Integrating geophysical and underwater robotic surveys with transport and geochemical modeling to assess the Solotvyno salt mine hazard in the Transcarpathia province of Ukraine

The Solotvyno salt mine in West Ukraine presents significant environmental and infrastructural risks due to uncontrolled salt dissolution following mine flooding, causing ground subsidence, sinkhole formation, and transboundary contamination of the Tisza River. This study employs an innovative multidisciplinary approach to characterize subsurface conditions and contamination pathways. The UX-1Neo autonomous underwater robotic platform conducted 14 dives in two flooded mine shafts, revealing intact main shaft structures but worrying blockages in horizontal passages, distinct haloclines at different depths (60 m in ventilation shaft 9, 140 m in shaft 10), and evidence of ongoing salt crystallization. Ground geophysical surveys such as electrical resistivity tomography, very low frequency radio-magnetotellurics and horizontal loop electromagnetics identified compromised protective clay layers (“pallag”) and potential air-filled voids, explaining accelerated dissolution and surface deformation. Hydrodynamic and contaminant transport modeling quantified salt fluxes (~ 2000 m 3 /day through “pallag” zones, ~ 13–15 m 3 /day from direct mine leakage) and confirmed dissolution rates of approximately 5.0–5.5 m 3 /day of rock salt, driven by both anthropogenic influences and natural gradients toward the Tisza River. NETPATH mixing models revealed variable contamination throughout the system, with the Black Moor area showing highest vulnerability (up to 27.4% mine water contribution) while the Tisza River maintains relatively good water quality despite measurable contamination (1.97% mine water). The suggested approach provides critical insights into risk assessment and remediation planning, representing a significant advancement in monitoring and managing complex environmental hazards associated with abandoned salt mines.