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In the lateritic tract of West Bengal, India, perched water table conditions exist in the soil profile during the rainy season. Consequently, the agro-chemicals present in the vadose zone are convected downwards by the declining perched water table after the cessation of the rainy season, and thereby may pollute the freshwater aquifer. A field experiment was carried out to evaluate the water flow and fertilizer solute transport through the vadose zone in presence of shallow perched water table and to assess the risk of groundwater pollution due to the leaching of fertilizer solutes. An experimental plot was developed for this study and the hydraulic head, matric suction and perched water table were monitored daily at different soil depths. In order to explore the dynamics of the NO3- and K+ leaching, KNO3 was applied in the slug form and it was displaced by applying a constant depth of water in the experimental plot. Leachates from the soil-water samplers, and the water samples from the nine observation wells were collected initially at short intervals and thereafter at relatively long intervals and were subject to chemical analysis. The analysis of the hydraulic head and the total water potential data at different depths suggested the presence of a relatively impermeable layer at about 1 m depth. The water flux through four soil layers was estimated ranging between 0.05 and 1.62 cm per day, with significantly high values in the upper layers compared to the lower layers. The accelerated transport of NO3- and K+ in the experimental plot just 1 day after the initiation of miscible displacement, indicated the presence of macropores in soil profile and their role in accentuating the groundwater contamination by fertilizer leaching. However, the mobility of K+ was found to be lower than that of NO3-. The present study also revealed that a substantial amount of NO3- was apparently convected from the upper soil profile and added to the lower soil profile during the recession of perched water table. Finally, it is concluded that the regional groundwater is under a serious threat of NO3- contamination. Future, long-term studies on a macro-scale are recommended to develop strategies for controlling or avoiding NO3- contamination in the Kharagpur region.
Author Keywords: Vadose zone; Perched water table; NO3- and K+ leaching; Miscible solute displacement; Groundwater pollution
