Mapping observations using AUV and numerical simulations of leaked CO2 diffusion in sub-seabed CO2 release experiment at Ardmucknish Bay

Abstract A controlled sub-seabed CO 2 release experiment (QICS: quantifying and monitoring potential impacts of geological carbon storage) was conducted in Ardmucknish Bay, Scotland, in 2012, to quantify the effect of leaked CO 2 on marine environments. In this experiment, CO 2 was injected beneath the seafloor, and closely monitored as the CO 2 was allowed to leak into the overlying water column. We performed mapping observations using an autonomous underwater vehicle (AUV) equipped with sensors to monitor physical and chemical conditions in the vicinity of the CO 2 leakage. We also numerically simulated the behavior of the low-pH plume caused by the CO 2 leakage, within the calculated tidal current in the bay. The results of AUV mapping observations showed that physical and chemical characteristics of the water mass in Ardmucknish bay are sensitive to tidal variations (low or high tide), and the ascent speed of the leaked CO 2 bubble and the pH decrease in the vicinity of the CO 2 leakage area are predominantly controlled by the tidal phase in the bay. The observations using an acoustic Doppler current profiler (ADCP) installed in the AUV indicated that the current in the bay flows westwards at high tide and eastwards at low tide. The simulated tidal current agreed well with the tidal current observed by ADCP. Numerical simulation suggested that the pH decrease due to the leaked CO 2 is restricted to the vicinity of the CO 2 leakage area, and fluctuates greatly with tidal variations. In flood tide periods, diffusion of the low-pH plume caused by the leaked CO 2 is barely detectable because low-pH seawater intrudes into the bay from the bay mouth.