Transfer processes of potentially toxic elements (PTE) from rocks to soils and the origin of PTE in soils: A case study on the island of Santiago (Cape Verde)

The understanding of rock-soil geochemical fluxes provides an important tool for evaluating natural environmental risk. In this research we compare the geochemical composition of soils (n = 249) collected on Santiago Island (Cape Verde) with their parent rocks (n = 96). Basalt and basanites units and overlying soils are relatively rich in Co, Sc, Mg, Ca, Fe, V, Ti, Ni, Cr, and Cu, whilst phonolitic-trachytic units and overlying soils are relatively rich in Zn, P, Sr, Mn, Ba, La, As, Th and Pb. An enrichment of potentially toxic elements (PTE; As, Cd, Co, Cr, Cu, Fe, La, Mn, Ni, Pb, Sc, Th, V) was observed in soils relative to their parent rocks. This enrichment of PTE in Santiago soils results mainly from the removal of mobile elements that are common in primary minerals (e.g., Ca, Na, Mg, K) and coprecipitation and/or adsorption by Fe-oxy-hydroxides and clay minerals (mostly smectite and palygorskyte) under the influence of high pH values. Potentially toxic element contents were found to increase with weathering; oxidation is a major weathering mechanism under a semi-arid climate, and a high correlation between Fe and PTE was found, suggesting that the mobility of PTE is controlled by the iron phases. The level of enrichment of PTE in a specific site does not reflect the bulk concentration and so is not a good indicator of the actual geochemical environmental quality. The extent of contamination is not limited to the vicinity of the source, as contaminated material may be physically remobilized, particularly in high runoff conditions, causing dispersion of geo-pollutants over many kilometres from the source. The geochemical flux clarify the lithological origin of these elements, refuting the hypothesis of an anthropogenic contamination, as the PTE enrichment in Santiago soils is not a local phenomenon, but, on the contrary, occurs over the entire island.