Preferential water and solute fluxes in a model macropored porous medium as a function of flow rate (original) (raw)
Related papers
Models developed for solute transport vary in their assumptions on macropore continuity and tortuosity. It is unclear how much simplification can be made in computer models to characterize macropore effects on water and solute transport through soils. The objectives of this study were to assess how the importance of macropore continuity and tortuosity varies (1) with various initial and boundary conditions (this paper) and (2) with simplifying model assumptions for macropore description (companion paper). The above assessments were made with a computer model based on 2- D Galerkin finite element solution of Richards’ equation for water flow and convective–dispersive equation for solute transport. The model can simultaneously handle macropores of varying length, size, shape, and continuity. Model predictions were in agreement with laboratory data for different macropore shapes and continuities under transient flow conditions. Simulations for various initial and boundary conditions showed that surface connected macropores under ponded conditions and under high intensity rainfalls favored the rapid transport of solutes. However, solute transport was delayed if the solute was initially incorporated in the soil even when macropores were connected to the soil surface. Macropores not connected to the soil surface only slightly accelerated solute transport for any boundary conditions. Macropore tortuosity did not influence breakthrough curves as much as the continuity but greatly influenced solute distribution in the profile. The importance of macropore continuity and tortuosity on preferential transport increased with an increase in solute retardation. General guidelines for simplifying continuity and tortuosity for modeling solute transport are presented for various initial and boundary conditions.
Analysis of Flow Rate Dependency of Solute Transport in an Undisturbed Inceptisol
Vadose Zone Journal, 2011
We inves gate the fl ow rate dependency of solute transport within an undisturbed monolithic soil core, collected in an Incep sol. Through a series of nine controlled steady-state solute breakthrough experiments, fl ow rate dependency of solute transport was elucidated using the general transfer func on (GTF) modeling theory. We fi rst observed that the apparent dispersion coeffi cient increases with depth and fl ow rate. We also observed that the fl ow regime is rather a convec ve-dispersive (CD) process at low fl ow rates and a stochas c-convec ve (SC) process at high fl ow rates. At intermediate fl ow rates, the fl ow regime could not be described with either CD or SC processes. To be er understand the mechanisms of altering fl ow regime at intermediate fl ow rates, a dye tracer experiment was conducted. Results show that preferen al fl ow is ini ated at intermediate fl ow rates. We hypothesize that the mixing of solutes between stream tubes decreases when fl ow rate increases, due to the decrease of the tortuosity of solute fl ow paths and the ini a on of preferen al fl ow through macropores. Abbrevia ons: BTC, breakthrough curve; CD, convec ve-dispersive; CDE, convec ve-dispersive equa on; CLT, convec ve lognormal transfer; EC, electrical conduc vity; GTF, general transfer func on; pdf, probability density func on; PVC, polyvinyl chloride; SC, stochas c-convec ve; TDR, me domain refl ectometry. Knowledge of the lateral mixing regime of surface-applied solutes is key for describing vertical solute transport in undisturbed soils (Jury and Flühler, 1992; Flühler et al., 1996). To evaluate the mixing processes, a mixing time t* was introduced by Dagan and Bresler (1979). Th ey defi ned the mixing time as the time interval during which a solute particle travels with constant velocity. If the mean solute travel time is much lower than t*, then the solute molecules disperse longitudinally by virtue of diff erences in convective velocities, resulting in a solute apparent dispersion increasing linearly with depth. In this case, the solute transport processes can be modeled with a SC transport model (Simmons, 1986; Dagan, 1984). In contrast, if t* is much lower than the mean solute arrival time, the apparent dispersion remains constant with depth, and the process is better described with a CD transport model (Khan and Jury, 1990). Solute transport in a natural soil cannot always be conceptualized as following either a CD or SC process. Indeed, the CD model considers full mixing in a homogenous matrix, while the SC assumes a vertically homogenous soil within the stream tubes. Th e stream tube model represents the fi eld by a set of vertical soil columns in which one-dimensional CD transport takes place, and which is parameterized by a distribution of advection velocities and dispersion coeffi cients (e.g., Toride and Leij, 1996a,b). Both models will therefore fail to describe transport in soils having signifi cant vertical heterogeneity. Th e vertical heterogeneity (e.g., in terms of diff erent pedogenic soil layers) has a considerable eff ect on solute transport (Leij and Dane, 1991). To be able to model CD, SC, and intermediate transport regimes, the GTF model was proposed by Zhang (2000). Th e GTF parameters, which can be obtained from measured solute breakthrough curves (BTC) at diff erent depths, allow identifi cation of the mixing regime in a comprehensive way (Javaux and Vanclooster, 2003a).
Interplay between Molecular Diffusion and Advection during Solute Transport in Macroporous Media
Vadose Zone Journal, 2019
Solute transport in soils is known to differ from solute transport in homogeneous porous media. Nonequilibrium processes, like those induced by the presence of macropores, can strongly influence the breakthrough of solute in soils. Breakthrough experiments and effective models are often combined to study the physicochemical processes involved in solute transport. However, the complexity of flow pathways and the diversity of possible processes is challenging. In this work, the influence of flow rate and viscosity of the carrying liquid on nonreactive solute transport is investigated under saturated conditions in a macroporous synthetic medium. As expected, solute transport is strongly affected by physical nonequilibrium induced by the preferential flow within the macropore. Breakthrough occurs early, and the shape of the breakthrough curve is influenced both by the flow rate and the coefficient of molecular diffusion of the solute. We show that when the mean residence time of the solute in the macropore is small enough, solute transport in a macroporous column can be considered as isolated within the macropore. The increase of the residence time strongly affects the shape of the breakthrough, and, eventually, a plateau appears during the ascent of the breakthrough curve. We demonstrate experimentally that the existence of this plateau, which is not predicted by classical effective models, is related to the relative importance of molecular diffusion versus advection. Indeed, this plateau can become unobservable if the coefficient of molecular diffusion is reduced through the use of a sufficiently viscous carrying liquid.
Unsaturated flow effects on solute transport in porous media
Journal of Hydrology, 2021
A major contaminant transport process in soils is hydrodynamic dispersion by affecting the spreading and arrival of surface-applied pollutants at underlying groundwater reservoirs. When a soil is unsaturated, hydrodynamic dispersion is very much affected by soil water saturation. Centimeter-and decimeter-scale column experiments were carried out to explore the effects of fluid saturation and particle size on the unsaturated solute dispersivity. Measured in-situ breakthrough curves were analyzed in terms of both classical advection-dispersion and dualporosity (mobile-immobile) type transport equations. A clear non-monotonic relationship was found between the dispersivity and soil water saturation. The extent of non-monotonicity was more pronounced for a relatively coarse-textured sand compared to a finer sand. This finding has been reported rarely before; it explains some of the inconsistencies of saturation-dispersivity relationships in the literature.
Relations between macropore network characteristics and the degree of preferential solute transport
Hydrology and Earth System Sciences
The characteristics of the soil macropore network determine the potential for fast transport of agrochemicals and contaminants through the soil. The objective of this study was to examine the relationships between macropore network characteristics, hydraulic properties and state variables and measures of preferential transport. Experiments were carried out under near-saturated conditions on undisturbed columns sampled from four agricultural topsoils of contrasting texture and structure. Macropore network characteristics were computed from 3-D X-ray tomography images of the soil pore system. Non-reactive solute transport experiments were carried out at five steady-state water flow rates from 2 to 12 mm h−1. The degree of preferential transport was evaluated by the normalised 5% solute arrival time and the apparent dispersivity calculated from the resulting breakthrough curves. Near-saturated hydraulic conductivities were measured on the same samples using a tension disc infiltrometer...
The effect of water content on solute transport in unsaturated porous media
Water Resources Research, 1999
The effect of water content on NaCl transport in unsaturated porous media was investigated under steady state flow conditions for water contents ranging between full saturation and 15% by volume. The experiments were conducted in a 25 cm column packed with homogeneous sand. Results of the experiments indicate that solute transport in unsaturated porous media is subject to greater velocity variations and slower solute mixing than one in saturated media. As a result, NaCl breakthrough curves (BTCs) show earlier initial arrival and greater tailing and variance as the average water content decreases. These results suggest that transport processes in our experiments have not fully developed to the Fickian regime at lower water contents. Because the classical convectiondispersion equation does not adequately describe the movement of solutes under the pre-Fickian regime, a mobile-immobile model was employed to reproduce the BTCs obtained under unsaturated conditions. In general, the results indicate that at lower water contents the medium has a greater fraction of immobile water, higher dispersion, and slower mass transfer between the mobile and immobile regions. A power law relationship between dispersion and water content-normalized velocities was found to exist for our experiments and other experiments reported in the literature using different porous media. Thus we suggest dispersivity is not only a function of properties of the media but also of water content.
The impact of macropore description on solute transport predictions in soils is not well understood. A 2-D Galerkin finite element model was used to compare different approaches for describing macropore flow in soil. The approaches were: a modification of the hydraulic conductivity function (Hydraulic function), the lumping of all macropores into one single straight macropore (Lumping), the use of an exchange factor between microporosities and macroporosities that occupy the same area (Dual porosity), and a detailed description of each macropore (Full description, base case). Simulated breakthrough curves were obtained with domains that contained one or more macropores of different shapes under both steady state and transient flow conditions. The Hydraulic function approach was not sensitive to macropore continuity and tortuosity. When the macropores were open at the soil surface and the solute was surface applied, the first three approaches underestimated both breakthrough curves and solute distribution in the profile compared to the Full description approach. When the solute was initially incorporated in the soil, the first three approaches overestimated the breakthrough curves compared to the Full description approach. The first three approaches also underestimated the heterogeneity of solute distribution in the profile compared to the Full description approach, mostly when the macropores were tortuous. The differences between predicted breakthrough curves with different approaches decreased with an increase in tortuosity and a decrease in surface continuity. To simplify macropore description, the Dual porosity approach was the better of the first three approaches for predicting breakthrough.
Investigating the impact of exit effects on solute transport in macroporous media
Hydrology and Earth System Sciences, 2021
The effect of macropore flow on solute transport has spurred much research over the last forty years. In this study, non-reactive solute transport in water-saturated columns filled with porous media crossed by a macropore was experimentally and numerically investigated. The emphasis was put on the study of exit effects, whose very existence is inherent to the finite size of any experimental column. We specifically investigated the impact of a filter at the column outlet on water flow and solute transport in macroporous systems. Experiments involving breakthrough measurements and magnetic resonance imaging (MRI) showed that solute transport displayed some significant nonunidirectional features, with a strong mass exchange at the interface between the macropore and the matrix. Fluid dynamics and transport simulations indicated that this was due to the non-unidirectional nature of the flow field close to the outlet filter. The flow near the exit of the column was shown to be strongly impacted by the presence of the outlet filter, which acts as a barrier and redistributes water from the macropore to the matrix. This impact was apparent on the breakthrough curves and the MRI images. It was also confirmed by computer simulations and could, if not properly taken into account, impede the accurate inference of the transport properties of macroporous media from breakthrough experiments.
Preferential Solute Transport through Macropores in Large Undisturbed Saturated Soil Columns
Journal of Environmental Quality, 1991
Six large undisturbed soil columns (61 cm in length, 15 cm in diameter) were collected from three no-till and three conventional tillage field plots. The side walls of these columns were sealed with either plaster of paris or paraffin wax to eliminate wall effects. After these columns were saturated with CaSO4 (0.005 M), CaCI2 (0.005 M) was applied at the surface and the effluent was collected at the bottom. Effluent samples were later analyzed for CI (chloride) concentrations. The C! breakthrough-curves (relative CI concentration vs. relative pore volume) were developed and the degree of preferential flow analyzed. Shape and other breakthrough-curve parameters, such as immobile pore-water fraction and initial breakthrough, indicated the occurrence of preferential flow through all columns. The degree of preferential flow, however, was greater in no-till than in conventional tillage columns (average immobile pore-water fraction was 56% for no-till and 49% for conventional tillage columns).