Groundwater bodies (horizontes)

In order to manage the groundwater, the Flemish subsurface is divided into several three-dimensional units: the groundwater bodies. Within these bodies of groundwater, environmental objectives are assessed and measures are imposed if necessary. The definition of groundwater bodies is mandatory in the European Water Framework Directive 2000/60/EC, which defines a body of groundwater as ‘a separate body of water in one or more aquifers’. The classification of the subsurface into different groundwater bodies is mainly based on the physical characteristics of the groundwater reservoirs and on the regional groundwater flow. The sequence of aquifers (sand, gravel, chalk, solid rock,...) and regionally occurring non-aquifers (clay,...) and the prevention of clear barriers to the groundwater flow (thick clay layers, fractures, groundwater separations, highly draining rivers, salination limits, etc.) are the main starting points. The delimitation of groundwater bodies was therefore based on the Hydrogeological Coding of the Underground of Flanders (HCOV coding) and the classification of the subsurface of Flanders into groundwater systems. The groundwater bodies are three-dimensional and can occur side by side and above each other. In total, there are 42 groundwater bodies. They are very different from each other: * surface area from 50 km² to 6 000 km^2 * thickness from 20 m to 1 000 m * Kh from 0.0000005 to 2 300 m/day * from sweet to salt * made up mainly of limestone, sand, gravel, peat, silt,... * *... The groundwater bodies have both a name and a code. The code is structured as follows: The naming of a groundwater body is always based on the HCOV code of the main aquifer. Each groundwater body has also received a meaningful code ‘GWS_HCOV_GWL_NR’. The code consists of an abbreviation of the groundwater system in which the groundwater body is located (e.g. CFS, Central Flemish System), followed by the HCOV code, which corresponds to a main aquifer layer (e.g. 0600 stands for the Ledo-Paniselian-Brussels Aquifer system). Then the abbreviation ‘GWL’ is added, after which a serial number NR indicates the further spatial classification of the aquifer in different regions. Finally, in some cases the letters ‘s’ and ‘m’ were added, indicating that a groundwater body was split into a part that can be located on the one hand in Scheldt district or on the other in the Meuse district. To help clarify the three-dimensional aspect of the groundwater bodies, they were split into horizons, i.e. the order in which groundwater bodies occur in depth. In a given place, each groundwater body occurs maximum in one horizon and there is a maximum of one groundwater body in each horizon. “1” means that the groundwater body appears at the top, “2” that it occurs as a second groundwater body, “3” as a third,... Conversely, there are 4 groundwater bodies above a groundwater body with horizon “5”.