Geological and hydrogeological characterisation of a new source of water for Grand Belfort

The need

Following a preliminary review of the available literature by BRGM, the Jurassic limestone aquifer was identified as a potential water resource. AGB thus asked BRGM to characterise the aquifer and its potential in more detail.

Results

In order to delineate aquifers and identify geological structures conducive to the circulation of groundwater, a geological map of the limestone was made. 

The geological study revealed the presence of outcropping limestone and underlain limestone, forming a vast, flat-bottomed syncline. These limestone formations have faults that allow surface water to infiltrate when they are not covered by impermeable ground. The weathering/dissolution of the rock, which forms decimetre- to metre-sized cavities, bears witness to this seepage and the probable presence of karst formations.

At the same time, a hydrogeological study of the outcropping limestone, based on field measurements (flow and conductivity measurements of springs and watercourses), confirmed the results of the geological investigations. The measurements taken indicate areas where river flow is lost near fault networks, and high variability in flow between low and high water, which is characteristic of karst contexts.

The groundwater in the outcropping limestone is sensitive to weather variations and surface pollution, making it less sustainable in terms of both quantity and quality. It therefore seems less suitable than underlain limestone, where the groundwater is potentially more resilient to drought episodes and better protected against surface pollution. The results of this study therefore tend to favour underlain limestone for the ongoing programme to search for new water resources.