CHARACTERIZATION AND MOBILITY OF HEXAVALENT CHROMIUM IN SOILS AND SEDIMENTS

Asopos river basin is located in the Region of Sterea Ellada, about 100 km north from Athens. Asopos River receives waste discharges from metallurgies and metal furnishing plants, industries related to textile and dyes production, as well as food and chemical industries. Asopos River has a serious chromium contamination problem since the concentrations of Cr(VI) in the surface and groundwater are above the quality standards. In addition, there is a geogenic origin hexavalent chromium from the weathering of ultramafic rock fragments incorporated in the neogene and alluvial deposits that contributes to groundwater and soil contaminations. The objectives of this study are threefold: a) to assess the physicochemical and geochemical characteristics of soils and sediments in the area, b) to assess the processes affecting the mobility of hexavalent chromium in soils and groundwater and c) to understand the processes that control the fate of chromium in the subsurface. The experimental design was the following. A field campaign (sediment, soil and water monitoring survey) was carried out in order to assess the natural ability of soils and sediments to reduce and immobilize hexavalent chromium as well as the probability of river sediments to become secondary source of contamination once the primary sources (all industrial effluents in the river) cease to exist. The sampling campaign was designed in such way as to capture the spatial variability of chromium occurring in the sediments and soils in the different lithological units in the area. Soil samples were collected from the neogene and alluvial formations and the ultramafic rock (serpentinite) and sediment samples and cores were collected from the main course of Assopos River. Physicochemical characterization of all samples included the determination of soil moisture, pH, conductivity, cations, heavy metals, total organic carbon, and total nitrogen. XRF analysis, alkaline digestion, redox capacity analysis and particle size distribution were also conducted. The results were analyzed using principal component analysis in order to determine the factors affecting clustering. The second phase involved mobility studies in the laboratory to quantify the rates of sorption and release of total and hexavalent Cr from the soils and the capacity of soil to adsorb chromium. The analysis of heavy metals and the study of the local geology support as the main source the geogenic origin of chromium in the sediments and soils. Geochemical evidence suggested two sources capable of producing detectable amounts of Cr(VI); the Neogene and alluvial sediments and the serpentine soil. The mobility studies showed that the process of leaching was very fast and the sorption capacity was significant.