Spatial and temporal dynamics of preferential bromide movement towards a tile drain

A tracer experiment was conducted to study the spatial and temporal dynamics of preferential Br- movement toward a 1-m-deep subsurface tile drain. Potassium bromide solution was sprayed onto the soil surface on a 0.30- by 10-m strip located at 1 m distance (parallel) to a drain, followed by irrigation of the plot area of 11.8 by 10 m between two adjacent drains for 4 h at 2 mm h(-1). Because irrigation intensity was too low to initiate preferential flow, the Br- application was repeated 3 mo later with two 7 mm h(-1) irrigations, each lasting 4 h, with a 12-h break in between. During the second irrigation, a concentration peak containing Br- mainly from the first application was observed in the drain effluent. Resident Br- concentrations were measured at 42 locations in a 1.1- by 1-m trench excavated across one end of the Br- strip before the second application, and at 108 locations in a 5.9- by 1-m trench at the opposite end of the strip at the end of the experiment. The spatial Br- concentration distributions suggested predominantly diagonal Br- transport from the application strip toward the tile drain. The experiment was numerically simulated with a two-dimensional Richards' and convective-dispersive model (CDM) and with a two-dimensional mobile-immobile model (MIM). Model analysis of Br- concentrations in the drain effluent revealed preferential flow since the main peak was reproduced with the MIM but not with CDM. The MIM analysis of the spatial Br- distribution in the soil showed that physical nonequilibrium transport was limited to periods of high intensity irrigation and rainfall, while convective-dispersive transport was prevalent at other times. This study showed that preferential flow as reflected by effluent concentrations from tile-drained field soils cannot be fully understood without considering two-dimensional spatial flow dynamics.