Around 90 % of Berlin's heat supply is based on fossil fuels, accounting for more than 40 % of the city's total CO₂ emissions. A central challenge of decarbonization lies in the temporal mismatch between heat supply and demand. Surplus heat is mainly available during the summer months, whereas demand is particularly high during the cold season. High-Temperature Aquifer Thermal Energy Storage (HT-ATES) provides a seasonal storage technology capable of storing large amounts of heat in the subsurface while requiring only little space on the surface. This makes HT-ATES particularly well suited to be a part of the heat transition in urban areas. Several HT-ATES pilot and test projects have been carried out since the 1970s, but most were abandoned – either because suitable heat sources were missing, integration into district heating networks proved uneconomic, or the underlying geochemical processes were not sufficiently understood. Heating the formation water to typically 40–95 °C shifts the natural chemical equilibrium between fluid and aquifer and can trigger partly irreversible reactions that affect both storage operation and environmental impact. These include efficiency losses from scaling and corrosion, permeability losses from pore clogging, and the mobilization of trace elements that may impact water quality. Implementing HT-ATES therefore requires reliable baseline data, obtained through comprehensive geochemical characterization of the reservoir. This is essential for ensuring long-term stable operation, preventing lasting damage to the aquifer, and identifying potential risks at an early stage. The objective of this work is to develop and apply methods for the geochemical characterization of potential HT-ATES formations in the North German Basin (NGB) in order to enable a well-founded risk assessment and early-stage site evaluation. On-site analytics, batch experiments under HT-ATES conditions, and geochemical equilibrium modeling were developed and applied at two Berlin sites: Jurassic sandstones (221–400 m bgl) and the Triassic Schaumkalk (514–558 m bgl). In the methodological part of this work, a closed-loop fluid monitoring system (FluMoMax) was developed for both representative sampling and continuous monitoring during operation. It records pH, redox potential, electrical conductivity, dissolved oxygen, temperature, turbidity, and density at temperatures up to 120 °C and pressure up to 10 bar. In addition, a built-in sighting tube indicates whether scale is forming. This allows the progress of well development and the optimal sampling time to be determined, and changes in fluid chemistry to be tracked in real time. In addition, a handheld XRF spectrometer (hXRF) was calibrated using a raw-data-based modeling approach. The method was successfully applied on-site for porosity determination on sandstone cores (adj. R² = 0.976 dry; R² = 0.82–0.94 moist) and for quantifying scaling-relevant ions (Cl, K, Ca, SO₄, Sr) in saline formation waters of up to 266 g L-1 TDS (R² = 0.80–0.99). FluMoMax and hXRF reduce analytical processing time from days or weeks to minutes or hours and enable geochemical risk assessment directly at the drill site. The hXRF was also used to characterize 200 m of drill core and cuttings from two HT-ATES exploration wells in the Jurassic sandstone. For the five aquifers encountered, an element-based, semi-quantitative assessment was carried out covering reactive phases (carbonates, pyrite, reactive iron), heterogeneity (grain-size proxies Ti, Rb, Al₂O₃), and permeability (Cl). The first Jurassic sandstone below the Rupelian Clay (221–234 m) has the lowest content of reactive phases but is limited to a thickness of 13 m. The fourth aquifer (359–400 m) is favored as the HT-ATES target horizon. Despite its heterogeneity, it combines a substantial thickness with low contents of reactive phases. Anoxic batch experiments were conducted to assess how HT-ATES operation might affect rock–water interactions. Two sandstone samples from the favored Hettangian aquifer were exposed to 0.5 mol L⁻¹ NaCl solution at 30, 45, 60, and 80 °C for reaction times of 1 to 10 days. Three temperature-induced processes were observed: reductive Fe-hydroxide dissolution, endothermic Fe sorption, and incongruent silicate dissolution. Ca, Mg, Si, Sr, Ba, B, and Li accumulated in the fluid with increasing temperature and reaction time, while Fe – and to a lesser extent Mn – decreased markedly after an initial rise, attributed to endothermic sorption onto mineral surfaces. At 80 °C, up to 27% of the total Ca was leached within 10 days into the carbonate-undersaturated fluid, likely originating from structural Ca in Fe-hydroxides, adsorbed Ca, and finely dispersed carbonates. Given the low CaO content of the aquifer material (< 0.1 wt %), this mobilizable pool may gradually become depleted over successive HT-ATES cycles, leaving less dissolved calcium available for carbonate precipitation. The fractured Rüdersdorf Schaumkalk (> 96 % calcite) in Berlin is the first deep carbonate aquifer in the NGB to be hydrochemically characterized for HT-ATES application. The saline, Na-Cl-dominated formation water (130 g/L, pH 6–7, 3 % gas content) is of marine origin and contains both sulfate-reducing bacteria and methanogenic archaea. Geochemical modeling of ten HT-ATES cycles shows pronounced calcite precipitation in the heat exchanger (up to 31 mg/kgw), as well as a significant dissolution potential in reservoir zones with calcite-undersaturated fluid (up to 21 mg/kgw). HT-ATES in siliciclastic sandstones already benefits from initial international operating experience, with research now focusing on water treatment, monitoring, and long-term behavior. For carbonate aquifers, however, fundamental design and operating concepts still need to be developed. The Jurassic sandstones studied here will serve as the storage horizon at the GeoSpeicher Berlin project. Drilling is scheduled to start in 2026, and at a storage temperature of 95 °C, the site will be one of Europe's hottest HT-ATES systems – demonstrating the potential of seasonal heat storage for Berlin's heat transition.
