Modelling leakage and groundwater pollution in a hypothetical CO2 sequestration project

The hypothetical injection of CO2 into the ≥1250 m deep Triassic Volpriehausen sandstone (smV1) at Birkholz-Beeskow, Germany, is simulated in a TOUGH2-MP flow model with 510,607 elements. A hypothetical leakage zone connects the Triassic smV1 horizon with a Miocene potable-water aquifer (tmi). The leakage zone has a transmissibility (product of width and permeability) of 5 × 10−14 m3 to 5 × 10−11 m3. Carbon dioxide rises up to the tmi horizon if the transmissibility of the leakage zone is ≥1.7 × 10−13 m3. The output of the TOUGH2-MP flow model is used for defining flow conditions for a TOUGHREACT reactive-transport model with 28,409 elements. The models does not only consider the chemical effects produced by intruding CO2 but also the intruding aqueous liquid phase, which accompanies the rise of CO2. The concentrations of hazardous As and Se in the tmi groundwater remain below the regulatory limits (10 μg/L) in all calculated pollution scenarios. The concentrations of U remain below the limit (2 μg/L) if adsorption is neglected and the dissolution of uranium oxide is assumed to be slow. The concentrations of Ni remain below the limit (20 μg/L) if adsorption is neglected. The concentrations of Pb remain below the limit (10 μg/L) only under the unrealistic assumption that the carbonates are Pb-free provided that adsorption is neglected.