Distribution of dissolved arsenic in a sedimentary environment from the near-surface to a depth of 2500Â m, and factors controlling distribution

The studied subsurface water samples represent different depositional environments from the Late Miocene lacustrine and fluviatile (delta-slope, delta-plain) facies to the Pliocene and Quaternary lacustrine, fluviatile and terrestrial ones in the upper 2500 m of the sediments, in the central part of the Pannonian Basin. The study area covers about 25,000 km2 and comprises two sub-basins and an area above uplifted basement. Spatial distribution of dissolved arsenic indicates the highest range of the arsenic concentrations (0-180 μg/l) in the groundwater from the Quaternary fluviatile aquifers in both sub-basins. Comparing the measured pH and redox potential to the stability diagrams in the As-O-H, Fe-CO2-H2O, and semiquinone-hydroquinone systems it was observed that in the pe-pH diagram, most samples from the Quaternary and Pliocene sediments appear in the stability field of HAsO42−, FeOOH, and semiquinone (Q3−
- ), and are grouped along a line with a slope similar to those of the redox boundaries between HAsO42−and H3AsO30, and also semiquinone (Q3−
- ) and hydroquinone (H2Q2−) species. The samples from the shallowest, unconfined Quaternary aquifers are above the line, and whereas those from the deepest, Upper Miocene aquifers are below the redox boundary of the standard-state systems. The mobilising processes in the different depositional environments are explained on the basis of the position of the samples in the pe-pH stability diagrams. The spatial distribution of arsenic concentrations in groundwater is attributed to the availability of mobilisable arsenic in the sediments rather than to the other elements of the mobilising processes, such as the type of the chemical reactions, redox circumstances, or temperature.