Estimating pressure and impact of nitrates on groundwater at national level: New methodological developments
Résumé
Within the water framework directive the assessment of the impact on groundwater of the pressure from nitrate from agriculture origin is requested. At basin and national levels other methods should be developed in order to provide a uniform view of the pressure/impact issue.
This presentation is based on a project which was initiated with the objective of proposing national maps of pressure and impact based on the combination of various thematic layers using a geographical information system.
The first step was the development of a national approach for the calculation of the nitrogen pressure note for agricultural sources. To attribute the cultural pressure note, several notes have been attributed according to an expert judgment before attributing the final note ("previous crop" N surplus; "following crop" capacity of absorption).
Using the existing map of the French plots (about 6 millions plots have been treated), it was thus possible to obtain a national map for the nitrogen pressure from agriculture.
To validate the approach the pressure map has been compared with the nitrate concentration measured at monitoring points. Median of concentrations from the French Groundwater database was used. A quite good matching appears between cultural notes (today’s pressure) and median of NO3 concentration in groundwater (today’s impact)
Due to different and sometimes long transfer and residence time of water and associated solutes it is necessary to take into account “delays” when comparing pressure and impact.
Nitrates transfer estimation depends mainly on lithology. For sedimentary basin, unsaturated zone depth and existing data on nitrate transfer velocity were coupled in order to obtain information on NO3 delay in the unsaturated zone. For other lithologies, only data on “chemical and isotope tracer” ages were used to get an estimation of the residence time of the contaminant in the saturated zone.
The various information layers will be combined in order to obtain a spatial distribution of the nitrate recharge ages (Wang et al., 2012) and link the today’s impact to the pressure at the origin of the impact (without the time delay).
The proposed approach will give an homogeneous national view of the pressure / impact approach for nitrate and will permit highlighting the sectors where today’s pressure may lead to a degradation of groundwater quality and that will need local studies in order for the Water Manager to propose adequate program of measures