Laser-induced fluorescence logging as a high-resolution characterisation tool to assess LNAPL mobility

High-resolution characterisation tools such as Laser-Induced Fluorescence (LIF) logging represent a step forward towards a more effective management of sites contaminated by light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. In this paper, the applicability of LIF response as an indicator of LNAPL mobility at one site with an unconsolidated aquifer was investigated. LIF profiles were logged adjacent to twin coring locations and wells with LNAPL transmissivity (T_n) measurements in a heterogeneous gasoline contaminated site in Western Australia. LIF response was correlated to T_n to a greater extent than LNAPL saturation (S_n) measurements from coring. In particular, LIF signal maxima were a better indicator of T_n than the integral LIF signal. Furthermore, LIF allowed rapid identification of areas with long-term near-immobile LNAPL (entrapped and residual) because of the multi-wavelength waveforms associated with distinct subsurface characteristics. It was also demonstrated that the delineation of presumably less-mobile intervals could be enhanced by using the relative LIF response in the 350 nm wavelength channel. Thus, this work gave evidence that LIF logging provides valuable information about LNAPL distribution and mobility in commonly found subsurface settings, despite generally poor correlations with S_n measurements. LIF probes can be successfully used to guide the installation and application of more costly conventional methods in addition to the development of existing site models.