Identification of groundwater quality trends in a chalky aqufieer threatened by intensive agriculture
As a response to the continuous increase of use on fertilizers and pesticides in last decades related to agricultural practices, the European Union (EU) has adopted in 1991 the Nitrate Directive 91/676/EEC (EU 2000). More recently the “Water Framework Directive 2000/60/EC” (WFD) has been published, stating that a good status of water is required for all EU members. The methodology used in this research mostly follows the works of Grath et al. (2001) who proposed particular algorithms and techniques for the identification of pollutants trends in ground water. As suggested by Hirsch et al. (1991), a three-step procedure is considered: 1) normality test of the dataset (Conover 1980, Helsel and Hirsch 1995); 2) trend detection (Mann 1945; Kendall 1975); and 3) trend estimation (Hirsch et al., 1991; EPA 2000). The study was performed in the Hesbaye chalky aquifer, located in the Geer river basin, at the North-West of the Liège city (Belgium). The Hesbaye aquifer is an important ground water resource for drinking water supply for the city of Liège and its suburbs, which means around 30 million cubic meters per year (Brouyère et al., 2004a). The land use in the basin is characterized by a dominant agricultural component, covering about 65% of the catchment area, the remaining space remaining being divided between pastures (15%), housing (13%) and forests (7%) (Broers et al., 2005). Nitrate datasets used in this study come mainly from the Nitrate Survey Network (NSN) established by the Walloon Region water authorities and from VMW (Vlaamse Maatschappij voor Watervoorziening), the Flemish water supply company. From groundwater points used (24), a total of 17 sampling points are characterized by an upward trend (71 % of the points), the annual increase of nitrate concentration ranging between 0.3 and 0.8 mg•l-1•year-1. The remaining points (7 over 24), which do not show any evidence of upward or downward trend, generally corresponds to sampling points with limited nitrate records, irregularly distributed in time. The transit time of nitrates across the thick unsaturated formations (from 10 to 70 m) have been deduced at approximately 1 m•year-1 to reach the groundwater (Brouyère et al., 2004b). This means that measures taken today will have an observable effect with a delay of 10 to 70 years. It gives thus more than time to react. Because changes in agricultural practices have started recently, it can be considered that the trends observed nowadays are likely to remain for years. Based on this assumption, a “simple” trend extrapolation is relevant in order to estimate the time remaining before ground water be unusable for public water supply. A rough estimation of the time remaining before the threshold concentration of 50 mg•l-1 would be reached in various parts of the chalk aquifer, has been calculated based on a point-by-point extrapolation of nowadays nitrate contamination levels using nitrate trend estimates in this research.