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.

The effects of water table fluctuation on LNAPL deposit in highly permeable porous media: A coupled numerical and experimental study.

Light Non-Aqueous Phase Liquid (LNAPL) flow on the water table is highly mobile and is sensitive to the fluctuation of groundwater. This process is highly complex and involves the migration of three immiscible phases (i.e. water, LNAPL and air) which need the explicit definition of multiple parameters. A coupled experimental and numerical simulation methodology is performed by using Time Domain Reflectrometer (TDR) and multiphase simulation of a controlled environment to mimic the water table fluctuation and its effect on the LNAPL residual saturation. TDR probes are installed in different locations of a 2D tank (i.e. a cuboid box with relatively low off-plane thickness) and the bulk permittivity of the phases are measured through artificially imposed boundary conditions. The bulk permittivity is then translated into saturation of the three different phases. The translated residual saturations along with the previously measured porous media properties (e.g. porosity and saturated permeability) are then inserted into the numerical simulator (i.e. COMSOL Multiphysics®) and the migration of the three phase in porous media is simulated. The numerical exponents and entry pressures needed for the simulation of the multiphase flow are estimated using the temporal experimental values. The exponents of water LNAPL relative permeability were estimated to be around 2 while the exponents gas LNAPL relative permeability were estimated to be closer to 3. The results, simulated with the optimized parameters, are then evaluated with pictures taken from the transparent face of the 2D tank different stages of the experiment. The temporal evolution of different phase saturation has been compared and validated between the experimental results obtained and interpreted by the TDR probe measurements and the simulations. The relative error stays in the 5 % confidence level for most reported points and only in the highly dynamic flow time steps the error reaches around 12% which are discussed in the text and is accepted due to the highly nonlinear nature of the problem.

DNAPL flow and complex electrical resistivity evolution in saturated porous media: A coupled numerical simulation.

Induced Polarization (IP) is a non-intrusive geophysical method to monitor Dense Non-Aqueous Phase Liquid (DNAPL) contamination and remediation processes underground. In this study, an advanced numerical code simulating DNAPL flow and complex electrical resistivity is presented. The model was validated against existing IP results and image measurements that were carried out previously in a series of 2D tank experiment. Multiphase flow modeling in porous media is coupled with electrical current modeling to simulate the process of DNAPL migration and the associated IP response. This brings a broader view of the contamination in space and time compared to surface and borehole measurements, especially when the results are supported by field measurements or laboratory experiments. The simulations are developed in 3D and are performed in COMSOL Multiphysics®. The simulations using petrophysical relationships for in-phase and quadrature resistivity and the results of the experiments are in complete accordance with each other in the parts of the tank where the saturation of DNAPL is relatively low (i.e. especially in the cone of depression in the pumping scenario). However, the parts associated with high saturation of DNAPL show high errors between the in-phase resistivity simulations and the results from experiments. The present work can be regarded as a preliminary study toward further applications of coupled IP-multiphase flow for more accurate detection and monitoring of DNAPLs. It is suggested that the choice of tool/approach in this study be extended to larger-scale studies for further investigation.

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.