Immobile Water Content and Mass Exchange

Coefficient of a Field Soil

 F. X. M. Casey,*  S. D. Logsdon, R. Horton, and D. B. Jaynes
 

 
F. X. M. Casey and R. Horton, Department of Agronomy, Iowa State University, Ames, Iowa  50011, S. D. Logsdon and D. B. Jaynes, USDA-Agricultural Research Service (ARS), National Soil Tilth Laboratory, 2150 Pammel Dr., Ames, Iowa  50011. Journal Paper No. J-16868 of the Iowa Agriculture and Home Economics Experimental Station, Ames, Iowa, Project No. 3287, and supported by Hatch Act and State of Iowa funds.  *Corresponding Author.

Immobile Water Content and  Mass Exchange Coefficient of a Field Soil

 Abstract

        Determining the preferential flow characteristics of a soil is important because agrichemicals can contaminate groundwater via preferential flow pathways.  A model that predicts solute transport due to preferential flow is the mobile/immobile solute transport model, which partitions the total water content ( , m3 m-3) into a mobile fraction (m) and an immobile fraction (im).  Recently, an in situ method was proposed for determining the mobile/immobile model parameters of im and mass exchange coefficient ( ) between the fractions, by using a tension infiltrometer to apply a series of four fluorobenzoate tracers.  The objective of this study was to test the in situ technique at 47 sites along a transect in a ridge-till corn field of Nicollet soil (fine-loamy, mixed, mesic Aquic Hapludoll).  The immobile fraction (im/) ranged from 0.394 to 0.952 with a median of 0.622.  The mass exchange coefficient ranged from 0.000237 min-1 to 0.00481 min-1 with a median of 0.00123 min-1.  These values are similar in magnitude and range to values reported by other investigators, and they follow the same relationships.  The values of im/ and  along the transect indicated no obvious spatial trends, nor spatial correlations.  Significant linear correlations did exist between  and soil water flux,  and im, and  and im.
Figure 1 - Jaynes et al. (1995) method used to estimate immobile water content and mass exchange coefficient at 48 sites in situ.  Here are the linear regressions used to the estimate the transport parameters with the Jaynes et al. (1995) method.
Figure 1a - The flux values along the transect.
Figure 1b - Distribution of measured flux values.

Figure 2a - Immobile water fraction values along
                                                                                        the measurement transect.
Figure 2b - Distribution of immobile water fraction.

Figure 3a - Mass exchange coefficient values along
                                                                                 the measurement transect.
Figure 3b - Distribution of mass exchange coefficient.
Figure 4 - Graph from Kookana et al. (1992) that shows the plot of log of  the mass exchange coefficient with log of pore water velocity.  These values come from seven separate laboratory studies.  Here we plotted our values of mass exchange coefficient estimated by the Jaynes et al. (1995) method to show that they follow the same relationship as found by Kookana et al. (1992) and that the values of mass exchange coefficient fall within the measured values from the seven different other laboratory studies.