NO3-N AND METOLACHLOR CONCENTRATIONS IN THE SOIL-WATER AS AFFECTED BYWATER-TABLE DEPTH

Experiments were conducted in lysimeters to study the effect of shallo w water table (WT) depths on the transport of two commonly used agricu ltural chemicals, nitrate-N and metolachlor to shallow groundwater Gro undwater samples were collected from 0.20-, 0.40; and 0.60-m depths us ing suction tubes during the growing season. The results showed signif icant reductions in both nitrate-N and metolachlor concentrations in t he groundwater by maintaining shallow WT depths. Lowest concentrations of nitrate-N and metolachlor in the groundwater were observed when WT were maintained at 0.15-m depth. Generally, nitrate-N concentrations were increased with the soil depth while metolachlor concentrations de creased with the sampling depth during the growing season. Analysis of drainage outflow data at the end of WT treatment period also provided the evidence of the effectiveness of shallow WTs in reducing chemical s losses to shallow groundwater systems. The results of this study ind icated that nitrate-N and metolachlor concentrations in the drainage o utflow were 54 and 45%, respectively, lower for the 0.15-m WT treatmen t than the 0.60-m WT treatment. Regression analysis showed a strong ne gative linear relationship between metolachlor concentration and days after planting (DAP). Soybean yield was significantly reduced with the rise in WT depth. Average soybean yield obtained for the 0.15-m WT de pth was 42% lower than the 0.60-m WT depth. It can be concluded from t he overall results of this study that shallow WTs can be used effectiv ely to reduce the nitrate-N and metolachlor losses to the shallow grou ndwater.