In situ redeposition of trace metals mobilized by CO2-charged brines

Mobilization of contaminants by CO2-charged brines is one concern relating to injection of CO2 as part of carbon capture and storage projects. This study monitors the mobility of trace metals in an exhumed CO2-charged aquifer near the town of Green River, Utah (USA), where CO2-charged brines have bleached red sandstones, and concentrated trace metals at the bleaching reaction front. Mass balance calculations on the trace metal enrichments are used to calculate time-integrated fluid fluxes and show that a significant fraction of the metals mobilized by the CO2-rich brines are redeposited locally. A sequential extraction procedure on metal-enriched samples shows that these metals are incorporated into secondary carbonate and oxide phases which have been shown to grow at the CO2-promoted bleaching reaction front. We argue that while CO2-charged brines are capable of mobilizing trace metals, local metal redeposition implies that the potential for contamination of overlying freshwater aquifers is low.