Multiple Lines of Evidence Reveal Rapid, Seasonal Watershed Responses to Enhanced Weathering

Abstract Enhanced rock weathering (ERW) is a natural carbon dioxide removal (CDR) approach that captures CO2 by accelerating silicate weathering using crushed rocks. A major question on the efficacy of ERW is how fast and efficient it is at transporting the products of weathering to drainage networks, and ultimately the ocean. Using a novel whole watershed experiment, we report multiple lines of evidence of rapid and pronounced streamwater chemistry responses within weeks of basalt application (20 t ha-1) to 15% of a 59-ha temperate, headwater catchment. Lines of evidence include an immediate streamwater alkalinity increase of more than 550 μeq L⁻1, a shift in elemental ratios to silicate endmembers, and a concomitant change in silicon and lithium chemistry reflective of basalt weathering. Finally, our difference-in-differences analysis revealed strong, recurring seasonal ERW treatment effects. ERW contributed 7–17% of observed alkalinity in summer and fall, but much less in winter and spring, reflecting the critical role of the near-surface and stream-proximal zones in alkalinity export, and the effects of precipitation and temperature on ERW rates. Over two years, 9.5–11% of the theoretical CDR potential was exported from the watershed in the form of alkalinity, with an average rate of 34.72 t CO2 km⁻2 yr⁻1. This work demonstrates rapid, seasonal watershed responses to ERW and its promise for CDR monitoring, reporting, and verification (MRV), and highlights how climate and hydrological variability set fundamental boundaries on ERW effectiveness across landscapes.