Groundwater residence times and hydrogeochemical processes in a shallow limestone aquifer overlying a deep-well CO2 injection demonstration site

South-western Victoria, Australia Sequestration of CO 2 is required to reduce greenhouse gas emissions. Multi-megatonne-scale CO 2 storage projects are essential for net zero emissions particularly. A demonstration carbon capture, utilisation and storage (CCUS) project has been operating since 2003 at the Otway International Test Centre (OITC) in south-western Victoria, Australia by CO2CRC Limited. This paper presents new insights from a sub-set of the hydrogeochemical data focused on the 2020–2022 sampling campaigns, which included major ions, the stable isotopes of hydrogen, oxygen and sulfur, and also sulfur hexafluoride (SF 6 ). A 500-fold improvement in analytical detection limits for SF 6 enabled mean residence times (MRTs) to be estimated for the first time for groundwater within the Port Campbell Limestone (PCL). MRTs were within the range of ∼54 to ∼1380 years while the vertical hydraulic conductivity of this strata was estimated to be between 1.8 × 10 −1 and 6.1 × 10 −3 m/day. The stable isotope data indicate upward groundwater flow along an on-site, steeply dipping fault. The δ 34 S and δ 18 O isotopic composition of dissolved sulphate in most of the shallow groundwater samples most likely represents a mixture of rainfall and, to a lesser degree, sea spray, pyrite oxidation and bacterial sulphate reduction. The relatively high hydraulic conductivity values, low MRTs and similarity of stable isotope ratios for most samples relative to meteoric water suggest that groundwater flow rates within the PCL are relatively high. • Mean Residence Times ranged from 54 to 1380 years. • The vertical hydraulic conductivity of the aquifer was estimated using SF 6 data. • Stable isotope results suggest upward groundwater flow along an on-site fault. • 34 S and 18 O-SO 4 were utilised to interpret sulphate concentrations.