Acceptance of Subsurface Heat Storage in Germany—Assessing the Impacts on Groundwater Quality

Subsurface heat storage in terms of building climatisation and storage of excess solar thermal energy has become a meaningful subject to recent energy management concepts. However, for administrative guideline elaboration and their integration into the European Water Framework Directive reliable findings about the influences of heating on ground water quality are required. As shallow subsurface heat storage in Germany is mainly subject to urban areas, transferable knowledge, particularly of the influence of temperature on subsurface redox systems, including contaminants such as LHKW and BTEX, is essential.

Our presentation will quantify changes in main solution constituent concentrations due to a temperature increase, studied in column experiments with aquifer sediment and tap water at 10, 25, 40, and 70°C. Here, no long-term changes were found with heating; effects on pH and major cations found at 70°C were limited to 50 days or less. Further, effects of heating on redox-processes and microbial population will be evaluated. A shift of redox zoning was found in the columns from oxic conditions at 10°C towards nitrate and iron reducing conditions at 25 and 40°C and rudimental sulfate reduction at 70°C, attributed to (a) higher microbial activity and (b) an enhanced C_org release from the sediment at increased temperature. The constant addition of sodium acetate in a second experimental step led to the development of sulfate reduction at all temperatures, with the highest reduction rates at 40°C. Temperature adopted populations established, with higher degradation efficiencies compared to an in situ ground water temperature of 10°C.

The results point out, that redox systems and their associated microbial populations sensitively react on subsurface temperature increase. If and how far these effects can be utilized to actually combine contaminated aquifer remediation with subsurface heat storage in urban areas is subject of our current studies; the results will also be presented.