The use of redox and hydrodynamic characteristics of groundwater in the control of nitrates.

The hydrodynamics and hydrochemistry of magmatic and metamorphic rocks, fissured, altered and with low relief, such as the Armorican massif (France), and this only for such conditions, leads to a redox organization of groundwater, which results in beyond 30-50 m deep, depending on the organization of the hydrogeosystems, there is a quasi-generalized reducing level of nitrates. This is due to the organization of underground hydrodynamic flows, the transport conditions and the redox stability range of nitrates.

Also, the modification of the distribution of nitrogen inputs in the areas to be protected as a priority (perimeters of protection of catchments along rivers, coastal watersheds, etc.) or more generally, provides a solution to the nitrate pollution of water. This consists in the interdiction or the strict control of nitrogen inputs in the parts of the catchment basins close to the rivers, where the deep denitrification of the nitrated groundwater cannot be done, because the hydraulic current lines reaching to rivers remain in the superficial oxidizing zone. The manuring of inputs would then only be admissible far enough from rivers so that the passage of streamlines through the deep denitrifying zone is inevitable (Fig. 1).

An experimentation or the practice should make it possible to best approach the delimitation of these preferential manuring zones. Numerical simulations show that significant improvements are possible in less than 5 years, the polluting nitrate stocks being mainly located in the oxidizing zone close to rivers, the best renewed part of the water tables (Fig. 2).

Finally, the constraints generated by this approach and the management measures intended to facilitate the deep infiltration of water and to prevent surface runoff as much as possible, such as plowing perpendicular to the slopes of the hillsides, the removal of drains in sewage fields, etc., would be minimal and have no significant impact on agricultural production. This should facilitate their application, especially since they would be accompanied by other types of measures relating to the modification and the amelioration of agricultural practices.