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Title: IMMOBILE WATER CONTENT AND MASS EXCHANGE COEFFICIENT OF A FIELD SOIL

Author
item CASEY, FRANCIS - IOWA STATE UNIVERSITY
item Logsdon, Sally
item HORTON, ROBERT - IOWA STATE UNIVERSITY
item Jaynes, Dan

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Nitrate is abundant in agricultural soils, and often leaches out of the soil to pollute surface and groundwater. Nitrate levels that are above the drinking water standard may cause harm, especially to infants. Water movement and nitrate transport may be rapid through certain regions of the soil, but slow or insignificant through other regions of the soil. The soil regions with rapid movement are called "mobile," and the regions with no movement are called "immobile." The exchange of nitrate and other chemicals between these two regions is called "mass exchange." Currently there is no proven field method for measuring the extent of mobile regions in soil nor the rate of mass exchange. This research demonstrated the feasibility of a field procedure that measures both the extent of the mobile region and the mass exchange rate. We expect this technique to allow soil professionals to make direct measure of these soil properties thus improving estimates of nitrate leaching and impact on water supplies. This method will be of interest to soil hydrologists active in both research, modeling, and regulatory activities.

Technical 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 into a mobile fraction and an immobile fraction. Recently, an in situ method was proposed for determining the mobile/immobile model parameters of immobile fraction and mass exchange coefficient between the fraction 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 ranged from 0.288 to 0.952 with a mean of 0.618 and standard deviation of 0.148. The mass exchange coefficient ranged from 0.000235 min**-1 to 0.0112 min**-1 with a mean of 0.00164 min**-1 and standard deviation of 0.00161 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 immobile fraction and mass exchange coefficient along the transect indicated no obvious spatial trends nor spatial correlates. Significant correlations did exist between mass exchange coefficient and soil water flux, mass exchange coefficient and immobile fraction, and total water content and immobile fraction.