COUPLING GIS WITH HYDROGEOLOGICAL MODELING, CASE STUDY: CHALK AQUIFER OF NORTHERN FRANCE

摘要

In the north of France (Nord -Pas de Calais), nitrate concentrations measured in the chalk groundwater exceed the drinking water limit of 50 mg/L fixed by the European Nitrates Directive (91/676/EEC). Lens Lievin's drinking water (North of France) is provided mainly by groundwater abstraction in the same sector. The physico-chemical quality of water is poor. This research is concerned with meeting the needs for drinking water in Lens-Lievin. The study focuses on two drinking water wells located within the chalk aquifer of Bethune (see figure 1). The chalk in this zone is covered by sand and clay formations of the Cenozoic, leading to the captivity of the aquifer. This confined aquifer allows the existence of a natural denitrification process, which leads to the rapid degradation of nitrates; therefore there is a good water quality in natural conditions for the B&hune's aquifer. The aim of this hydrogeological survey is to determine the optimal abstraction rates which allows at the same time, supplying the Lens-Lievin sector and keeping the Bethune aquifer's captivity. A HBDS (Hypergraph-Based Data Structure) model has been created to structure and to simplify this hydrogeological problem. Then a physical data model was carried out using ARCGIS9.3 software, and input data was implemented in GIS using the model builder tool to perform a spatial distribution of input data used in the modeling phase. Two numerical simulations were performed using Visual Modflow's software, first a steady state simulation was implemented in order to calibrate the model and second a transient flow simulation from 1972 to 2008 was set to simulate additional abstraction consequences on water table level. The results of these simulations have been treated by the GIS allowing thus the analysis of the impact of these additional abstractions on the denitrification phenomenon and choosing the optimal abstraction rates.