Bromide and chloride tracer movement in macroporous tile-drained agricultural soil during an annual climatic cycle

This study investigates the dynamics of agricultural tile drain flow and solute capture during normal and wet hydrologic conditions over an annual climatic cycle. In November, bromide (Br) and chloride (Cl) tracers were applied as solutes at 0–2.3 m, and 2.3–4.6 m from a tile drain, respectively, prior to a 9 h simulated precipitation event on a macroporous, silt loam soil. Tracer concentrations were monitored in the tile over the course of the year following tracer application. Results show that the tile drain captured 8% of the applied Br mass within 48 h and 27% within 21 days of tracer application. During a major winter melt event, the tile captured an additional 25% of the total Br mass in less than 10 days. Seven months after application, about 95% of the Br tracer had been captured; however, nearly all the Cl appeared to remain in the soil. Detailed hydrologic monitoring during the melt event indicated that a saturation threshold existed, beyond which, tile effluent tracer concentrations no longer rose in conjunction with discharge but instead dropped as the tile was inundated with dilute event water transmitted through the primarily vertical macropore network within the overlying soil. Although the tile’s rapid capture zone was relatively small, conservative solutes applied post-harvest, and within approximately 2 m of the tile, readily moved to surface water prior to the following growing season. Seasonal melts were identified as the most influential climatic events for solute mobilization.