A SIMPLE MIXING LAYER MODEL PREDICTING SOLUTE FLOW TO DRAINAGE LINES UNDER PREFERENTIAL FLOW

Concern about the environmental effects of agricultural chemicals thro ugh preferential flow has risen in recent years. A simple model is pre sented describing the processes involved in preferential transport of both soil-adsorbed and non-adsorbed solutes. The model assumes that wa ter and solutes are mixed into an upper soil layer. The water and solu tes then flow through macropores to the groundwater. Analysis of the s olute breakthrough curves in subsurface drainage effluent makes it pos sible to calculate the depth of the mixing layer or the adsorption des orption partition coefficient. Data from a drainage experiment with ch loride, 2.4-D and atrazine were used to test the model. The study was performed on a no-till and a conventionally tilled plot. The model and experimental results indicate that only a fraction of the field area participates in transport to the macropores. Differences between break through curves from the conventionally tilled and no-till plots are ex plained well by the mixing of solutes and water in the upper layer. Th is simple, physically based model can help us to understand and estima te the environmental threats of herbicides shortly after application.