Quantifying Preferential Flow by Breakthrough of Sequentially Applied Tracers Silt Loam Soil

Field experiments were conducted on tile‐drained plots at the South East Purdue Agricultural Center in Butlerville, Indiana, to quantify contaminant transport via preferential flow paths in a silt loam soil. Breakthrough patterns of three fluorobenzoic acids (pentafluorobenzoic acid [PFBA], o‐trifluoromethylbenzoic acid [o‐TFMBA], and 2,6‐difluorobenzoic acid [2,6‐DFBA]) in a preliminary study indicated that they were transported as conservatively as is bromide (Br−). These four tracers were then sequentially applied, in an adjacent plot, during simulated precipitation (3 mm h−1 intensity, 10‐h duration). Bromide was sprayed shortly before irrigation started, while PFBA, o‐TFMBA, and 2,6‐DFBA were applied at 2, 4, and 6 h thereafter, respectively. Tile flow began increasing at around 3 h, and Br− appeared in tile drain flow ≈4 h after irrigation started, yet benzoic acids, PFBA, o‐TFMBA, and 2,6‐DFBA, were detected in the tile drainage at 102 min, 42 min, and 18 min after their applications, respectively. Tracer mass recovery from tile drainage was Br− (7.04%), PFBA (13.9%), o‐TFMBA, (18.7%), and 2,6‐DFBA (19.7%) of applied mass. The faster arrival time and greater recovery of sequentially applied tracers confirmed that water movement and contaminant transport shifts toward increasingly larger pores of the preferential flow paths as soil becomes wet during a precipitation event. The breakthrough patterns of these tracers can be used to quantify the water flux distributions of preferential paths. Because ≈90% of the chemical leached from this precipitation event occurred during the first day, it was critical to intensively monitor contaminant transport during the first 24 h after a rainfall. A soil sampling protocol based on collecting soil cores at random locations once every several days is unsuitable for determining the deep leaching under field conditions.