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United States Department of Agriculture

Agricultural Research Service

Title: Comparing Field Methods That Estimate Mobile-Immobile Model Parameters

Authors
item Casey, F - IOWA STATE UNIVERSITY
item Jaynes, Dan
item Horton, R - IOWA STATE UNIVERSITY
item Logsdon, Sally

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 16, 1998
Publication Date: N/A

Interpretive Summary: Concerns with the presence of agrochemicals in groundwater have drawn attention toward the processes of chemical movement in soils. Of specific concern is the rapid or preferential movement of agrochemicals through soil. To date, our ability to quantify the preferential flow characteristic of soil has been especially limited. Two methods have been proposed in the literature for measuring the preferential flow tendencies of soil, a sequential tracer and a sequential sampling method, but no information exists as to how the two methods compare. This study evaluated the two methods in a side-by-side field study for their ability to accurately measure the preferential flow tendencies of soil. Of the two, the sequential tracer method gave internally consistent estimates and the measurements were less variable. The sequential sampling method was highly erratic in its performance and the results could not be used to estimate the interaction rate between preferential flow paths and the main soil pore space. This method also gave consistently higher estimates of the volume of preferential flow paths in the soil compared to the sequential tracer method. This study documented that the sequential tracer method gives a much more reliable estimate of the preferential flow characteristics of a soil. The method is easy to use and should be of value to soil scientists and others trying to estimate travel times to groundwater for agrochemicals applied to the soil.

Technical Abstract: Recent studies have used field techniques that estimate soil hydraulic and solute transport parameters. These methods utilize a tension infiltrometer to infiltrate either a single tracer or a series of tracers to estimate immobile mater content (W) and mass exchange coefficient of the mobile/immobile solute transport model. The objective of this study was to compare the single tracer and the multiple tracer methods on the same field soil locations, estimating hydraulic conductivity W, and C. Research was done at five inter-row sites in a ridge-tilled corn field and the soil was mapped as a Nicollet series. The values of W and C estimated by the multiple tracer method compared well with previously measured values using the same technique on the same field. The W values for the multiple tracer technique were larger than values derived from the single tracer technique. The single tracer technique did not take into consideration mass exchange between W and the mobile water domain. Also, the C values were less variable for the multiple tracer methods. Values of immobile water fraction for the multiple and single tracer techniques ranged from 0.30 to 0.52 and 0.24 to 0.35, respectively. The values of C for the multiple and single tracer techniques ranged from 0.06 to 0.9 d-1 and 0.03 to 60 d-1 respectively. The volumetric water content changed considerably over the course of the experiment for the estimation of C using the single tracer method; thus, the assumptions of this technique were compromised. There was evidence that the single tracer method underestimated W and this method considers C = 0 during the tracer application.

Last Modified: 10/25/2014
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