Radiometric dating and biodegradation assessment of Light Non-Aqueous Phase Liquids (LNAPL): Results from two case studies

Abstract

This study introduces a comprehensive methodology that integrates radiometric dating with gas chromatographic (GC) analyses of light non-aqueous phase liquids (LNAPL) recovered from monitoring wells at different filling stations. The aim is to evaluate the temporal evolution of LNAPLs in two distinct scenarios: natural attenuation (at site 1) and ongoing remediation activities (at site 2). The study period spans one year and four months, from June 2020 to October 2021. Throughout this time frame, the study monitored the progress of petroleum hydrocarbon biodegradation by analysing selected compounds: Total Petroleum Hydrocarbons (TPH), Σ n-alkanes (C₁₃ to C₁₈), Σ isoprenoids (C₁₅ to C₂₀), C₁₇/Pristane and C₁₈/Phytane ratios, and a specific biodegradation diagnostic index. The age determination provided reliable results for a single-point model release scenario (site 1). However, at site 2, where multiple LNAPL spills occurred, the age determination resulted in only an apparent age, due to the mixing of different fluids. Nevertheless, the unlike spatial distribution of the fluids offers some chances to estimate the age of the end members. The trend of the biodegradation in LNAPLs recovered from the two sites seems similar, pointing to a nearly complete removal of isoprenoids, independent of the initial values and whether or not there is an active remediation process. The main difference between the sites is the apparent tendency to the reduction of the heaviest (TPH) and more persistent compounds (such as pristane and phytane), probably triggered by the electrokinetic oxidation system taking place in the second site. The importance of these initial experimental findings lies in their ability to evaluate the site-specific response to pollutants, thereby enhancing the effectiveness of remediation efforts over time.