Field Investigation of Water Movement and Nitrate Transport under Perched Water Table Conditions

Increasing amounts of potentially hazardous chemicals such as nitrate (NO3−) arising from various agricultural operations are polluting soil and water ecosystems worldwide. In the present study, the perching process and the simultaneous movement of water and nitrate through lateritic soils under perched water table conditions were investigated through two field experiments. The piezometric head and soil matric suction at five depths as well as the perched water table fluctuations in nine vadose zone wells were monitored daily. Also, a 20 mm pulse of calcium nitrate was displaced in two experimental plots (called ‘plot I’ and ‘plot II’) by maintaining a constant depth of water throughout the leaching period, and the leaching of nitrate was monitored at six soil depths and in all the observation wells at suitable time intervals. The results of saturated hydraulic conductivity and bulk density at different depths in plot II suggested that the soil profile is heterogeneous. The analysis of hydraulic head and soil matric suction data revealed that the top 1·25 m of the soil profile responds rapidly to rainfall in both the plots, with plot I more permeable than plot II. Relatively long perching conditions at 1·71 m depth in both the plots indicated the presence of a more effective water-resisting layer at this depth. The maximum nitrate concentration in soil water was found after 8–12 h of leaching at shallower depths (up to 0·33 m) and after 12–18 h at greater depths. The enhanced rate of nitrate leaching in both the plots indicated the presence of macropores in the soil profile. Lateral flow due to perching conditions was also found to considerably affect the contamination process in lateritic soils. Furthermore, the dispersion coefficients of nitrate based on the breakthrough curves were found to range between 0·004 and 1·67 m2/day, suggesting heterogeneity in the soil profile. Finally, it is concluded that the relatively shallow aquifers of the lateritic region are vulnerable to nitrate contamination due to the current practice of nitrogenous fertiliser application in agricultural fields. Long-term and large-scale field studies under actual field conditions, together with simulation modeling, are recommended to generalise the flow and transport behaviour in lateritic soils.