Groundwater pollution potential of additives used in borehole heat exchanger fluids

Ground-based heat exchanger systems need to be evaluated in terms of potential effects on groundwater quality due to the risk of leakage of borehole heat exchanger fluids. They contain glycols or inorganic brines as major components, but typically also various additives. As the identity of these additives is proprietary information, the major objective of this study was to obtain an overview of the identities and properties of individual additive compounds used in borehole heat exchanger fluids. Leading companies in the production and distribution of these fluids were invited to participate in a confidential data survey. A wide variety of additive compounds were assembled, which can be grouped into ten chemical classes: triazoles, carboxylic acids/carbonates, phosphonates, alcohols, aldehydes, ethers borates, silicates, nitrates and hydroxides. Among these substances some are also used in other applications, e.g. aircraft de-icing fluids and engine antifreeze formulations, and are of potential concern for groundwater quality. Based on their abundance in the investigated borehole heat exchangers and their concerning environmental behaviour in the subsurface, benzotriazoles, tolyltriazoles and some carbonates were selected for further biodegradation experiments.