Exploitation of high-yields in hard-rock aquifers: Downscaling methodologycombining GIS and multicriteria analysis to delineate field prospecting zones

Based on research work in the Truyere River catchment of the Massif Central (Lozere Department, France), a methodology has been developed for delineat ing favorable prospecting zones of a few square kilometers within basement areas of several hundred, if not thousand, square kilometers for the purpos e of siting high-yield water wells. The methodology adopts a functional app roach to hard-rock aquifers using a conceptual model of the aquifer structu re and of the functioning of the main aquifer compartments: the alterites ( weathered and decayed rock), the underlying weathered-fissured zone, and th e fractured bedrock. It involves an economically feasible method of mapping the thickness and spatial distribution of the alterites and the weathered- fissured zone, on which the long-term exploitation of the water resource ch iefly depends. This method is used for the first time in hydrogeology. The potential ground water resources were mapped by GIS multicriteria analy sis using parameters characterizing the structure and functioning of the aq uifer, i.e., lithology and hydrogeological properties of the substratum, na ture and thickness of the alterites and weathered-fissured zone, depth of t he water table, slope, fracture networks and present-day tectonic stresses, and forecasted ground water quality. The methodology involves a coherent process of downscaling that, through ap plying methods that are increasingly precise but also increasingly costly, enables the selection of sites with diminishing surface areas as the work a dvances. The resulting documents are used for ground water exploration, alt hough they can also be applied to the broader domain of land-use management .