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

In order to study the movement of agrichemicals in a flooded system under c ontrolled conditions, we performed two tracer tests on a 3- by 8-m plot tha t was deliberately flooded twice during the study: once in fall 1996 and ag ain in fall 1997. Each test was monitored through the summer following floo ding. Hydraulic head data continuously collected while the site was hooded indicate that a downward vertical hydraulic gradient predominated throughou t the entire depth monitored (4.5 m), although there was also a horizontal gradient. Atrazine, bromide, and nitrate enriched in N-15 were sprayed at t he 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 analy sis seven times throughout the flooding cycle. The atrazine, bromide, and N -15 moved rapidly after flooding, in a nearly vertical direction, greater t han 4.5 m in less than 24 h. After 24 h, concentrations of the three applie d chemicals decreased with time,N-15 most rapidly and atrazine least rapidl y. 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 gr ound water contamination when hood events coincide with atrazine presence i n 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.