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.

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

Natural source zone depletion (NSZD) is the main process of LNAPL (Light Non-Aqueous Phase Liquid) removal under natural conditions. The NSZD rates assessed ranged from 0.55 to 11.55 kg·m-2·a-1 (kilograms per square meter per year) in previous studies. However, most of these data were obtained from sandy sites, with few clayey sites. To gain knowledge of NSZD in clayey soil sites, the study assessed the NSZD of a petroleum hydrocarbon-contaminated clayey soil site in China, combining the concentration gradient method with metagenomic sequencing technology. The results show that the abundance of methane-producing key enzyme mcrA gene in the source zone was more abundant than in background areas, which suggests that there was methanogenesis, the key process of NSZD. The concentration gradients of oxygen and carbon dioxide existed only in shallow soil (<0.7 m), which suggests that there was a thin methane oxidation zone in the shallow zone. The calculated NSZD rates range from 0.23 to 1.15 kg·m-2·a-1, which fall within the moderate range compared to previous NSZD sites. This study expands the knowledge of NSZD in clayey soil and enriches the attenuation rate data for contaminated sites, which is of significant importance in managing petroleum contaminants.

Vapor-phase biodegradation and natural attenuation of petroleum VOCs in the unsaturated zone: A microcosm study.

Traditional natural attenuation studies focus on aqueous process in the saturated zone while vapor-phase biodegradation and natural attenuation in the unsaturated zone received much less attention. This study used microcosm experiments to explore the vapor-phase biodegradation and natural attenuation of 23 petroleum VOCs in the unsaturated zone including 7 monoaromatic hydrocarbons, 6 n-alkanes, 4 cycloalkanes, 3 alkylcycloalkanes and 3 fuel ethers. We found that monoaromatic hydrocarbon vapors were easily attenuated with significantly high first-order attenuation rates (9.48 d-1-43.20 d-1) in live yellow earth, of which toluene and benzene had the highest rates (43.20 d-1 and 28.32 d-1, respectively). The 13 aliphatic hydrocarbons and 3 fuel ethers all have relatively low attenuation rates (<0.54 d-1) in live soil and negligible biodegradation contribution. We explored the effects of soil types (black soil, yellow earth, lateritic red earth and quartz sand), soil moisture (2, 5, 10, and 17 wt%) contents and temperatures (4, 15, 25, 35 and 45 °C) on the vapor attenuation. Results showed that increasing soil organic matter (SOM) content, silt content, porosity and soil microorganism numbers enhanced contaminant attenuation and remediation efficiency. Increasing moisture content reduced the apparent first-order biodegradation rates of monoaromatic hydrocarbon vapors. The vapor-phase biodegradation had optimal temperature (∼25 °C in yellow earth) and increasing or decreasing temperature slowed down biodegradation rate. Overall, this study enhanced our understanding of vapor-phase biodegradation and natural attenuation of petroleum VOCs in the unsaturated zone, which is critical for the long-term management and remediation of petroleum contaminated site.

Effects of freeze-thaw cycles on methanogenic hydrocarbon degradation: Experiment and modeling.

Cold regions are warming much faster than the global average, resulting in more frequent and intense freeze-thaw cycles (FTCs) in soils. In hydrocarbon-contaminated soils, FTCs modify the biogeochemical and physical processes controlling petroleum hydrocarbon (PHC) biodegradation and the associated generation of methane (CH4) and carbon dioxide (CO2). Thus, understanding the effects of FTCs on the biodegradation of PHCs is critical for environmental risk assessment and the design of remediation strategies for contaminated soils in cold regions. In this study, we developed a diffusion-reaction model that accounts for the effects of FTCs on toluene biodegradation, including methanogenic biodegradation. The model is verified against data generated in a 215 day-long batch experiment with soil collected from a PHC contaminated site in Ontario, Canada. The fully saturated soil incubations with six different treatments were exposed to successive 4-week FTCs, with temperatures oscillating between -10 °C and +15 °C, under anoxic conditions to stimulate methanogenic biodegradation. We measured the headspace concentrations and 13C isotope compositions of CH4 and CO2 and analyzed the porewater for pH, acetate, dissolved organic and inorganic carbon, and toluene. The numerical model represents solute diffusion, volatilization, sorption, as well as a reaction network of 13 biogeochemical processes. The model successfully simulates the soil porewater and headspace concentration time series data by representing the temperature dependencies of microbial reaction and gas diffusion rates during FTCs. According to the model results, the observed increases in the headspace concentrations of CH4 and CO2 by 87% and 136%, respectively, following toluene addition are explained by toluene fermentation and subsequent methanogenesis reactions. The experiment and the numerical simulation show that methanogenic degradation is the primary toluene attenuation mechanism under the electron acceptor-limited conditions experienced by the soil samples, representing 74% of the attenuation, with sorption contributing to 11%, and evaporation contributing to 15%. Also, the model-predicted contribution of acetate-based methanogenesis to total produced CH4 agrees with that derived from the 13C isotope data. The freezing-induced soil matrix organic carbon release is considered as an important process causing DOC increase following each freezing period according to the calculations of carbon balance and SUVA index. The simulation results of a no FTC scenario indicate that, in the absence of FTCs, CO2 and CH4 generation would decrease by 29% and 26%, respectively, and that toluene would be biodegraded 23% faster than in the FTC scenario. Because our modeling approach represents the dominant processes controlling PHC biodegradation and the associated CH4 and CO2 fluxes, it can be used to analyze the sensitivity of these processes to FTC frequency and duration driven by temperature fluctuations.

The role of predicted chemotactic and hydrocarbon degrading taxa in natural source zone depletion at a legacy petroleum hydrocarbon site.

Petroleum hydrocarbon contamination is a global problem which can cause long-term environmental damage and impacts water security. Natural source zone depletion (NSZD) is the natural degradation of such contaminants. Chemotaxis is an aspect of NSZD which is not fully understood, but one that grants microorganisms the ability to alter their motion in response to a chemical concentration gradient potentially enhancing petroleum NSZD mass removal rates. This study investigates the distribution of potentially chemotactic and hydrocarbon degrading microbes (CD) across the water table of a legacy petroleum hydrocarbon site near Perth, Western Australia in areas impacted by crude oil, diesel and jet fuel. Core samples were recovered and analysed for hydrocarbon contamination using gas chromatography. Predictive metagenomic profiling was undertaken to infer functionality using a combination of 16 S rRNA sequencing and PICRUSt2 analysis. Naphthalene contamination was found to significantly increase the occurrence of potential CD microbes, including members of the Comamonadaceae and Geobacteraceae families, which may enhance NSZD. Further work to explore and define this link is important for reliable estimation of biodegradation of petroleum hydrocarbon fuels. Furthermore, the outcomes suggest that the chemotactic parameter within existing NSZD models should be reviewed to accommodate CD accumulation in areas of naphthalene contamination, thereby providing a more accurate quantification of risk from petroleum impacts in subsurface environments, and the scale of risk mitigation due to NSZD.

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.

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.

Advanced methods for RNA recovery from petroleum impacted soils.

Microbially-mediated hydrocarbon degradation is well documented. However, how these microbial processes occur in complex subsurface petroleum impacted systems remains unclear, and this knowledge is needed to guide technologies to enhance microbial degradation effectively. Analysis of RNA derived from soils impacted by petroleum liquids would allow for analysis of active microbial communities, and a deeper understanding of the dynamic biochemistry occurring during site remediation. However, RNA analysis in soils impacted with petroleum liquids is challenging due to: (A) RNA being inherently unstable, and (B) petroleum impacted soils containing problematic levels of polymerase chain reaction (PCR) inhibitors that must be removed to yield high-purity RNA for downstream analysis. A previously published soil wash pretreatment step and a commercially available DNA extraction kit protocol were combined and modified to be able to purify RNA from soils containing petroleum liquids.•A key modification involved reformulation of the pretreatment solution via replacing water as the diluent with a commercially-available RNA preservation solution.•Methods were developed and demonstrated using cryogenically preserved soils from three former petroleum refineries. Results showed the new soil washing approach had no adverse effects on RNA recovery but did improve RNA quality, by PCR inhibitor removal, which in turn allows for characterization of active microbial communities present in petroleum impacted soils.•In summary, our method for extracting RNA from petroleum-impacted soils provides a promising new tool for resolving metabolic processes at sites as they progress toward restoration via natural and/or engineered remediation.

Quantifying natural source zone depletion at petroleum hydrocarbon contaminated sites: A comparison of 14C methods.

Surficial CO2 efflux surveys have been used to delineate hydrocarbon source zones in contaminated aquifers and provide estimates of hydrocarbon biodegradation rates. This approach requires distinguishing between CO2 derived from petroleum degradation and CO2 produced from natural soil respiration. To this end, radiocarbon has been used to differentiate between 14C-depleted CO2 from hydrocarbon degradation and 14C-enriched CO2 from natural soil respiration to effectively quantify the contribution of each source to total CO2 efflux, and by deduction natural source zone depletion (NSZD) rates. In this study, a systematic method comparison has been conducted to evaluate available approaches for collecting CO2 gas samples for radiocarbon analysis used to correct total CO2 efflux measurements for quantifying natural source zone depletion rates. Gas samples for radiocarbon analysis were sampled from (i) dynamic closed chambers (located at ground surface), (ii) static chambers (also at ground surface), (iii) shallow soil gas probes (0.3 m bgs), and (iv) soil gas monitoring wells (~0.6 m below ground surface) during a CO2 efflux survey conducted at the site of a historical pipeline rupture near Bemidji, MN. The mean fraction of radiocarbon (F14C) obtained from samples overlying the source zone were (i) 0.93 ± 0.01, (ii) 0.73 ± 0.03, (iii) 0.71 ± 0.04, and (iv) 0.41 ± 0.06, for the four methods respectively. These F14C values were used to apportion total CO2 efflux measurements into contributions of contaminant-derived CO2 efflux and natural soil respiration to evaluate natural source zone depletion processes. Results suggest that the method of radiocarbon sampling has a significant effect on the calculated fraction of the CO2 efflux originating from contaminant-related soil respiration, with contributions ranging between 27% and 59% of total soil respiration. Results indicate that radiocarbon sampled from static chambers and shallow soil gas probes methods offer the best compromise between CO2 sample yield and sample representativeness, providing the most reliable estimates of CO2 effluxes originating from contaminant degradation. However, the results also show that for this study, all methods agree within a factor of <2.3 regarding the inferred NSZD rates.

Real-time soil and groundwater monitoring via spatial and temporal resolution of biogeochemical potentials.

Oxidation-reduction potentials (ORP) govern the transformation of organic compounds in water and soils. Standard methods for measurements of ORPs in subsurface setting are deeply flawed due to heterogeneous samples from wells, failure to capture weakly poised redox couples, and biases with ex-situ measurements. In this study, we developed a real-time in-situ ORP sensor system that continuously measures biogeochemical electrical potentials using vertically distributed point sensing electrodes in direct contact with the soil. Three hundred thousand data points, providing a full range of aqueous ORP values (+ 600 to - 600 mV vs. Ag/AgCl) were collected over 513 days to spatially and temporally resolve subsurface biogeochemical processes at a former petroleum refinery. Water quality and microbial community data support the validity of the ORP data. In locations impacted by petroleum light non-aqueous phase liquids (LNAPLs), barometric pumping and ebullition events drive near-daily cycles of ORP changes in the vadose zone of 400 mV. When only dissolved phase hydrocarbons are present, near-daily redox cycles are absent and values for ORP indicate methanogenic conditions immediately about the water table. When hydrocarbons are not present, redox conditions are more oxidizing by + 400 to + 700 mV. The embedded electrodes revealed variations in hydrocarbon biodegradation in time and space that cannot be resolved by collection and analysis of conventional samples of groundwater and soil gas.

Biogeochemical gradients above a coal tar DNAPL.

Naturally occurring distribution and attenuation processes can keep hydrocarbon emissions from dense non aqueous phase liquids (DNAPL) into the adjacent groundwater at a minimum. In a historically coal tar DNAPL-impacted site, the de facto absence of a plume sparked investigations regarding the character of natural attenuation and DNAPL resolubilization processes at the site. Steep vertical gradients of polycyclic aromatic hydrocarbons, microbial community composition, secondary water quality and redox-parameters were found to occur between the DNAPL-proximal and shallow waters. While methanogenic and mixed-electron acceptor conditions prevailed close to the DNAPL, aerobic conditions and very low dissolved contaminant concentrations were identified in three meters vertical distance from the phase. Comprehensive two-dimensional gas chromatography-mass spectrometry (GC×GC-MS) proved to be an efficient tool to characterize the behavior of the present complex contaminant mixture. Medium to low bioavailability of ferric iron and manganese oxides of aquifer samples was detected via incubation with Shewanella alga and evidence for iron and manganese reduction was collected. In contrast, 16S rDNA phylogenetic analysis revealed the absence of common iron reducing bacteria. Aerobic hydrocarbon degraders were abundant in shallow horizons, while nitrate reducers were dominating in deeper aquifer regions, in addition to a low relative abundance of methanogenic archaea. Partial Least Squares - Canonical Correspondence Analysis (PLS-CCA) suggested that nitrate and oxygen concentrations had the greatest impact on aquifer community structure in on- and offsite wells, which had a similarly high biodiversity (H' and Chao1). Overall, slow hydrocarbon dissolution from the DNAPL appears to dominate natural attenuation processes. This site may serve as a model for developing legal and technical strategies for the treatment of DNAPL-impacted sites where contaminant plumes are absent or shrinking.