NO3-N and Metolachlor Concentrations in the Soil Water as Affected by Water Table Depth

Abstract

Experiments were conducted in lysimeters to study the effect of shallow water table (WT) depths on the transport of two commonly used agricultural chemicals, nitrate-N and metolachlor, to shallow groundwater. Groundwater samples were collected from 0.20-, 0.40-, and 0.60-m depths using suction tubes during the growing season. The results showed significant reductions in both nitrate-N and metolachlor concentrations in the 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 decreased 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 chemicals losses to shallow groundwater systems. The results of this study indicated that nitrate-N and metolachlor concentrations in the drainage outflow were 54 and 45%, respectively, lower for the 0.15-m WT treatment than the 0.60-m WT treatment. Regression analysis showed a strong negative 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 depth was 42% lower than the 0.60-m WT depth. It can be concluded from the overall results of this study that shallow WTs can be used effectively to reduce the nitrate-N and metolachlor losses to the shallow groundwater.