In-situ recovery is a common method of mining U ore. This technique involves injecting an oxidant to oxidize U deposited as insoluble U(IV) to soluble U(VI) and a complexing agent to form mobile U(VI) compounds, which are extracted from mining waters. Unfortunately, U concentrations often remain elevated in groundwater following this mining process. One promising technique to decrease U concentrations to acceptable levels is in-situ remediation where a reductant is injected into the mined area to reduce U(VI) back to insoluble U(IV). Here we assess the efficacy of the chemical reductant dithionite to reduce U(VI) concentrations. While dithionite can reduce environmental Cr(VI), dithionite has previously never been tested for reducing U(VI) in the environment. We determine how effectively dithionite remediates U(VI) contamination using aqueous U concentration and δ238U data. U concentrations and δ238U decrease during the dithionite experiment, demonstrating that dithionite induces U(VI) reduction. As dithionite is removed by pumping, U concentrations and δ238U increase towards background levels, but U(VI) reduction continues for an extended period. Measurements demonstrate that re-oxidation and remobilization does not appear to occur due to the reducing condition of groundwater. •Dithionite induces reduction of U(VI) in a previously mined area.•Dithionite may be utilized to remediate U contamination in other settings.•U(VI) and δ238U provide complementary information on extent of U(VI) reduction.•Chemical reductant dithionite impacts the geochemistry of sulfur and iron species.