Contribution to the biogeochemical evaluation of the impacts related to the geothermal exploitation of the near-surface aquifers: Experiments and simulations on a pilot and real installation scale

The use of shallow groundwater as heat source for heat pump is very coveted for air-conditioning of building or industrial facilities. Their intensive use during many years combined with the redeployment of the geothermal industry these last years, raise concerns about the safeguarding of the water resources. In such a context, the present study aims to evaluate the impact of local variations of temperature on the physicochemical and microbiological quality of groundwater systems on the basis of (I) field investigation and, (II) experiments on a pilot (named BIOTHERMEX) making it possible to reproduce, in perfectly supervised conditions, the effect of the propagation of a thermal plume in a reduced aquifer model. Within the temperature recorded on site, principal outcomes showed that thermal impacts are confined in the immediate vicinity of the installation, and were able to deteriorate, up to about ten degrees, the stenothermy of the groundwater systems. The evolution of the physicochemical parameters did not reveal any significant disturbances over the monitoring period, this being also predicted by hydrogeochemical modeling. On the other hand, a significant influence was raised about some microbiological indicators-descriptors (namely activity, diversity of the total microflora). Lastly, the experiments undertaken on the laboratory scale made it possible to finely apprehend the reactional behavior of the system and to define a critical temperature of re-injection beyond which, potential disorders are expected. (author)