Abstract
The energy available via microbial sulfate reduction was estimated for a Quaternary aquifer in northern Japan that is a candidate site for low temperature aquifer thermal energy storage. In evaluating whether microbial sulfate reduction proceeded or ceased, it was assumed that electron donor/acceptor concentrations were unchanged by temperature increase. The estimated energy availability via microbial sulfate reduction at 9 °C with no thermal disturbance was 37, 51, and 53 kJ·(mol SO42−)−1. The low estimate of 37 kJ·(mol SO42−)−1 was attributed to low concentration of SO42−. Excluding the sampling site with low concentration of electron acceptors, energy availability was estimated 52 and 54 kJ·(mol SO42−)−1 at 20 °C; 54 and 57 kJ·(mol SO42−)−1 at 40 °C; and 57 and 59 kJ·(mol SO42−)−1 at 60 °C. These results indicate that possible energy availability via microbial sulfate reduction at approximately 40–60 °C exceeded the range of available energy (compiled from previous studies) at which sulfate reduction would cease. Thus, microbial sulfate reduction at this site may proceed at approximately 40–60 °C.
Highlights
Aquifer thermal energy storage, which is the subsurface storage of excess solar thermal energy and exhaust heat from air conditioners, is an effective way to bridge hot summer months [1]
These results indicate that possible energy availability via microbial sulfate reduction at approximately 40–60 °C exceeded the range of available energy at which sulfate reduction would cease
The study examines whether microbial sulfate reduction is likely to proceed or cease at a candidate site for low temperature aquifer thermal storage
Summary
Aquifer thermal energy storage, which is the subsurface storage of excess solar thermal energy and exhaust heat from air conditioners, is an effective way to bridge hot summer months [1]. Groundwater contained within an aquifer is an important resource for the world’s agricultural, industrial, and drinking-water requirements [2]. Thermal energy storage may often be implemented in the same aquifers that supply drinking water. The preservation of groundwater requires an understanding of the quality of groundwater [3]. The effects of aquifer thermal energy storage on groundwater quality must be identified [4,5]. The redox reactions are significant geochemical processes that determine the quality of natural groundwater [6,7]. Microbial sulfate reduction is a important redox reaction (see Equation (1), below), as it produces hydrogen sulfide—a highly toxic compound that can form in any aqueous system which contains both organic matter and sulfate [11]
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