Présentations Colloques

Oral Presentation
8.04
Session 8.04: Isotopic and residence time tracers
Vincent Aude
What is the impact of land use change on the behaviour of shallow aquifers ? A comparison of particle-tracking and lumped-parameter models for evaluating the groundwater age distributions
The Crau aquifer in the Mediterranean irrigation area (France) is a major resource for drinkable water supply. This aquifer, mainly recharged by gravity irrigation, is today threatened by environmental changes (expected changes in the irrigation pattern, urban expansion, increasing groundwater demand, climate changes). In order to evaluate the actual impact of land use changes on this resource, the groundwater behavior must be well defined. As irrigation water is clearly depleted in 18O and 2H compared to local rainfall, we propose to use the isotopic approach (18O 2H) associated to groundwater modeling to quantify (i) the respective influence of the different input variables contribution and (ii) the distribution of residence times of groundwater.**A monthly sampling of groundwater and recharge water has been carried out for four years, for chemical and isotopic analyses. First isotopic results confirmed that recharge resulted mostly from irrigation (80-85% under meadow) but variable in space and time. A lumped parameter model (exponential transfer function) applied to one observation well showed a mean residence time of three months. This approach will be applied to other wells in relation with land cover (irrigated area, steppe, urban area) in order to highlight the impact of land cover on residence time distribution. Baillieux et al. (2015) developed a 3D groundwater flow model of the Crau aquifer. To complete and validate the lumped approach, the distributed model will be used to run particle tracks to create a transit time distribution. Measured tracer concentrations will be used to validate or refine this model.**These first results open up new perspectives in the use of environmental tracers in highly transient hydrosystems highlihting the interest of coupling isotope monitoring and conceptual or numerical modeling.**Baillieux A. et al., (2015) Géologues 187, 85-92.
France