Indicators that large-amplitude water-table fluctuations control natural source zone pathways.

Natural source zone depletion (NSZD) has become central to the management of hydrocarbon-contaminated sites, and the methods established to quantify it rely on the gaseous or thermal expression of biodegradation in the vadose zone. These methods were developed in temperate settings, where the water table fluctuates modestly and typically where the LNAPL remains within reach of the air phase. Tropical and subtropical environments differ in two respects that bear directly on this premise: they experience large-amplitude seasonal water-table fluctuations, and their soils sustain high natural CO₂ partial pressures. Here we reinterpret fifteen years of investigation (2002 to 2017) at a jet-fuel-contaminated site in a subtropical climate, combining previously published hydrological, geochemical, isotopic and microbiological records with new analyses of the LNAPL n-alkane distribution, the joint dynamics of water table and LNAPL thicknesses, and laser-induced fluorescence profiles referenced to the water-table range. Seasonal fluctuation commonly reaches 2 m and multi-annual variation approaches 4 m, against 0.9 and 0.5 m at the low-rainfall sites on which much of the NSZD framework was built. The LNAPL-bearing interval extends over more than 3 m and its base lies below the lowest recorded water level, so that a large proportion of the oil remains submerged. A geochemical model reproduces the source-zone water, matching the measured ferrous iron and methane and accounting for its alkalinity to within about 18%, through the degradation of about 4 mg L-1 of toluene by methanogenesis and goethite reduction, and requires a CO₂ partial pressure matching the value independently inferred from the Keeling-plot intercept. Because aerobic degradation alters none of these parameters, this figure is a lower bound, and the aliphatic fraction is depleted without leaving any aqueous trace. Assessments based solely on vadose-zone CO₂ may therefore substantially underestimate the depletion at such sites.