Biogeochemical effects of high temperature storage on groundwater quality – field monitoring and modelling results

As part of the energy transition, aquifer thermal energy storage (ATES) systems are increasingly used to store thermal energy for heating or cooling purposes in the built environment. In the Netherlands, most ATES systems have a legally defined maximum temperature of 25°C with as justification that this temperature has a limited effect on groundwater. The water quality effects that do occur in this temperature range are mainly caused by the mixing of different types of water (e.g., fresh and brackish or salt water). Currently, there is increasing interest in the application of medium and high-temperature ATES (MT/HT-ATES) with storage temperatures between 25 °C and 90 °C. The advantage of this is that more energy can be stored in the same volume, enabling larger-scale seasonal storage.While various studies have modeled these effects and investigated them experimentally in laboratories (e.g. Luders et al, 2020; Bonte et al., 2013), the actual impact and potential risk on longer time-scales has not yet been investigated much in practice and needs to be better understood. In the Netherlands, only a few MT-ATES (25-60 °C) systems and one HT-ATES (60-90 °C) system are currently operational. In this study, field monitoring results, including geochemical and microbiological analysis of groundwater at two MT/HT-ATES are discussed. At the HT-ATES location, there is particular focus on the effects to the shallower overlying aquifer, as storing heat in the subsurface will not only increase the temperature of the groundwater at the injection depth, but can also affect shallower groundwater in overlying layers by conduction from the hot wells and the hot reservoir. Results for the shallow aquifer above the HT-ATES sites show that the observed effects are actually not directly temperature induced but result from upwards buoyancy flow caused by heat conduction from the hot well. In the storage aquifer at the MT-ATES site, mixing of water types due to pumping is shown to be the main driver of changes in groundwater composition.