Carbon dioxide removal during dissolution of granular basalt: A mass balance test of enhanced rock weathering at the hillslope scale

• CDR in basalt weathering is limited by gas diffusion and mineral precipitation. • Hillslope discharge chemistry reveals solute retention and incongruent dissolution. • HCO 3 − and non-hydrolyzing cations exhibit clockwise hysteretic C-Q relationships. Enhanced rock weathering (ERW) is proposed as a carbon dioxide removal (CDR) strategy that sequesters carbon through the carbonic acid-promoted dissolution of ground silicate rocks. Studies have explored the efficacy of ERW through geochemical models and bench-scale reactors, but field-scale experimentation is limited. A year-long, replicated study was conducted at the Landscape Evolution Observatory (LEO) at Biosphere 2 to quantify basaltic CDR at the hillslope scale. LEO comprises three mesoscale surfaces (each 330 m 2 ) with 1 m depth of granular basalt. We subjected these structures to three 30 d irrigation events followed by progressively lengthened dry periods. Aqueous discharge was collected bihourly for major and trace chemistry, and subsurface interactions were observed at 15 min intervals through distributed sensors enabling continuous monitoring of P CO2 , volumetric water content, and total hillslope mass. This approach enabled closing of the carbon and water mass balance of the system for the duration of the experiment. CDR was quantified through direct monitoring of bicarbonate (HCO 3 − ) concentrations as validated through the charge balance of non-hydrolyzing cations and strong-acid anions. Concentration-discharge relations for HCO 3 − showed dilution trends with clockwise hysteresis, while a decrease in CO 2 uptake occurred with increased hillslope water saturation ( S hydro ). The CDR rate, normalized to the specific surface area of the basalt, was -13.45 log 10 moles C m −2 s −1 , while other studies report CDR rates from -14 to -10 log 10 moles m −2 s −1 . We found that basalt CDR rates were impacted by depletions of P CO2 upon hydrologic infiltration, variable S hydro , and incongruent dissolution.