Implementing a geochemical, hydrogeological and microbiological monitoring strategy for an open-loop low-temperature geothermal system in a shallow coastal aquifer

Open-loop Ground Water Heat Exchangers (GWHEs) coupled with heat pumps are a common type of domestic and industrial air conditioning system. These systems consist of extracting groundwater which is used to exchange heat with the geothermal heat pump, before being usually re-injected into the aquifer. This operation can affect, albeit usually mildly, the thermal and chemical features of the groundwater and the local ecology. The impacts vary depending on the physico-chemical characteristics of the water, local geology, hydraulic conductivity, and the water volumes extracted and discharged by the GWHE. The purpose of this study is to implement a monitoring plan for an open-loop low-temperature geothermal system located in the Fano Municipality (Central Italy). The urban area lies on the Metauro river alluvial plain, which mostly consists of gravel, sand and silty-clay, with intercalated lenses of sand and clay. These deposits host a phreatic aquifer that can reach a maximum thickness of about 40 m, and whose temperature is generally comprised between 15 and 17°C. Water sampling will be carried out for at least one hydrological year on the injection and extraction wells of the geothermal system and on two wells located up- and down-hydrogeological flow of the selected GWHE. Changes in pressure and groundwater temperature may cause variations of solubility of minerals, redox processes, and sorption-desorption of dissolved components, besides the formation of carbonate and silica mineral scaling. These possible alterations of the natural state of the aquifer will likely be constrained through a monthly-based monitoring that will be aim at the evaluations of the: i) main physico-chemical parameters (i.e., temperature, pH, electrical conductivity, redox potential, and dissolved oxygen content), ii) piezometric levels, and iii) main dissolved ions concentrations (i.e., HCO3, SO4, Cl, NO3, Br, Ca, Mg, Na, K). These measurements will be coupled by bimonthly analysis of selected trace elements concentrations (e.g., Al, Cd, Fe, Mn, Si). In addition, as the modifications of the thermal state of the groundwater might enhance the proliferation of microbial community such as the mesophilic bacteria which start to appear at about 20°C, the dynamics within the aquifer in terms of microbiology will be also evaluated. This study will help to understand possible criticisms for the installation of GWHEs and will provide a support for the definition of a regulatory framework, often lacking at local level to enhance a sustainable use and management of this type of renewable in a fragile environment like that of the groundwaters.