Solute Transport in the Unsaturated Zone – an Experimental Approach

Abstract

Human activities cause release of various sort of chemical and contaminants at the earth surface. How these contaminants flow downwards in soil towards our underlying subsurface reservoirs is a serious and critical issue. This knowledge is needed for remediation of the contaminated soils as well as soil usage management. Therefore, transport of solutes into the top soil layer, which is an unsaturated zone, is the aim of this study. Here, we used the multistep-flux transport (MS-T) approach to measure solute dispersivity as a function of saturation, complemented with the gamma-ray transmission method to measure the breakthrough (i.e., arrival time) curves of pulse-injected solutes. Our set-up consists of a small column (3cm by 3cm by 2cm) containing a sand sample. The results of the ADE-model show that our porous medium has a saturated dispersivity of around 0.3-0.5 cm. The unsaturated dispersivity increases with decreasing saturation to a maximum of 1.54 cm (at Sw = 0.51). The dispersivity value then begins to decrease until it gets to 0.283 cm (at Sw = 0.28). We have established a relationship between dispersivity and soil water saturation, which is in agreement with some previous studies (Toride et al., 2003; Raoof and Hassanizadeh, 2013), while it is in disagreement with some others (Maciejwski (1993) and Kanzari et al. (2015)). The MIM-model has a similar trend to the ADE-model, however dispersivity starts to decrease at a higher saturation with respect to ADE (0.60 to 0.51) and generally has lower dispersivity values. This research confirms the presumption that dispersivity decreases beyond a certain critical saturation level.