Integrated probabilistic risk assessment and site-specific target level determination for groundwater contamination associated with phosphogypsum stacks

Phosphogypsum (PG) is an industrial by-product of phosphoric acid production and is typically disposed of in stacks. Predicting contaminant migration and assessing groundwater risks under different leakage modes is essential for PG stack management. In this study, we established a three-dimensional groundwater flow and solute transport model for a closed PG stack in Yichang City, China, to simulate the migration of soluble phosphorus (P) and fluoride (F) over a 10-year period. To account for hydrogeological parameter uncertainty, Monte Carlo simulations were coupled with deterministic models. The results indicate that complete failure of the anti-seepage system (Scenario 5) caused catastrophic groundwater contamination. Under Scenario 5, the peak P and F concentrations in the downstream drinking water wells reach 6204.13 and 78.25 mg/L, respectively. Conversely, implementing surface ecological restoration (Scenario 4) significantly mitigates runoff-driven pollution, reducing peak pollutant concentrations by nearly half. Site-specific target levels (SSTLs) for source control were derived reversely. For soluble phosphorus, the SSTL under ecological restoration (Scenario 4) is determined to be 29.7–41.5 mg/L. This threshold is significantly higher than that under Scenario 5 (0.2–0.3 mg/L), where the complete failure of containment measures strips the aquifer of its natural buffering capacity. These findings demonstrate that risk management must transition from static limits to scenario-specific early warning.