Movement of bromide, nitrogen-15, and atrazine through flooded soils

In order to study the movement of agrichemicals in a flooded system under controlled conditions, we performed two tracer tests on a 3- by 8-m plot that was deliberately flooded twice during the study: once in fall 1996 and again in fall 1997. Each test was monitored through the summer following flooding. Hydraulic head data continuously collected while the site was flooded indicate that a downward vertical hydraulic gradient predominated throughout the entire depth monitored (4.5 m), although there was also a horizontal gradient. Atrazine, bromide, and nitrate enriched in ¹⁵N were sprayed at the surface in the center of the plot prior to each flooding. Water samples were taken from multi-level samplers to depths up to 4.6 m for tracer analysis seven times throughout the flooding cycle. The atrazine, bromide, and ¹⁵N moved rapidly after flooding, in a nearly vertical direction, greater than 4.5 m in less than 24 h. After 24 h, concentrations of the three applied chemicals decreased with time, ¹⁵N most rapidly and atrazine least rapidly. Most of the atrazine (65%) remained bound in the top 50 cm of soil, but the rapid movement of the dissolved atrazine indicates the potential for ground water contamination when flood events coincide with atrazine presence in soils. The presence of dissolved sulfide indicated suboxic conditions in the submerged soil, which were conducive to denitrification reactions, thus nitrate appears to be of less concern than atrazine.