SOIL WATER FLOW AND NITRATE TRANSPORT STUDY THROUGH HETEROGENEOUS VADOSE ZONE

Abstract

Nitrate, because of its intensive use as fertilizers/manure to increase the agricultural productivity of soils, is necessary to fulfil the demand of growing population. However, its high mobility in vadose zone is making it as a potential groundwater contaminant worldwide. In agricultural based economies, nitrate enrichment of groundwater has been appearing as a major threat to the local inhabitants especially in intensively cultivable areas. A high nitrate concentration in groundwater in these areas is due to heavy use of nitrogenous fertilizers in crop lands. In general, agricultural activities and dairy operations are identified as a major source of nitrate. The dairies and agricultural activities comprise a complex accumulation of multiple potential points and diffuse sources for nitrate contamination of vadose zone and underlying groundwater. The vadose zone is the layer of soil present between the earth surface and water table, and is of utmost physical, chemical and biological importance. This zone is also associated with the recharge of groundwater, degradation and uptake of contaminants including nitrate, and thus, preventing them to percolate to the groundwater table. Conventionally vadose zone is considered as a homogeneous region for analyzing soil moisture flow and solute transport. But in reality, heterogeneity is invariably present in the vadose zones, and hence, non-uniform flow takes place leading to the non-equilibrium flow conditions. Most of the studies on nitrogen laid more focus on root zone nitrate dynamics and reach ability of nitrate to the adjoining water bodies from surrounding surface and shallow aquifers. Further, nitrate movement in vadose zone is mostly investigated considering uniform moisture flow and solute transport mechanisms and studies performed on nitrate dynamics through vadose zone considering the realistic approaches are at nascent stage. Therefore, the main focus of this study is to investigate the soil water flow and nitrate transport in heterogeneous subsurface at different scales using a series of laboratory and simulation experiments. The aim of the conducting practical experiment was to characterize the flow and transport behavior of nitrate in subsurface under varying environmental conditions. Numerical runs were also conducted to predict the nitrate load to underlying groundwater resources of heterogeneous vadose zone using HYDRUS simulator. At plot scale, numerical simulation was performed considering the vadose zone heterogeneity for predicting accurate nitrate flux loading to groundwater under different rates of fertilizer applications. The experimental data of San Joaquin valley, California were used for these simulations. The nitrate leaching from the surface to the underlying water resources was simulated using dual porosity approach. Subsequently, a field scale study on the Nebraska ii Management Systems Evaluation Area (MSEA) site, located within the Central Platte Natural Resources District (CPNRD) of the Platte River Valley, was performed for quantifying corn yield and nitrate leaching to the underlying groundwater resources. A lithologic framework was developed for the MSEA using Rock works considering the observed soil properties of the field site. Future climatic scenarios were also considered for predicting the nitrate leaching and the associated crop yield. The observed data at the laboratory, plot scale and field scale were compared with the breakthrough curves obtained by numerical simulation. The results of this study may help in protection of groundwater resources from vadose zone return flows and can be used directly in planning better fertigation scheduling of farmlands.