Infiltration into unsaturated compacted mine tailings under extreme rainfall events for use in tailings storage facilities

• Percolation in tailings cover exceed by 18 times under 100-year rainfall ponding. • Bentonite-amended tailings reduce infiltration under 100-year rainfall ponding. • Hydrophobized tailings barrier resists infiltration under 500-year rainfall events. • Tailings show strong hysteresis near surface; effects lessen with depth. Mining operations in Nordic countries face increasing scrutiny due to environmental incidents stemming from inadequate tailings storage, including groundwater contamination during and after mine operations. With climate-induced extremes in wetting and drying, there is a pressing need to develop and evaluate effective unsaturated hydraulic barrier systems for tailings storage facilities (TSFs) using both conventional and alternative materials. This study investigates the infiltration behaviour of compacted unmodified and modified tailings for their potential application as hydraulic cover systems in TSFs. A series of one-dimensional instrumented column tests were conducted under controlled laboratory conditions to simulate a 100-year rainfall return period ponding scenario. The primary objectives were to evaluate the infiltration response of unsaturated tailing materials and to compare the performance of two hydraulic barrier systems: a conventional bentonite-amended tailings mixture and a novel hydrophobized tailings barrier. Experimental results revealed that compacted tailings alone are insufficient to restrict percolation under extreme rainfall events. The observed cumulative percolation rate (535 mm/year) exceeded the regulatory limit (30 mm/year) by over 18 times, emphasizing the environmental risk posed by unmodified tailings under climate extremes. Soil-water retention curves exhibited hysteresis effects that were more pronounced at shallow depths (up to 320 mm), diminishing with depth, likely due to increased overburden stress and associated pore structure alteration. In terms of hydraulic barrier performance, the bentonite-amended system limited water infiltration effectively within the top 70 mm after 24 h. However, the hydrophobized tailings barrier showed complete resistance to infiltration, even under a 500-year rainfall event, indicating a significantly higher water entry head.