(Bio)geochemical reactions in aquifer material from a thermal energy storage site

The mobilization of organic compounds and the release of CO 2 was studied in aquifer material from a site chosen for thermal energy storage (ETS). These processes have been measured aerobically and anaerobically within a temperature range of 4–95°C in sediment samples consisting of either quartz-rich coarse sand or peaty clay. At temperatures above 45°C organic carbon compounds, including fulvic acids, were mobilized from both sediments resulting in an increased chemical oxygen demand of the water phase. Complexation of calcium and magnesium by fulvic acids resulted in the supersaturation of the water phase with regard to calcite and dolomite and thus prevented the precipitation of these carbonates. The highest rates of CO 2 release were observed during the first 4 days. Aerobically, the maximum velocity for CO 2 formation varied between 35 and 800 (sand) or 15 and 150 (peaty clay) μmol CO 2 per gram volatile solids per day. Anaerobically, similar rates were observed, namely 25–500 (sand) and 10–110 (peaty clay) μmol CO 2 per gram volatile solids. At temperatures above 55°C, CO 2 was produced purely chemically.