Origin of hexavalent chromium in groundwater: The example of Sarigkiol Basin, Northern Greece N. Kazakis a , N. Kantiranis b, ⁎, K. Kalaitzidou c , E. Kaprara c , M. Mitrakas c , R. Frei d , G. Vargemezis e , P. Tsourlos e , A. Zouboulis f , A. Filippidis b a Aristotle University of Thessaloniki, School of Geology, Lab. of Engineering Geology and Hydrogeology, 54124 Thessaloniki, Greece b Aristotle University of Thessaloniki, School of Geology, Dept. of Mineralogy, Petrology and Economic Geology, 54124 Thessaloniki, Greece c Aristotle University of Thessaloniki, Department of Chemical Engineering, 54124 Thessaloniki, Greece d University of Copenhagen, Department of Geoscience and Natural Resource Management Nord CEE, Denmark e Aristotle University of Thessaloniki, School of Geology, Applied Geophysics Lab., 54124 Thessaloniki, Greece f Aristotle University of Thessaloniki, Department of Chemistry, Lab. of General and Inorganic Chemical Technology, 54124 Thessaloniki, Greece HIGHLIGHTS • High concentrations (up to 120 μg/L) of Cr(VI) were locally observed in the groundwaters of Sarigkiol/Kozani area. • Unconfined aquifers are more vulnera- ble to Cr(VI) pollution from ophiolitic rocks, as well as from dispersed fly ash. • Fly ash produced from a power plant burning lignite, was found to contribute to groundwater pollution with Cr(VI). • Positively fractionated δ 53 Cr values in groundwater and fly ash leachate point to partially anthropogenic origin of Cr(VI). • The natural (geochemical) contribution, regarding Cr (VI) pollution, was found up to 60 μg/L. GRAPHICAL ABSTRACT abstract article info Article history: Received 29 December 2016 Received in revised form 13 March 2017 Accepted 13 March 2017 Available online xxxx Editor: D. Barcelo Hexavalent chromium constitutes a serious deterioration factor for the groundwater quality of several regions around the world. High concentrations of this contaminant have been also reported in the groundwater of the Sarigkiol hydrological basin (near Kozani city, NW Greece). Specific interest was paid to this particular study area due to the co-existence here of two important factors both expected to contribute to Cr(VI) presence and groundwater pollution; namely the area's exposed ophiolitic rocks and its substantial fly ash deposits originating from the local lignite burning power plant. Accordingly, detailed geochemical, mineralogical, hydro-chemical, geophysical and hydrogeological studies were performed on the rocks, soils, sediments and water resources of this basin. Cr(VI) concentrations varied in the different aquifers, with the highest concentration (up to 120 μgL -1 ) recorded in the groundwater of the unconfined porous aquifer situated near the temporary fly ash disposal site. Recharge of the porous aquifer is related mainly to precipitation infiltration and occasional surface run-off. Nevertheless, a hydraulic connection between the porous and neighboring karst aquifers could not be delineated. Therefore, the presence of Cr(VI) in the groundwater of this area is thought to originate from both the ophiolitic rock weathering products in the soils, and the local leaching of Cr(VI) from the diffused fly ash located in the area surrounding the lignite power plant. This conclusion was corroborated by factor analysis, Keywords: Cr(VI) presence Ophiolitic rocks Dispersion of fly ash Electric resistivity tomography Factor analysis Chromium isotopes Science of the Total Environment 593–594 (2017) 552–566 ⁎ Corresponding author. E-mail address: kantira@geo.auth.gr (N. Kantiranis). http://dx.doi.org/10.1016/j.scitotenv.2017.03.128 0048-9697/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv