Residual NAPL Occupancy in Pore Networks of Unconsolidated Sand Using SMT

Groundwater contamination due to petroleum products can severely impact the environment and halt human activities in the affected region. Cleanup efforts can take long periods and cost tremendous resources. Non-Aqueous phase liquids (NAPL) include (Oils, Tar, Gasoline, etc.), NAPL can seep into the subsurface and contaminate soil and water reserves. NAPL is a main source of pollution for water aquifers made of unconsolidated sand. After initial cleanup efforts, residual NAPL quantities can remain within the pores of sandy aquifers and can be a source of long-term contaminant release. Hence, understanding the distribution of residual NAPL at the pore scale in unconsolidated sand is fundamental to develop feasible and rapid techniques for remediation. The research aims to measure the quantity of residual NAPL within pores of unconsolidated sand sediments and link the occupancy percentage of oil inside the pores with the pore radius and coordination number for every pore in the system. Multiphase flow experiments of fifteen unconsolidated sand specimens were 3D scanned using x-ray synchrotron microcomputed tomography (SMT) at Argonne National Laboratory (ANL). The specimens varied between silica sand, quartz sand, and mixtures of both types, with different sand grain diameters. At the beginning of each experiment, specimens were flooded with water, followed by sterol NAPL, then flushed with water, to simulate contamination and cleanup of an aquifer. The 3D images reconstructed the systems at 9 µm3 resolution, resolving the pores and the different phases. Findings show that residual NAPL after water flushing does not occupy more than 80% of the pore volume. The majority of pores in the unconsolidated sand systems that had residual NAPL have an occupancy of around 20%. As the pore radius increases the NAPL occupancy drops below 10% of the pore volume. Pores with coordination number above four had occupancies with gap distribution of either above 50% or below 10%. The methods and findings can be used to develop groundwater remediation techniques that take advantage of micro-scale observations to determine the contaminant quantities in polluted regions and to test new methods to further reduce the residual NAPL in sandy aquifers.