An investigation of the fate of petroleum contaminants (LNAPL) under fluctuating groundwater table levels induced by climate change

International audienceThe global water demand for industrial, agricultural, and drinking water needs increases but so is the number of soil and groundwater contaminations. Light Non-Aqueous Phase Liquids (LNAPLs) which come from the accidental release of refined petroleum products (diesel or fuel…) represent 40% of these contaminations. After their infiltration in the unsaturated zone, part of LNAPLs reaches and accumulates above the top of the water table as a mobile liquid phase able to be displaced by water table level variations. Hence, pump-induced, or seasonal variations in the groundwater level led to the significant vertical spreading of these light petroleum hydrocarbon contaminants at the capillary fringe that can temporarily favor their release into the air and groundwater. In the coming decades, an intensification of these groundwater level variations is expected in response to extreme climatic events and variations in precipitations patterns and water needs. This context may strongly impact the mobilization of these organic contaminants, impacting their release in the air and water and their attenuation rates. It is, therefore, essential to better understand the impact of the groundwater level fluctuation patterns on the fate of LNAPLs. To this end, an innovative system was developed this year at the GISFI station (Homécourt, France - http://gisfi.univ-lorraine.fr). This device is composed of two lysimetric columns of soil equipped with in-situ sensors that combine indirect geophysical (complex electrical conductivity, permittivity), physical-chemical (pH, Eh, temperature), and geochemical measurements. This device allows to assess and compare the evolution of the LNAPL saturation distribution in soil, the nature and the amount of LNAPLs release into the air and groundwater during two groundwater level fluctuations and precipitation scenarios of 14 months: one based on the current rainfall and groundwater fluctuation patterns calculated after the last ten years regional climate records; the other based on the most extreme predictions of IPCC climatic models. The remobilized hydrocarbons will be collected from lysimeters via suction cups (dissolved phase) and gas collection chambers (vapor phase) and regularly analyzed (µGC, GC-MS, FTIR) using the analysis protocol established in the laboratory for these pollutants. To prepare these experiments, laboratory tests are also conducted on laboratory decametric columns and 2D tanks of soil under controlled soil saturation of air/LNAPL, water/LNAPL, and air/water to calibrate the signals of in-situ sensors. The complementarity of these monitoring techniques should provide a better understanding of the current and future behavior of these organic pollutants and the evolution of the associated environmental risks (water/air quality) at contaminated sites under various climatic conditions. Preliminary results concerning the migration of the pollution through the unsaturated zone of the columns after their release will be presented to illustrate the capability of this new instrumental system