Field data and modelling of water and nitrate movement through deep unsaturated loess

The load and transfer time of nitrogen within the unsaturated zone must be determined to predict the evolution of nitrate concentration in groundwater. The main objectives of the study were to characterize the transfer of nitrate in deep unsaturated loess and to evaluate the capacity of the model. Agriflux, coupled with a convective transfer below the first meter of soil, to simulate this transfer. Grain size, gravimetric water content, and tritium and nitrate in the pore water of solids from the unsaturated zone were determined for three profiles (15-17 m deep) collected in an agricultural plot for which detailed data on land uses and fertilization practices were available. All these data were used to identify hydrodynamic process in the unsaturated zone and to provide inputs for the modelling exercise. The transfer appeared governed by convection-dispersion processes with an estimated average transfer rate of around 20-26 cm per year that appeared very homogeneous at the plot scale. The model, received limited calibration of some nitrogen cycle parameters (humus mineralization constant, fraction of litter that passes to humus) and adjustment of some input data relative to fertilization. It subsequently generated in the vertical profiles tritium peaks that were less than 100 cm off the H-3 peak observed between 8 and 9 m below soil level (bsl), and nitrate peaks less than 50 cm off the NO3 peaks observed at 2.5 and 5 m bst. The maximum model values for NO3 are within 10% of the measured values. Furthermore, the model helped identify factors influencing nitrate concentrations along the profiles. In the hydrodynamic context of loess soils, the modelling approach used in the study is a very useful tool to evaluate impacts of variable agricultural practices on nitrate concentrations in the unsaturated zone. (c) 2007 Elsevier B.V. All rights reserved.