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 gC8H18/gCH4 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.

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.

At contaminated sites, groundwater monitoring results are commonly used (quantitatively or qualitatively) to predict remediation timeframes. If results are predictive, then there should be a strong positive correlation between attenuation rates for the first half of a temporal monitoring record and attenuation rates for the second half of the same record. We utilized the GeoTracker database to evaluate the power of historical groundwater monitoring results to predict future attenuation rates. For two data sets (petroleum and chlorinated solvent), we found a small negative correlation between the first-order concentration vs. time attenuation rate observed during the earlier part of the monitoring record and the later part of the monitoring record: benzene-correlation coefficient (r) = -0.11, methyl tert-butyl ether (MTBE)-r = -0.12, trichloroethene (TCE) = -0.12. For each data set, a small negative correlation between the first-order attenuation rate observed during the earlier part of the monitoring record and the later part of the monitoring record was also observed for a subset of monitoring records exhibiting the best model fits (R-2 > 0.8), a subset with a statistically significant (p < 0.05) positive attenuation rate for the first half of the monitoring record. For the TCE data set, this negative correlation was also observed for a subset of monitoring records with no change in site remedy during the monitoring period (r = -0.22). Our analysis suggests that the historical concentration vs. time attenuation rate for a contaminant at an individual site or monitoring well is a poor predictor of the future rate.

Currently, there are over 62,000 petroleum-contaminated sites undergoing remediation in the United States. Understanding attenuation rates for dissolved contaminant concentrations and factors that influence these rates is important to effectively manage these sites and move them towards closure. The GeoTracker and California Department of Public Health databases provide more than 15 years of groundwater monitoring results for tens of thousands of monitoring wells and public water supply wells in California. First-order maximum concentration attenuation rate constants (k(c-max)) were determined for sites with 5 or more years of monitoring data over the time period of 2002 to 2017. k(c-max) values vary by constituent; across more than 5000 sites, the median k(c-max) value ranged from 0.30 year(-1) for methyl tert-butyl ether (MTBE) (half-life of 2.3 years) to 0.076 year(-1) for naphthalene (half-life of 9.1 years). A focused evaluation of remediation technologies at 50 petroleum release sites indicates that dissolved contaminant attenuation typically increased during the time period of remediation. Further, the analysis indicates that biodegradation is an important attenuation process even during periods without active remediation (i.e., periods of natural attenuation). There was little evidence of petroleum and fuel oxygenate impacts to public water supply wells. Overall, the analyses indicate that both remediation and natural biodegradation contribute to site remediation and that these attenuation processes have been sufficient to prevent widespread impacts to public water supply wells.

This paper describes the proceedings of a Special Session on Natural Source Zone Depletion (NSZD) at the AquaConSoil conference, held virtually in June 2021. It spans research across Europe, the United States of America and Australia. NSZD has been described as the ‘combination of processes that reduce the mass of LNAPL light non-aqueous phase liquid) in the subsurface’. LNAPL NSZD research and investigations have been focused on a range of hydrocarbon products, such as gasoline, diesel, jet fuel, as well as crude oil. Key NSZD processes include aerobic biodegradation, fermentation and methanogenesis of LNAPL constituents, dissolution of LNAPL constituents into groundwater and volatilization of LNAPL constituents into the unsaturated zone. In turn the generated methane, carbon dioxide and dissolved/volatilized constituents can be cycled and biodegraded in the unsaturated and saturated zones. Importantly these physical, chemical, and biological processes can act without human intervention to reduce the NAPL mass and toxicity. Over time NSZD can both reduce LNAPL mass, and change its chemical composition resulting in risk reduction, and ultimately source depletion. Methanogenesis of organic materials has long been recognized in municipal landfills and natural anoxic environments, such as peat and wetlands. Recognition of similar processes in LNAPL source zones in the past decade along with high rates of aerobic biodegradation observed in unsaturated zones above LNAPL-impacted areas, has significantly revised the conceptual model of LNAPL source zone behaviour and persistence. Several NSZD monitoring approaches have been developed and are being applied in field studies. While the quantitative NSZD rates derived can vary between techniques, they all demonstrate that NSZD LNAPL removal can exceed that delivered by engineered LNAPL recovery techniques, particularly for mature LNAPL bodies.

Groundwater remediation and no-further action decision making at petroleum underground storage tank (UST) sites has largely been based on an understanding of plume length, plume stability, and attenuation rates for key hydrocarbon constituents. Regulatory guidance to support and guide such decisions is based in part on plume studies involving individual hydrocarbon constituents, namely benzene and methyl tert-butyl ether (MTBE). Questions remain regarding whether current guidance is applicable to chemical mixtures such as gasoline range organics (GRO), diesel range organics (DRO), and oxygen containing organic compounds (OCOCs) resulting from hydrocarbon biodegradation. To help address this concern, data from California's GeoTracker database were used to estimate maximum plume lengths, plume stability, and attenuation rates of DRO (which can be used as an analytical surrogate for OCOCs) and GRO relative to benzene and MTBE. The distributions of maximum plume lengths were similar for the four constituents with medians ranging from 27 to 32 m. The fraction of monitoring wells with a decreasing concentration trend ranged from 19% for DRO to 40% for MTBE, while fewer than 7% of the wells had an increasing concentration trend for any of the constituents. Median attenuation rates ranged from 0.10% day(-1) for DRO to 0.17% day(-1) for MTBE. The results suggest attenuation based risk management is appropriate for DRO and GRO plumes at most petroleum UST sites.

At sites impacted by volatile organic compounds (VOCs), vapor intrusion (VI) is the pathway with the greatest potential to result in actual human exposure. Since sites with VI were first widely publicized in late 1990s, the scientific understanding of VI has evolved considerably. The VI conceptual model has been extended beyond relatively simple scenarios to include nuances, such as biological and hydrogeological factors that may limit the potential for VI and alternative pathways, such as preferential pathways and direct building contact/infiltration that may enhance VI in some cases. Regulatory guidance documents typically recommend initial concentration- or distance-based screening to evaluate whether VI may be a concern, followed by a multiple-lines-of-evidence (MLE) investigation approach for sites that do not screen out. These recommendations for detailed evaluation of VI currently focus on monitoring of VOC concentrations in groundwater, soil gas, and indoor air and can be supplemented by other lines of evidence. In this Critical Review, we summarize key elements important to VI site characterization, provide the status and current understanding, and highlight data interpretation challenges, as well as innovative tools developed to help overcome the challenges. Although there have been significant advances in the understanding of VI in the past 20 years, limitations and knowledge gaps in screening, investigation methods, and modeling approaches still exist. Potential areas for further research include improved initial screening methods that account for the site-specific role of barriers, improved understanding of preferential pathways, and systematic study of buildings and infrastructure other than single-family residences.

Spills and releases of hydrocarbons may result in zones of nonaqueous phase liquids (NAPL) within soils and groundwater. The NAPL will change, or "weather" over time due to a range of physical, chemical, and biological processes. Hydrocarbon constituents in environmental samples collected from NAPL-impacted groundwater wells, sediments, or soils vary in composition over time due to weathering. The changing composition can be used to estimate mass depletion rates and trends for the bulk NAPL and for individual constituent chemicals relative to marker constituents which are less susceptible to weathering. Methods for the selection of marker constituents and for quantitatively and conservatively estimating NAPL depletion rates and trends over time are shown. Estimates are included for two sites with different NAPL mixtures present (crude oil and gasoline/diesel-range product), for which depletion of half the initial total NAPL is estimated at 13.6 +/- 2.9 years and 7.3 +/- 1.8 years respectively, and with no active NAPL remediation at either site. Similar methods for oil or NAPL depletion estimates have often relied on a prior-identified suite of presumed-conserved marker constituents. The method presented here includes steps which identify the best set of analyzed candidate marker constituents in a NAPL mixture. This can confirm prior-selected markers but is particularly useful for NAPL mixtures in which no prior-identified marker constituents are present.

If volatile organic compound (VOC)-contaminated soil exists underneath a building, vapors may migrate upwards and intrude into the interior air of the building. Most previous models used to simulate vapor intrusion (VI) were developed by assuming that the source was constant, although a few recent models, such as the Risk-Based Corrective Action (RBCA) Tool Kit (TK) model, have been developed to consider source depletion (SD). However, the RBCA TK model ignores the effects of building characteristics due to its assumption that the ground is not covered by the actual building it models, which leads to incorrect results since the presence of the building affects the SD. In this study, a SD model is developed based on the three processes of VI while considering the impact of key building parameters on SD. The proposed model (i.e., the SD model) still follows the law of mass conservation, and the sensitivity analysis shows that the soil-building pressure differential (dP) is an important building characteristic that affects SD. Taking trichloroethylene (TCE) for simulation in the case of a soil concentration below the saturation concentration, as the soil permeability decreases, the differences in the results between the SD model and RBCA TK model decrease; as the Peclet number decreases, the effect of the dP on the results of the SD model decreases. The new model only accounts for the migration of contaminants at the source of depletion; therefore, the model is more applicable for these contaminants, which are considered to have low-biodegradable characteristics. Furthermore, since the model emphasizes the impact of buildings on the source, it is applicable when there is a considerable building area above the source, such as large commercial buildings or residential communities with underground parking lots, which exist in most cities.

The Quest Carbon Capture and Storage (CCS) project in Alberta, Canada, is a fully integrated project, as it involves the capture, transport, injection, storage of CO2, and a measurement, monitoring and verification (MMV) program. The MMV program has two key objectives: a) to ensure containment and b) to ensure conformance. Prior to the start of CO2 injection at the end of August 2015, a number of projects were undertaken to gather data from various domains, namely the atmosphere, biosphere, hydrosphere and geosphere, to provide input to the Quest MMV program. The focus of this paper is on monitoring activities undertaken in relation to the atmosphere and biosphere domains. Activities undertaken across the Quest sequestration lease area (SLA) included an eddy covariance system, soil gas probes, soil flux chambers, and walk-over surveys. In conclusion, understanding the spatial and temporal variability of CO2 levels prior to start of CO2 injection represents an important activity of a CCS MMV program. It provides technical input to the development of such a program, but also provides knowledge for communication to and awareness of project stakeholders (e.g. landowners) regarding CO2 levels within the atmosphere and biosphere across a SLA.

Soil cleanup criteria define acceptable concentrations of organic chemical constituents for exposed humans. These criteria sum the estimated soil exposure over multiple pathways. Assumptions for ingestion, dermal contact, and dust exposure generally presume a chemical persists in surface soils at a constant concentration level for the entire exposure duration. For volatile chemicals, this is an unrealistic assumption. A calculation method is presented for surficial soil criteria that include volatile depletion of chemical for these uptake pathways. The depletion estimates compare favorably with measured concentration profiles and with field measurements of soil concentration. Corresponding volatilization estimates compare favorably with measured data for a wide range of volatile and semivolatile chemicals, including instances with and without the presence of a mixed-chemical residual phase. Selected examples show application of the revised factors in estimating screening levels for benzene in surficial soils. Integr Environ Assess Manag 2017;13:861-869. © 2017 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Ethanol-blended fuel releases usually stimulate methanogenesis in the subsurface, which could pose an explosion risk if methane accumulates in a confined space above the ground where ignitable conditions exist. Ethanol-derived methane may also increase the vapor intrusion potential of toxic fuel hydrocarbons by stimulating the depletion of oxygen by methanotrophs, and thus inhibiting aerobic biodegradation of hydrocarbon vapors. To assess these processes, a three-dimensional numerical vapor intrusion model was used to simulate the degradation, migration, and intrusion pathway of methane and benzene under different site conditions. Simulations show that methane is unlikely to build up to pose an explosion hazard (5% v/v) if diffusion is the only mass transport mechanism through the deeper vadose zone. However, if methanogenic activity near the source zone is sufficiently high to cause advective gas transport, then the methane indoor concentration may exceed the flammable threshold under simulated conditions. During subsurface migration, methane biodegradation could consume soil oxygen that would otherwise be available to support hydrocarbon degradation, and increase the vapor intrusion potential for benzene. Vapor intrusion would also be exacerbated if methanogenic activity results in sufficiently high pressure to cause advective gas transport in the unsaturated zone. Overall, our simulations show that current approaches to manage the vapor intrusion risk for conventional fuel released might need to be modified when dealing with some high ethanol blend fuel (i.e., E20 up to E95) releases.

A detailed seasonal study of soil vapor intrusion at a cold climate site with average yearly temperature of 1.9 °C was conducted at a house with a crawlspace that overlay a shallow dissolved‐phase petroleum hydrocarbon (gasoline) plume in North Battleford, Saskatchewan, Canada. This research was conducted primarily to assess if winter conditions, including snow/frost cover, and cold soil temperatures, influence aerobic biodegradation of petroleum vapors in soil and the potential for vapor intrusion. Continuous time‐series data for oxygen, pressure differentials, soil temperature, soil moisture, and weather conditions were collected from a high‐resolution monitoring network. Seasonal monitoring of groundwater, soil vapor, crawlspace air, and indoor air was also undertaken. Petroleum hydrocarbon vapor attenuation and biodegradation rates were not significantly reduced during low temperature winter months and there was no evidence for a significant capping effect of snow or frost cover that would limit oxygen ingress from the atmosphere. In the residual light nonaqueous phase liquid (LNAPL) source area adjacent to the house, evidence for biodegradation included rapid attenuation of hydrocarbon vapor concentrations over a vertical interval of approximately 0.9 m, and a corresponding decrease in oxygen to less than 1.5% v/v. In comparison, hydrocarbon vapor concentrations above the dissolved plume and below the house were much lower and decreased sharply within a few tens of centimeters above the groundwater source. Corresponding oxygen concentrations in soil gas were at least 10% v/v. A reactive transport model (MIN3P‐DUSTY) was initially calibrated to data from vertical profiles at the site to obtain biodegradation rates, and then used to simulate the observed soil vapor distribution. The calibrated model indicated that soil vapor transport was dominated by diffusion and aerobic biodegradation, and that crawlspace pressures and soil gas advection had little influence on soil vapor concentrations.

Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening-level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil-gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16-year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil-gas screening levels of 30, 50, and 100 mu g/m3 and nonparametric Kaplan-Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are 30 mu g/m3 at any distance above a dissolved-phase hydrocarbon source. Dissolved-phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous-phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are 30 mu g/m3 for vertical screening distances of 13ft (4m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100ft (10 to 30m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.

Fuel ethanol releases can stimulate methanogenesis in impacted aquifers, which could pose an explosion risk if methane migrates into enclosed spaces where ignitable conditions exist. To assess this potential risk, a flux chamber was emplaced on a pilot-scale aquifer exposed to continuous release (21 months) of an ethanol solution (10% v:v) that was introduced 22.5 cm below the water table. Despite methane concentrations within the ethanol plume reaching saturated levels (20-23 mg/L), the maximum methane concentration reaching the chamber (21 ppm(v)) was far below the lower explosion limit in air (50,000 ppm(v)). The low concentrations of methane observed in the chamber are attributed to methanotrophic activity, which was highest in the capillary fringe. This was indicated by methane degradation assays in microcosms prepared with soil samples from different depths, as well as by PCR measurements of pmoA, which is a widely used functional gene biomarker for methanotrophs. Simulations with the analytical vapor intrusion model "Biovapor" corroborated the low explosion risk associated with ethanol fuel releases under more generic conditions. Model simulations also indicated that depending on site-specific conditions, methane oxidation in the unsaturated zone could deplete the available oxygen and hinder aerobic benzene biodegradation, thus increasing benzene vapor intrusion potential. Overall, this study shows the importance of methanotrophic activity near the water table to attenuate methane generated from dissolved ethanol plumes and reduce its potential to migrate and accumulate at the surface.

Vapor intrusion is associated with subsurface sources of both chlorinated VOCs and petroleum VOCs; however, petroleum vapor intrusion has been reported to occur under a narrower range of hydrogeologic settings. Research conducted over the last several years including field studies, evaluation of large datasets, and modeling studies, has provided an improved understanding of the differences in vapor intrusion associated with chlorinated VOCs and petroleum VOCs. When oxygen is present in the vadose zone, aerobic biodegradation typically results in rapid attenuation of petroleum VOCs diffusing upwards from deeper sources. At many building sites, wind-driven advection and/or building pressure fluctuations provide sufficient oxygen transport below the foundation to support this aerobic biodegradation. In such cases, there is limited potential for vapor intrusion from dissolved sources of petroleum VOCs unless preferential migration pathways are present. These findings support a framework for the evaluation of vapor intrusion at petroleum hydrocarbon sites that involves simple screening for preferential pathways at sites with sufficient vertical separation between the building and a dissolved source (e.g., 3 m) or a LNAPL source (e.g., 10 m), but a more intensive investigation at sites with petroleum sources in closer proximity to the building.

This chapter contains sections titled: Building Wakes and Stack Downwash Gravity-Driven Flows and the Effects of Terrain Aerosol Rainout Fanning Plumes and Subsidence

This chapter contains sections titled: Source Specification Source Area Region Stably-Stratified Region Passive Dispersion Region