-W-pH; U-Na-Li-B-bicarbonate; Ca-Mg-sulfate-E.C. The occurrence of trace elements seems to be controlled by two mobilization mechanisms: (i) desorption of As and related elements in alkaline waters (pH>8.5), increased by high concentrations of competing anions (mainly sulfate), and (ii) dissolution of carbonated minerals, abundant in the basin, releasing alkaline elements and uranyl ions. The intensive use of groundwater in agriculture and the high evaporation rates are worsening the quality of groundwater in the Region.