Soil wetting processes studied by magnetic resonance imaging: Correlated study of contaminant uptake

The wetting of an air-dried sample of an organic rich soil, Mossy Point, ha s been studied by proton magnetic resonance imaging. Spin density maps demo nstrate heterogeneity of the sample at the level of the volumes sampled by each image unit despite general procedures designed to produce homogeneous samples. Heterogeneity in transverse relaxation times can be shown in a his togram. One component of the histogram shows relaxation times that shift to shorter values over a period of many days. This indicated the slow penetra tion of some of the water into an environment of reduced water mobility. Th e water may be entering into, and perhaps forming, micropores or a water sw ollen gel. Study of the kinetics of uptake of two different pesticides (2,4 -D and Atrazine) in slurries of the air-dried soil in aqueous solutions of the pesticides requires at least two kinetic components. The slower entry o f both takes place over a time period comparable to, or a little less than, the slow water uptake. Despite significant difference in the pesticide str uctures, both have a slow uptake rate near that of slow water penetration.