Modelling of Thermal Use of Shallow Groundwater in an Urban Aquifer: a Case Study in Nice

The Var low valley and its unconfined alluvial aquifer is the main water resource for around 600,000 inhabitants who live in the French cities and towns near the river mouth, such as Nice and St Laurent du Var. Several commercial and residential complexes are planned to be built in the downstream floodplain areas of the Var River. This new city is one of the biggest development plans in France intending to host more than 40,000 inhabitants. As part of its legal commitment to reducing CO2 emissions, the city has outlined a roadmap for significant increases in the use of ground source energy for heating and cooling buildings. This paper presents an aquifer scale modeling approach to study the impacts of geothermal activities planned for several large commercial buildings in the districts of Meridia and Grand Meridia. Due to its short distance to the pumping stations used for water supply, therefore, it is very important to study the impacts of these projects on the groundwater and consequently on the supply of drinking water. The groundwater flow was simulated using MIKE FEFLOW numerical model for the entire aquifer, which covers an area of 146.45 km(2), with a river length of 22.4 km. Vertically, the model contains the layers of recent alluvium, alluvial terraces. Pliocene conglomerate, Pliocene marls, impermeable layer from Miocene to Cretaceous, and the Jurassic limestone. The model was able to simulate the hydraulic head with good precision (the Nash Sutcliffe Efficiency coefficient of 0.7 and higher). The model was then used to estimate the changes in the groundwater table under different scenarios for the planned geothermal projects for thirty years period. For the same period, a map of thermally affected areas was also developed to check the sustainability of these projects. There were several proposals of reinjection sites, and the best solutions were ranked based on the simulation results to minimize the impacts on the nearby pumping stations. The model can be used to study the overall impacts on the groundwater table for similar projects in the future and to assess development plans on a larger scale rather than to focus on each project separately. Therefore, better management of groundwater resources will be achieved.