Groundwater pollution by nitrates in a heterogeneous agricultural watershed: 1. Evaluation of inputs to the water table (Agriflux model)
In agricultural regions, groundwater contamination by nitrogen compounds originating from fertilizers is one of the most significant environmental problems. Along with in situ monitoring, simulation models have been developed for non point pollution (nitrates, pesticides) in order to evaluate both the level and the extent of the contamination. Simulation models, originally intended for research purposes in relation to the dynamics of agricultural systems, have been adapted and applied to environmental management in order to quantify water volumes and contaminant masses likely to reach groundwater systems. Recently, mechanistic models such as Agriflux (Banton et al., 1993) have been developed for use in the field. Agriflux is based on a mechanistic approach to the processes and incorporates a stochastic analysis that takes into account the spatial variability of the parameters. lt. calculates nitrate concentrations as well as water fluxes in the unsaturated zone. In the present study, environmental management principles integrating heterogeneity in soils and agricultural practices were applied to an agricultural watershed in Poitou (France). Preliminary simulations were carried out in order to estimate the influence of various parameters on the nitrate and water fluxes. First, a three-year wheat mono-crop was simulated using the same fertilization rate for each year. The calculated nitrate concentrations follow a trend opposite to that of the seasonal growing crop. To estimate the influence of the soil characteristics on the nitrate concentrations, the four types of soil in the watershed were simulated using the same three-year crop rotation. The results show that the soil type directly influences the amount of nitrate leaching. Under different soils types, the evolution of the concentrations over time follows the same pattern, but the concentration levels are significantly different. To quantify the impact of crops on the nitrate concentrations, the main crop rotations were simulated for the same type of soil. This set of simulations underlines the environmental differences between winter and spring crops. lt. also shows the differences induced by the presence of residues. The La Jannerie watershed was divided into homogeneous zones for soil and crop characteristics. During a four-year period, seasonal and annual nitrate concentrations were calculated for each homogeneous zone from the daily water and nitrate fluxes simulated with Agriflux. The results demonstrate the influence of the agricultural practices on the calculated concentrations. Overall, nitrate levels remain quasi-constant during the periods when the crops are active but vary considerably during the winter when the crops are absent or inactive. This winter period corresponds to a peak in nitrate leaching because of the excess rainfall and the absence of nitrogen uptake by the plants. The incorporation of crop residues in the soil in the autumn generates a high production of nitrates during winter due to the mineralization of the organic nitrogen. Two different environmental approaches can be used jointly to evaluate agricultural practices. The first consists of a comparison between the nitrate flux that can reach the saturated area and the fertilizer rate. This approach provides an estimate of the amount of nitrogen lost to the aquifer. Simulations with Agriflux show that the nitrate fluxes are highest during the autumn when plant uptake is non-existent, except in fields with winter crops. The second approach compares the calculated nitrate concentrations that may occur in the aquifer with recognized water quality criteria. lt. is interesting and important to note that, during the simulated period, the calculated concentrations in the leach were often much lower than the water quality criterion (50 mg NO3/L). This result indicates that the fertilization practices applied in the watershed during this period tended to approach the real crop requirements (minimal requirements) and were more environmentally adequate (environmental optimum) than those used previously.