Anomalously high concentration of Ni as sulphide phase in sediment and in water of a mountain catchment with serpentinite bedrock

Abstract

Potentially toxic elements (i.e., PTEs, e.g., Ni, Cr, Co, Cu) are a major concern for environmental quality and human health, and soils derived from ultramaphic bedrock are a main natural source of these elements. For a good comprehension of the chemical availability and possible dissolution of these elements in areas with high natural background concentrations, it is important to evaluate their speciation in soils and sediments and their concentrations in waters. The speciation and the total load of PTEs in serpentinite-derived glacial sediment were evaluated through acid digestion and sequential extractions, together with the analysis of water samples in an alpine catchment during the summer of 2014 and 2015. Our results revealed a peculiar Ni speciation in the sediment samples, appointing to the presence as sulphide and differentiating from results of previous works usually indicating a main source bond to Fe and Mn oxides. These data probably revealed an important amount of sulphide minerals and were related to a relatively low weathering of glacial sediment. Moreover, water analysis showed a relatively high Ni concentration consistent with an enhanced dissolution. Enrichment due to atmospheric deposition is excluded by the temporal trend of Ni concentrations in water samples. Our results showed anomalies in Ni speciation and dissolution in water different from previous studies on the same bedrock type, raising new issues regarding the primary pedogenetical phase of serpentine soils and the dissolution of Ni in high mountain catchments.