Enhanced rock weathering: the overlooked hydrodynamic trap

Enhanced rock weathering (ERW) is a technique proposed to remove large amounts of CO2 from the atmosphere (i.e. a negative emission technology) in which finely fragmented silicate rocks such as basalts (ground basalt) are distributed over agricultural or other land plots. The weathering process involves trapping CO2 but will also typically ameliorate soil properties (pH, soil moisture retention, cation exchange capacity, availability of Si), and can therefore be expected to positively affect plant and microbiological activity. This technique has been proposed in different modified forms over the past decades. In its current format, mainly its potential for near global application (e.g. Beerling et al. 2020) is stressed, and its acceptance is helped by the positive reception by e.g. nature organisations that already apply it as a technique for ecological restoration. Two main and largely separated processes result in trapping of CO2. The first is precipitation of carbonates, often as nodules, in the soil. The second is increased CO2 solubility in groundwater and eventually ocean water due to an increase of the pH value, referred to as the pH-trap. Most of the pH-trapping schemes are built on the assumption that CO2 is dissolved in infiltrating and shallow ground water, then discharged into surface water and consecutively transported to the seas and oceans. In that reservoir CO2 is expected to remain dissolved for centuries and possibly up to ten thousands of years, depending on surfacing times of deep oceanic currents. Another pathway that is systematically overlooked is that of groundwater fluxes that recharge deeper groundwater bodies. Depending on the regional geology, a significant fraction of infiltrating water will engage in deeper and long-term migration. For Belgium, the contribution of hydrodynamic trapping, depending on the hydrogeological setting, could be any part of the 15 to 25% of precipitation that infiltrates. Once infiltrating water enters these cycles, it will not come into contact with the atmosphere for possibly fifty thousand years. In this model, the long-term impact of ERW as a climate mitigation measure rests on a good understanding of the larger hydrogeological context, which encompasses infiltration and the deeper aquifers. Deep aquifers, as well as the migration paths towards them, are strictly isolated and residence times are much longer than for oceans. Recharge areas for deeper aquifer systems may therefore become preferential sites for ERW application, becoming an additional evaluation factor for siting ERW locations that is currently based on surface factors alone.

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