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Preventing the global average temperature of the Earth’s atmosphere from rising more than 2 C above pre-industrial levels is a global goal adopted by 187 nations through the 2015 Paris Agreement (UNFCCC 2015). Meeting this target requires that CO2 emissions from fossil fuel use and industrial processes must be curtailed (UNFCCC 2015). This can only be achieved through the combination of switching to zero-carbon energy sources and the use of large-scale CO2 mitigation strategies (Rogelj et al., 2018). Carbon capture and storage (CCS) is an industrial scale, cost-effective mitigation strategy for reducing anthropogenic CO2 from entering the atmosphere (IPCC 2005; Scott et al., 2012). The technology consists of CO2 capture at the point source of emission, transport of this captured CO2 to an engineered storage site and the secure storage of the injected CO2 within the subsurface (IPCC 2005). For CCS to be routinely deployed, an understanding of the potential migration pathways that could lead to the contamination of the overlying shallow groundwaters and how CO2 migration can be monitored is needed (Alcalde et al., 2018; Ju et al., 2019). This understanding can be provided through the study of CO2 storage and migration in pre-existing engineered storage sites, such as CO2 enhanced oil recovery fields (Györe et al., 2015; 2017; Stalker et al., 2015) and in purpose-built test sites (Kikuta et al., 2005; Spangler et al., 2010; Martens et al., 2013; Smith et al., 2013; Jones et al., 2014; Serno et al., 2016; Feitz et al., 2018; Ju et al., 2020; Michael et al., 2020). Pilot CO2 injection sites provide a unique opportunity to establish a geochemical …