Présentations Colloques

Oral Presentation
Session 7.03: Agricultural and sanitation contaminants and implications for water services and health
Faillat Jean-pierre
A methodology to reduce the intake of nitrate to rivers and coastal regions in fissured and altered hard-rocks
The hydrodynamic and hydrochemical natural functioning of fissured and altered igneous or metamorphic basement, in areas of low slope gradient as the Armorican massif (France), results in redox groundwater organization, which is specific to such geomorphic conditions. Indeed, beyond a 30-50 m depth, a reductive layer in relation to nitrates always exists, depending on the type of rock, structure of the aquifers and hydrodynamic. In areas of priority protection (catchments with the stream for drinking water supply, coastal watersheds, etc.), a change in the distribution of nitrogen inputs into the aquifers could provide a solution to the nitrate pollution of surface waters by taking into account, in addition to the model of groundwater flow, both the flow conditions of nitrate transport and the redox stability domain of nitrates.**The solution would be implemented as the following guideline. Application of chemical and organic fertilizers in the vicinity of streams, where the flow lines reaching the rivers are entirely included within the shallow oxidizing zone and natural denitrification of groundwater cannot occur, would be prohibited or strictly controlled. Fertilizer or manure would be allowed only around interfluves, where hydraulic flow lines go through deep denitrifying level, thus giving a better approach to the delimitation of preferential manuring areas. Numerical simulations show that significant improvement is possible in less than 2-5 years, mainly because the stocks of nitrates are located in the most renewed parts of the aquifer. Management measures would be applied to facilitate deep infiltration and prevent shallow nitrate transfer, such as ploughing lines perpendicular to slopes of hillsides, or removal of drains in nitrogen spreading areas. Finally, the stresses generated by this approach would be minimal and with no significant impact on agricultural production, which should facilitate its application.**