Of the importance of d-excess in understanding recharge process in two case studies of France

Stable isotopes of water (δ 2 H and δ 18 O) are classical tools for determining hydrogeological parameters ea. recharge area, flow directions, groundwater-surface water exchanges,… (Mook et al. 2008). However the successful use of the isotope techniques depends on local or regional hydrogeological characteristics. Meteorological and hydrological conditions leading to contrasting isotope composition of the water bodies are required. The relation between these two elements in precipitation from various part of the world is δ 2 H = 8xδ 18 O + 10. The so called Global Meteoric Water Line (GMWL) is characterised by a slope of 8 and an intercept with the 2 H axis of 10‰. This intercept is more generally called d-excess and may vary from 10 depending on the humidity and the temperature of the evaporation region. Then depending on the existing conditions during the formation of precipitation (=evaporation) and mostly at the site of sea-air interaction the d-excess may vary. The d-excess is then highly variable from one precipitation station of another and may reflect various so-called "effect" (continental, seasonal, altitude effects) as well as other processes such as re-evaporation of rain droplets… In two recent studies carried out in south of France and in the Martinique French overseas the d-excess has proven to be a valuable tool giving additional and more evident information than the stand alone δ