Tracking NSZD mass removal rates over decades: Site-wide and local scale assessment of mass removal at a legacy petroleum site

Long-term estimates of natural source zone depletion (NSZD) rates for petroleum LNAPL (light non-aqueous phase liquid) sites are not available. One-off measurements are often thought valid over the lifetime of LNAPL sites. In the context of site-wide LNAPL mass estimates, we report site-specific gasoline and diesel NSZD rates spanning 21-26 years. Using depth profiles of soil gases (oxygen, carbon dioxide, methane, volatiles) above LNAPL, NSZD rates were estimated in 1994, 2006 and 2020 for diesel and 1999, 2009 and 2020 for gasoline. Each date also had soil-core mass estimates, which together with NSZD rates allow estimation of the longevity for LNAPL presence. Site-wide coring (in 1992, 2002, 2007) estimated LNAPL mass reductions of 12,000 t. For diesel NSZD, the ratio of NSZD rates for 2006 (16,000-49,000 L/ha/y) to those in 2020 (2600-14,000 L/ha/y) was ~3-6. By 2020, the 1994 diesel NSZD rates would have predicted the entire removal of measured mass (16-42 kg/m²). For gasoline, NSZD rates in 1999 were extremely high (50,000-270,000 L/ha/y) but 9-27 times lower (5800-10,000 L/ha/y) a decade later. The gasoline NSZD rates in 1999 predicted near complete mass removal in 2-12 years, but 10-11 kg/m² was measured 10 and 21 years later which is 26% of the initial mass in 1999. The outcomes substantiate the need to understand NSZD rate changes over the lifetime of LNAPL-impacted sites.

Unravelling Microbial Communities Associated with Different Light Non-Aqueous Phase Liquid Types Undergoing Natural Source Zone Depletion Processes at a Legacy Petroleum Site

Petroleum contaminants are exposed to weathering when released into environment, resulting in the alteration of their chemical composition. Here, we investigated microbial communities through the soil profile at an industrial site, which was exposed to various petroleum products for over 50 years. The petroleum is present as light non-aqueous phase liquid (LNAPL) and is undergoing natural source zone depletion (NSZD). Microbial community composition was compared to the contaminant type, concentration, and its depth of obtained soil cores. A large population of Archaea, particularly Methanomicrobia and Methanobacteria and indication of complex syntrophic relationships of methanogens, methanotrophs and bacteria were found in the contaminated cores. Different families were enriched across the LNAPL types. Results indicate methanogenic or anoxic conditions in the deeper and highly contaminated sections of the soil cores investigated. The contaminant was highly weathered, likely resulting in the formation of recalcitrant polar compounds. This research provides insight into the microorganisms fundamentally associated with LNAPL, throughout a soil depth profile above and below the water table, undergoing NSZD processes at a legacy petroleum site. It advances the potential for integration of microbial community effects on bioremediation and in response to physicochemical partitioning of LNAPL components from different petroleum types.

Natural source zone depletion of LNAPL: A critical review supporting modelling approaches

Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) includes partitioning, transport and degradation of LNAPL components. NSZD is being considered as a site closure option during later stages of active remediation of LNAPL contaminated sites, and where LNAPL mass removal is limiting. To ensure NSZD meets compliance criteria and to design enhanced NSZD actions if required, residual risks posed by LNAPL and its long term behaviour require estimation. Prediction of long-term NSZD trends requires linking physicochemical partitioning and transport processes with bioprocesses at multiple scales within a modelling framework. Here we expand and build on the knowledge base of a recent review of NSZD, to establish the key processes and understanding required to model NSZD long term. We describe key challenges to our understanding, inclusive of the dominance of methanogenic or aerobic biodegradation processes, the potentially changeability of rates due to the weathering profile of LNAPL product types and ages, and linkages to underlying bioprocesses. We critically discuss different scales in subsurface simulation and modelling of NSZD. Focusing on processes at Darcy scale, 36 models addressing processes of importance to NSZD are investigated. We investigate the capabilities of models to accommodate more than 20 subsurface transport and transformation phenomena and present comparisons in several tables. We discuss the applicability of each group of models for specific site conditions.

Evaluating the reliability of equilibrium dissolution assumption from residual gasoline in contact with water saturated sands

Understanding dissolution dynamics of hazardous compounds from complex gasoline mixtures is a key to long-term predictions of groundwater risks. The aim of this study was to investigate if the local equilibrium assumption for BTEX and TMBs (trimethylbenzenes) dissolution was valid under variable saturation in two dimensional flow conditions and evaluate the impact of local heterogeneities when equilibrium is verified at the scale of investigation. An initial residual gasoline saturation was established over the upper two-thirds of a water saturated sand pack. A constant horizontal pore velocity was maintained and water samples were recovered across 38 sampling ports over 141days. Inside the residual NAPL zone, BTEX and TMBs dissolution curves were in agreement with the TMVOC model based on the local equilibrium assumption. Results compared to previous numerical studies suggest the presence of small scale dissolution fingering created perpendicular to the horizontal dissolution front, mainly triggered by heterogeneities in the medium structure and the local NAPL residual saturation. In the transition zone, TMVOC was able to represent a range of behaviours exhibited by the data, confirming equilibrium or near-equilibrium dissolution at the scale of investigation. The model locally showed discrepancies with the most soluble compounds, i.e. benzene and toluene, due to local heterogeneities exhibiting that at lower scale flow bypassing and channelling may have occurred. In these conditions mass transfer rates were still high enough to fall under the equilibrium assumption in TMVOC at the scale of investigation. Comparisons with other models involving upscaled mass transfer rates demonstrated that such approximations with TMVOC could lead to overestimate BTEX dissolution rates and underestimate the total remediation time.

Dissolution of multi-component LNAPL gasolines: the effects of weathering and composition

The composition of light non-aqueous phase liquid (LNAPL) gasoline and other petroleum products changes profoundly over their life once released into aquifers. However limited attention has been given to how such changes affect key parameters such as the activity coefficients which control partitioning of components of petroleum fuel into groundwater and are used to predict long-term risk from fuel releases. Laboratory experiments were conducted on a range of fresh, weathered and synthetic gasoline mixtures designed to mimic the expected changes in composition in an aquifer. Weathered gasoline created under controlled evaporation and water washing, and naturally weathered gasoline, were investigated. Equilibrium concentrations in water and molar fractions in the gasoline mixtures were compared with equilibrium concentrations predicted by Raoult's law assuming ideal behaviour of the solutions. The experiments carried out allowed the relative sensitivity of the activity coefficients of key risk drivers such as benzene, toluene, ethylbenzene and xylene (BTEX) compounds to be quantified with respect to the presence of other types of compounds and where the source LNAPL had undergone different types of weathering. Results differed for the mixtures examined but in some cases higher than predicted dissolved equilibrium concentrations showed non-ideal behaviour for toluene, benzene and xylenes. Comparison of the activity coefficients showed that the naturally weathered gasoline and a 50% evaporated unleaded gasoline present a similar range of values varying between 1.0 and 1.2, suggesting close to ideal partitioning between the LNAPL and water. The fresh and water-washed gasoline had higher values for the activity coefficient, from 1.2 to 1.4, indicating non-ideal partitioning. Results from synthetic mixtures demonstrated that these differences could be due to the different molar fractions of the nC5 and nC6 aliphatic hydrocarbons acting on the molecular interactions, while differences in molar volumes seemed to have less of an influence on ideality.

A conservative vapour intrusion screening model of oxygen-limited hydrocarbon vapour biodegradation accounting for building footprint size

Petroleum hydrocarbon vapours pose a reduced risk to indoor air due to biodegradation processes where oxygen is available in the subsurface or below built structures. However, no previous assessment has been available to show the effects of a building footprint (slab size) on oxygen-limited hydrocarbon vapour biodegradation and the potential for oxygen to be present beneath the entire sub-slab region of a building. Here we provide a new, conservative and conceptually simple vapour screening model which links oxygen and hydrocarbon vapour transport and biodegradation in the vicinity and beneath an impervious slab. This defines when vapour risk is insignificant, or conversely when there is potential for vapour to contact the sub-slab of a building. The solution involves complex mathematics to determine the position of an unknown boundary interface between oxygen diffusing in from the ground surface and vapours diffusing upwards from a subsurface vapour source, but the mathematics reduces to a simple relationship between the vapour source concentration and the ratio of the half slab width and depth to the vapour source. Data from known field investigations are shown to be consistent with the model predictions. Examples of 'acceptable' slab sizes for vapour source depths and strengths are given. The predictions are conservative as an estimator of when petroleum hydrocarbon vapours might come in contact with a slab-on-ground building since additional sources of oxygen due to advective flow or diffusion through the slab are ignored. As such the model can be used for screening sites for further investigation.

Multiple lines of evidence to demonstrate vinyl chloride aerobic biodegradation in the vadose zone, and factors controlling rates

A field-based investigation was conducted at a contaminated site where the vadose zone was contaminated with a range of chlorinated hydrocarbons. The investigation consisted of groundwater and multilevel soil-gas monitoring of a range of contaminants and gases, along with isotope measurements and microbiology studies. The investigation provided multiple lines of evidence that demonstrated aerobic biodegradation of vinyl chloride (VC) was occurring in the vadose zone (i) above the on-site source zone, and (ii) above the downgradient off-site groundwater plume location. Data from both the on-site and off-site locations were consistent in showing substantially greater (an order of magnitude greater) rates of VC removal from the aerobic vadose zone compared to more recalcitrant contaminants trichloroethene (TCE) and tetrachloroethene (PCE). Soil gas VC isotope analysis showed substantial isotopic enrichment of VC (δ¹³C -5.2 to -10.9‰) compared to groundwater (δ¹³C -39.5‰) at the on-site location. Soil gas CO₂ isotope analysis at both locations showed that CO₂ was highly isotopically depleted (δ¹³C -28.8 to -33.3‰), compared to soil gas CO₂ data originating from natural sediment organic matter (δ¹³C= -14.7 to -21.3‰). The soil gas CO2 δ¹³C values were consistent with near-water table VC groundwater δ¹³C values (-36.8 to -39.5‰), suggesting CO₂ originating from aerobic biodegradation of VC. Bacteria that had functional genes (ethene monooxygenase (etnC) and epoxyalkane transferase (etnE)) involved in ethene metabolism and VC oxidation were more abundant at the source zone where oxygen co-existed with VC. The distribution of VC and oxygen vadose zone vapour plumes, together with long-term changes in soil gas CO₂ concentrations and temperature, provided information to elucidate the factors controlling aerobic biodegradation of VC in the vadose zone. Based on the overlapping VC and oxygen vadose zone vapour plumes, aerobic vapour biodegradation rates were independent of substrate (VC and/or oxygen) concentration. The high correlation (R=0.962 to 0.975) between CO₂ concentrations and temperature suggested that aerobic biodegradation of VC was controlled by bacterial activity that was regulated by the temperature within the vadose zone. When assessing a contaminated site for possible vapour intrusion into buildings, accounting for environmental conditions for aerobic biodegradation of VC in the vadose zone should improve the assessment of environmental risk of VC intrusion into buildings, enabling better identification and prioritisation of contaminated sites to be remediated.

Biodegradation of chlorobenzene, 1,2-dichlorobenzene, and 1,4-dichlorobenzene in the vadose zone

Much of the microbial activity in nature takes place at interfaces, which are often associated with redox discontinuities. One example is the oxic/anoxic interface where polluted groundwater interacts with the overlying vadose zone. We tested whether microbes in the vadose zone can use synthetic chemicals as electron donors and thus protect the overlying air and buildings from groundwater pollutants. Samples from the vadose zone of a site contaminated with chlorobenzene (CB), 1,2-dichlorobenzene (12DCB), and 1,4-dichlorobenzene (14DCB) were packed in a multiport column to simulate the interface of the vadose zone with an underlying groundwater plume. A mixture of CB, 12DCB, and 14DCB in anoxic water was pumped continuously through the bottom of column to an outlet below the first sampling port to create an oxic/anoxic interface and a capillary fringe. Removal to below the detection limits by rapid biodegradation with rates of 21 ± 1 mg of CB • m(-2) • d(-1), 3.7 ± 0.5 mg of 12DCB • m(-2) • d(-1), and 7.4 ± 0.7 mg of 1.4 DCB • m(-2) • d(-1) indicated that natural attenuation in the capillary fringe can prevent the migration of CB, 12DCB, and 14DCB vapors. Enumeration of bacteria capable of degrading chlorobenzenes suggested that most of the biodegradation takes place within the first 10 cm above the saturated zone. Biodegradation also increased the upward flux of contaminants and thus enhanced their elimination from the underlying water. The results revealed a substantial biodegradation capacity for chlorinated aromatic compounds at the oxic/anoxic interface and illustrate the role of microbes in creating steep redox gradients.

Integrating spatial and temporal oxygen data to improve the quantification of in situ petroleum biodegradation rates

Accurate estimation of biodegradation rates during remediation of petroleum impacted soil and groundwater is critical to avoid excessive costs and to ensure remedial effectiveness. Oxygen depth profiles or oxygen consumption over time are often used separately to estimate the magnitude and timeframe for biodegradation of petroleum hydrocarbons in soil and subsurface environments. Each method has limitations. Here we integrate spatial and temporal oxygen concentration data from a field experiment to develop better estimates and more reliably quantify biodegradation rates. During a nine-month bioremediation trial, 84 sets of respiration rate data (where aeration was halted and oxygen consumption was measured over time) were collected from in situ oxygen sensors at multiple locations and depths across a diesel non-aqueous phase liquid (NAPL) contaminated subsurface. Additionally, detailed vertical soil moisture (air-filled porosity) and NAPL content profiles were determined. The spatial and temporal oxygen concentration (respiration) data were modeled assuming one-dimensional diffusion of oxygen through the soil profile which was open to the atmosphere. Point and vertically averaged biodegradation rates were determined, and compared to modeled data from a previous field trial. Point estimates of biodegradation rates assuming no diffusion ranged up to 58 mg kg(-1) day(-1) while rates accounting for diffusion ranged up to 87 mg kg(-1) day(-1). Typically, accounting for diffusion increased point biodegradation rate estimates by 15-75% and vertically averaged rates by 60-80% depending on the averaging method adopted. Importantly, ignoring diffusion led to overestimation of biodegradation rates where the location of measurement was outside the zone of NAPL contamination. Over or underestimation of biodegradation rate estimates leads to cost implications for successful remediation of petroleum impacted sites.

Review of unsaturated-zone transport and attenuation of volatile organic compound (VOC) plumes leached from shallow source zones

Reliable prediction of the unsaturated zone transport and attenuation of dissolved-phase VOC (volatile organic compound) plumes leached from shallow source zones is a complex, multi-process, environmental problem. It is an important problem as sources, which include solid-waste landfills, aqueous-phase liquid discharge lagoons and NAPL releases partially penetrating the unsaturated zone, may persist for decades. Natural attenuation processes operating in the unsaturated zone that, uniquely for VOCs includes volatilisation, may, however, serve to protect underlying groundwater and potentially reduce the need for expensive remedial actions. Review of the literature indicates that only a few studies have focused upon the overall leached VOC source and plume scenario as a whole. These are mostly modelling studies that often involve high strength, non-aqueous phase liquid (NAPL) sources for which density-induced and diffusive vapour transport is significant. Occasional dissolved-phase aromatic hydrocarbon controlled infiltration field studies also exist. Despite this lack of focus on the overall problem, a wide range of process-based unsaturated zone - VOC research has been conducted that may be collated to build good conceptual model understanding of the scenario, particularly for the much studied aromatic hydrocarbons and chlorinated aliphatic hydrocarbons (CAHs). In general, the former group is likely to be attenuated in the unsaturated zone due to their ready aerobic biodegradation, albeit with rate variability across the literature, whereas the fate of the latter is far less likely to be dominated by a single mechanism and dependent upon the relative importance of the various attenuation processes within individual site - VOC scenarios. Analytical and numerical modelling tools permit effective process representation of the whole scenario, albeit with potential for inclusion of additional processes - e.g., multi-mechanistic sorption phase partitioning, and provide good opportunity for further sensitivity analysis and development to practitioner use. There remains a significant need to obtain intermediate laboratory-scale and particularly field-scale (actual site and controlled release) datasets that address the scenario as a whole and permit validation of the available models. Integrated assessment of the range of simultaneous processes that combine to influence leached plume generation, transport and attenuation in the unsaturated zone is required. Component process research needs are required across the problem scenario and include: the simultaneous volatilisation and dissolution of source zones; development of appropriate field-scale dispersion estimates for the unsaturated zone; assessment of transient VOC exchanges between aqueous, vapour and sorbed phases and their influence upon plume attenuation; development of improved field methods to recognise and quantify biodegradation of CAHs; establishment of the influence of co-contaminants; and, finally, translation of research findings into more robust practitioner practice.

Microbial growth with vapor-phase substrate

Limited information exists on influences of the diffusive transport of volatile organic contaminants (VOC) on bacterial activity in the unsaturated zone of the terrestrial subsurface. Diffusion of VOC in the vapor-phase is much more efficient than in water and results in effective VOC transport and high bioavailability despite restricted mobility of bacteria in the vadose zone. Since many bacteria tend to accumulate at solid-water, solid-air and air-water interfaces, such phase boundaries are of a special interest for VOC-biodegradation. In an attempt to evaluate microbial activity toward air-borne substrates, this study investigated the spatio-temporal interplay between growth of Pseudomonas putida (NAH7) on vapor-phase naphthalene (NAPH) and its repercussion on vapor-phase NAPH concentrations. Our data demonstrate that growth rates of strain PpG7 were inversely correlated to the distance from the source of vapor-phase NAPH. Despite the high gas phase diffusivity of NAPH, microbial growth was absent at distances above 5 cm from the source when sufficient biomass was located in between. This indicates a high efficiency of suspended bacteria to acquire vapor-phase compounds and influence headspace concentration gradients at the centimeter-scale. It further suggests a crucial role of microorganisms as biofilters for gas-phase VOC emanating from contaminated groundwater or soil.

Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone

Analytical models were developed that simulate stable isotope ratios of volatile organic compounds (VOCs) near a point source contamination in the unsaturated zone. The models describe diffusive transport of VOCs, biodegradation and source ageing. The mass transport is governed by Fick's law for diffusion. The equation for reactive transport of VOCs in the soil gas phase was solved for different source geometries and for different boundary conditions. Model results were compared to experimental data from a one-dimensional laboratory column and a radial-symmetric field experiment. The comparison yielded a satisfying agreement. The model results clearly illustrate the significant isotope fractionation by gas phase diffusion under transient state conditions. This leads to an initial depletion of heavy isotopes with increasing distance from the source. The isotope evolution of the source is governed by the combined effects of isotope fractionation due to vaporisation, diffusion and biodegradation. The net effect can lead to an enrichment or depletion of the heavy isotope in the remaining organic phase, depending on the compound and element considered. Finally, the isotope evolution of molecules migrating away from the source and undergoing degradation is governed by a combined degradation and diffusion isotope effect. This suggests that, in the unsaturated zone, the interpretation of biodegradation of VOC based on isotopic data must always be based on a model combining gas phase diffusion and degradation.

Biodegradation of hydrocarbons vapors: Comparison of laboratory studies and field investigations in the vadose zone at the emplaced fuel source experiment, Airbase Vaerløse, Denmark

The natural attenuation of volatile organic compounds (VOCs) in the unsaturated zone can only be predicted when information about microbial biodegradation rates and kinetics are known. This study aimed at determining first-order rate coefficients for the aerobic biodegradation of 13 volatile petroleum hydrocarbons which were artificially emplaced as a liquid mixture during a field experiment in an unsaturated sandy soil. Apparent first-order biodegradation rate coefficients were estimated by comparing the spatial evolution of the resulting vapor plumes to an analytical reactive transport model. Two independent reactive numerical model approaches have been used to simulate the diffusive migration of VOC vapors and to estimate degradation rate coefficients. Supplementary laboratory column and microcosm experiments were performed with the sandy soil at room temperature under aerobic conditions. First-order kinetics adequately matched the lab column profiles for most of the compounds. Consistent compound-specific apparent first-order rate coefficients were obtained by the three models and the lab column experiment, except for benzene. Laboratory microcosm experiments lacked of sensitivity for slowly degrading compounds and underestimated degradation rates by up to a factor of 5. Addition of NH3 vapor was shown to increase the degradation rates for some VOCs in the laboratory microcosms. All field models suggested a significantly higher degradation rate for benzene than the rates measured in the lab, suggesting that the field microbial community was superior in developing benzene degrading activity.