Time Variance Analysis of Preferential Solute Movement at a Tile-Drained Field Site

Preferential flow has been identified as an important secondary process to transport through the soil matrix in controlling solute movement in soils. However, it remains unknown to what extent a fast-transporting flow regime governs solute leaching and whether it persists for various seasons at field scale. For 3 yr, KBr was applied in late autumn to the surface of a tile-drained field site (0.5 ha in size) and monitored in drain outflow for 5 mo each year. All three seasonal breakthrough curves (BTC) were dominated by an early first concentration maximum and intensive tailing, indicating preferential flow to be a strong intrinsic soil property at the experiment site. A bimodal probability density function (pdf) was adequate to describe the concentration courses of the first 2 yr of investigations. The coefficient of determination was not satisfying when the model was fitted to the BTC of the last season, which was the only one to have a distinct secondary (matrix) peak. The optimized weighting factor combining the two pdfs suggested that preferential transport mechanisms governed the flow process to about 60% during all 3 yr. Simple linear regression analysis among now rates and solute concentrations showed that both measures increased and decreased simultaneously during the early stages of the first test, which was interpreted as a confirmation of the dominance of preferential flow. The same method, however, revealed that Br - was leached predominantly through the soil matrix during the corresponding observation period of the last season.