BROMIDE TRANSPORT DETECTION IN TILLED AND NONTILLED SOIL - SOLUTION SAMPLERS VS. SOIL CORES

Experimental validation of solute transport models is often met with unsatisfying or ambiguous results. Aside from the inherent difficulty in conducting these experiments, the lack of a standard set of methodologies employed in these studies may be an impediment. This study was conducted to determine if the method employed in the sampling of soil solutes could influence the results of model validation attempts. A narrow pulse of KBr was uniformly applied to two 25-m(2) plots, one tilled and the other undisturbed, on a level Ulm clay loam (fine, montmorillonitic, mesic Ustollic Haplargid). Solute movement was then monitored for the next 388 d using replicated ceramic cup solution samplers at depths of 0.15, 0.30, 0.80, 1.20, 1.60, and 2.00 m and by the periodic removal of replicate soil cores in 0.05-m increments to a maximum depth of 3.70 m. Overall, the methods yielded consistent information regarding the mean convective transport of Br-. Both methodologies indicated deeper and more rapid movement of Br- in the nontilled soil, although the shallow solution samplers appear to miss the most rapidly moving solute. By the conclusion of the experiment, the difference between the centers of mass of the Br - plumes in the tilled and nontilled soil was at least 0.6 m. The soil core and solution sampler methods appear to yield inconsistent measures of the solute dispersion but we attribute the differences to the effects of incomplete mass recovery and, we argue, follow naturally from the instrument window of the sampling methodology relative to the observed soil heterogeneity.