Influence of transient flow on the mobility of strontium in insaturated sand columns

Titre du congrès :Migration '07 Ville du congrès :Munich Date du congrès :27/08/2007

The prediction of a contaminant's fate and its behaviour in soils and the vadose zone is important for the environmental evaluation and possible rehabilitation of soils occupied by industrial installations in operation or during dismantling. Up to now, reactive transport of contaminants in the vadose zone was very simplified owing to the complexity of the hydro-geochemical phenomena. Field and laboratory experiments are in progress world-wide to improve prediction models. Some recent experiments at IRSN were focussed on the transport strontium 90 in sand columns both in saturated and unsaturated steady state hydraulic conditions [1]. The soil used was an eolian sand [2] taken near the experimental platform of IRSN in the Chernobyl area. For constant geochemical conditions, it has been shown that the mean migration depth of 90Sr can be described by a retardation factor (R) depending on the sorption coefficient (Kd), the bulk density (d) and the water content (). However, the assumption of a constant water content might hide physical and chemical phenomena that could occur in natural conditions. The purpose of this study was to investigate the influence of transient unsaturated hydraulic conditions on the mobility of strontium 90 in the same sand. First, we studied the hydrodynamic properties of the porous medium using a gamma attenuation system to determine the local porosity and the water content of the sand. Special care was taken to determine the water retention h() and the hydraulic conductivity K() curves with drainage and infiltration experiments. Then, experiments were conducted by injecting a series of infiltrations in an initially drained column. The gamma attenuation system allowed us to follow the evolution of water content with time at fixed depths along the column during infiltrations. Experimental data were successfully modelled with HYDRUS-1D code. Reactive transport of strontium was then conducted on a drained sand column. A first infiltration flow event using a solution spiked with 85Sr was injected at the top of the column. We then followed the redistribution of water in the column for 24 hours until hydrodynamic equilibrium was reached. The profile of 85Sr was then monitored. A series of infiltration - redistribution events using a solution free of 85Sr was then injected. The migration profile of 85Sr was then monitored and successfully modelled with a Convection/Dispersion model using the Hydrus 1-D code. A comparison with an equivalent steady state unsaturated hydraulic flow was also performed. We also analysed the mean migration depth of 85Sr as a function of the equivalent water column that had passed through the column. A linear relationship was derived. The slope of the linear function is dependant only on the bulk density and the sorption coefficient of strontium.