『Abstract
　Regional degradation of groundwater resources by nitrate has become one of the main challenges for water managers worldwide. Regulations have been defined to reverse observed nitrate trends in groundwater bodies, such as the Water Framework Directive and the Groundwater Daughter Directive in the European Union. In such a context, one of the main challenges remains to develop efficient approaches for groundwater quality assessment at regional scale, including quantitative numerical modelling, as a decision support for groundwater management. A new approach combining the use of environmental tracers and the innovative ‘Hybrid Finite Element Mixing Cell’ (HFEMC) modelling technique is developed to study and forecast the groundwater quality at the regional scale, with an application to a regional chalk aquifer in the Geer basin in Belgium. Tritium data and nitrate time series are used to produce a conceptual model for regional groundwater flow and contaminant transport in the combined unsaturated and saturated zones of the chalk aquifer. This shows that the spatial distribution of the contamination in the Geer basin is essentially linked to the hydrodynamic conditions prevailing in the basin, more precisely to groundwater age and mixing and not to the spatial patterns of land use or local hydrodispersive processes. A three-dimensional regional scale groundwater flow and solute transport model is developed. It is able to reproduce the spatial patterns of tritium and nitrate and the observed nitrate trends in the chalk aquifer and it is used to predict the evolution of nitrate concentrations in the basin. The modelling application shows that the global inertia of groundwater quality is strong in the basin and trend reversal is not expected to occur before the 2015 deadline fixed by the European Water Framework Directive. The expected time required for trend reversal ranges between 5 and more than 50 years, depending on the location in the basin and the expected reduction in nitrate application. To reach a good chemical status, nitrate concentrations in the infiltrating water should be reduced as soon as possible below 50 mg/l; however, even in that case, more than 50 years is needed to fully reverse upward trends.

Keywords: Nitrate; Groundwater; Modelling; Chalk; Environmental tracers; European Water Framework Directive』

1. Introduction
2. Regional conceptual model for nitrate in the Geer basin
　2.1. Geographical, geological and hydrogeological context
　2.2. Time and spatial distribution of nitrate in groundwater
　2.3. Tritium survey
3. Regional scale groundwater transport modelling in the Geer basin
　3.1. Modelling concepts
　3.2. Conceptual model and discretisation choices for the Geer basin
　3.3. Calibration of the groundwater flow model
　3.4. Calibration of the solute transport model
　3.5. Forecasting nitrate trends using the numerical model
4. Conclusions
Acknowledgements
References