Location of protection zones along production galleries: An example of methodology

The Hesbaye aquifer, consisting of Chalk, is located in the northeastern part of Belgium. It provides about 60 000 m³ day^-1 of potable groundwater for Liege and its suburbs. This aquifer is developed by gravity or pumping, from 45 km of galleries. Despite a protective cover of 5-20 m of superficial loess deposits, groundwater quality is threatened by intensive agriculture and, in the southeastern part, by industrial activities. In the Walloon region of Belgium, protection zones around galleries or well fields are defined on the basis of pollutant transfer time in the saturated part of an aquifer. 'Zone IIa' and 'Zone IIb' areas are respectively limited by isochrone lines corresponding to an advection-dispersion travel time of 24 h and 50 days. To delineate zone IIa along the Hesbaye galleries, five sites were selected. At each site, four or five new wells were drilled. Drilling sites were located according to geomorphological studies and geophysical investigations. Pumping and tracing tests were performed and interpreted using deterministic flow and transport groundwater models. Then, geophysical data and geomorphological observations were used to extrapolate results from these five sites to other areas. This has lead to zone IIa extensions varying from 1560 m to 30 m according to the weathering or fracturing of the Chalk, but also to the regional groundwater flow direction and gradient.