Hydrological partitioning controls rainfall-driven mobilisation and attenuation of residual petroleum hydrocarbons in a karst spring system

Petroleum leakage in karst regions poses persistent groundwater risks because rainfall-driven recharge can rapidly remobilise residual hydrocarbons retained within heterogeneous conduit–fracture networks. Yet the event-scale mechanisms governing rapid hydrocarbon flushing, delayed release, and persistence remain poorly constrained. We investigated a crude-oil-contaminated karst spring system in southwestern China, using eight months of high-frequency monitoring of rainfall, spring discharge, and petroleum hydrocarbon concentrations, complemented by tracer tests and concentration–discharge (C–Q) hysteresis analysis. Of 139 monitored rainfall events, 59 generated hydrologically effective recharge and measurable spring responses, exporting 26.25 tonnes of petroleum hydrocarbons. Cumulative export over the monitoring period reached 44.37 tonnes when artificial water-injection responses and intervening background-flow periods were included. Tracer breakthrough and C–Q hysteresis revealed a dual-domain transport pattern. Clockwise hysteresis occurred in 67.8% of the hydrologically effective events and was consistent with rapid conduit-dominated flushing through preferential-flow pathways. The remaining 32.2% exhibited anticlockwise hysteresis, indicating delayed, storage-controlled release from secondary fracture–storage zones, prolonged concentration tailing, and persistent low-level contamination. Log-transformed event-load analyses further supported the contrasting event-scale contaminant-export patterns associated with the two recharge–transport domains. The results indicate that short-term apparent attenuation mainly reflected source removal, rainfall-driven flushing, and progressive depletion of readily mobilisable residual hydrocarbons rather than uniform intrinsic degradation or self-purification. This dual-domain framework advances mechanistic understanding of contaminant persistence and provides a basis for rainfall-event-based monitoring, risk assessment, and emergency management of petroleum leakage in karst environments.