Methane soil gas gradient method for quantifying natural source zone depletion rates at petroleum contaminated sites

This study presents a novel method that relies on the methane gradient in soil gas for estimating natural source zone depletion (NSZD) rates of light non-aqueous phase liquids (LNAPL) in the subsurface. Methane generation via methanogenesis at the LNAPL source, followed by methane oxidation in the unsaturated zone, is typically the rate-limiting degradation pathway and can, therefore, serve as a reliable indicator for NSZD rate estimation of bulk LNAPL. Considering that methanogenesis associated with natural soil respiration processes is often negligible, this method can be used to directly convert methane fluxes into NSZD rates. Unlike other methods that focus on O₂, CO₂ or volatile organic compounds (VOCs), this approach is based on an analytical model that incorporates both diffusion and advection-driven transport of methane in soil gas. The application of this model supports the general assumption that diffusion dominates methane transport in the air-connected vadose zone, except in scenarios with high-pressure gradients (e.g., 10 Pa/m) and high soil permeability (e.g., sandy soils), where advection becomes significant relative to diffusion. Additionally, the analysis shows that the overall methane velocity in the aerobic oxidation zone, in most cases, falls within the range of 0.1-1 m/d. By multiplying this velocity by the maximum methane concentration in soil gas and the stoichiometric coefficient of the reference hydrocarbon compound (e.g., 1.14 g_C8H18/g_CH4 for octane), a reliable estimate of the NSZD rate can be derived. When applied to typical soil gas concentrations, this methane gradient method yields NSZD estimates consistent with values reported in the literature, validating its use as a simplified screening approach.