Preferential Solute Transport through Macropores in Large Undisturbed Saturated Soil Columns (original) (raw)
Related papers
Transport of Chloride Through an Unsaturated Field Soil
Water Resources Research, 1991
A chloride tracer was applied to the surface of a vegetable field and then leached downward by rainfall and irrigation. Tracer concentrations in a vertical two-dimensional region down to a depth of 2.4 m were monitored with suction cups that were installed horizontally from a tunnel. The uniformly applied tracer pulse split into a slowly moving main pulse and a series of fast pulses. The first of the fast pulses reached a depth of 2.2 m after an infiltration of just 31 mm of natural rainfall, whereas the peak of the main pulse was still at a depth of 0.84 m by the end of the experiment after an infiltration of 0.853 m. The movement of the main pulse can be described by a convection-dispersion process in a homogeneous medium, provided that time is replaced by cumulative infiltration. However, the values of the parameters that produce a maximum agreement between the model and the observed main pulse have no physical basis, and consequently prediction of solute movement, based on measurements of soil properties, is not possible.
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
Analysis of steady state chloride transport through two heterogeneous field soils
Water Resources Research, 1998
Chloride transport was investigated in a loamy soil and a silty-loam soil at the field scale under steady state flow conditions using a water flux of 2.84 cm d -• for the loamy and 1.5 cm d -• for the silty-loam soil. The solute plume movement was recorded by means of horizontally installed time domain reflectrometry (TDR) probes at 5 depths up to 90 cm below the soil surface and 24 locations along a transect of 8 m. The measurements consisted of solute resident concentrations every 2 hours in the loamy soil for a total period of 42 days and every 4 hours for the silty-loam soil for a period of 65 days. Parameters of the convective-dispersive equation (CDE) and the lognormal stochastic-convective transport model (CLT) were determined using time-normalized resident concentration breakthrough curves crt*(z, t). In addition, temporal moments of crt*(z, t) were related to travel time moments and transport parameters for the two transport processes. At both sites the breakthrough curves at different depths were better described by the CLT than by the CDE. However, early solute breakthrough was underestimated at most depths. Mean travel time and dispersivity were estimated using the temporal moments of crt*(z, t) with the assumption of a stochastic-convective transport process. In the loamy soil, solute was traveling from a heterogeneous, macroporous top soil toward a subsoil containing significantly fewer macropores. The flow of solutes through the macropores is not detected by the TDR probes, resulting in a larger observed mean travel time compared with the expected mean travel time based on the piston flow model and no increase in dispersivity. In contrast, in the subsoil the observed and expected mean travel times were in good agreement, and dispersivity increased with depth. In the silty-loam soil, mean travel times derived from concentration measurements were larger than the expected mean travel times based on the piston flow model, implying temporal storage of solutes in stagnant water zones. Dispersivity also showed deviations from the expected linear increase with depth, probably because of changing soil properties with depth. show a great variety in their downward movement such as significant horizontal redistribution of solutes [Schulin et al., 1987; Hills et al., 1991], preferential movement of solutes [Roth et al., 1991; Flury et al., 1994; van Weesenbeeck and Kachanoski, 1994], and expansion-compression of the solute plume [Butters et al., 1989; Ellsworth et al., 1991].
Hydrology and Earth System Sciences, 2012
Preferential flow is a widespread phenomenon that is known to strongly affect solute transport in soil, but our understanding and knowledge is still poor of the site factors and soil properties that promote it. To investigate these relationships, we assembled a database from the peer-reviewed literature containing information on 733 breakthrough curve experiments under steady-state flow conditions. Most of the collected experiments (585 of the 733 datasets) had been conducted on undisturbed soil columns, although some experiments on repacked soil, clean sands, and glass beads were also included. In addition to the apparent dispersivity, we focused our attention on three indicators of preferential solute transport: namely the 5 %-arrival time, the holdback factor, and the ratio of piston-flow and average transport velocities. Our results suggest that, in contrast to the 5 %-arrival time and the holdback factor, the piston-flow to transport velocity ratio is not related to preferential macropore transport but rather to the exclusion or retardation of the applied tracer. Confirming that the apparent longitudinal dispersivity is positively correlated with the travel distance of the tracer, our results also illustrate that this relationship is refined if the normalized 5 %-tracer arrival time is also taken into account. In particular, we found that the degree of preferential solute transport increases with apparent dispersivity and decreases with travel distance. A similar but weaker relationship was observed between apparent dispersivity, 5 %-tracer arrival time, and lateral observation scale, such that the degree of preferential transport increases with lateral observation scale. However, we also found that the travel distance and the lateral observation scale in the investigated dataset are correlated, which makes it difficult to distinguish their influence on these transport characteristics. We also found that the strength of preferential transport increased at larger flow rates and water saturations, which suggests that macropore flow was a more important flow mechanism than heterogeneous flow in the soil matrix. Nevertheless, our data show that heterogeneous flow in the soil matrix also occasionally leads to strong preferential transport. Furthermore, we show that preferential solute transport under steady-state flow depends on soil texture in a threshold-like manner: moderate to strong preferential transport was found to occur only for undisturbed soils that contain more than 8 % clay. Preferential flow characteristics were also absent for columns filled with glass beads, clean sands, or sieved soil. No clear effect of land use on the pattern of solute transport could be discerned, probably because the available dataset was too small and too strongly affected by cross-correlations with experimental conditions. Our results suggest that, in developing pedotransfer functions for solute transport properties of soils, it is critically important to account for travel distance, lateral observation scale, and water flow rate and saturation.
Influence of macroporosity on preferential solute and colloid transport in unsaturated field soils
Journal of Contaminant Hydrology, 2009
Transport of solutes and colloids in soils, particularly those subject to preferential flow along macropores, is important for assessing the vulnerability of shallow groundwater to contamination. The objective of this study was to investigate flow and transport phenomena for dissolved and colloid tracers during large infiltration events in partially saturated, macroporous soils. Controlled tracer infiltration tests were completed at two field sites in southern Ontario. A tension infiltrometer (TI) was used to infiltrate water with dissolved Brilliant Blue FCF dye simultaneously with 3.7 µm and 0.53 µm diameter fluorescent microspheres. Infiltration was conducted under maximum infiltration pressure heads ranging from − 5.2 to − 0.4 cm. All infiltration test sites were excavated to examine and photograph dye-stained flow patterns, map soil features, and collect samples for microsphere enumeration. Results indicated that preferential transport of dye and microspheres via macropores occurred when maximum pressure heads were greater than − 3.0 cm, and the corresponding infiltration rates exceeded 2.0 cm h − 1. Dye and microspheres were detected at depths greater than 70 cm under the highest infiltration rates from both sites. Microsphere concentrations in the top 5-10 cm of soil decreased by more than two orders of magnitude relative to input concentrations, yet remained relatively constant with depth thereafter. There was some evidence for increased retention of the 3.7 µm microspheres relative to the 0.53 µm microspheres, particularly at lower infiltration pressures where straining and attachment mechanisms are most prevalent. Microspheres were observed within dye stained soil matrix surrounding individual macropores, illustrating the significance of capillary pressures in controlling the vertical migration of both tracers in the vicinity of the macropores. Overall, microsphere distributions closely followed the dye patterns, with microsphere concentrations at all depths directly related to the intensity (or concentration) of dye staining. It is concluded that the flow system influenced transport to a much greater degree than differences between dissolved and colloidal species, and hence a dye tracer could serve as a reasonable surrogate for colloid distributions in the vadose zone following individual infiltration events.
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.
Solute Movement in an Intact Soil Core Under Different Ponding Heights
Abstract—Contaminant transport to groundwater systems through the unsaturated zone follows, in most cases, preferential pathways. The understanding and assessment of this preferential flow mechanism is important to the prevention and solution of resulting problems. A column study was carried out where a NaCl-tracer solution (33 to 37 mmohs/cm of conductivity) was passed through an intact soil core under saturated conditions and three different ponding heights ranging from 5 to 30 mm. Outflow rates varied from 13 to 103 ml/min during the three tests performed. A subsequent dye tracing and slicing of the soil column was performed. Electrical conductivity measurements assessed indirectly the chloride content of the water outflow. Breakthrough curves (BTCs) showed short threshold times (less than 20% pore volume time) and asymmetry with respect to the point defined by C/Co = 0.5 and pore volume time. The BTCs successfully represent the flow measurements through the soil column, this fact is supported by the subsequent horizontal slicing (5 cm thick) of the intact core.
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
Non-reactive solute diffusion in unconfined and confined specimens of a compacted soil
Waste Management, 2009
The effect of specimen confinement on the determination of the effective diffusion coefficients, D * , for chloride, a non-reactive (non-adsorbing) solute, diffusing in a compacted soil was evaluated. The diffusion tests were performed by placing an acetic acid/sodium acetate buffer solution containing ZnCl 2 (pH $ 4.8) in a reservoir in contact with unconfined and confined specimens of a compacted sand-clay mixture for test durations of 7 or 14 d. The concentrations of chloride in the reservoir were measured as a function of time during the test, as well as a function of depth within the specimen at the end of the test. The resulting concentration distributions were analyzed using two models to Fick's second law for non-reactive solute diffusion in porous media, viz., (1) an analytical model assuming the porosity distribution could be represented by a single, weighted mean porosity and (2) a commercially available model, POLLUTE, that directly accounted for the measured porosity distribution. The D * for unconfined specimens based on the analytical model tended to be overestimated by a factor ranging from 1.13 to 1.59 relative to the D * using POLLUTE, whereas the D * values based on both methods for confined specimens typically were more consistent. In addition, the D * for unconfined specimens was greater than the D * for confined specimens when soil concentrations were used for the analysis, presumably due to the higher porosity for the unconfined specimens relative to the confined specimens. Analyses based on reservoir concentrations were inconsistent and contradictory in some cases, suggesting that the D * values based on soil concentrations were more reliable.
Solute and Bacterial Transport through Partially-Saturated Intact Soil Blocks
1998
Steady-state transport of water, chloride and bacteria was measured through intact blocks of Maury and Cecil soils, under partially saturated conditions. Major objectives were to determine if transport occurs uniformly or via preferential flow paths, and if soil physical properties could be used to predict breakthrough. The blocks were instrumented with TDR probes and mounted on a vacuum chamber containing 100 cells that collected eflluent. After each experiment the blocks were sampled for soil physical properties. The fluxes showed no spatial autocorrelation and the eflluent variance was not statistically different between soils. Less than 3% of the influent bacteria appeared in the effluent. Maximum bacterial breakthrough occurred after 0.25 water-filled pore volumes had been leached, and was greater for Cecil soil than for Maury soil. The chloride breakthrough curves were fitted to the convection dispersion equation. The best predictor of dispersivity was volumetric water content...