Factors controlling arsenic and selected potentially toxic elements in stream sediment-soil and groundwater-surface water systems of a hydrologically modified semi-closed basin (Uluova) in Elazig Province, Eastern Turkey

摘要

The Uluova basin aquifer system (UBAS), comprised of fractured/karstic rock and multilayer basin-fill aquifers, provides water for both domestic and agricultural needs in the Elazig province (Eastern Turkey). Although, the UBAS has been subject of large-scale hydrological modifications and controversial water management practices since late 1950s, thus far no studies have examined basinwide implications of such human perturbations in an integrated manner. In this study, GIS, geostatistics, R-mode factor analysis (R-mFA) and geochemical modeling techniques were employed to unravel the factors controlling the distribution and sources of arsenic (As) and selected potentially toxic elements (PTEs) in the stream sediment-soil and groundwater-surface water systems of the UBAS. As revealed by the results from R-mFA, three geogenic factors (Fl: Clay minerals and Fe-Mn oxyhydroxides, F2: Weathering of parent materials, and F3: Sulfide oxidation in mineralized zones) govern the geochemical dynamics of the PTEs in the stream sediment/soil media. In stream sediment and soil samples, especially Ni, Cr, and Co presented significant enrichment relative to upper continental crust average composition, whereas As contents were relatively low, varying from 0.3 to 13 mg kg(-1). Factors extracted from the combined water dataset (Fl: Groundwater salinization and arsenic mobilization, F2: Clay minerals and Fe-Mn oxyhydroxides, F3: pH and redox conditions, and F4: Aquifer oxygenation and nitrate contamination) accounted for 72.59% of the total variance. Water-rock interaction (e.g. sulfide oxidation, carbonate dissolution, silicate hydrolysis, adsorption-desorption, ion exchange, and evaporite dissolution), dilution/mixing with fresh/saline water components, evapoconcentration, and human induced perturbations causing internal salinization and oxygenation of the UBAS were the key mechanisms controlling the chemistry of waters and mobilization of As. In the UBAS, majority of As-rich water samples are confined to central-northem half of the basin and typically display high levels of dissolved O-2, inorganic oxyanions (HCO3, SO42- and of Si, B, Mo, Sb and V) and alkaline pH. Oxidation of sulfides (e.g. pyrite and arsenopyrite) found within the highly fractured Elazig magmatics in the upland areas at north and subsequent competitive adsorption-desorption processes occurring under alkaline, oxidizing and high ionic strength aquifer conditions along the downgradient groundwater flow path play a pivotal role in the As-enrichment in the UBAS. As concentrations ranged between 0.02 and 367.2 mu g L-1 in groundwater, 0.13-4842 mu g L-1 in spring water, and 0.04-31.1 mu g L-1 in the stream water samples, of which 20.83% exceeded the WHO provisional guideline value. In the water samples, As occurs mainly as As(V) species (HAsO42- and H2AsO4), indicated by the Eh-pH diagram and speciation calculations. The results of this study have shown that As enrichment in the UBAS can be attributed to both geogenic processes and anthropogenic activities that have modified the basin hydrology/hydrochemistry.