TRANSPORT OF ATRAZINE, 2,4-D, AND DICAMBA THROUGH PREFERENTIAL FLOWPATHS IN AN UNSATURATED CLAYPAN SOIL NEAR CENTRALIA, MISSOURI

The objectives were to determine how atrazine oro-4-(ethylamino)-6-(is opropylamino)-s-triazine), dicamba (3-6-dichloro-2-methoxybenzoic acid ), and 2,4-D (2,4-dichlorophenoxy-acetic acid) move through claypan so ils (fine montmorillonitic, mesic Udollic Ochraqualf Mollic albaqualf, Mexico silty loam) at the Missouri Management System Evaluation Area (MSEA) near Centralia in Boone County, Missouri, and the role of prefe rential flowpaths in that movement. Twelve intact soil cores (30 cm di ameter by 40 cm height), were excavated sequentially, four from each o f the following depths: 0-40 cm, 40-80 cm, and 80-120 cm. These cores were used to study preferential flow characteristics using dye stainin g experiments and to determine hydraulic properties. Six undisturbed e xperimental field plots, with a 1 m2 surface area (two sets of three e ach), were instrumented at the Missouri MSEA on 11 May 1991: 1 m2 zero -tension pan lysimeters were installed at 1.35 m depths in Plots 1-3 a nd at 1.05 m depths in Plots 4-6. Additionally, each plot was planted with soybeans (Glycine max L.) and instrumented with suction lysimeter s and tensiometers at 15 cm depth increments. A neutron probe access t ube was installed in each plot to determine soil water content at 15 c m intervals. All plots were enclosed with a raised frame (of 8 cm heig ht) to prevent surface runoff, and were allowed to equilibrate for a y ear before data collection. During this waiting period, all suction an d pan lysimeters were purged monthly and were sampled immediately prio r to herbicide application in May 1992 to obtain background concentrat ions. Atrazine, 2,4-D, and dicamba moved rapidly through the soil, pro bably owing to the presence of preferential flowpaths. Staining of lab oratory cores showed a positive correlation between the per cent area stained by depth and the subsequent breakthrough of Br- in the laborat ory and leaching of field-applied herbicides owing to large rainfall e vents. Suction lysimeter samples in the field showed increases in conc entrations of herbicides at depths were laboratory data indicated grea ter percentages of what appeared to be preferential flowpaths. Concent rations of atrazine, 2,4-D, and dicamba exceeding 0.50, 0.1, and 0.15 mug ml-1 were observed with depth (45-135 cm, 60-125 cm and 60-135 cm) after several months following rainfall events. Preferential flowpath s were a major factor in transport of atrazine, 2,4-D, and dicamba at the site.