Enhanced mobility of non aqueous phase liquid (NAPL) during drying of wet sand

Groundwater and soil contamination by LNAPL: State of the art and future challenges

Contamination by Light Non-Aqueous Phase Liquids (LNAPL) represents a challenge due to the difficulties encountered in its underground assessment and recovery. The major risks arising from subsoil LNAPL accumulation face human health and environment, gaining a social relevance also in the frame of a continuously changing climate. This paper reports on a literature review about the underground contamination by LNAPL, with the aims of providing a categorization of the aspects involved in this topic, analyzing the current state of the art, underlying potential lacks and future perspectives. The review was focused on papers published in the 2012-2022 time-interval, in journals indexed in Scopus and WoS databases, by querying "LNAPL" within article title, abstract and/or key words. 245 papers were collected and classified according to three "key approaches" -namely laboratory activity, field based-data studies and mathematical simulations- and subordinate "key themes", so to allow summarizing and commenting the main aspects based on the application setting, content and scope. Results show that there is a wide experience on plume dynamics and evolution, detection and monitoring through direct and indirect surveys, oil recovery and natural attenuation processes. Few cues of innovations were found regarding both the use of new materials and/or specific field configuration for remediation, and the application of new techniques for plume detection. Some limitations were found in the common oversimplification of the polluted media in laboratory or mathematical models, where the contamination is set within homogeneous porous environments, and in the low number of studies focused on rock masses, where the discontinuous hydraulic behavior complicates the address and modeling of the issue. This paper represents a reference for a quick update on the addressed topic, along with a starting point to develop new ideas and cues for the advance in one of the greatest environmental banes of the current century.

Magnetic resonance imaging of enhanced mobility of light non aqueous phase liquid (LNAPL) during drying of water wet porous media

Visualization of NAPLs in multiphase systems in porous media is important for determining contaminant transport in the environment. In this study, magnetic resonance imaging (MRI) was used to confirm the recent observations of mobilisation of a light non aqueous phase liquid (LNAPL) trapped in wet sand under natural drying conditions of the wet porous medium. Visualization of LNAPL (motor oil) and water mobility during the drying of wet glass beads (0.5 mm) in a cylindrical glass column (15 mm ID, 45 mm long) was obtained using spin echo-based NMR microimaging performed at 500 MHz, corresponding to a field of ca. 11.75 T. Sagittal and axial images of LNAPL and water in the porous medium were obtained at a spatial resolution of 59 μm/pixel at different time intervals. A rise of 15-20 mm was observed in the presence of evaporation of water as compared to a 2-3 mm rise in the absence of evaporation in a time span of about 1400 min. The spatio-temporal MRI scans of the water and LNAPL in the glass column reveals that LNAPL rise occurs when the water evaporation front reaches the LNAPL layer. This implied that the enhanced LNAPL rise was strongly linked to the process of water evaporation. A linear correlation of the MRI signal intensities of LNAPL and water with reference to different saturation levels of LNAPL and water in the porous media was obtained. This calibration information was used to quantify the saturation levels of the LNAPL and water during the drying process. These findings show the application of non-invasive techniques such as MRI in quantifying and understanding the mechanism of fate and transport of LNAPLs in porous media, towards effective environmental quality assessment.