Groundwater circulation and mixing inferred from age dating with dissolved gas tracers in a complex Mediterranean karstic and thermal aquifer (Thau lagoon area, Montpellier, France).

Groundwater in coastal area is a strategic but fragile resource since it undergoes high anthropogenic pressure that can lead to saltwater intrusion. Therefore the use of coastal groundwater needs a thorough understanding of the groundwater flow and mixing to assure a suitable management of the resource.

The coastal and thermal karstic hydrosystem of the Thau basin (South of France) shows a good example of the pressure that can undergoes coastal groundwater as it is a strategic resource for drinking water, spa activities as well as shellfish aquaculture. In this aquifer, groundwater originates from 3 different naturally sources with contrasted mean residence time (MRT): 1) cold and young karstic water, 2) warm and mineralized thermal waters with long MRT, and 3) marine waters (Thau lagoon and/or seawater). In this context, age dating tracers can be valuable tools for the characterization of the groundwater flow circulations, the estimation of their residence time but also of the mixing which can affect the thermal system.

We used dissolved gases (CFCs and SF6) and ³H age dating tracers to characterize the young end-member, as these tracers are particularly suitable for identifying and quantifying water mixing of different ages (Newman et al., 2010). Strategic locations representative of each component of the system (surficial and deep karst, springs and thermal boreholes) were sampled during different hydrogeological contexts (high flow/base flow).

The first results show that as expected, in general, the thermal component has a very low level of dissolved gas indicating long MRT and few mixing whereas karstic springs show high contents of dissolved gas. However, some thermal wells show important and variable gas content indicating mixing with the karstic component and rapid circulation in some parts of the system. These data will contribute to determine the groundwater transfer model(s) in the Thau system and to estimate the contribution of the current karst water to the Balaruc thermal system. These results will in turn, be used within the framework of the “Dem’Eaux Thau project” to develop tools for groundwater resources management allowing decision-makers to take on the challenges of this region.