Méthodologie de dimensionnement des zones de protection des captages d'eaux souterraines contre les polluants chimiques persistants

This research proposes a general methodology allowing the delimitation of protection areas for abstracted groundwater against persistent chemical contaminants for the setting up of: (1) remediation programmes in the event of contamination and (2) prevention programmes, in a general way. Since such substances are mainly transported by water circulation, this approach suggests delineating protection areas on the basis of the complete groundwater flow cycle, from the surface of the catchment zone to the groundwater source itself. Areas with the highest contribution rate to the groundwater source will be protected first and foremost. The method under consideration is essentially applied to diffuse contaminants, such as pesticides or nitrates released by agriculture. If pollution is local, a standard method must be applied to treat the problem at the root. The methodology proposes the following steps, which start from the groundwater source and are widened, upstream, to the catchment zone: Identifying the aquifer portion that supplies the groundwater source. Defining, on the ground surface, the catchment zone of the groundwater source. Quantifying the recharge and discharge processes. Calculating the contribution to the groundwater abstraction rate of any point i on the catchment zone surface (Ci) by the following equation:          Ci = Ii(1-ei)Pi [m/s]    Ii : Infiltration of the precipitation or stream losses at point i [m/s].    ei : Discharge coefficient of water before reaching the main groundwater table (agricultural draining, perched water table, etc.), 0 ≤ e ≤ 1 [-].    Pi : Probability of a particle of water that reaches the groundwater table from point i at the surface going to the groundwater source, 0 ≤ P ≤ 1 [-]. Checking the results (water and mass balances). When the results are not satisfactory, the steps will have to be reiterated. Delineating protection areas (areas with the highest C will be primarily protected). With this approach, we propose not to confine ourselves taking the totality of the catchment zone as protection area against persistent contaminants (protection defined in most of the countries), but to define a smaller area of protection by selecting only the most contributive surfaces. As the acreage of the protection area can be considerably reduced, it will be possible to apply more severe regulations to it, which could never be done on the scale of the entire acreage of the catchment zone. Protection areas defined on this basis moreover allow target remediation solutions(1) which optimize the efficiency/cost ratio. They are, by their very concept, independent of the flux of contaminants at any given time and provide a tool for sustainable development and land planning. For each groundwater source, a rapid calculation based on the contribution field can be made to estimate the potential efficiency of the method and determine the remediation surface that is necessary to reduce contamination. In the same way, the effects of the potential development of the region can be evaluated. The contribution isoline that delimits the protection area can be proposed on the basis of this result. Three actual cases have been treated in order to test the method's applicability. The results are convincing in various porous or fractured aquifers. In the three sites, the results show that, with a similar remediation surface, remediation is more efficient when acting according to the concept of the methodology than when operating without spatial differentiation. ---------- (1) This study uses the concept of a "natural" remediation surface whereby contaminant leaching is reduced by a change in cultivation or in proceedings. The conversion of cultivated areas into meadows is, e.g., one of the most effective methods to reduce nitrates leaching.