Evaluation of the Cr(VI) and other toxic element contamination and their potential sources: The case of the Thiva basin (Greece)

An integrated approach was performed on the soil, plant-crops and groundwater system at the Thiva basin, to evaluate the extent and intensity of the heavy metal contamination, the percentage of metals transferred into plants/crops (bio-accumulation) and the potential sources of contamination.
Heavy metal contents (230–310 Cr, 1200–2200 Ni, 50–80 Co, 880–1150 Mn and 44,500–66,200 Fe all in ppm ) in the Thiva soils are higher than those in the Assopos basin. An increasing trend in the heavy metal contents, from the Assopos to the Thiva basin (Mouriki), indicates that the metal source is probably the Ni-laterites and ophiolites, located towards north. On the other hand, groundwater samples from domestic and irrigation wells throughout the Thiva basin exhibit relatively low (8–37 ppb) concentrations in Cr compared to the Assopos (up to 150 ppb). The plots of Cr versus Mg/Ca, Mg/Na versus Ca/Na, B versus Si/(Si + Na) and Mg/Si versus Ca/Si ratios revealed a low degree of salinization in the Thiva compared to that in the Assopos (Avlida) basin. The low Cr concentration (average 23 ppb, during dry period) in the Thiva wells may be related with their depth (>120 m), in contrast to shallow wells in the central Euboea (hundreds ppb Cr) and points to a solution of a crucial environmental problem in Euboea and Assopos basin by using the deep karst-type aquifer instead the shallow-Neogene one.
The average Cr contents (dry weight) ranges from 1.7 to 4.6 ppm (average 2.2) in carrots, potatoes and onions. The percentage of soil metals Crtotal, Cr(VI), Ni, Mn and Fe ranges between 0.06 and 3.2 (average 0.53 ± 0.4), whilst that for Cu and Zn ranges from 16 to 81 (average 36 ± 24). The Cr transfer to plants is low, due to the high resistance of chromite (main host of Cr), but the determined Cr contents in plant-crops in the Thiva basin are higher than normal or sufficient values. Although minimal uptake for growth and production for Cr, Ni, Zn and Cu is still lacking, furthermore research is required in order to restrict heavy metal accumulation and ensure sustainability.