Impacts of Shallow Geothermal Energy Production on Redox Processes and Microbial Communities

Abstract

Shallow geothermal systems are increasingly being used to store or harvest thermal energy for heating or cooling purposes. This technology causes temperature perturbations exceeding the natural variations in aquifers, which may impact groundwater quality. Here, we report the results of laboratory experiments on the effect of temperature variations (5-80 °C) on redox processes and associated microbial communities in anoxic unconsolidated subsurface sediments. Both hydrochemical and microbiological data showed that a temperature increase from 11 °C (in situ) to 25 °C caused a shift from iron-reducing to sulfate-reducing and methanogenic conditions. Bioenergetic calculations could explain this shift. A further temperature increase (>45 °C) resulted in the emergence of a thermophilic microbial community specialized in fermentation and sulfate reduction. Two distinct maxima in sulfate reduction rates, of similar orders of magnitude (5 × 10(-10) M s(-1)), were observed at 40 and 70 °C. Thermophilic sulfate reduction, however, had a higher activation energy (100-160 kJ mol(-1)) than mesophilic sulfate reduction (30-60 kJ mol(-1)), which might be due to a trade-off between enzyme stability and activity with thermostable enzymes being less efficient catalysts that require higher activation energies. These results reveal that while sulfate-reducing functionality can withstand a substantial temperature rise, other key biochemical processes appear more temperature sensitive.

An Estimate of Energy Available via Microbial Sulfate Reduction at a Quaternary Aquifer in Northern Japan considered for Low Temperature Thermal Energy Storage

“…In addition, sulfate reduction can cause anaerobic corrosion of ferrous metals, and to a lesser degree stainless steel, thereby reducing the lifespan of the thermal storage system [10]. In recent studies of the effects of aquifer thermal storage, Jesußek et al [1] and Bonte et al [10] reported a shift from iron reduction to sulfate reduction at 70 °C and 25 °C respectively.…”

“…In recent studies of the effects of aquifer thermal storage, Jesußek et al [1] and Bonte et al [10] reported a shift from iron reduction to sulfate reduction at 70 °C and 25 °C respectively. Both studies employed laboratory experiments.…”

“…Jesußek et al [1] identified that redox zoning shifted from oxic conditions toward nitrate and iron (III) reducing conditions at 25 and 40 °C, and toward sulfate reducing conditions at 70 °C in column experiments with Tertiary lignite sand at 10, 25, 40, and 70 °C. On the other hand, Bonte et al [10] reported that a temperature increase from 11 °C to 25 °C caused a shift from iron-reducing to sulfate-reducing and methanogenic conditions in column experiments at 5, 11, 25, and 60 °C with sediments collected from an unconsolidated sandy aquifer that was deposited during the Early to Middle Pleistocene. The large difference in temperature at which the shift in redox conditions occurred might be associated with the rate of supply of an electron donor for sulfate reduction caused by the difference in sedimentary age or the reactivity of organic matter, i.e., the release of organic carbon from the experimental sediment to solution was small except in the laboratory experiment at 70 °C in Jesußek et al [1].…”

“…The redox reactions are significant geochemical processes that determine the quality of natural groundwater [6,7]. Previous studies on that issue have examined redox chemistry [8][9][10]. Microbial sulfate reduction is a particularly 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].…”

The element-release mechanisms of two pyrite-bearing siliciclastic rocks from the North German Basin at temperatures up to 90 °C under oxic and anoxic conditions

“…Consistent elementrelease patterns were only partially observed, e.g., for silicium and potassium. Adaptations of the microbial communities to temperature changes in both composition and quantity were found in field-monitoring campaigns (Vetter et al 2012;Würdemann et al 2014) and in the laboratory (Bonte et al 2013b).…”