| Nitrate has been recognized as one of the most widespread sources of surface and groundwater deterioration and associated ecological problems in California due to its intensive use in agriculture, its high mobility and persistence in the subsurface. Experimental studies of nitrogen cycling processes and nitrogen budget estimation have focused on the characterization of the uppermost soil horizon exclusively or on the immediate root zone. Limited empirical evidence exists about the amount of residual nitrate in the deep vadose zone below the root zone and its effect on nitrate leaching to groundwater.; A comprehensive assessment of nitrate fate and transport in a 16-m deep alluvial vadose zone was undertaken to quantify the role of the deep, highly heterogeneous vadose zone in evaluating the long-term impacts of agricultural practices on groundwater quality. The site is a former "Fantasia" nectarine orchard located at the Kearney Research Center, Fresno, California. Over 1,000 core samples were obtained from an extensive core drilling campaign under three nitrate fertilizer subplots with an annual fertilizer rate of 0, 110, and 365 kg N ha-1. Samples were analyzed for soil texture, soil hydraulic and chemical properties. Three modeling approaches, each representing a different subsurface scale of heterogeneity (field, lithofacies, and Darcy's scale), provided three different levels of risk analyses of nitrate leaching to water table in response to the long-term site irrigation and nitrate management practices. None of the three models implemented herein is capable of sufficiently explaining the low nitrate levels observed at the site. The model estimated deep vadose zone nitrate mass is at least four times larger than the measured deep vadose zone nitrate mass. Our findings suggest that, given the highly variable soil texture, soil hydraulic properties and nitrate concentrations observed at the site, preferential flow paths can lead to rapid, highly localized nitrate transport to water table. The significant degree of stratigraphic layering can enhance lateral flow and nitrate exchange among the adjacent subplots. Under the site conditions, denitrification in the deep vadose zone is limited, thus cannot account for the low nitrate levels observed at the site. |
