Does the release of toxic metals due to subsurface CO2 storage in basalts pose an environmental hazard?

• The concentrations of most trace and toxic elements in CarbFix1 and CarbFix2 waters were low. • These concentrations were substantially lower than that estimated from stoichiometric basalt dissolution. • Low concentrations of trace and toxic elements are consistent with secondary mineral precipitation of carbonates and sulfides. • Results suggest negligible risk of substantial water contamination from dissolved CO 2 and H 2 S injection into basalts. Carbon dioxide storage through the carbonation of subsurface basaltic rocks is currently being explored to limit carbon emissions to the atmosphere. Basaltic rocks, however, contain trace and toxic metals that could potentially be mobilized by the carbonation process. This study reports the degree to which selected trace and toxic metals were mobilized during CarbFix1 and CarbFix2 projects. CarbFix1 injected 175 tons of CO 2 -charged water followed by 73 tons of CO 2 /H 2 S-charged water into basalts at 35 °C, whereas CarbFix2 continuously injected CO 2 /H 2 S-charged water into basalts at >250 °C. In most cases dissolved concentrations of Ba, Sr, Mo, Cu, Cr, Ni, Cd, and Pb in monitoring well fluids remained low. Although these fluids are not intended for human consumption, the aqueous trace element concentrations were generally below the WHO, EU, and Iceland drinking water standards, except for Fe and Mn in CarbFix1. Aluminum and As concentrations exceeded these standards during CarbFix2, but were elevated before injection. The low concentrations of most trace and toxic metals are consistent with their removal by secondary processes, particularly co-precipitation into carbonate and sulfide minerals formed during gas-water-basalt interaction. Solid precipitates recovered from CarbFix1 show strong enrichment of transition metals in calcite, consistent with natural and engineered analogues. As the two CarbFix injections bound the lower and upper temperature ranges of likely mineral carbon storage efforts, these results suggest limited risk of water contamination due to toxic and trace element release from subsurface basalts due to the injection of dissolved CO 2 and H 2 S.