Natural background levels in the groundwater bodies of Brussels Capital Region

Natural background levels are important for setting regulatory limits. The Water Framework Directive (WFD 2000/60/EC) and its daughter directive (2006/118/EC) aims at achieving good status of groundwater in the EU (initially by 2015). To achieve good chemical status, Member States must set threshold values for pollutants and indicators of pollution. Some substances may occur both naturally and/or as a result of human activities. For this reason, it is necessary to determine natural background levels (NBL).In the Brussels-Capital Region, 5 groundwater bodies are defined. They have different lithology (sand, sand-clay, chalk and Paleozoic basement), some are unconfined and others are confined. Defining the NBL of these groundwater bodies is a challenge because the surface area of the Brussels-Capital Region is highly urbanized, and it is also quite small, so there are few monitoring sites.The NBLs were defined following the European methodologies (Bridge project, Baseline project and guidance note). Hydrochemical data used correspond to the sampling sites reported in the monitoring programmes, linked to the Water Framework Directive, and supplementary monitoring sites of Brussels Environment for the period 2006-2017. Additional data from neighbouring groundwater bodies in the Walloon and Flemish Regions have been added to complete the dataset and to place the studied groundwater bodies in a larger regional hydrogeological and hydrochemical context. Data quality was examined through the calculation of the ionic balance, the analysis of limits of quantification and the analysis of the presence of pollutants to exclude some monitoring stations from the data set. To understand the processes influencing the geochemical composition of groundwater in each groundwater body, different tools were used (Piper diagram, spatial and temporal changes in concentrations, Na/Cl ratio and saturation indices of calcite, halite and gypsum to understand the origin of salinity, study of atypical values, correlation matrix, multivariate analysis). On the basis of all these analyses, several sampling stations were selected for the calculation of the NBLs of each groundwater body as the Percentile 90 of the selected samples. The NBLs could then be used by water authorities to set/adapt threshold values"Caractérisation de la concentration de référence de certains paramètres chimiques présents naturellement dans les masses d’eau souterraine captives du Socle et du Crétacé (BR01) et du Landénien (BR03) en Région de Bruxelles-Capitale" et "Mise en évidence des concentrations géochimiques de référence dans les nappes phréatiques tertiaires et du Socle en zone d’alimentation