Sediment characterization and its implications for arsenic mobilization in deep aquifers of eastern Croatia

Sediments from two boreholes drilled in the Drava and Sava depressions were analyzed to determine the origin of the elevated arsenic (As) concentrations in groundwaters in eastern Croatia. The composition of the sediments was investigated by total chemical analysis and sequential extraction. In addition, groundwater samples were taken from eight producing wells, and hydrochemical and trace element and As speciation analyses were performed. The concentrations of As in the sediment were high (up to 471 and 65.1 mg/kg, in the < 0.065 mm fraction in the boreholes in the Drava and Sava depression respectively) supporting the hypothesis proposed in a previous hydrochemical study that the Middle and Upper Pleistocene sediments are the main source of As in eastern Croatian groundwaters. Mineralogical and chemical analyses showed that sediments from the Drava and the Sava boreholes differed, supporting different origins of the sediments in the boreholes concerned: the provenance of PVc-3 sediments being the Alps, and of G1 sediments, the Bosnian mountains. In both boreholes, the large amount of As was bound to amorphous Fe and Mn (hydr)oxides (on average 32% and 60% in boreholes PVc-3 and G1 respectively). Sediment and groundwater analyses indicate that reductive dissolution of iron (hydr)oxides and concomitant release of sorbed or co-precipitated As into the groundwater is ready to occur. Since the groundwaters in eastern Croatia are generally anoxic and reducing conditions, in which S2 − and Fe2 + are stable, prevail the concentration of As released by reductive dissolution of iron (hydr)oxides may be controlled by the secondary formed sulfides which could act as a sink rather than as a source of As in the groundwater. Depending on local geomorphological and hydrogeochemical conditions, in the zones with a longer groundwater retention time, pH dependant As desorption from iron (hydr)oxides can occur and other trace elements such as Mo may become mobile.

► Pleistocene sediments are the main source of As in eastern Croatian groundwaters.
► Important amount of As was bound to amorphous Fe and Mn (hydr)oxides.
► Reductive dissolution of Fe (hydr)oxides dominant mechanisms of As mobilization
► pH can locally influence As mobility.