Datação de águas subterrâneas por CFC e SF6

Responsible entity

The CONDATE-EAU Platform is the entity responsible for this innovative management practice. It is a platform of the University of Rennes 1 that offers services in hydrogeology and residence time estimation to communities, engineering agencies, and other academic partners.

Institutional setting

Since 2000, the Water Framework Directive (WFD) has set ambitious objectives in terms of restoring the quality of water resources (whether they are intended for drinking water supply or not). Within the framework of this directive, the member states of the European Union must act in particular to protect their drinking water catchments in order to reduce the treatments applied to the water extracted and to fight against the deterioration of the quality of the resource. The water bodies used for water catchments for human consumption (or which may be used in the future) are listed as protected areas. In France, the law on water and aquatic environments (LEMA, n°2006-1772, article 21) and the decree of 14 May 2007 (n°2007-888) have reinforced the existing regulatory tools. These texts have made it possible to use the “Environmentally Restricted Areas” (ERA) system on catchments. This device can be used on the scale of the catchment area (AAC is the French term) presenting a particular challenge for the current or future supply of drinking water (quantitative and qualitative protection of drinking water catchments). Subsequently, the Grenelle Environment Forum confirmed the importance of the protection of water catchments intended for drinking water supply. The implementation of the Grenelle conclusions (article 27 of the law n°2009-967 of August 3, 2009) thus provides for the protection of a little more than 500 catchments among those most threatened by diffuse pollution as of 2012.

Geographical setting

The practice is carried out on all types of aquifers on an international scale. Several examples of the use of this practice have been carried out in France, Brazil, India, Quebec, etc. There are about 30 projects per year on average of groundwater dating by CFC and SF6, of which 15% are carried out on an international scale.

Detailed explanation

The estimation of the mean turnover time of groundwater is a geochemical analysis methodology that allows the dating of groundwater from CFC and SF6 gases. The principle is that these tracers tell us the time it took for the water to travel between its point of infiltration and its point of withdrawal. Indeed, once the gases are dissolved in the water table, they isolate themselves and keep their atmospheric signature.

However, a catchment is fed by a multitude of water drops that have different ages, so we can only estimate the average renewal time of the water table as a whole, we thus speak of average age or apparent age.
The understanding of aquifer functioning is thus achieved via circulation models observed in the environment, which represent either open aquifers, confined aquifers, or a mixture of both. The average age of the water table is then interpreted on the basis of 3 simple hydrogeological models: (i) piston, (ii) continuous recharge, and (iii) binary mixing.
For the piston model, all water lines have the same age and therefore all tracers are consistent. The recharge zone is then localized and isolated during the groundwater flow.

For the exponential or continuous recharge model: the recharge is carried out on the whole catchment area at a given point, i.e. a mixture of water of all ages, the distribution thus corresponds to an exponential distribution. The time obtained is the average age for which 2/3 of the water table has been renewed.
For the binary mixing model, it is a mixture between two distinct and restricted recharge zone water masses in its simplest form. It can be infinitely complexified.