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Le gaz dit de schistes – ou plus justement de roche-mère- est une ressource potentielle, diffuse, largementdistribuée. La mise en exploitation d’un hydrocarbure non conventionnel soulève des questions nouvelles, même si laplupart des techniques sont connues sinon maîtrisées : forage profond, cimentation, développement des discontinuitésnaturelles de la roche pour la chenaliser avant mise en production.Les impacts préoccupants sont au sol : chantiers de forage, conflits d’usages, infrastructure de production, voies d’accès,sécurité des ouvrages vieillissant. Les impacts profonds sont a priori moins à craindre faute d’enjeu, les forages n’y rencontrantque rarement des ressources potentiellement exploitables. Les impacts éventuels sur les eaux souterraines sontplus indirects que directs. Le coût d’ouvrages à une telle profondeur imposera de jumeler exploration et expérimentationsen vraie grandeur.La ressource potentielle a été estimée approximativement par l’administration américaine : la France serait le 2° paysd’Europe le mieux doté, mais elle doit engager sa propre estimation.Le code minier n’est plus adapté pour traiter de ressources non conventionnelles, dans l’ignorance des objectifs de laDCE, de la loi sur l’eau, du code de l’environnement, sans impliquer les parties prenantes. Perçu très tard, ce point aconduit à interdire inconsidérément la fracturation hydraulique, pourtant pratiquée depuis des dizaines d’années, pénalisantdu même coup d‘autres branches professionnelles.Le sujet doit être repensé des points de vue réglementation, communication, partage des décisions, conformité et suividans le temps des ouvrages, validation des techniques.

The 2000-2001 high water levels in the Somme catchment highlited the possible contribution of groundwaters to surficial floods, with considerations which could be extrapolated to a great number of sedimentary geological provinces displaying a plateau landscape, those mostly located in the northern half territory of France. Successively an interministerial Commission, the Regional Directorate for Environment of Picardie, and the General Council of Somme asked Brgm to analyse the underground geo-structures and seepage as part of the fluxes and exchanges which took place during 2000-2001 winter, and thus to create a forecasting tool based on mathematical simulations. A flood is integrating several components, ranking from the rainfall to the river flux, the relative weight of which being affected by the morphology, the surficial hydrology, land uses, as well as seasonal conditions such as frozen soils. One peculiar component is the groundwater outlet which may be fairly significant from a chalk body, such as the general substratum of the Somme watershed. The hydrodynamic rules which apply to seepage, allowed an extension of modelling techniques, commonly used in groundwater resources management, to the seasonal events computation in the Somme region, in order to partly provide an explanation of the groundwater contribution within the water balance. Several models were created, lumped up and deteministic ones. From july 2001 on, a complete rebulting of the geological scheme was performed and data series computed. The first deterministic model was available in december 2001. Both models showed that the successive sub-catchement contributions to the river flow were increasing from up- to downstream. Seasonaly, the total groundwater contribution, close to 100% in low water periods, remains at a significant level, even at flood time : not less than 80%. The isotopie facies gave a confirmation of the relatively high contribution of groundwater into the river flow. The forecasting tool built up in 2001 helps computing the variations of flow at the city of Abbeville. It is operated every fortnight in winter and spring time (from november to may) from 2002 on. The deterministic model is operated once a year, a posteriori, to bring up the local relative contributions of groundwater into stream flow, over the last one year period.

Floods in the Somme district during 2001 were studied with regard to the water chemistry (major and trace elements) and isotopic tracing (oxyg?ne, hydrogene, tritium and strontium). The chemical and isotopic composition of river and ground-waters are constrained by the weathering of the surrounding lithologies and the anthropogenic influences. The coupled using of water chemistry and isotopic tracing allows deciphering the nature of the water fluxes, e.g. runoff and groundwater inputs, during the period of maximum flooding (april 2001) and during another period of lower river discharge (october 2001). The use of isotopic systematics strengthens and gives new insight to the hydrodynamic analysis of groundwater induced flood processes.

In the last two decades, a growing number of actors in charge of water resources took steps to improve their management. The purpose was generally twofold : (1) to insure preservation of the resource through space and time, and (2) to optimize exploitation processes. Several methods have been developed and applied in France and abroad. Their concept, their dimension and their technical structure vary a lot. To be fully efficient, they must be adapted to the local context and be correctly scaled to the problem at stake. This article briefly describes three of these methods built on different concept, with increasing complexity. They were implemented within the framework of an environmento-economical optimization approach, in three different contexts in terms of size, and both structural and operational characteristics. These resulted with positive impacts on the environment and on the social context, and sound balances between the different water uses could be achieved. Associated short, middle and long term economic benefits are manifold and variable. They depend among other things, (1) on the starting conditions, (2) on the characteristics of the context, and (3) on the criteria and methods used to compute environmental and exploitation values. Generally, however, economic benefits are important, and sometimes even, very important.