Fractionation and potential mobility of trace metals in Danube alluvial aquifer within an industrialized zone

Thirty-five alluvial sediments of the River Danube and 12 groundwater samples were taken within the Pančevo Oil Refinery (Serbia). The results for groundwater samples exceed European primary drinking water standards for Fe (obtained results, >200 μg/l) and Mn (obtained results, >50 μg/l), while the levels of the trace metals are below the thresholds for drinking water quality. Sediments were treated by sequential extraction procedure with five different solutions, each having a higher extraction capacity than the previous one. We also wanted to determine the possible relationships among trace metals and between sediment properties and elemental concentrations. These solutions partitioned metals into CH₃COONH₄ extractable (F1); HCl carbonate extractable and NH₂OH·HCl easily reducible (F2); (NH₄)₂C₂O₄/H₂C₂O₄ moderately reducible (F3); H₂O₂-HNO₃ organic/sulfide extractable fractions (F4); and HCl acid-soluble residue (F5). The sum of trace metals Ni, Pb, Cu, and Zn associated with the first two fractions (exchangeable, carbonate, and easily reducible) is significant and extremely important because it represents the proportion of heavy metals that can be easily remobilized by changes in environmental conditions such as pH, redox potential, salinity, etc. Sediments located nearer the groundwater flow are exposed to stronger groundwater fluctuation and had a higher quantity of amorphous and less stable substrates of trace metals. Principal component analysis was used to understand and visualize the associations between the trace metals and certain geological forms within analyzed sediments. The observed association between Cr with total sulfur and Mn from the acid-soluble residue could indicate that Cr is in the form of reduced, less toxic Cr(III), which is from the ecochemical point of view very important.