A comparison of three methods to assess natural source zone depletion at paved fuel retail sites

Abstract

Natural Source Zone Depletion (NSZD) encompasses all processes that result in petroleum hydrocarbon light non-aqueous phase liquid (LNAPL) mass loss. Vertical gas transport between the subsurface and atmosphere is a key component of NSZD. Gas exchange with the atmosphere may be restricted at sites with ground cover which is typical for European fuel retail sites. This raises questions of whether, and to what extent, the generic NSZD conceptual model applies at these sites. Here, we present a study that evaluated how concrete and asphalt pavement affected NSZD processes and data interpretation for three NSZD assessment methods: soil gas concentration gradient, biogenic heat, and carbon dioxide traps.All methods demonstrated that NSZD was occurring and NSZD rates were generally within the low-end of values reported in literature for unpaved sites. However, there was considerable variability in the rates, which highlights the need for careful examination of the conceptual site model and potential interferences for each method. The results demonstrate the viability of soil gas and temperature data collected from existing monitoring wells screened into the unsaturated zone without the need for additional, intrusive subsurface installations. The results also provide useful guidance for developing optimal long-term NSZD monitoring approaches, where necessary.

Incorporating the Soil Gas Gradient Method and Functional Genes to Assess the Natural Source Zone Depletion at a Petroleum-Hydrocarbon-Contaminated Site of a Purification Plant in Northwest China

“…All A–C soil gas profiles displayed more apparent O 2 usage and CO 2 and CH 4 production in the deeper subsurface. The concentrations of VOCs were about 2 to 4 orders lower than CO 2 and CH 4 , which suggested that nonmethane hydrocarbon contaminants were scarcely depleted through volatilization, but were rather potentially biodegraded by functional microorganisms in the source zone, consistent with previous studies [ 4 , 13 , 14 , 15 , 16 , 17 , 19 ]. For the background (uncontaminated) samples, no VOCs and CH 4 were generated.…”

“…Gradient methods were then used to estimate NSZD rates in various LNAPL-contaminated sites, including crude oil, diesel, jet fuel, and gas condensate, etc. [ 4 , 13 , 14 , 15 , 16 , 17 ].…”

Rapid Field Approach to Evaluating Natural Source Zone Depletion for a Range of Light Non‐Aqueous Phase Liquid Sites

“…This, however, cannot be done for the monitoring well analysis as only a single depth is available at the base of the vadose zone. Recently, Smith et al (2020) have shown that a second soil‐gas sample can be collected in monitoring wells at the top of the screened interval to apply the gradient method. We did not do this and decided to determine if a gradient could be estimated from the shallow soil‐gas measurements as not all monitoring wells are screened at shallow depths.…”

Assessing the Natural Source Zone Depletion of a Petroleum-Contaminated Clayey Soil Site in Southern China Combining Concentration Gradient Method and Metagenomics

“…According to a large number of field case data [ 1 , 3 , 5 , 11 , 12 , 20 , 29 , 36 ], there are generally two situations in the profile gas concentration. One is that, within a considerable depth range from the surface to the contaminated source area, oxygen and carbon dioxide have a constant concentration gradient, and methane is almost non-existent within this range.…”

“…The methods currently used to assess NSZD rates of petroleum hydrocarbon-contaminated sites mainly rely on subsurface gas fluxes and thermal fluxes associated with natural degradation processes [ 10 ]. Commonly used methods include the gas concentration gradient method, dynamic chamber method, carbon capture method, and thermodynamic gradient method [ 10 , 11 , 12 ]. Many studies have used different methods to evaluate the natural attenuation of contaminated sites, even making improvements and corrections to these methods [ 10 , 13 , 14 , 15 , 16 , 17 , 18 ].…”