Role of macropore continuity and tortuosity on solute transport in soils: 2. Interactions with model assumptions for macropore description (original) (raw)
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Role of macropore continuity and tortuosity on solute transport in soils
Journal of Contaminant Hydrology, 2002
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 0169-7722/02/$ -see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 -7 7 2 2 ( 0 2 ) 0 0 0 3 4 -7 * Corresponding
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.
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...
Water and solute movement in soil as influenced by macropore characteristics 1. Macropore continuity
In most contaminant transport modeling studies, only the macropores that are visible at the soil surface are considered. Furthermore, it is assumed that these macropores are straight and continuous throughout the soil profile. Little is known on the importance of other types of macropore continuity and tortuosity on preferential movement of contaminants through soils. This paper describes the results of a laboratory study dealing with macropore continuity effects on breakthrough curves BTCs. and solute distribution in a Forman loam fine loamy mixed Udic Haploborolls. soil. BTCs were obtained under a constant hydraulic head of 0.08 m from a 2-D column slab. containing artificial macropores. The input solution contained 1190 mg ly1 KBr, 10 mg ly1 Rhodamine WT, and 100 mg ly1 FD&C Blue a1. The continuity types studied were: macropore open at the soil surface–open at the bottom of the column O O., open–closed O–C., closed–open C–O., and closed–closed C–C.. A treatment without macropore served as a control. As expected, the solution in the O–O treatment reached the bottom of the macropore about 100 times faster by bypassing most of the soil matrices. As a result, the breakthrough time for O–O treatments was much faster than any other continuity treatments. Both the O–O and O–C type macropores favored earlier breakthrough, smaller apparent retardation coefficient RX ., deeper center of mass, and higher anisotropy in tracer concentrations in the horizontal direction than the C–O, C–C, and the Control treatment. The C–C macropore was favored in deeper penetration of tracer only when another macropore was present nearby. The importance of macropore continuity increased with an increase in the adsorption coefficient of the tracers.
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).
Water and solute movement in soil as influenced by macropore characteristics
Journal of Contaminant Hydrology, 2000
The paper describes the results of a laboratory study on the effects of macropore tortuosity on Ž breakthrough curves BTCs and solute distribution in a Forman loam fine loamy-mixed Udic . Ž . Haploborolls soil. BTC were obtained using 2-D columns slab containing artificial macropores of five different tortuosity levels. The BTCs were run under a constant hydraulic head of 0.08 m over an initially air dry soil. The input solutions contained 1190 mg l y1 of potassium bromide, 10 mg l y1 of Rhodamine WT, and 100 mg l y1 of FD & C Blue a1. A soil column without macropores served as a control. The displacement of a non-adsorbed tracer was not affected by the tortuosity level. An increase in macropore tortuosity progressively increased the breakthrough Ž X . time, increased the apparent retardation coefficient R , decreased the depth to the center of mass of a given adsorbed tracer, and increased the anisotropy in tracer distribution profile. The relative importance of macropore tortuosity increased with an increase in the adsorption coefficient of the tracer. Compared to macropore continuity, the macropore tortuosity had greater impact on solute distribution profile than in its leaching. q
International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1991
A detailed mechanistic model ofwater movement and transport of non-reactive solute in a macroporous soil is described. One important feature of the model is that it may be run in either one or two flow domains using the same values for the hydraulic properties characterizing the soil. Water and solute movement in the micropores is calculated with the Richards and convection-dispersion equations and, in two domains, this is coupled to fluxes of water and solute in the macropores by empirical interaction terms. These interaction terms are redundant in the one-domain model, which simply reduces to the nonsteady state convection-dispersion equation. A sensitivity analysis is presented showing how it is possible to identify conditions under which a macropore flow domain may need to be considered. In part I1 (Jarvis et al., 199 I), the model is evaluated under field conditions in chloride breakthrough experiments in soil monolith lysimeters.
Water and solute movement in soil as influenced by macropore characteristics 2. Macropore tortuosity
The paper describes the results of a laboratory study on the effects of macropore tortuosity on breakthrough curves BTCs and solute distribution in a Forman loam fine loamy-mixed Udic Haploborolls. soil. BTC were obtained using 2-D columns slab. containing artificial macropores of five different tortuosity levels. The BTCs were run under a constant hydraulic head of 0.08 m over an initially air dry soil. The input solutions contained 1190 mg l-1 of potassium bromide, 10 mg l-1 of Rhodamine WT, and 100 mg l-1 of FD&C Blue a1. A soil column without macropores served as a control. The displacement of a non-adsorbed tracer was not affected by the tortuosity level. An increase in macropore tortuosity progressively increased the breakthrough time, increased the apparent retardation coefficient RX ., decreased the depth to the center of mass of a given adsorbed tracer, and increased the anisotropy in tracer distribution profile. The relative importance of macropore tortuosity increased with an increase in the adsorption coefficient of the tracer. Compared to macropore continuity, the macropore tortuosity had greater impact on solute distribution profile than in its leaching. q2000 Published by Elsevier Science