Temporal consistency of solute transport in a heterogeneous field plot

Although the spatial variability of solute transport in field soils has been examined intensively, far less information is available about temporal changes in transport processes. If there are changes in transport over time, simply calibrating a model with a breakthrough curve for a single time period may not be adequate. Therefore, this paper examines the consistency in solute transport measurements in a field soil using two in situ nondestructive techniques: fiber optic miniprobes (FOMPs) and time domain reflectometry (TDR) probes. Miscible displacement studies were performed in a small field plot containing 20 FOMPs and 16 TDR probes placed at two soil depths. Whereas there was moderate consistency in transport response measured by the TDR probes, the FOMP data suggested that solute transport converged into fewer flow pathways over time with repeated leaching. The 5-cm-long TDR probes also provided evidence of increased lateral flow in the first 50 mm of soil with time. The temporal variability was surprisingly similar between the FOMPs and the TDR probes, even though their sampling volumes differ by more than four orders of magnitude. Relationships between probe responses within the plot were examined using a Spearman's rank test, confirming that transport response pattern may not be temporally stable. This paper demonstrates that solute breakthrough may not be consistent in heterogeneous field soils subjected to regular leaching.