Experimental insights into the role of running waters in Enhanced Rock Weathering

Rivers transport and transform products of chemical weathering, yet the processes governing carbonate precipitation, biotic transformation and fixation of enhanced rock weathering products remain poorly constrained. We conducted a controlled stream flume experiment manipulating simulated basalt-amended runoff across six ERW levels (0–100 t ha⁻¹) in groundwater- and river-fed systems to investigate calcite saturation state (Ω)  dynamics and ecosystem responses to alkalinity enhancement. Water chemistry and ecosystem responses were monitored for four weeks following alkalinity addition. Across all treatments and water types, flumes were generally calcite-supersaturated (Ω > 1), indicating baseline conditions favourable for carbonate precipitation. ERW treatments substantially increased supersaturation, and high temperatures combined with low flows led to Ω frequently exceeding 10 (a proposed precipitation threshold) and reaching > 50 under high-input scenarios. Relationships between Ca, CO₃²⁻ and Ω differed between groundwater- and river-fed systems, indicating that ERW effects on saturation state were mediated not only by cation supply but also by buffering capacity and carbonate speciation. Effects on benthic communities were weak, with invertebrate richness, abundance, and biomass being largely unaffected, though community composition contracted at the highest ERW level. Microbial respiration and primary production were generally only higher in groundwater-fed systems, while leaf litter and cellulose degradation were unchanged. Overall, these results indicate minimal ecological risk and demonstrate that thermokinetic conditions in small streams can support highly supersaturated calcite states during warm, low-flow periods without immediate carbonate precipitation. However, elevated supersaturation states may reduce carbonate solubility and increase the potential for secondary precipitation under changing hydrological or thermal conditions, representing a potential constraint on ERW-derived alkalinity transport efficiency within river networks.