Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/28/1998
Publication Date: N/A
Interpretive Summary: Public concerns with the presence of agrochemicals in surface and ground waters have focused research attention on the process of chemical leaching in soils. Of specific concern is the rapid or preferential leaching of agrochemicals that can result in rapid movement of soil-applied chemicals to the underlaying ground water. To date, our ability to characterize the preferential leaching potential of soils has been severely limited. In earlier work, we proposed a simple technique for measuring the preferential flow tendencies of soil by infiltrating a sequence of chemical tracers into soil. In this study, we evaluated the applicability of this method for a wide range of soil and climatic conditions. Experimental results enabled us to show that the primary assumption underlying the proposed method is valid for many transport conditions and that the proposed method can give accurate estimates of preferential-flow characteristics of soil. However, we also demonstrated that under some soil and weather conditions, the proposed method does not perform well. These findings will give soil scientists and others interested in estimating fate and behavior of agrochemicals in soil, confidence in using the technique to measure the preferential flow characteristics of soils.
Technical Abstract: A field method has been proposed for measuring the mobile water fraction (beta) and solute exchange rate (omega) for the mobile-immobile solute transport model. Two critical assumptions made in the development of the method are that the effect of hydrodynamic dispersion, D, on the resident tracer concentration is small after the solute front has moved past the depth of sampling and that the tracer concentration in the mobile domain is equal to the input tracer concentration at short distances from the infiltrometer. We evaluated the impact of these assumptions by comparing known values of omega and beta with estimates calculated using the proposed method and synthetic solute concentration data. We found the assumption of negligible D to be valid for Peclet numbers (P) greater than 100 and any omega and for omega < 0.01 and any P. For P < 0.3, D is too large to assume negligible and obtain accurate parameter estimates for most omega values. At omega > 1, the solute transport system effectively degenerates into a single, mobile domain and the approach performs poorly for most P values. At intermediate P and omega conditions, the accuracy of the method depends on the specific transport conditions determined by P, omega, and beta. The log-linear solution to the MIM equations provided worse estimates of beta and omega than the complete solution with a small preset value of D for much of the range of omega and P reported in the literature. Results document that the method is applicable to some typically encountered transport conditions.