Environmental risk assessment of underground concentrated brine reservoir with solute transport model: A case study of a coal mine in Northwest China

Underground reservoirs have emerged as a promising technology for storing and reusing saline wastewater from coal mines. However, the high salinity and potential leakage risks pose environmental threats needing assessment. This study developed an integrated solute transport and relative risk modeling framework to evaluate the environmental risks of an underground concentrated brine reservoir (UCBR) in Northwest China. The solute transport model simulated subsurface brine migration under long-term penetration, sudden leakage, and complete leakage scenarios, parameterized by field data. The relative risk model integrated source, receptor, and exposure analyses to quantify normalized risk levels. Results revealed that risks incrementally rose over longer storage durations, with 70 years posing high risk under 10m hydraulic gradients in the long-term scenario. The risk of sudden leakage is controllable within 30 days but dramatically elevated by 60 days. Complete leakage was predicted to affect a stable 3km2 zone. The integrated modeling provides an improved methodology for assessing UCBR risks and highlights the critical influence of bottom barrier permeability, groundwater gradients, and storage time. Targeted strategies are proposed, including reinforcing bottom barriers, managing hydraulic gradients, controlling storage durations, implementing early warning systems, and contingency planning to promote sustainable mining water management.