Présentations Colloques

Urban subsoil provides space, groundwater, geothermal energy and geomaterials, and the importance underlying the management of this strategic space is well-known. In the context of vertical urban growth, groundwater management, is a crucial issue. First, the aim of this talk is to synthesize the state of the art dealing with the impact of underground structures (US) on groundwater flow. Secondly, feedback on deterministic approaches to the interaction between US and urban groundwater will be presented in order to propose recommendations dedicated to urban planners. Understanding of the role played by numerous types of US is improved at the decision-making scale, resulting in a methodology to define the potential state of an urban groundwater flow. The influences of several kinds of US are assessed. US involving pumping devices are shown to be responsible for the fragmentation of flow systems, which can modify the urban groundwater regime. Under low gradient conditions, impervious structures are confirmed to have a nonsignificant effect on water table elevation. Regarding quality, groundwater age theory is applied to assess the influence of US, integrating the advective and dispersive part of groundwater flow. A dispersive piston effect is shown to occur under the US, favoring the vertical spreading of pollutants via a mixing process between shallow and deep groundwater. Consequently, a ‘dispersive trap’ under US is described. In addition, the impacts of US on groundwater temperature and, by consequence, on the geothermal potential of urban groundwater bodies are investigated. The annual heat from US is computed deterministically, highlighting the significant thermal affected zones. On the one hand, zones subject to thermal impacts caused by US should be taken into account to avoid conflicts of use with groundwater heat pumps. On the other hand, the relevance of extracting this energy from the zone affected is shown. Finally, the impacts of US on groundwater flow quality and temperatures and on urban groundwater management are investigated to prevent risks and disturbances- flood risk, degradation of groundwater quality, jeopardizing geothermal potential.