Formatted Title
Identification of Mobile, Residual and Entrapped LNAPL Using Laser-Induced Fluorescence as a Line of Evidence
Background/Objectives
Effective management of fuels such as gasoline, diesel and jet fuel released into the subsurface requires correct delineation of the entrapped, residual and free light non-aqueous phase liquid (LNAPL) fractions. An important advance towards this goal has been the development of direct sensing technologies such as laser-induced fluorescence (LIF). However, LIF response can be influenced by several factors, including geological and LNAPL properties. The objective of this work is to explore the use of LIF logs as a line of evidence to identify mobile, residual and entrapped LNAPL under field conditions.
Approach/Activities
The employed LIF system was the Ultra-Violet Optical Screening Tool (UVOST®) by Dakota Technologies, Inc. (USA), which incorporates a 308-nm excimer laser source and records fluorescence intensity and lifetime at four wavelengths: 350, 400, 450 and 500 nm. Data from multiple sites across Australia were collected, including 20 UVOST® logs, three cores and more than 50 baildown tests at a service station where confined and unconfined LNAPL conditions were observed. LIF intensity values and multi-wavelength waveforms were compared to LNAPL transmissivity (Tn) estimates, historical in-well fluid levels and LNAPL saturation values derived from physical samples. Multi-wavelength waveforms were analyzed to discriminate between LNAPL contamination and natural fluorescence from the geological medium as well as investigating the influence of factors like oxygen presence and natural source zone depletion processes.
Results/Lessons Learned
At the service station site, LIF response exhibited better correlation with Tn than with LNAPL saturation measurements in adjacent sampling locations. This could be explained by the subsurface heterogeneous conditions and the relatively consistent LNAPL composition found across the site. Improved delineation of LNAPL-impacted intervals could be performed by analyzing the multi-wavelength waveforms and lifetime data since very low LIF intensity values may still reflect LNAPL presence. Examples from different sites showed that LIF logging could be a valuable tool to rapidly identify areas of long-term confined, entrapped or residual LNAPL conditions. These aspects may be difficult to detect even after long monitoring campaigns through conventional methods. High-resolution delineation of LNAPL source zones using LIF methods can greatly contribute to building more robust conceptual site models and design more effective and sustainable management plans.