Evaluation of LNAPL distribution and mobility in heterogeneous porous media under water table fluctuations

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This research is focused on examining LNAPL distribution and mobility in heterogeneous porous media under the effect of water table fluctuations. Although it has been recognised that the potentiometric surface (Z𝑎𝑤) changes may play a crucial role on LNAPL transmissivity (T𝑛), the related field based published work that has directly address it is limited and provides little insight regarding the impact of geological heterogeneity and complex NAPL distributions. This research work encompasses field studies in a heterogeneous gasoline contaminated site that presents seasonal water table fluctuations of 2 - 3 m. LNAPL distribution and hydrogeological conditions were evaluated using diagnostic gauge plots, hydrostratigraphs, contaminant concentrations in cores, high resolution characterisation methods, LDRM simulations and baildown testing. T𝑛 was evaluated using bail-down testing during both seasonal water table fluctuations and during induced hydraulic conditions (skimming, water-enhanced skimming, vacuum-enhanced skimming and water- and vacuum-enhanced skimming). Collectively these tools and approaches have given useful insight into NAPL distribution and mobility under both natural and induced hydraulic conditions during seasonal changes and mass recovery applications. Results indicated that the NAPL hydrogeological conditions were related to the behavior of Z𝑎𝑤 and both confined and unconfined conditions can be found in the same well seasonally. Additionally, in heterogeneous settings the stratigraphy may override typical smearing patterns related to homogeneous systems. T𝑛 ranged from 0.03 m²/day to 2.13 m²/day under unconfined LNAPL conditions, exhibiting a strong spatial variability. An inverse relationship between the Z𝑎𝑤 and T𝑛 was found under natural conditions. The area with a wider NAPL distribution and higher NAPL saturations was less sensitive to changes in Z𝑎𝑤 during the skimming trials. The calculated T𝑛 values from bail- down and skimming testing were in a close agreement and showed similar trends in most of the situations (less than a factor of 2 difference). However, in areas with narrow NAPL distribution and low NAPL saturations the discrepancy between both methods presented clearly higher differences (by a factor up to 7.3) and was related to Z𝑎𝑤. Under stable Z𝑎𝑤, T𝑛 was found to be a relatively reliable metric for LNAPL recoverability. In contrast, variable water table conditions affected the evolution of Tn. Consequently, remediation practitioners, researchers and regulators should account for the nexus between T𝑛, LNAPL distribution, geological setting and temporal effects for a more efficient management of NAPL contaminated sites.
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