Effect of water content on transport of water tracer and strontium in compacted clay-rich soil column

Mechanisms involved in the radionuclide mobility in water-saturated environments have been extensively studied in order to predict their migration. However, in natural environments, a partially water-saturated zone occurs between the soil surface and the water table. It is well known that a decrease in water content reduces the porosity available for flow. Various studies have reported a increase or reduction of the contaminants residence time in porous media, explained by the flow paths complexity increase, a preferential path by the macroporosity and a reduced accessibility to reactive sites [1]. This study aims to understand the influence of water content on transport parameters such as dispersivity, porosity and chemical reactivity. This study investigates the effect of water content by comparing column transport experiments performed with inert (enriched-HDO water) and reactive (strontium) tracers on water-saturated and partially saturated soil.

Transport experiments were carried out on columns filled with the 300-400 µm fraction extracted by dry sieving from a sedimentary alluvium. This material was then compacted inside a glass column to reach the same density as that measured in the field (1.47 g.cm^-3). Transport experiments were performed under water saturated and partially saturated conditions corresponding to 0.43 to 0.19 water content, with a CaCl₂ solution equilibrated with calcite at pCO₂ atm. At steady flow, tracers were introduced into the system by an injecting loop, passed through the material and was collected in sequenced fractions. Sensors placed at both inlet and outlet of the column [2] allowed pH and electrical conductivity to be continuously controlled. HDO and Sr were measured with a deuterium analyser and an ICP MS respectively. HDO and Sr breakthrough curves were interpreted with HYDRUS-1D coupled with PhreeqC softwares. A multi-site ion exchange model was implemented in PhreeqC [2]. Flowrate and porosity were experimentally measured while dispersivity was determined by inverse modelling. To compare the different experiments, results were expressed in dimensionless units: relative concentration (C/C₀) and pore volume passed through the column normalized to the column pore volume (V/V_pore).

Based on experiments carried out in water-saturated media with HDO, the dispersivity in the material was estimated at 0.1 cm^-1. The Sr residence time was tenfold more than HDO (from 2.5 to 30 V/V_pore), which confirms that chemical retention drives the cation migration into porous media. Three HDO experiments carried out at various water contents (0.24 to 0.19 cm.cm^-1) revealed a regular dispersivity increase with decreasing water content from 0.1 to 0.2 cm^-1. For Sr experiments, decreasing water content led to the increase of the breakthrough curve intensities and a tailing effect, meaning that Sr would be less retained and more spread with reduced water content.

These results show that reducing the water content in porous media leads to reduce the porosity accessibility to flow and to increase the dispersivity. This suggests that the water content decrease constrains the water flow path, this is intensified with the desaturation. The Sr transport behaviour change with desaturation may be explained by the reduction in the accessibility to the sorption sites.