Impact of groundwater level variations induced by climate change on the mobilization of light refined petroleum hydrocarbon contaminants (LNAPLs)

Light Non-Aqueous Phase Liquids (LNAPLs) are one of the most important sources of soil and groundwater contamination worldwide. When they infiltrate through the unsaturated zone, part of the LNAPLs remains trapped by capillary forces. The others accumulate above the top of the water table, forming a floating ‘free’ phase able to generate a long-term dissolved LNAPL plume that durably alters the quality of the water resource. Seasonal variations in the groundwater level lead to significant vertical spreading of these light petroleum hydrocarbon contaminants at the capillary fringe, favoring their release into the air and groundwater. In the climate change context, the IPCC predicts an intensification of these groundwater level variations over the next century in response to variations in rainfall intensity and frequency, whose effects are increased by the use of water resources. This context may strongly impact the mobilization of these organic contaminants and their release to the environment. To study these phenomena, it is, therefore, essential to better understand the impact of the groundwater level fluctuation patterns on the LNAPLs mobilization processes. To this end, an original experimental system combining indirect geophysical (complex electrical conductivity, permittivity), in-situ physical-chemical (pH, Eh, temperature), and geochemical measurements was developed at the GISFI station (Homécourt, France). This device allows the assessment and the comparison of the amount and nature of LNAPLs release into the atmosphere and water from contaminated soil during two groundwater level fluctuations scenarios: one corresponding to the ‘actual’ rainfall pattern based on regional climate records; the other based on the predictions of the most extreme IPCC scenario. This study will be conducted at different scales (laboratory decametric columns and 2 m³ lysimeters) and on soils of different geological complexity. The remobilized hydrocarbons will be collected via suction cups and gas collection chambers as the groundwater table fluctuates and will be regularly analyzed (GC-MS, FTIR). The complementarity of the monitoring methods aims to provide a better understanding of the fate of these organic pollutants at contaminated sites and the evolution of the associated environmental risks in the coming years, under the expected effect of climate change. Preliminary results concerning the hydrocarbon pollution migration through the unsaturated zone and the distribution of the LNAPL will be presented to illustrate the capacity of this new instrumental system.

This work is partly funded by the DEEPSURF project "Lorraine Université d’Excellence", ANR-15-IDEX-04-LUE".