Climate-driven hydrological extremes increase subsurface mobility of petroleum hydrocarbons: an experimental study under semi-continental climate conditions

While the influence of groundwater table fluctuations on the fate of Light Non-Aqueous Phase Liquids (LNAPLs) in soils is recognised, the impact of these fluctuations' intensity was never quantified and remains poorly understood. Yet, in the climate change context, these factors may alter their long-term environmental risks and challenge existing management strategies. To address this gap, a lysimetric column (2 m³) was developed to measure in situ the soil moisture, soil weight, pH, and redox potential and investigate the fate of a LNAPL-contaminated sandy soil exposed to controlled extreme rainfall and water table level fluctuations over a 120-day period. This scenario simulated the most extreme effects of climate change predicted by the IPCC for 2100 in the Grand Est Region, France. Molecular analyses of the free, gaseous and dissolved LNAPL phases (GC-TQD, micro-GC) revealed that, compared to current local climatic conditions (low water table fluctuation intensity), the extreme scenario led to a 50 % increase in the smear zone thickness, 30 % greater depletion of the free LNAPL phase, and a 2.8-fold rise in BTEX volatilisation. Additionally, dissolved LNAPL concentrations increased by 25 % and the natural attenuation rate reached 37.5 g/m²/day, 50 % higher than under current hydrological conditions. This work provides the first quantification of the effect of groundwater level fluctuation intensity on the multi-phase redistribution of LNAPL in sandy soils. This study demonstrates that the intensity of groundwater level fluctuations is a key driver of LNAPL saturation redistribution, enhancing their short-term releases but reducing their persistence.