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

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.

A critical review of the influence of groundwater level fluctuations and temperature on LNAPL contaminations in the context of climate change.

The intergovernmental panel on climate change (IPCC) predicts significant changes in precipitation patterns, an increase in temperature, and groundwater level variations by 2100. These changes are expected to alter light non-aqueous phase liquid (LNAPL) impacts since groundwater level fluctuations and temperature are known to influence both the mobility and release of LNAPL compounds to air and groundwater. Knowledge of these potential effects is currently dispersed in the literature, hindering a clear vision of the processes at play. This review aims to synthesize and discuss the possible effects of the increase in temperature and groundwater level fluctuations on the behavior of LNAPL and its components in a climate change context. In summary, a higher amplitude of groundwater table variations and higher temperatures will probably increase biodegradation processes, the LNAPL mobility, and spreading across the smear zone, favoring the release of LNAPL compounds to the atmosphere and groundwater but decreasing the LNAPL mass and its longevity. Outcomes will, nevertheless, vary greatly across arid, cold, or humid coastal environments, where different effects of climate change are expected. The effects of the climate change factors linked to soil heterogeneities, local conditions, and weathering processes will govern LNAPL behavior and need to be further clarified.

Impact of Oil-Prone Sedimentary Organic Matter Quality and Hydrocarbon Generation on Source Rock Porosity: Artificial Thermal Maturation Approach.

This work investigates the relation between the molecular composition of the organic matter (OM), hydrocarbon generation, and porosity of artificially matured mudstones from the Kimmeridge Clay formation. Anhydrous thermal maturations, geochemical characterization (gas chromatography-mass spectrometry, gas chromatography with a thermal conductivity detector), scanning electron microscopy observations, and nitrogen adsorption measurements were carried out. The results were compared to the calculated OM porosity after maturation. Our results reveal that samples richer in phytoplanktonic OM generated more abundant oils enriched in alkanes and lighter aromatic hydrocarbons (i.e., biphenyls and naphthalenes) that are more able to fill the adjacent pores of the mineral matrix during maturation. Both the calculated and the observed OM porosity increases during maturation, but the measured rock pore volume shows a non-linear evolution related to the amount of gas generated and the ensuing ability of the rock to preserve the pores. The secondary cracking of highly oil-prone samples generated larger amounts of C1-C5 gases but lower pore volumes, less well preserved. OM composition and its ability to generate oil and gas seem therefore to affect the pore volume. These variations are nevertheless small compared to the effect of thermal maturity, which remains the major process controlling the evolution of porosity. During gas generation, high C2-C5 concentrations are generated compared to methane. The short thermal maturation duration of our experiments may have delayed the conversion of C2-C5 hydrocarbons to methane. In addition, slight differences in the concentrations of saturated and aromatic components and markedly different pore volumes and pore size distributions exist between artificially and naturally matured rocks. The conditions of artificial maturations may thus impact the thermal transformations of OM, emphasizing the necessity to investigate the role of the artificial maturation kinetics on OM and porosity.