An experimental multi-method approach to better characterize the LNAPL fate in soil under fluctuating groundwater levels

Abstract

Light-Non-Aqueous phase liquids (LNAPLs) are important soil contamination sources, and groundwater fluctuations may significantly affect their migration and release. However, the risk assessment remains complex due to the continuous three-phase fluid redistribution caused by water table level variations. Hence, monitoring methods must be improved to integrate better the LNAPL multi-compound and multi-phase aspects tied to the groundwater level dynamics. For this purpose, a lysimetric contaminated soil column (2 m³) combining in-situ monitoring (electrical permittivity, soil moisture, temperature, pH, Eh), direct water and gas sampling and analyses (GC/MS-TQD, μGC) in monitoring well, gas collection chambers, and suction probes) were developed. This experiment assesses in an integrated way how controlled rainfalls and water table fluctuation patterns may affect LNAPL vertical soil saturation distribution and release. Coupling these methods permitted the investigation of the effects of rainwater infiltration and water table level fluctuation on contaminated soil oxygen turnover, LNAPL contaminants' soil distribution and remobilization towards the dissolved and the gaseous phase, and the estimate of the LNAPL source attenuation rate. Hence, 7.5% of the contamination was remobilized towards the dissolved and gaseous phase after 120 days. During the experiment, groundwater level variations were responsible for the free LNAPL soil spreading and trapping, modifying dissolved LNAPL concentrations. Nevertheless, part of the dissolved contamination was rapidly biodegraded, leaving only the most bio-resistant components in water. This result highlights the importance of developing new experimental devices designed to assess the effect of climate-related parameters on LNAPL fate at contaminated sites.