The protection of a karst water resource from the example of the Larzac karst plateau (south of France): a matter of regulations or a matter of process knowledge?

摘要

Wide karst plateaus extend in limestone and dolostone of the Grands Causses area north of Montpellier. They are surrounded with a hard rock basement that feeds important rivers, such as the Tarn River and its tributaries, crossing the plateaus in deep gorges. The Larzac plateau, the most southern of them, makes the bond with the Mediterranean region, traversed by one of the main national roads, presently doubled with a speedway under construction from Clermont-Ferrand to Montpellier and Spain. The karst water resource, through several springs, is used for the water supply of the Millau area (30,000 people). It also recharges surface rivers, mainly during the summer and fall low stage, in a region well known for water recreation activities, fish farming and, above all, sheep farming and Roquefort cheese production. The fundamental question is, "Should the karst and its water resource be fully protected by strict regulations, for example considering the plateau in the same way as a nature reservation, or may we foresee a land management plan of the plateau based upon the knowledge of its hydrogeological functioning and of its karst structure?" Support decisions concerning about a 500-km~2 area were developed from detailed hydrogeological studies, including natural and artificial tracing, landscape analysis, risk inventory and mapping. Recommendations were proposed to local and regional decision-makers in order to define what are the preference shares in terms of water resource protection and water quality restoration. Following such an approach is not easy for hydrogeologists and for decision-makers. In non-karstic regions, protection zones currently extend over small areas of few square kilometers, which is easily under the control of a small municipality. In the present example, the main water supplies are two springs with recharge areas of 100 and 110 km~2, respectively. Concerning several municipalities, some of them are used as water supply, others for pollution risks and land management projects. The recent French Water Act, which considers water as a common heritage, allows management of water and the terrain in which it flows in a common way, by all users, in the same area. The regulation tools seem very well suited to karst region management. However, time is necessary for educating users and decision-makers, with the help of karst environment scientists, to work together in a frame other than the usual municipal limits.