Implications of the Riverine Response to Enhanced Weathering for CO2 removal in the UK

Enhanced silicate weathering, the application of crushed calcium and magnesium-rich rocks to arable cropland, has been proposed as a potential negative emissions technology for the drawdown of atmospheric CO2. Previous estimates have suggested that enhanced silicate weathering (EW) has the potential to remove significant quantities of CO2, as much as 6-30 Mt CO2 yr(-1) for the UK. However, if secondary carbonates are precipitated during the riverine transport of the products of EW, a portion of this CO2 will be re-released, lowering the net carbon dioxide removal (CDR) potential of the mitigation strategy. Here, we assess the fluvial response to EW in the UK by calculating the expected riverine carbonate precipitation due to the dissolution of 10-50 t ha(-1) yr(-1) of silicate rock on available arable cropland in major UK catchments. Increases in calcite saturation due to the export of soluble cations sourced from EW from soil to rivers are simulated to cross thresholds for spontaneous carbonate precipitation in several major UK river catchments (e.g. the Great Ouse, Thames). Catchments most susceptible to potential secondary carbonate precipitation are those with a high ratio of cropland to water discharge. On average, carbonate precipitation for major UK catchments is expected to make EW 16% (10 t ha(-1)), 21% (20 t ha(-1)) or 27% (50 t ha(-1)) less effective at removing CO2 than that predicted in the absence of such precipitation. Furthermore, when placing strict silica limitations on weathering reactions within soils, the CDR potential of EW is reduced considerably, to 0.23 Mt CO2 yr(-1). Although reducing the CDR potential of EW, we suggest that under rapid weathering conditions, carbonate precipitation in UK rivers will prevent pH increases over the safe level for freshwater ecosystems (pH > 9). Together, the simulations suggest that ambient hydrological conditions may make certain UK agricultural areas less effective for CDR by EW and call for methods to quantify secondary carbonate precipitation in response to EW treatments and to further investigate the role of silica saturation in suppressing weathering reactions.

Automated summary

Enhanced silicate weathering is a potential negative emissions technology that involves applying crushed calcium and magnesium-rich rocks to cropland to reduce atmospheric CO2. However, the presence of secondary carbonate precipitation during the riverine transport of the weathering products may release CO2, thus reducing the net carbon dioxide removal potential of this mitigation strategy.