Transport of nonreactive and volatile Solutes in unsaturated porous media under wetting and draining conditions

Abstract

The effect of water content and soil-water hysteresis on transport of unreactive water-tracers and volatile organic compounds (VOCs) in porous media is investigated under steady-state water-flow conditions. Specifically, this research addresses the effect on dispersive and mass transfer processes affecting the movement of NaC1 and trichloroethene (ICE) and how these processes influence the approach to Fickian flux conditions. Transport experiments were conducted in a 25-cm column packed with silica sand. Based on the results, it is concluded that water content (0), pore-water velocity, and flow history affect the average movement and spread of water-tracers and VOCs. It is suggested that non-volatile solutes in unsaturated media travel longer distances or times to achieve a Fickian state. Consequently, a greater number of averaged heterogeneities are encountered and solute flux is characterized by a greater dispersion coefficient (D). A power (n) law relationship (D(m) = η(v(m)/ θ(m))ⁿ), found between mobile dispersion coefficients (D(m_), pore-water velocity (v(m)), and water content (θ(m)) for different porous media, indicates that dispersivity (η) is not only a function of the media, but also of θ(w). TCE transport is controlled by advection processes for Ow greater than 50% saturation. Lower θ(w) result in greater TCE dispersion, retardation, mass-transfer resistance, vapor diffusion, and spreading. Consequently, VOCs reach the Fickian regime at shorter distances than unreactive solutes in water. Although VOC transport is influenced by multiple rate-limited mass transfer, the mechanisms controlling the overall mass-transfer resistance vary as a function of θ(w). The hysteretic behavior of solute transport parameters is attributed to a greater degree of irregular flow paths and entrapped air, higher air-water interfacial areas, and thicker water-films for wetting than draining scenarios. Consequently, wetting conditions result in slower mixing (up to 98% lower mass-transfer coefficients) of dissolved solutes. Since TCE transport at low water contents and wetting conditions is dominated by diffusion and dispersion mechanisms, the TCE velocity distribution in the liquid phase is normalized by velocity distributions in the gas-phase and becomes closer to Fickian conditions.