The effect of preferential flow on the short and long-term spatial distribution of surface applied solutes in a structured soil

We measured the lateral distribution of Br in a well-structured soil (clayey, kaolinitic, thermic Typic Kanhapludults) over a period of 40 days following a leaching event with Br-tagged water (Event-l) and the subsequent vertical transport of solute that occurred following a second leaching event (Event-2). The objectives were to determine the time required for solutes to become uniformly distributed laterally after Event-1, and the effect that degree of spatial distribution had on the subsequent vertical movement induced by Event-2. Six field plots, 1.2 by 1.2 m, were exposed to Event-1. Three of the plots were sampled 1, 5, or 40 days later. The three remaining plots were exposed to Event-2 either 1, 5, or 40 days following Event-1 and they were sampled the day following Event-2. A total of 319 samples were taken from a 38.5 by 38.5 cm area in the center of each plot at each of five depths ranging from 5 to 60 cm. Bromide was practically uniformly distributed in the surface layer immediately following Event-1, as indicated by a coefficient of variation (CV) of 25% among the 319 samples, but became progressively more non-uniformly distributed as depth increased. At 60 cm, the CV had increased to 240%. The longer we delayed sampling following Event-1, the more uniform was the lateral distribution of Br. During the 40 days following Event-1, Br became practically uniformly distributed across all layers, and the center of mass moved from 8 to 16 cm below the surface. Event-2 caused Br to move deeper within the soil profile when it occurred the day following Event-1 than when it occurred later, but because drainage occurred during the 40-day delay, the center of Br mass was located 26-27 cm below the surface after Event-2 regardless of the delay time. (C) 1999 Elsevier Science B.V. All rights reserved.