Effects of thermal energy storage on shallow aerobic aquifer systems: temporary increase in abundance and activity of sulfate-reducing and sulfur-oxidizing bacteria

Aquifer thermal energy storage may result in increases in the groundwater temperature up to 70 degrees C and more. This may lead to geochemical and microbiological alterations in the aquifer. To study the temperature effects on the indigenous microbial community composition, sediment column experiments at four different temperatures were carried out and the effluents were characterized geochemically and microbiologically. After an equilibrium phase at groundwater temperature of 10 degrees C for 136 days, one column was kept at 10 degrees C as a reference and the others were heated to 25, 40 and 70 degrees C. Genetic fingerprinting and quantitative PCR revealed a change in the bacterial community composition and abundance due to the temperature increase. While at 25 degrees C only slight changes in geochemical composition and gene copy numbers for bacteria were observed, increasing concentrations of total organic carbon in the 40 degrees C column were followed by a strong increase in bacterial abundance. Thermophilic bacteria became dominant at 70 degrees C. Temporary sulfate reduction took place at 40 and 70 degrees C and this correlated with an increased abundance of sulfate-reducing bacteria ( SRB). Furthermore, a coexistence of SRB and sulfur-oxidizing bacteria ( SOB) at all temperatures indicated an interaction of these physiological groups in the sediments. The results show that increased temperatures led to significant shifts in the microbial community composition due to the altered availability of electron donors and acceptors. The interplay of SRB and SOB in sedimentary biofilms facilitated closed sulfur cycling and diminished harmful sulfur species.