Impacts of Temperature Variations on Geochemical Processes in an HT-ATES System: A Field Site Assessment

Aquifer Thermal Energy Storage (ATES) is an effective solution to store thermal energy but its climate protection potential remains largely unused. Concern about negative changes in groundwater chemistry is often cited as an obstacle especially for HT-ATES (high-temperature ATES), although the underlying results are almost exclusively based on laboratory tests. Increasing temperature in this context up to 80 °C can impact subsurface processes like carbonate precipitation, silicate dissolution, trace element mobility, release of DOC, redox processes and microbial activity including biodegradation. However, to date there is only one HT-ATES field test in which the geochemical impacts on groundwater quality was investigated in detail showing that the geochemical changes were relatively small compared to the variability of the baseline monitoring and were smaller than in associated laboratory tests. However, it is still unclear whether or to what extent these results can also be representative for other sites. In order to expand the basis for assessing hydrochemical impacts, e.g., also for deriving regulatory measures, it is therefore necessary to carry out similar field tests at different sites.

Therefore a HT-ATES field test system was performed in a near-surface quaternary aquifer contaminated with chlorinated hydrocarbons (CHCs < 3 mg/l) in Leipzig, Germany consisting of coarse sands, and gravels and is overlain by a glacial till. The field test system includes 13 monitoring wells, as well as a cold, a warm, and a control well with an injection rate of 0.6 m³/h targeting the horizon at 11-14 mbgs with a temperature of ˜70°C. Before being injected, water was treated using activated carbon filters, zeolites, water softening, and deferrization. Samples were collected during 3 baseline measurements, 3 injection, 3 extraction phases, an interim and a post-operational phase. Samples were analyzed for major ions, trace elements, total inorganic carbon, non-purgeable organic carbon and NH⁺₄. Temperature in the aquifer was 12-62°C during operation.

The concentrations of almost all trace elements during the warm water injection remained within the baseline concentration range. Arsenic, which was below the detection limit at all measuring points during baseline monitoring, increased during the warm water injection to still low values ​​between 2 and 5 µg/L exclusively at one measuring point with the highest temperatures (45-62 °C). The arsenic concentrations decreased from 5 to 2 µg/L between the first and the third warm water injection suggesting that only a very small amount of arsenic can be eluted. Nickel concentrations decreased by up to 30% during warming. Al, Pb, Cd, Cr, Co, Cu, Mo, Se, Tl, V, and Sn stayed below the detection limit and all other elements remained below the no effect levels (GFS/LAWA).

Mobilisation of CHCs from the sediment as a result of the temperature increase was not observed. Instead, dissolved CHC concentrations decreased by up to 90 %. This study reinforces findings from previous research that such temperature variations do not cause critical hydrochemical effects on groundwater quality.

Acknowledgements: This study is part of the KONATES project funded by the German Federal Ministry of Education and Research (03G0916B).