Natural source zone depletion (NSZD) insights from over 15 years of research and measurements: A multi-site study

Intercomparison of in-well, depth profile and online soil gas measurements for estimating petroleum natural source zone depletion rates: Will in-well gas samples suffice?

Natural Source Zone Deletion (NSZD) is a viable long-term management option for sites impacted by petroleum hydrocarbon fuels. NSZD rate estimation methods for petroleum mass losses often use soil gas gradients of oxygen, carbon dioxide, methane or vapour concentrations through the vadose zone. Seeking greater efficiencies, we investigated if existing short-screened wells are reliable for representative sampling of soil gases in a vadose zone undergoing NSZD. At gasoline, diesel, aviation-gasoline, crude oil and background locations, we compared soil gas compositions from three methods: (i) multilevel samplers (VZ) on three occasions over 18 months (three per location); (ii) oxygen sensors (GP) at multiple depths and tracked over 3 years (one per location); and (iii) short-screened monitoring wells (MW) across the water table (five per location) and sampled as per VZ. GP sensors showed seasonal and other variations, yielding variable NSZD estimates. Oxygen depth profiles from GP and VZ measurements showed good correspondence despite fluctuations. Oxygen, carbon dioxide and methane concentrations from the deepest sampling ports of VZ samplers correlated strongly with concentrations from adjacent MW installations, indicating in-well sampling in the field can provide reliable data. The advantages and limitations of using MW data in accelerating NSZD field assessments are outlined.

Natural Source Zone Depletion of crude oil, gasoline, diesel and aviation gasoline petroleum in the same soil/aquifer system - An intensive intercomparison field study and simulation

Petroleum biodegrades and naturally depletes. Natural Source Zone Depletion (NSZD) quantifies this at petroleum affected sites in support of management decisions for cessation of active remediation efforts. Whilst a range of NSZD estimates and methods are available, side by side comparison of NSZD rates across petroleum types in the same soil/groundwater system are lacking, especially linked to the weathering status of petroleum. At a former refinery site near Perth Western Australia, locations contaminated by crude oil, gasoline, diesel and aviation gasoline, have been intensively instrumented to enable (i) measurement of vadose zone major gas (O2, CO2, CH4), volatile organic compounds (VOCs) and temperature depth profiles, (ii) online near-continuous temperature, water level, O2 and VOC concentrations, (iii) depth profiles of groundwater parameters, and (iv) in-well gases, temperatures, water and LNAPL thickness. These measurements were compared to a background location with no history of contamination. Multiple coring events were also undertaken to determine LNAPL mass and its vertical distribution at each location. Additionally, LiCor and Eflux was conducted to measure CO2 fluxes at ground surface. NSZD rates were estimated from the measurement methods across the four petroleum types and the background site. Despite NSZD estimates that differed across some methods at sites (for example at the gasoline and diesel sites Eflux/LiCor estimates were consistently lower by a factor of 3-4 than those obtained using oxygen/temperature depth profile data) the minimum-maximum range of mean NSZD rates showed a distinct decreasing order across the fuel types: highest rates being aviation gasoline (69,000-91,000 L/ha/y), then gasoline, diesel and crude oil (2700-6200 L/ha/y). Reasons for differences are explored. Analysis of LNAPL in cores and from wells, historical data comparisons and simulations over 50 years, show that composition, age and weathering of the releases are critical to current and long-term NSZD mass losses and rate estimates.

Soil gas gradient method for estimating natural source zone depletion rates of LNAPL and specific chemicals of concern

This paper presents a simplified approach for the soil gas gradient method for estimating natural source zone depletion (NSZD) rates of specific contaminants of concern (COCs) at sites contaminated by light non-aqueous phase liquids (LNAPL). Traditional approaches to quantify COC-specific NSZD rates often rely on numerical or analytical reaction-transport models that require detailed site-specific data. In contrast, the proposed method employs simple analytical solutions, making it more accessible to practitioners. Specifically, it requires only the maximum soil gas concentration, the effective diffusion coefficient, and the diffusive reaction length calculated from vertical soil gas concentration profiles. The simplified approach was validated against a reactive transport numerical model reported in the literature, showing consistent results within the same order of magnitude for BTEX NSZD rates at a gasoline spill site in South Carolina. Further validation using a larger dataset involved comparing NSZD rate estimates for benzene and total petroleum hydrocarbons (TPH) against those obtained using BioVapor, utilizing empirical soil gas data from the USEPA Petroleum Vapor Intrusion Database. Results demonstrated a strong correlation between NSZD rates and maximum soil gas concentrations, allowing the development of a rapid screening approach based only on the measured soil gas concentrations and literature values for diffusion coefficients and diffusive reaction lengths. This approach aligned well with previous modeling studies and was consistent with literature values for TPH NSZD rates. Overall, both the simplified and screening approaches offer practical, easy-to-use tools for evaluating temporal variability in natural attenuation rates, supporting baseline assessments and ongoing performance evaluations of remediation at LNAPL sites.

Sorption and attenuation of petroleum VOCs in five unsaturated soils: Microcosms and column experiments

The fate of volatile organic compounds (VOC) vapors in the unsaturated zone is the basis for evaluating the natural attenuation potential and vapor intrusion risk. Microcosm and column experiments were conducted to study the effects chemical speciation and soil types/properties on the fate of petroleum VOCs in unsaturated zone. The biodegradation and total attenuation rates of the seven VOCs obtained by microcosm experiments in black soil and yellow earth were also generally higher than those in floodplain soil, lateritic red earth, and quartz sand. The VOC vapors in floodplain soil, lateritic red earth, and quartz sand showed slow total attenuation rates (<0.3 d-1). N-pentane, methylcyclopentane, and methylcyclohexane showed lower biodegradation rates than octane and three monoaromatic hydrocarbons. Volatilization into the atmosphere and biodegradation are two important natural attenuation paths for VOCs in unsaturated soil columns. The volatilization loss fractions of different volatile hydrocarbons in all five unsaturated soils were generally in the order: n-pentane (93.5%-97.8%) > methylcyclopentane (77.2%-85.5%) > methylcyclohexane (53.5%-69.2%) > benzene (17.1%-73.3%) > toluene (0-45.7%) > octane (1.9%-34.2%) > m-xylene (0-5.7%). The fractions by volatilization into the atmosphere of all seven hydrocarbons in quartz sand, lateritic red earth, and floodplain soil were close and higher compared to the yellow earth and black soil. Overall, this study illustrated the important roles chemical speciation and soil properties in determining the vapor-phase transport and natural attenuation of VOCs in the unsaturated zone.

Identification of carbon fixation microorganisms and pathways in an aquifer contaminated with long-chain petroleum hydrocarbons

Petroleum hydrocarbons (PHCs) can be biodegraded into CO2, and PHC-contaminated aquifers are always deemed as carbon sources. Fortunately, some carbon fixation microorganisms have been found in PHC-contaminated sites. However, most of the studies are related to volatile short-chain PHC, and few studies focus on long-chain PHC-contaminated sites. To reveal the carbon fixation microorganisms in these sites, in the study, a long-chain PHC polluted site in North China was selected. Through hydrochemical and metagenomics analysis, the structure and capacity of carbon fixing microorganisms in the site were revealed. Results showed that there were many kinds of carbon fixed microorganisms that were identified such as Flavobacterium, Pseudomonas. HP/4HB, rTCA, and DC/4HB cycles were dominated carbon fixation pathways. The long-chain PHC were weakly correlated with carbon fixation microorganisms, but it may stimulate the growth of some carbon fixation microorganisms, such as microorganisms involved in rTCA cycle. PRACTITIONER POINTS: The microorganisms with carbon fixation gene exist in the aquifer contaminated by long-chain petroleum hydrocarbon. Microorganisms that have the ability to degrade petroleum also have the ability to carbon fixation. Long-chain petroleum hydrocarbon may promote the growth of carbon fixation microorganisms.

Impacts of LNAPL types on mechanisms and rate of natural source zone depletion

Understanding the mechanisms of natural source zone depletion (NSZD) will support an improved understanding of the long-term sustainability of NSZD as a site remedy and how NSZD rates may change over time. This is the first study that has quantified and compared the rate of three NSZD mechanisms (methanogenesis, vaporization, and aqueous biodegradation) between two chemically distinct light non-aqueous phase liquid (LNAPL) source zones (aliphatic-rich naphtha for Zone #1 vs aromatic-rich pyrolysis gasoline for Zone #2) within the same geologic and climate conditions. The rates of NSZD attributable to vaporization (400 mg C/m2/d vs. 300 mg C/m2/d) and aqueous biodegradation (92 mg C/m2/d vs. 67 mg C/m2/d) were similar for Zone #1 and #2; however, the rate of methanogenesis NSZD was 6x higher in Zone #1 (1000 mg C/m2/d vs. 170 mg C/m2/d). These results suggest that the aliphatic hydrocarbons content in an LNAPL source may be a factor in the rate of methanogenesis NSZD. For both Zone #1 and #2, total NSZD rate determined using this "three mechanism" measurement method was in reasonable agreement with two other methods used to measure total NSZD rates (CO2 Gradient Method and Dynamic Closed Chamber Method), validating the "three mechanism" method as a tool to measure the total NSZD rate at a site and to provide an improved understanding of the predominant NSZD mechanism. Overall, this study highlights the importance of LNAPL type and chemical characteristics in determining source zone natural attenuation mechanism and its total rates.

Experimental research on the transport-transformation of organic contaminants under the influence of multi-field coupling at a site scale

Organic-contaminated shallow aquifers have become a global concern of groundwater contamination, yet little is known about the coupled effects of hydrodynamic-thermal-chemical-microbial (HTCM) multi-field on organic contaminant transport and transformation over a short time in aquifers. Therefore, this study proposed a quick and efficient field experimental method for the transport-transformation of contaminants under multi-field coupling to explore the relationship between organic contaminants (total petroleum hydrocarbon (TPH), polycyclic aromatic hydrocarbons (PAHs), benzene-toluene-ethylbenzene-xylene (BTEX) and phthalates acid esters (PAEs)) and multi-field factors. The results showed that hydrodynamics (affecting pH, p < 0.001) and temperature (affecting dissolved oxygen, pH and HCO3-, p < 0.05) mainly affected the organic contaminants indirectly by influencing the hydrochemistry to regulate redox conditions in the aquifer. The main degradation reactions of the petroleum hydrocarbons (TPH, PAHs and BTEX) and PAEs in the aquifer were sulfate reduction and nitrate reduction, respectively. Furthermore, the organic contamination was directly influenced by microbial communities, whose spatial patterns were shaped by the combined effects of the spatial pattern of hydrochemistry (induced by the organic contamination pressure) and other multi-field factors. Overall, our findings imply that the spatiotemporal patterns of organic contaminants are synergistically regulated by HTCM, with distinct mechanisms for petroleum hydrocarbons and PAEs.

On quantifying global carbon emission from oil contaminated lands over centuries

Petroleum releases into the subsurface contribute to global soil carbon emissions. Quantifying releases and changes in releases of carbon from soils over the lifetime of a spill is complex. Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) embodies all key mechanisms for transformation to carbon gases and their release from soils including partitioning, transport and degradation of petroleum components. Quantification of the interconnected behaviours of the soil microbiome, fluid flow, multi-component transport, partitioning, and biodegradation is crucial for understanding NSZD. Volatilization from LNAPL, aerobic biodegradation, methanogenesis, and heat production all lead to release of greenhouse gases to the atmosphere. To estimate carbon emissions, using a validated computational platform, we modelled the long term NSZD of four petroleum hydrocarbon types; crude oil, diesel, jet fuel and gasoline, to span the major products used globally. For two soil types, we estimated 150 years of carbon emissions from annual minor and 25 mostly major petroleum hydrocarbon land release incidents since 1950 - with an estimated released mass of ~9 million tonnes across the circumstances considered. Up to 2100 the mass of carbon emitted to the atmosphere is estimated to range from 4 to 6 Teragrams, with nearly 60 % currently released. Nomographs generated help predict the fate of LNAPL plumes and carbon emissions due to NSZD, which is crucially important to management of soil and groundwater contamination. The method provides a basis to include additionally identified and future petroleum releases. It is noted that the petroleum mixture composition, degradation rates, volatilization, and subsurface characteristics all can influence carbon emission estimations.

Groundwater and soil contamination by LNAPL: State of the art and future challenges

Contamination by Light Non-Aqueous Phase Liquids (LNAPL) represents a challenge due to the difficulties encountered in its underground assessment and recovery. The major risks arising from subsoil LNAPL accumulation face human health and environment, gaining a social relevance also in the frame of a continuously changing climate. This paper reports on a literature review about the underground contamination by LNAPL, with the aims of providing a categorization of the aspects involved in this topic, analyzing the current state of the art, underlying potential lacks and future perspectives. The review was focused on papers published in the 2012-2022 time-interval, in journals indexed in Scopus and WoS databases, by querying "LNAPL" within article title, abstract and/or key words. 245 papers were collected and classified according to three "key approaches" -namely laboratory activity, field based-data studies and mathematical simulations- and subordinate "key themes", so to allow summarizing and commenting the main aspects based on the application setting, content and scope. Results show that there is a wide experience on plume dynamics and evolution, detection and monitoring through direct and indirect surveys, oil recovery and natural attenuation processes. Few cues of innovations were found regarding both the use of new materials and/or specific field configuration for remediation, and the application of new techniques for plume detection. Some limitations were found in the common oversimplification of the polluted media in laboratory or mathematical models, where the contamination is set within homogeneous porous environments, and in the low number of studies focused on rock masses, where the discontinuous hydraulic behavior complicates the address and modeling of the issue. This paper represents a reference for a quick update on the addressed topic, along with a starting point to develop new ideas and cues for the advance in one of the greatest environmental banes of the current century.

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

An increasing number of studies have demonstrated that natural source zone depletion (NSZD) in the vadose zone accounts for the majority (90%~99%) of the natural attenuation of light non-aqueous phase liquid (LNAPL). Until now, 0.05 to 12 kg/a.m² NSZD rates at tens of petroleum LNAPL source zones have been determined in the middle or late evolution stage of LNAPL release, in which limited volatile organic compounds (VOCs) and methane (CH₄) were detected. NSZD rates are normally estimated by the gradient method, yet the associated functional microbial activity remains poorly investigated. Herein, the NSZD at an LNAPL-releasing site was studied using both soil gas gradient methods quantifying the O₂, CO₂, CH₄, and VOCs concentrations and molecular biology methods quantifying the abundance of the pmoA and mcrA genes. The results showed that the methanogenesis rates were around 4 to 40 kg/a.m². The values were greater than the rates calculated by the sum of CH₄ escaping (0.3~1.2 kg/a.m²) and O₂ consuming (3~13 kg/a.m²) or CO₂ generating rates (2~4 kg/a.m²), suggesting that the generated CH₄ was oxidized but not thoroughly to CO₂. The functional gene quantification also supported the indication of this process. Therefore, the NSZD rates at the site roughly equaled the methanogenesis rates (4~40 kg/a.m²), which were greater than most of the previously studied sites with a 90th percentile value of 4 kg/a.m². The study extended the current knowledge of the NSZD and has significant implications for LNAPL remediation management.