When heat meets water: Environmental trade-offs of underground thermal energy storage

The transition to net-zero energy systems has sparked increased interest in integrating renewable energy and implementing large-scale storage. Underground thermal energy storage (UTES) offers a promising approach, enabling the seasonal storage of heat within geological formations. However, its direct interaction with groundwater introduces environmental challenges, particularly related to thermal, hydrogeochemical, and microbiological processes. This review examines the environmental implications of UTES, with a focus on groundwater quality. Key parameters influencing water chemistry, such as pH, dissolved oxygen, major ions, trace elements, and microbial activity, are discussed as indicators of system impact. The review compares different UTES configurations, including aquifer thermal energy storage (ATES), borehole thermal energy storage (BTES), cavern thermal energy storage (CTES), and systems in flooded abandoned mines, and evaluates the effects of temperature shifts and water mixing on subsurface conditions. By consolidating current knowledge and identifying tiered monitoring strategies, this paper highlights the need for long-term assessment and mitigation frameworks to ensure that UTES can be deployed sustainably while protecting groundwater resources.