Underground storage of CO2 as part of Carbon Capture and Storage (CCS) provides an option for significant reduction of CO2 emissions from fossil fuel power generation and other industrial processes as well as facilitating direct air capture. Although CCS will mitigate climate change, international regulations and societal expectations require that storage integrity is assured and environmental issues of deployment are considered, in particular related to the unplanned release of CO2. Globally many storage sites are located offshore within continental shelves, necessitating both a marine environmental impact assessment and an appropriate offshore monitoring strategy coupled with appropriate communication to enhance public understanding. This thesis by papers contains a body of work addressing both impact and monitoring challenges, primarily mediated using modelling approaches but also describing a novel real-world CO2 release experiment. The experiment revealed the previously unconsidered complexity of CO2 flow through and reactivity within sediments and behaviour in the water column. This work established the challenges associated with monitoring and detection informing further research programmes focused on increasing the technology readiness levels of sensors and developing monitoring strategies. As a result of the body of work presented here it is possible to constrain potential impacts, relative to leakage rates and conclude that only large-scale catastrophic leaks would have the capacity to inflict significant environmental damage. This work also identifies a potentially cost efficient and highly sensitive detection/assurance methodology, based on an understanding of leakage dynamics and natural variability of the marine system.