Mobility of major-, minor- and trace elements in solutions of a planosolic soil: Distribution and controlling factors

Subsurface waters circulating in an unpolluted soil of a planosolic horizon (Massif Central, France) were studied in order to determine their physico-chemical characteristics. Three water sampling sites were chosen along a toposequence. For each site, two piezometers were placed above and in the gravelly and concretion-rich horizon (Fe- and Mn- oxyhydroxides). Concentrations of major-, minor- (cations, anions, Fe, Mn, P and Si) and trace elements (Al, Ba, Cd, Co, Cr, Cu, Ni, Pb, Rb, Sr, Zn and U) were monitored on bulk and filtered water (0.45 μm) to study both the particulate and the dissolved components, from 2004 to 2006, during the soil saturation period (i.e., from November to May). Chemical characteristics of soil solutions provide evidence for various chemical water compositions and for temporal variations of water quality, revealing that the hydrodynamic and chemical reactivity in the solution is different for the three sites. Calculations of pe values indicate a range of redox state of the soil solutions. The pe ranges are different for each piezometer but correspond to anoxic solution. For all piezometers, distribution between the dissolved and the particulate fraction and correlations between the various elements in the soil solutions indicate that: (i) Al and Fe show similar behaviour, (ii) Al is mainly present as oxyhydroxides and (iii) some trace metals are mainly associated with particles which have a mixed nature. The impact of a concretion-rich horizon is noticed both on the nature of particles and on the speciation of trace metals and could be explained by the hydrodynamic and chemical reactivity of the circulating solution. Very few correlations exist between elements in the dissolved phase.