Impacts of borehole heat exchangers (BHEs) on groundwater quality: the role of heat-carrier fluid and borehole grouting

This study investigated the role of two technical features of borehole heat exchangers (BHEs) in causing impacts on groundwater quality: the use of propylene glycol (PG) and other additives in heat-carrier fluids and the BHE grouting. The PG is the most diffused antifreeze compound used in heat-carrier fluids circulating into the ground loops. Laboratory-scale experiments were performed to simulate the release of PG into soil–water microcosms. PG itself is not toxic, but it is a potential organic nutrient able to stimulate the micro-organisms’ metabolism, thus resulting in O2 consumption and subsequent decrease in redox potential of the system. Indeed, a leaching of the main heavy metals was observed: Fe, Mn and Ni ions reached 102–104 μg l−1 concentrations after 60 days. Secondly, the role of BHE grouting in changing the hydraulic conditions of aquifers was investigated because defects and voids in the grout may increase the hydraulic conductivity along the boreholes, leading to inter-aquifer flux. A field-scale experiment was performed by grouting a 7-m-long pilot BHE, and many defects and void spaces were detected along the BHE. A simplified analytical model was thus applied to detect the magnitude of the inter-aquifer flux. Basing the calculations on typical hydraulic parameters of the alluvial aquifer of Piedmont Po Plain (NW Italy), the downward flux is up to 2.65 cubic metres per day in the worst scenarios with high head difference and thin separation layer between two superposed aquifers.