Origin, mineral speciation and geochemical baseline mapping of Ni and Cr in agricultural topsoils of Thiva Valley (central Greece)

Lithogenic Ni and Cr enrichment in agricultural topsoils of Thiva Valley (central Greece) is the result of transport of weathered parent rock fragments from upslope ultramafic sources. Chemical and mineralogical evolution of these potentially toxic elements between the trace metal bearing bedrock and the overlying soil horizon along two weathering profiles located in an area where the ultramafic rocks are exposed was accomplished to unravel the source and mineral speciation dynamics of Ni and Cr in the soil system of Thiva Valley. Within the soil horizons, Ni primarily occurs in serpentine (average 0.5 wt.% of NiO) and secondary weathering products like smectites (average 0.5 wt.% of NiO) and goethite (average 1.1 wt.% of NiO). Significant amounts of this element are also hosted in less abundant Mn (hydr)oxides. On the contrary, Cr is mainly bounded with inherited chromite (average 54.7 wt.% of Cr2O3) and to a lesser extent with Cr-magnetite (average 12.2 wt.% of Cr2O3) and Cr silicates like enstatite (average 0.8 wt.% of Cr2O3). As a result of the solid speciation and the applied decomposition method (aqua regia digestion), the soil geochemical anomalies within the valley are more pronounced with respect to Ni (up to 2640 mg/kg) compared to Cr (up to 856 mg/kg). The geochemical baseline maps produced by the ordinary kriging interpolation method reveal a strong tendency for large Ni and Cr amounts in the southern part of the area warning about the potential occurrence of high metal values in soils of Thiva town with an approximate number of 25,000 exposed population. Based on the mineralogical residence of Ni and Cr differences in their potential bioavailability and leaching to groundwater are anticipated.

► Chemical and mineralogical evolution between ultramafic bedrock and overlying soil
► Natural geochemical anomalies of Ni and Cr in alluvial topsoils of Thiva Valley
► Geochemical baseline maps of aqua regia extractable Ni and Cr concentrations
► Differences in mineralogical residence and chemical reactivity of Ni and Cr
► Implications for Ni and Cr bioavailability and potential leaching to groundwater.