Parameterisation of physically based solute transport models in sandy soils

Soil Research ◽  
2003 ◽  
Vol 41 (4) ◽  
pp. 771 ◽  
Author(s):  
Y. M. Oliver ◽  
K. R. J. Smettem
Keyword(s):  
Solute Transport ◽  
Water Content ◽  
Mass Exchange ◽  
Granular Media ◽  
Sandy Soils ◽  
Exchange Coefficient ◽  
Water Fraction ◽  
Immobile Water ◽  
Mass Exchange Coefficient ◽  
Tracer Experiments

Immobile water fractions of up to 40% had been reported in sands, and it was therefore relevant to determine if the convection-dispersion equation (CDE) or mobile-immobile model (MIM) should be used as the basic physical model for field studies of solute transport in sandy soils. A review of literature data for granular media indicated that for steady state flow, the dispersion coefficient could be estimated from the grain Peclét number and the mass exchange coefficient from the pore water velocity, but the immobile water fraction was poorly predicted from soil properties.In this study, performed on a sandy soil at Moora, Western Australia, the choice of model was determined after analysis of column effluent breakthrough curves (BTC), sequential tracer experiments, and single tracer experiments on the same core. The latter two methods have recently been introduced in an attempt to independently measure some of the MIM parameters (immobile water content and mass transfer exchange coefficient) in situ. Low immobile water content was found in this sand, with very rapid exchange between the mobile and immobile regions. All 3 techniques gave measured immobile water contents around 10%, which was consistent with most literature values for granular media. The single tracer experiment does not give the mass exchange coefficient α (h–1), but α determined by the sequential tracer technique could not be confirmed by the BTC technique due to the wide 95% confidence interval of the fitted parameter. Although the MIM behaviour was minor and inconsistent in the Moora sand, the choice of model may depend on the problem under consideration. For short column experiments, the CDE and MIM produced similar solute transport behaviour. However, for leaching below the root-zone, the MIM is recommended.

scholarly journals Immobile Water Content and Mass Exchange Coefficient of a Field Soil

1997 ◽  
Vol 61 (4) ◽  
pp. 1030-1036 ◽  
Author(s):  
F. X. M. Casey ◽  
R. Horton ◽  
S. D. Logsdon ◽  
D. B. Jaynes
Keyword(s):  
Water Content ◽  
Mass Exchange ◽  
Field Soil ◽  
Exchange Coefficient ◽  
Immobile Water ◽  
Mass Exchange Coefficient

Field measured mobile water fraction for soils of contrasting texture

Soil Research ◽  
2000 ◽  
Vol 38 (6) ◽  
pp. 1131 ◽  
Author(s):  
A. E. A. Okom ◽  
R. E. White ◽  
L. K. Heng
Keyword(s):  
Hydraulic Conductivity ◽  
Solute Transport ◽  
Water Content ◽  
Time Scale ◽  
Solute Transfer ◽  
Water Fraction ◽  
Mobile Water ◽  
Immobile Water ◽  
Mobile Fraction ◽  
Fertiliser Application

For the purpose of modeling solute transport, soil water has often been simply divided into an essentially mobile fraction, q m , which is active in solute transport, and an apparently immobile fraction, q im . Distinction between q m and q im was sought using the disc permeameter technique. This study examines unsaturated estimates of mobile water content at suction heads, h, of 20, 40, 80, and 120 mm for several soils ranging in texture from sand to clay. Following infiltration of 35 mm depth of 0.01 M KBr into initially dry soils, soil samples were collected from below the base of the disc permeameter and analysed for tracer concentrations which enabled partitioning of mobile and immobile water. Hydraulic conductivity and sorptivity were also derived from the infiltration data. The results show the expected non-linearity of hydraulic conductivity and sorptivity with suction. The mobile water expressed as a fraction, f, of the volumetric water content q (f = q m / q ) was generally found to range from 0.7 to 0.95, with an average of 0.85. The exception was one site for which f was ª 0.50. These values of f are comparable to those derived from leaching studies reported in the literature. An important finding of this work is that within the range of suctions measured, the mobile fraction was independent of suction. A possible explanation for this observation is that the soil capillary forces were dominant during the time scale of the experiment and therefore rapidly drew the invading solution. This finding could have important implications for fertiliser application. Furthermore, this result suggests that the assumption of a negligible solute transfer coefficient, a , between the mobile and immobile domains may be valid within the time scale of this method of measuring the mobile water content.

The measured mobile-water content of an unsaturated soil as a function of hydraulic regime

Soil Research ◽  
1995 ◽  
Vol 33 (3) ◽  
pp. 397 ◽  
Author(s):  
BE Clothier ◽  
L Heng ◽  
GN Magesan ◽  
I Vogeler
Keyword(s):  
Solute Transport ◽  
Water Content ◽  
Sandy Loam ◽  
Pure Water ◽  
Solute Concentration ◽  
Petroleum Engineering ◽  
Spatial Distance ◽  
Water Fraction ◽  
Hydraulic Regime ◽  
Single Mass

To account for observations of preferential solute transport through soil, increasingly models are used in which the total water content of the soil (�, m3 m-3) is partitioned into an essentially mobile phase (8,) and an apparently immobile fraction (�im). However, few methods exist for measuring this separation in the field. Here we use the recently proposed, disc permeameter technique. Following infiltration with a tracer, measurements of the resident solute concentration directly under the disc are used to infer the �m-�im partitioning. For Manawatu fine sandy loam in situ, we applied this technique at the three pressure heads (ho) of -20, -40 and -150 mm, in order to deduce the influence of the hydraulic regime on the soil water fraction �m/� that appears to be actively involved in solute transport during infiltration. When a depth I of between 15 and 25 mm of KBr tracer was added to soil already wet by pure water to h,, the measured mobile fraction �m/� rose from 0.41 at -20 mm, through 0.50 at -40, to 0.64 at the most unsaturated head of -150 mm. Thus, less evidence of preferential solute transport was recorded with decreasing ho. The spatial distance between the preferential paths was observed to range from 20 to 150 mm, the separation increasing and the pathways becoming more diffuse with decreasing h,. Depthwise dispersion of the invading solute thus increased with ho. At the two higher heads, when I =80mm of KBr was allowed to infiltrate, the �m/�, inferred from the resident concentration observed directly under the disc, now also became 0.65. For ho = -40 mm, the measured rise in the resident concentration under the disc, with I, could be predicted using a dispersivity, �, of 20 mm in the approximation provided by the 1-D form of the convective dispersion equation. When 15-25 mm tracer was applied directly to initially dry soil (� = 0.3), capillary forces drew the invading solute from the disc with much less dispersion, such that the resident concentration under the disc rose more rapidly with I. Now �m/� was found to be virtually 0.65 at all the heads. In several experiments at h, = -40 mm, ethanol was used as the solvent for the tracer. No change in the measured �m/� was observed. Thus hydrophobicity was deemed not to be a factor in our measurement of 8,/B being consistently about 0.65. An attempt was made to parameterize the diffusive-exchange scheme that the Coats and Smith (1964) model, taken from petroleum engineering, proposes as the link between the mobile and immobile domains. However, our observations at ho = -20 mm suggest that no single mass-transfer coefficient 5 can describe this solute exchange. Over the first few days, a � value of 0.5 day-1 seemed reasonable, but over the next fortnight there appeared no further interdomain exchange of solute between the two domains.

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