Carbon capture and storage (CCS) is a promising means of directly lowering CO2 emissions from fossil fuel combustion. However, concerns about the possibility of CO2 leakage are contributing to slow the widespread adoption of the technology. Research to date has failed to identify a cheap and effective means of measuring how CO2 injected underground is being stored. CO2 can be stored in four different ways: 1.Physically - where gaseous or liquid CO2 is trapped beneath an impermeable sealing cap rock. 2.Residually - where CO2 is trapped within individual and dead end spaces between rock grains (pores). 3.Solubility - where CO2 is dissolved into the formation water, which fills the pores between rock grains. 4.Mineralisation - where CO2 reacts with the host rock forming new carbonate minerals within the pores. Importantly, physically trapped CO2 is mobile and able to leak should a break form in the overlying sealing rocks. CO2 stored by the other three means is not mobile or buoyant, and hence will not migrate out of the CO2 storage site should the seal fail. It is therefore critical for reassurance to the public and regulators of CO2 storage that reliable ways to measure how much of the CO2 injected into the subsurface for storage is locked away in these secure means. Few research studies to date have quantified exactly how much CO2 is stored by residual and solubility trapping across an entire storage site. Estimations have been made from laboratory studies on rock core samples, but these only represent rocks from a small part of the CO2 storage site. Extending these results to infer how CO2 will be stored in the entire storage site is difficult as the rock cores do not represent the variation seen across the storage site. It is possible to use seismic waves to image the CO2 injected. This has proved to be a reliable means of imaging large amounts of CO2 but is unable to image thin layers of CO2 or % dissolved CO2 which makes it very difficult to quantify exactly how CO2 is being stored. Hence, there is a need to develop a reliable test which can be performed at a single CO2 injection well during assessment of a potential site for CO2 storage. This would allow the amount of CO2 which will be residually trapped in the storageformation to be determined. Such a test will lower the risk of mis-estimating the storage capacity of a site and provide a commercial operator with greater reassurance of the predictability of their proposed storage site. We will work with one of the world's leading research organisations focused on CCS, CO2CRC. They own and operate a dedicated research facility into CO2 storage, at Otway CO2 in Australia. This is uniquely suitable because in mid-2011 Otway undertook a successful experimental programme focused on determining residual trapping. Building on these experiments and in direct collaboration with CO2CRC we will use water geochemistry to establish the fate of CO2 injected into the Otway site by quantifying both the level of CO2 residually and solubility trapped and at what distance into the reservoir. This will be achieved using noble gas tracer injection and recovery, to determine residual trapping levels, and by independent oxygen stable isotope measurements to quantify the amount of CO2 dissolution. These tests will calibrate downhole geophysical techniques which CO2CRC will use. Grant number: UKCCSRC-C2-204.