Non-Conservative Transport of Cr(VI) and Potential Activation of Geogenic Sources in a Loess Gully Aquifer Under the Influence of Landfill Leachate Perturbation

Whether groundwater Cr(VI) enrichment near landfills results from direct leachate input or geogenic mobilization remains unresolved. Groundwater and leachate hydrochemistry, a Cl−-based mixing model, PHREEQC inverse modeling, sediment Cr fractionation, and correlation analysis were used to investigate a loess-gully landfill in Northwest China. The site was divided into four groundwater environmental units along the flow path. Cr(VI) ranged from below detection to 0.170 mg/L, and 35% of samples exceeded the Class III limit (0.05 mg/L). At the near-source groundwater drainage outlet (II-GEU), Cr(VI) was below detection despite an 8.22% apparent leachate contribution. Downgradient, the contribution decreased to 5.12%, yet Cr(VI) reached 4.17 times the conservative-mixing estimate. Thus, direct leachate mixing cannot explain downstream enrichment. Near the landfill, organic-rich leachate favored reducing conditions and Cr(VI) reduction. Downgradient, increasingly oxidizing conditions limited Cr(VI) reduction, while SO42− competed with chromate for mineral-surface adsorption sites, promoting Cr(VI) desorption and mobility. Leachate-induced mineral dissolution and cation exchange may further facilitate Cr(VI) migration by altering groundwater chemistry and mineral-surface properties. Landfill leachate therefore acts primarily as a geochemical trigger, inducing non-conservative transport and downstream release of geogenic Cr(VI), rather than as a direct Cr(VI) source. This mechanism has implications for monitoring and risk management in loess aquifers.