High arsenic contents in groundwater of central Spain

The chemical and microbiological characteristics of groundwater from two provinces of central Spain were studied. In some zones of this area, the concentrations of As in groundwater exceed the guideline concentrations, set internationally between 10 g/l and 50 g/l, reaching levels over 100 g/l. A narrow correlation between the contents of arsenic and HCO₃^– was observed. These data suggest a possible mechanism of the As mobilization from aquifer sediments to groundwater: the bicarbonate ions could displace HAsO₄^2– adsorbed on aquifer oxyhydroxides. Sediments containing relatively high contents of adsorbed arsenic are deposited in surface water environments with low carbonate concentrations. Subsequently, the sediments become exposed to groundwater with highly dissolved carbonate content, and arsenic can be mobilized by displacement from mineral surfaces. In addition, the presence of Pseudomonas genera bacteria, which secrete siderophores (Fe chelating agents) could mobilize As adsorbed on Fe oxides through their dissolution. These combined microbiological and chemical processes might have increased the natural mobility of As.