Impact of Preferential Flow on the Transport of Adsorbing and Non‐Adsorbing Tracers

Field experiments were conducted by using a tile drain monitoring facility to determine the impact of preferential flow on the transport of adsorbing and non‐adsorbing tracers. Simulated rainfall with 7.5 mm h −1 intensity and 7.5 h duration was applied to a 18‐ by 65‐m no‐till plot. After 72 min of irrigation, a pulse of Br − and rhodamine WT (water tracer) was applied through irrigation, and 4 h later, a second pulse of Cl − and rhodamine WT was applied. The breakthrough curves (BTC) of these tracers were measured by sampling the tile. The same experiments were repeated in an adjacent conventional‐till plot, except the rainfall intensity was reduced to 5 mm h −1 The results showed that both the non‐adsorbing and the adsorbing tracers applied in the same pulse arrived at the tile line at the same time and their BTC peaked at the same time. This suggested that water dynamics of preferential flow paths dominated the initial phase of the contaminant transport, regardless of the retardation properties of contaminants. The tracers from the second pulse were detected at only 13 min after application. Among the four tracer pulses in two plots, the BTC from the second pulse in the no‐till plot had the longest period in which the non‐adsorbing and adsorbing tracers had identical patterns. This indicated that the wetter the soil profile, the longer the water dynamics of preferential flow paths dominate the contaminant transport. The BTC from the second pulse applied to the two plots had identical arrival and peak times.