IMMOBILE WATER-CONTENT AND MASS-EXCHANGE COEFFICIENT OF A FIELD SOIL

Determining the preferential flow characteristics of a soil is importa nt because agrichemicals can contaminate groundwater via preferential now pathways, A model that predicts solute transport due to preferenti al now is the mobile-immobile solute transport model, which partitions the total water content (theta, m(3) m(-3)) into a mobile fraction (t heta(m)) and an immobile fraction (theta(im)). Recently, an in situ me thod was proposed for determining the mobile-immobile model parameters of theta(im) and mass exchange coefficient (alpha) bet treen the frac tions by using a tension infiltrometer to apply a series of four fluor obenzoate tracers, The objective of this study was to test the in situ technique at 47 sites along a transect in a ridge-till corn (Zea mays L.) field of Nicollet soil (tine-loamy, mixed, mesic Aquic Hapludoll) , The immobile fraction (theta(im)/theta) ranged from 0.394 to 0.952 w ith a median of 0.622., The mass exchange coefficient ranged from 0.00 0237 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 invest igators, and they Follow the same relationships. The values of theta(i m)/theta and alpha along the transect indicated no obvious spatial tre nds or spatial correlations, Significant linear correlations did exist between oi and soil water nux, alpha and theta(im), and theta and the ta(im).