Enhanced rock weathering (ERW) could make meaningful contributions to carbon-dioxide removal, yet quantifying its field performance remains difficult. This study shows that the ⁸⁷Sr/⁸⁶Sr ratio of strontium, a unique fingerprint of both the parent material and of the applied basalt, can trace basalt-derived solutes exported from agricultural catchments. After spreading 44.8 t ha⁻¹ of basalt across 75 ha of cropland in southern Virginia, we observed the stream draining the fields shift steadily from the native granitic signature toward that of the basalt. A two-end-member mixing model, paired with a stream flow model, converted this isotopic signal into time-resolved fluxes of dissolved basalt and, by stoichiometry, the associated CO₂ uptake. The approach lays a practical foundation for rigorous MRV of ERW projects.