In situ monitoring of LNAPL-contaminated aquifer remediation using ultraviolet light-induced fluorescence imaging

Groundwater contamination by light non-aqueous phase liquids (LNAPLs) represents a major environmental challenge, requiring efficient and adaptive remediation methods. This study proposes and validates an innovative implementation of an in situ monitoring technique for LNAPL remediation treatments, based on ultraviolet light-induced fluorescence imaging (UVIF), combined with automated image post-treatment, to enable in situ and real-time monitoring of decontamination processes. Specifically, a mini-camera integrated into a set of transparent wells embedded in the subsurface of LNAPL-contaminated zones was used to monitor both a surfactant injection remediation process and a skimming operation involving groundwater drawdown. The technique was first calibrated and validated through laboratory and pilot-scale experiments, then tested on a real site, which was contaminated with diesel. The pilot test revealed differences of less than 5 % between recovery factors obtained via gas chromatography (GC) of soil samples and those measured with the proposed imaging technique. Site-specific calibration correlated fluorescence intensity from endoscopic images in transparent tubes with GC-analyzed LNAPL content, showing a strong correlation (R² = 0.993) and relative errors below 10 %. This enabled accurate in situ estimation of LNAPL variations: content remained mostly unchanged without treatment (<5 % variation), decreased moderately during pumping and skimming (≈35 %), and dropped substantially during surfactant injection (≈60 %).This capability supports immediate treatment adjustments, thereby optimizing pollutant recovery rates. Beyond simple monitoring, the results demonstrate that UVIF can effectively guide remediation operations by providing rapid feedback (near real-time) on contaminant removal dynamics. This near real-time capability enables optimization of treatment parameters-such as surfactant dosage, injection timing, or pumping duration-thereby improving hydrocarbon recovery efficiency. Despite some limitations, such as the need for rigorous calibration and sensitivity to UV lighting conditions, UVIF offers an alternative to conventional drilling-based methods, while improving measurement reproducibility and eliminating the need for destructive sampling.