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 the 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.