Transport and fate of the herbicide diclofop-methyl in a large-scale physical model

A 3.6-m-thick unsaturated zone was constructed in a mesoscale model (2.4-m diameter by 4.6 m high, 65 tonnes) consisting of A, B and C profiles. The model was designed to be a homogeneous isotropic system. Assuming piston flow the estimated transit time for water through the unsaturated zone (3.3 m) under steady-state conditions was 25–30 days. Monitoring of major ions, metals, pH, alkalinity, CO2, N2, O2, CH4 and other parameters indicated that the system approached chemical steady state after 130–150 days. The herbicide diclofop-methyl was applied to ground surface on day 78. The migration of the herbicide was monitored at intervals using suction lysimeters to collect pore-water samples. Diclofop was detected in the 0.08-, 0.22-, 0.36- and 0.54-m-depth samplers. However, the timing of detection, at 0.08 m immediately after application, 0.54 m after 4 days, and 0.22 and 0.36 m after 12 days indicated the presence of preferential flow paths. Studies of the surface CO2 flux and water chemistry at 0.85 m further supported the existence of preferential flow. A fluorescein treatment was applied (day 345) to the surface and the model was systematically sampled and excavated (day 348), under ultra violet illumination, to further examine flow patterns and paths. Mapping of the distribution of fluorescein and examination of vertical sections confirmed that several pathways existed within the A and B horizons. These observations explained the observed distribution of diclofop within the system. The diclofop dissipated at all sample depths within 35 days of application. Further studies were carried out to determine the fate of 14C labeled diclofop-methyl in A, B and C horizon materials derived from the model. The results of these studies indicated that the maximum conversion of diclofop to CO2 in each of the horizons A, B and C was 1%, 59% and 75%, respectively. These studies indicated that inhomogeneities in the reconstructed soil profile were the major path for penetration of diclofop below the rooting zone. Further, the diclofop rapidly dissipated and degraded at all depths in the unsaturated zone. In addition, the results show that these large-scale physical models can exhibit variability similar to that observed at field scale.