Experimental investigation of hydrogeochemical consequences of gas leakages into shallow aquifers

The fluctuating energy production of renewable sources implies the necessity of energy storage. For the implementation of gas storage, a knowledge gap regarding gas-specific reactions to be expected in shallow aquifers following a leakage of compressed air, methane, or hydrogen existed. These reactions can change the composition of the groundwater, which potentially leads to conflicts between different ways of utilizing the subsurface. Experiments representing reactive hydrogeological environments characteristic to a shallow aquifer influenced by a gas plume were carried out. The applied experimental approach included flow-through column experiments using a sediment from a shallow aquifer percolated by the groundwater from the same aquifer. The water used to percolate the experimental sediment columns was saturated by the respective gas at partial pressures representing the conditions taking place within a dissolved gas plume in a shallow aquifer after a leakage of compressed air, methane, or hydrogen. Experiments on leakage of compressed air showed pyrite oxidation. The transfer function on reaction kinetics based on the experimental reaction rates for upscaling the results includes a new surface passivation term describing the inhibition of more than 90% of the pyrite reactivity compared to the reactivity expected based on previously published models. Fugitive methane was not oxidized and did not cause detectable changes in the groundwater within one year in the presented flow-through column experiments. This finding acknowledges earlier studies describing no methane oxidation if methane or its electron acceptors are newly introduced into an aquifer. In contrast to methane, elevated concentration of hydrogen immediately triggers a series of redox reactions, which have major effects on groundwater composition. Based on these observations, a descriptive reaction model was developed for further modeling applications