Temporal consistency of solute transport in a heterogeneous field plot

Although the spatial variability of solute transport in field soils has bee n examined intensively, far less information is available about temporal ch anges in transport processes. If there are changes in transport over time, simply calibrating a model with a breakthrough curve for a single time peri od may not be adequate. Therefore, this paper examines the consistency in s olute transport measurements in a field soil using two in situ nondestructi ve techniques: fiber optic miniprobes (FOMPs) and time domain reflectometry (TDR) probes. Miscible displacement studies were performed in a small fiel d plot containing 20 FOMPs and 16 TDR probes placed at two soil depths. Whe reas there was moderate consistency in transport response measured by the T DR probes, the FOMP data suggested that solute transport converged into few er 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 soi l 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 withi n the plot were examined using a Spearman's rank test, confirming that tran sport response pattern may not be temporally stable. This paper demonstrate s that solute breakthrough may not be consistent in heterogeneous field soi ls subjected to regular leaching.