Unexpected δ18O and δ2H variability of groundwater in the eastern Paris Basin

Abstract : The Paris Basin covers about one-third of the total surface area of France. In 2009 two campaigns of sampling 25 boreholes tapping Tertiary aquifers were carried out in the Basin. These aquifers are recharged at a similar altitude and the groundwater age is too young to have registered climate change. In the past, regional studies included the use of isotopes to understand groundwater origin and dynamics. Both δ18O and δ2H as well as ages (CFC/SF6) and chemical components were determined in all collected samples. A noticeable stable-isotope “anomaly” appears in the south-western part of the Basin, where the average δ18O and δ2H values are more depleted and do not fit the pattern given by the Continental effect in this area. A regional particularity of the spatial distribution of such isotopes in precipitation may be possible, but should be confirmed by additional work.