Analysis of 2,4-D residues in soil profile using HYDRUS-1D model

Despite the benefits of herbicides, their persistence in soil can harm other plants in next rotations and contaminate ground water. The main purpose of this research is to simulate 2,4-dichlorophenoxyaceticacid(2,4-D) movement in silty clay loam soil using HYDRUS-1D model in a corn root zone. The maximum contaminant level goal, MCLG, of 2,4-D in water resources is 10 ppb. The 3.5 kg a.i ha-1 of 2,4-D was applied to the experimental field under two irrigation treatments including normal and deficit irrigation. The presence of 2,4-D along with soil water content was measured in soil profile at different depths during the growing season at 8, 13, 23, 30, 37 and 57 days after application. Both measured and simulated data showed that 2,4-D concentrations reduced from top to bottom of soil profile, maximum 40 cm in both irrigation treatments. The 2,4-D residues in normal and deficit irrigation were used for calibration and validation of HYDRUS-1D model, respectively. In general, total measured and simulated 2,4-D concentration in normal irrigation treatment were 68.94 and 64.96 mg kg-1and in deficit irrigation treatment were 74.3 and 74.0 mg kg-1, respectively. Statistical parameters including NRMSE, CRM and d were used for comparison of measured and simulated data. These parameters were 0.267, 0.77 and 0.846 for normal irrigation and 0.189, -0.02 and 0.937 for deficit regime, respectively. Therefore, the results showed a good agreement between measured values of herbicide residue and the values simulated using HYDRUS 1D model. The numerical simulations for deficit irrigation regime were more precise than normal irrigation because of less microbial activity due to less soil moisture. The numerical models could not simulate microbial activity with acceptable accuracy.