Physical and chemical characterisation of superficial sediment of the Ribarroja Reservoir (River Ebro, NE Spain)

To determine the spatial heterogeneity of the physical and chemical characteristics of superficial sediments from the Ribarroja Reservoir, we have analysed particle size distribution and concentrations of major (Al, Fe, Si, K, Ca, Mg, Ti, Mn and P) and minor (Mo, Nb, Zr, Y, Sr, Rb, Th, Pb, Sn, Ce, Ga, Zn, W, Cu, Co, Ni and V) elements at sixteen sites distributed along the reservoir. Globally, the sediment was quite homogeneous: the dominant grain sizes were silt and clay, and the major element composition agreed with that expected for a mixture of carbonates and weathered silicates, with moderate levels of nitrogen and phosphorus. Concentrations of minor elements did not surpass the Probable Effect Levels, although the concentrations of nickel and chromium exceeded the Threshold Effect Level. A Principal Component Analysis performed on normalised minor elements indicated that the main processes determining the variation of sediment composition were the degree of alteration/weathering of materials, and the association of some metals with organic/clay aggregates.