Geochemical modeling of a sustained shallow aquifer CO₂ leakage field study and implications for leakage and site monitoring

A geochemical numerical modeling study was conducted to constrain processes occurring in field and laboratory experiments, simulating CO₂ leakage from geological storage on shallow potable aquifers. A leak was previously physically simulated in a shallow potable aquifer at Vrøgum plantation, Western Denmark by injection of 1600kg of gas phase CO₂ over 72 days. Here, a 1-dimensional reactive transport model was constructed based on field and laboratory results and subsequently used to explore the contributions of various geochemical processes to explain observed results from the carbonate free system. Finite gibbsite derived Al³⁺ driven cation exchange is able to explain the majority of water chemistry change observed at Vrøgum including: a pulse like effect showing a fast peak and return toward background levels for alkalinity and dissolved ion concentrations; and increasing and persistent acidification via buffering exhaustion. Model processes were supported further by simulation of a batch experiment conducted on the Vrøgum glacial sand, employing the same processes and sediment parameters. The fitted reactive transport model was subsequently used to extend predictions and explore various scenarios. Extended predictions suggest the pulse of elevated ions travels with advective flow succeeded by a zone of increasing acidification. Model runs at higher P_CO2 (implying greater depths) suggest amplification of effects, i.e., greater peaks and more rapid and severe acidification. Calcite limits acidification, however, induces additional Ca driven ion exchange giving rise to more significant chemistry change. Although a site specific model, results have significant implications for risks posed to water resources from CCS leakage and implementation of MMV programs.