AbstractHydrogeochemical processes associated with the potential seasonal storage of 90°C hot water in a Lower Jurassic aquifer (Lower Sinemurian/ Hettangian stages) in the city of Berlin, Germany, are characterized and evaluated to determine possible sources of mineral precipitation resulting in aquifer damage (clogging). Laboratory leaching tests with material from the sandstone aquifer and the pelitic hanging aquiclude obtained from the wellbore “Am Reichstag 2/98” were conducted over a period of 28 days under anoxic conditions. A hydrogeochemical batch reaction simulation of the leaching test was set up with the commercial software PHREEQC and matched to the experimental results. Laboratory experiments show a strong pH decrease and sulfur mobilization as well as precipitation of reddish-brown iron hydroxides. This is most likely the consequence of pyrite oxidation. PHREEQC simulations can reproduce acidification and hematite precipitation if a minor diffusion of oxygen into the system is assumed.