Impacts of source zone architecture on nonaqueous phase liquid dissolution and cleanup: A magnetic resonance imaging study

Nonaqueous phase liquid (NAPL) source zone duration and the efficiency of remediation depend to a large extent on porous media heterogeneities. The objective of this study is to investigate the impacts of porous media heterogeneity on source zone architecture, aqueous flow, rate-limited dissolution and surfactant-enhanced aquifer remediation. In contrast to previous studies, a direct visualization method using magnetic resonance imaging (MRI) was developed to characterize porous media heterogeneity, aqueous phase flowpaths, and NAPL distribution in three-dimensional (3D) heterogeneous experimental aquifers with different longitudinal correlation lengths.; 19F images of entrapped NAPL showed that 90% of NAPL mass was in the coarsest sand. Most NAPL was trapped as pools not residual ganglia, NAPL saturation increased with depth, and NAPL dissolution front moved vertically from top to the bottom of the aquifers. NAPL effluent concentrations initially increased due to development of flow in zones with decreasing NAPL saturation. 1H images of water doped with a paramagnetic tracer (MnCl2) showed that water preferentially flowed through coarse sands before NAPL release. After NAPL release, flow bypassed NAPL zones and bypassing was positively related to NAPL saturation.; A one-dimensional (1D) model based on interfacial area (a t) estimated from MR images of NAPL or an empirical mass transfer correlation, and a multiple analytical source superposition technique were tested. Only the 1D model using a constant intrinsic mass transfer coefficient and at was able to predict the initial increase in effluent concentrations in experiments with high initial NAPL spill volumes.; Alternate flushing with water and two surfactant pulses (5∼6 pore volumes) resulted in 63% of NAPL mass removal from both aquifers. However, NAPL mass flux reduction was lower from aquifer 2 (68%) with a shorter correlation length than from aquifer 1 (82%). Local effluent concentrations from aquifer 2 increased by as high as 34 times after surfactant flushing. 3D MR images of NAPL revealed that NAPL migrated downward and created additional pools in previously uncontaminated areas at the bottom of the aquifers. The additional NAPL pools were created in the direction transverse to the flow in aquifer 2, which explains the higher mass flux relative to aquifer 1.