A GWB is defined in WFD Art. 2 as a distinct volume of groundwater within an aquifer or aquifers, whereas an aquifer is defined as a geological layer with significant groundwater flow. This definition of a GWB allows a wide scope of interpretations.
The submission of GWB data to the Commission by the EU Member States was accomplished via the Reportnet platform, as a part of the dataflow for WFD. Art. 13 reporting. GWB data includes spatial data as GIS polygons and GWB characteristics in an XML schema. GWBs are registered to so-called horizons, which represent distinct vertical layers of groundwater resources.
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please refer to the file "GW_reference_layer_status_v3.ETCICM.pdf"
European Topic Centre on Inland, Coastal and Marine waters (ETC/ICM) in particular by Federal Institute for Geosciences and Natural Resources based on country deliveries.