Identification of transport parameters of chlorides in different soils on the basis of column studies (original) (raw)
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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].
Chloride migration in heterogeneous soil: 1. Experimental methodology and results
Water Resources Research, 1994
The statistics of chloride transport through 29 undisturbed soil monoliths (20 cm in diameter and 1 rn long, sampled from a field plot of 15 x 175 m 2) were evaluated in a controlled laboratory experiment. Although the field soil {loamy sand) was relatively homogeneous with regard to texture, the individual monoliths showed large and irregular variability in their soil characteristics and in their flow and transport properties. The distribution of the specific discharge of water could be quantified by a birnodal distribution; the horizontal correlation length for the specific discharge of water was estimated to about 10 m. The ratio between the arrival time of the peak in chloride concentration and the water residence time given by the measured flow parameters indicated a mobile water content that was smaller than the measured water content. The large variability in hydraulic conductivity in both the vertical and horizontal direction, and the agreement between the present hydraulic conductivity measurements and earlier measurements in the field area, indicate that the partition of the measured water content between mobile and relatively immobile water was an effect of the soil structure rather than a boundary effect. van der Zee, S. E. A. T. M., and W. H. van Riemsdijk, Transport of reactive solute in spatially variable soil systems, Water Res•ur.
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 Chloride Transport in a Fine-Textured Soil: Field Experiments and Modeling
Soil Science, 2000
Cattle feeding in feedlot pens produces large amounts of manure and animal urine. Manure solutions resulting from surface runoff are composed of numerous chemical constituents whose leaching causes salinization of the soil profile. There is a relatively large number of studies on preferential flow characterization and modeling in clayed soils. However, research on water flow and solute transport derived from cattle feeding operations in fine-textured soils under naturally occurring precipitation events is less frequent. A field monitoring and modeling investigation was conducted at two plots on a fine-textured soil near a feedlot pen in Argentina to assess the potential of solute leaching into the soil profile. Soil pressure head and chloride concentration of the soil solution were used in combination with HYDRUS-1D numerical model to simulate water flow and chloride transport resorting to the concept of mobile/immobile-MIM water for solute transport. Pressure head sensors located at different depths registered a rapid response to precipitation suggesting the occurrence of preferential flow-paths for infiltrating water. Cracks and small fissures were documented at the field site where the % silt and % clay combined is around 94%. Chloride content increased with depth for various soil pressure head conditions, although a dilution process was observed as precipitation increased. The MIM approach improved numerical results at one of the tested sites where the development of cracks and macropores is likely, obtaining a more dynamic response in comparison with the advection-dispersion equation.
2013
Two-dimensional diffusive and advective-diffusive chloride transport through clay and silt, respectively, was investigated using laboratory models. The observed chloride concentration plumes in the soil samples as well as the chloride concentration-versus-time profiles in the source reservoirs were predicted using the computer code MIGRATEv9. Equivalent horizontal and vertical chloride diffusion coefficients reasonably predicted the observed concentration plumes and concentration-versus-time indicating that soils were homogeneous and isotropic. The predicted diffusion coefficients were in the range of the reported values for similar soils. The effect of downward Darcy flux in the advection-diffusion tests on the chloride concentration plumes was assessed by comparing the shapes of the plumes with those in the pure diffusion tests. The downward Darcy flux caused the concentration plumes in the advection-diffusion tests to migrate further in the vertical direction compared to that in ...
Water Resources Research, 1998
Saturated groundwater flow and tracer experiments using fluorescent dye, chloride, and the herbicides mecoprop and simazine were carried out in the laboratory using three large-diameter (0.5 m) undisturbed columns of fractured clayey till. Hydraulic conductivity of the columns ranged from 10 -s m/s in the shallowest column (1 rn depth) to 10 -7 m/s in the deepest column (4 rn depth) and were similar to field-measured values for these deposits. Results of the tracer experiments are consistent with a conceptual model of advective transport along the fractures combined with diffusion into the finegrained matrix between the fractures. Arrival of the chloride tracer in the effluent was highly retarded relative to fracture flow velocities calculated on the basis of the cubic law and measured values of fracture spacing and hydraulic conductivity. The herbicides were more strongly retarded than the chloride at low flow rates, but at higher flow rates the herbicides arrived with the chloride, indicating the influence of nonequilibrium sorption of the herbicides to fracture walls and the matrix solids. The columns were dismantled following the tracer experiments and mapping under UV light showed that nearly all of the visible, weathered fractures (and the few root holes in the case of the shallowest sample) were active transport pathways, with the dye appearing mainly on the fracture surfaces and as a "rim" in the adjacent matrix. Concentration profiles measured perpendicular to the fracture surfaces showed that the herbicides had also moved into the matrix, apparently by diffusion. Simulations of solute transport with a discrete fracture flow/matrix diffusion model showed that the simulations could be "fit" to the data if all of the visible fractures were hydraulically active, but could not be fit if all or most of the flow was channelled through just the primary fractures (defined by prominent oxidation stains). Simulations with an equivalent porous media (EPM) model could not fit the data using the measured total porosity as the effective porosity. The simulations could likely be fit with a smaller value of effective porosity, but this would limit applicability to field situations because fitted effective porosity is expected to change with physical scale and residence time of the solute in the soil. clayey glacial deposits [Johnson et al., 1989; Desaulniers et al., 1981]. The migration of dissolved contaminants in clayey glacial deposits is mainly controlled by advective transport through the fractures and biopores (such as burrows and root casts) combined with diffusion of solutes into the relatively immobile pore water in the clayey matrix between fractures. The diffusion process, which influences both reactive and nonreactive solutes, can attenuate solute migration relative to the velocity of flow in the fractures and is often referred to as matrix diffusion [Freeze and Cherry, 1979]. The migration of contaminants, such as pesticides, can also be influenced by chemical or biochemical decomposition and by sorption to soil materials. The attenuating effect of these reactions depends on the nature of the sediment, the contaminant and the geochemical environment. In cases with high hydraulic gradients (0.1-1.0) such as those 539 540 JORGENSEN ET AL.: EVALUATION OF CHLORIDE AND PESTICIDE TRANSPORT that could occur during seasonally high groundwater levels and rain storms, the flow velocity in fractures can be in the order of tens to hundreds of meters per day [McKay et al., 1993b; JOrgerisen, 1995; Hinsby et al., 1996; JOrgerisen et al., 1998b]. At these high flow velocities there would likely be insufficient residence time for decomposition or for sorption of the contaminants to reach local equilibrium. Hence it is possible that contaminants could move rapidly with only small declines in concentration. In field experiments in clay-rich soils rapid leaching of pesticides, which were expected to be essentially immobile because of their high distribution coefficients (Ka values), have been observed and are believed to result from rapid transport in fractures and biopores [e.g., Brown et al., 1995; Traub-Eberhard et al., 1995]. Assessment and prediction of groundwater flow and contaminant transport in fractured or biopore dominated soils or clay-rich deposits have proven to be very uncertain because of difficulties in measuring critical parameters such as hydraulic conductivity, fracture/biopore aperture, and degree of interconnection. Field-and laboratory-measured values of hydraulic conductivity at several fractured clay sites in Canada and Denmark have shown varying degrees of sensitivity to scale of measurement and disturbance, particularly borehole smearing, which closes fractures and reduces field-measured conductivity values [Hendry, 1982; Keller et al., 1986; DMstous et al., 1989; Fredericia, 1990, McKay et al., 1993a]. Even in cases where efforts were taken to minimize smearing in the boreholes it was found that there was still uncertainty in hydraulic conductivity values because of the influence of the physical scale of measurement.
Solute Transport Under Water Table Fluctuations in a Fine Sand and a Sandy Clay Loam Soil
Drainage contributes to removing the excess of water from cultivated land. In general, most common drainage systems lead to loss of fertilizers applied for plant growth and yield improvement. Thus, the main objective of this work was to study experimentally the redistribution of solutes within the soil profile caused by water table fluctuations by making use of a non-reactive tracer in laboratory studies on soil columns. Two soil materials, namely fine sand and a sandy clay loam soil, were used in this study. Use was made of potassium chloride as a non-reactive tracer. Profiles of chloride redistribution as a function of depth caused by a first drainage of a saturated column with chloride in the surface layer, sub-irrigation and a second drainage were obtained for the fine sand columns with draining water levels at depths 25.5 and 44.5 cm, and for the sandy clay loam columns with a draining water level at depth 44.5 cm. Although only results for water table depth at 44.5 cm for both soils materials are presented. The redistribution of chloride in both the fine sand and the sandy clay loam columns was dominantly attributed to convective movement of solutes and was considered to be little influenced by diffusion. Preferential flow could have taken place through large sized pores of the fine sand columns. The knowledge and data of this study is a contribution towards that needed to define operation strategies for sub-irrigation-drainage systems which can lead to optimize fertilizers use by crops.
Chloride migration in heterogeneous soil: 2. Stochastic modeling
Water Resources Research, 1994
The observed statistics of chloride breakthrough presented by Sassner et al. (this issue) were compared with predictions of a stochastic-advective modeling approach. The stochastic-advective model based on the observed spatial distribution of flow rates and on transport parameters consistent with parameter values obtained from local breakthrough curves (BTCs) agreed well with the observations. Alternative models that agreed well with the local BTCs failed to predict the large-scale BTC with realistic parameter values. The results support the assumption that compared to advection variability, local dispersion within the mobile water will often have a secondorder effect on field scale solute transport. The stochastic-advective model is robust with regard to the rate of mass transfer between mobile and immobile water zones. This robustness implies that order of magnitude estimates may be sufficient for providing useful predictions of both field scale solute transport and the associated prediction uncertainty. In contrast, accurate estimation of the statistics of solute advection at the scale of interest for the transport problem is necessary.
Prediction and experimental validation of liquid-phase diffusion resistance in unsaturated soils
Journal of Contaminant Hydrology, 1995
Determination of liquid-phase diffusion through unsaturated soils is important for estimating contaminant transport in soils and design of remediation processes for contaminated soils and groundwater. Liquid-phase diffusion through unsaturated soils is governed by both the pore size distribution and water distribution of the soil matrix. Diffusion tube experiments were carried out using several soils packed to field densities to determine the effective diffusivity of chloride ion as a function of soil moisture content. Chloride was selected to serve as a non-volatile, non-sorbing tracer species. A transport model was developed to predict liquid-phase tortuosity as a function of readily obtained soil parameters, including density, moisture content, particle size distribution and pore size distribution. The basis of the model was parallel diffusion resistances in the inter-and intra-particle pore regimes. The new model was found to provide an accurate prediction of observed experimental results. Previously reported models did not agree with experimental results over significant portions of the experimental domain investigated. * Corresponding author. 0169-7722/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-7722(95)00020-8
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.