Estimating Groundwater time residence at drinking water abstraction wells for a better protection of water ressources : a case study in Rhône-Mediterranean Basin

Abstract : One of the main objectives of the water framework directive (WFD 2000/60/CE) is to reach good chemical status for groundwater bodies by 2015. The Water Directive also imposed to Member States to study pollutants trends and to plan a programme of measures for improving water quality. Transfer time of diffuse pollutants from soils to aquifer may be quite long (several years), an early detection of groundwater quality decrease is therefore necessary in order to set up protection measures in due time. At the opposite, when water renewal times are short, water quality at abstraction wells can change quickly after setting up the programme of measures. The apparent age data is then important for evaluating the efficiency of the action plans implemented in the drinking water protection area. Geochemical dating tools used for young water (superior to 1950), CFC and SF6, allowed, in various hydrogeological contexts, to improve knowledge on water and solutes transfer modes. Estimating time of the renewal of the groundwater is also an interesting tool for water manager and more particularly to restoring water quality in the Grenelle Environment framework. After a short presentation of the age dating tools, the use of these gas tracers for understanding time evolution of nitrate and pesticides concentrations at groundwater wells in protecting areas where programme of measures have been implemented since some years is described for some specific case studies of drinking water -wells of the Rhône-Mediterranean basin.