LNAPL transmissivity as a remediation metric in complex sites under water table fluctuations

Water table fluctuations affect the recoverability of light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. LNAPL transmissivity (T_n) is being applied as an improved metric for LNAPL recoverability. In this paper, the applicability of T_n as a lagging and leading metric in unconsolidated aquifers under variable water table conditions was investigated. T_n values obtained through baildown testing and recovery data-based methods (skimming) were compared in three areas of a heterogeneous gasoline contaminated site in Western Australia. High-resolution characterisation methods were applied to account for differences in the stratigraphic profile and LNAPL distribution. The results showed a range of T_n from 0 m²/day to 2.13 m²/day, exhibiting a strong spatial and temporal variability. Additionally, observations indicated that T_n reductions may be more affected by the potentiometric surface elevation (Z_aw) than by the application of mass recovery technologies. These observations reflected limitations of T_n as a lagging metric and a Remedial Endpoint. On the other hand, the consistency and accuracy of T_n as a leading metric was affected by the subsurface conditions. For instance, the area with a larger vertical LNAPL distribution and higher LNAPL saturations found T_n to be less sensitive to changes in Z_aw than the other two areas during the skimming trials. T_n values from baildown and skimming tests were generally in a close agreement (less than a factor of 2 difference), although higher discrepancies (by a factor up to 7.3) were found, probably linked to a preferential migration pathway and Z_aw. Under stable Z_aw, T_n was found to be a relatively reliable metric. However, variable water table conditions affected T_n and caution should be exercised in such scenarios. Consequently, remediation practitioners, researchers and regulators should account for the nexus between T_n, LNAPL distribution, geological setting and temporal effects for a more efficient and sustainable management of complex contaminated sites.