A conceptual model for groundwater circulation using isotopes and geochemical tracers coupled with hydrodynamics : a case study of the Lez karst system, France

Geochemical and isotopic tracers (delta O-18, delta H-2, Sr-87/Sr-86 and delta C-13(TDIC)) were used to constrain origins and chemical evolution of groundwater in a Mediterranean karst system. The Lez spring is the main perennial outlet of this karst system and supplies the metropolitan area of Montpellier (southern France) with drinking water. Groundwater samples were collected at the Lez spring and surrounding springs and wells under different hydrodynamic conditions during two hydrological years, from June 2008 until May 2010. The results show that multiple hydrological compartments interact through an important network of fractures and faults. They notably reveal connections between the main Jurassic limestone aquifer and the overlying Cretaceous (Valanginian) compartment, and between the surface and deep levels of the karst system. Isotopic tracers provided information about atmospheric recharge origins, lithological signatures and chemical evolution of waters. Long residence-time groundwaters, issued from deep layers have a Triassic hydrochemical fingerprinting, being enriched in delta C-13(TDIC) and characterized by high concentrations in Cl- as well as high Sr/Ca, Mg/Ca and Sr-87/Sr-86 ratios. Evidences suggest that these waters mix with waters from the lower layers of the main Jurassic aquifer constitute an intermediate storage compartment prone to rise through piston-flow mechanism. A two-end member hydrograph separation based on EC-TDS was used to determine the proportion of the deep compartments contribution to the Lez spring outflow. On average over the study period, the main aquifer compartment and the deep aquifer compartment are estimated to contribute 92.6% and 7.4% of groundwater flow at the Lez spring, respectively