Reactive transport modeling for impact assessment of a CO_2 intrusion on trace elements mobility within fresh groundwater and its natural attenuation for potential remediation

摘要

A leakage of CO_2 from a geological storage site into an aquifer exploited for human use could raise concerns about potential changes of groundwater quality. We use the reactive transport code TOUGH2-ECO2N/TOUGHREACT in order to apprehend the possible geochemical mechanisms that could affect a freshwater aquifer. Our aquifer model is based on the mineralogical and geochemical features of an existing glauconitic-sandstone aquifer, to which trace elements bearing minerals are added in average proportions found in similar mineralogies. The code computes kinetic precipitation/dissolution processes. 2D sensitivity tests on: i) intrusion position (at the ceiling and at the floor) of pure CO_2 - ii) intrusion rates - iii) natural groundwater flow, were performed, exhibiting several dynamics. Dissolution of C_O2 decreases the pH to 5.0 and leads to dissolution of trace elements bearing minerals, enriching the waters in trace elements including Cd, Pb, Zn. Their concentrations may locally exceed quality parameters. Gaseous CO_2 migrates upwards and water enriched in dissolved CO_2 moves downwards due to the density gradient. The geometric impact extends and the total amounts of mobilized health-significant-elements increase with increasing intrusion rates. Groundwater flow may have an impact on these parameters too. An interruption of the CO_2 stream is immediately followed with decreasing tendencies of these parameters. A 3D test at an intrusion rate of 0.1 g/s/m~2 during 1 year shows that, at this rate, the initial shape of the plume is much more extended horizontally than vertically. A strong limitation of the model is currently that sorption processes on clay minerals as well as on oxy-hydroxydes have not been taken into account, which most certainly means that our computations are conservative, as these mechanisms could strongly and quickly inhibit the mobilization of traces that would follow acidification and dissolution processes.