Aquifer heat storage: sulphate reduction with acetate at increased temperatures

Aquifer thermal energy storage in urban and industrial areas can lead to an increase in subsurface temperature to 70 A degrees C and more. Besides its impacts on mineral and sorption equilibria and chemical reaction kinetics in an aquifer, temperature sensitively influences microbial activity and thus redox processes, such as sulphate reduction. Microorganism species can only operate within limited temperature ranges and their adaptability to temperature is a crucial point for the assessment of the environmental consequences of subsurface heat storage. Column experiments with aquifer sediment and tap water at 10, 25, 40, and 70 A degrees C showed that under the constant addition of acetate sulphate reduction could be initiated after 26-63 pore volumes exchanged at all temperatures. Fastest initiation of sulphate reduction with the highest reduction rates was found at 40 A degrees C. Maximum rate constants during experimental run-time were 0.56 h(-1) at 40 A degrees C and 0.33, 0.36, and 0.25 h(-1) at 10 and, 25, and 70 A degrees C, respectively. Hence, microbial activity was enhanced by a temperature increase to 40 A degrees C but was significantly lowered at 70 A degrees C. At 25 A degrees C methane was found in solution, indicating the presence of fermenting organisms; at 10, 40, and 70 A degrees C no methane production was observed. It could be shown that redox processes in an aquifer generally can adapt to temperatures significantly higher than in situ temperature and that the efficiency of the reduction process can be enhanced by temperature increase to a certain limit. Enhancement of sulphate reduction in an aquifer due to temperature increase could also allow enhanced degradation of organic ground water contaminants such as BTEX, where sulphate is an important electron acceptor.