Bioventing in Shallow Tundra Overlying Permafrost

The purpose of this research was to investigate the feasibility of suction bioventing for treatment of contaminated tundra soil. Two laboratory-scale venting reactors were prepared with tundra from Arctic Alaska and operated, one for 32?d and the other for 52?d. For each rectangular reactor, suction was applied to a central well screened at mid-depth, while opposite ends of the reactor were screened to serve as air intake zones. The volume of liquid and gas recovered from the suction well was quantified daily. Numbers for heterotrophic organisms, pH, and dissolved organic carbon were quantified in the recovered liquid. The suction pump held a full vacuum (i.e., 101?kPa vac) for the duration of both experiments, indicating continuous obstruction of pneumatic and hydraulic conductivity. In both reactors, the soil in the proximity of the suction well separated from the bulk of the soil, precluding hydraulic communication. Furthermore, the soil nearest the well screen compacted, forming a barrier to appreciable pneumatic conductivity. At the end of operation, the soil was removed and sampled for moisture content, pH, and numbers of heterotrophic organisms at various locations. The results of this study showed that for suction bioventing to be successful in tundra, consolidation of the soil around the well screen must be prevented, as it will cause well isolation and limit both pneumatic and hydraulic conductivities.     

Degradation Rates for Petroleum Hydrocarbons Undergoing Bioventing at the Meso-Scale

ABSTRACT

Bioventing can be effective for the remediation of soil contaminated with petroleum hydrocarbons. However, implementing laboratory results in field scenarios is difficult due to the lack of scale-up factors. Accordingly, laboratory bioventing experiments were undertaken at the meso-scale and then compared with previously completed micro-scale tests to evaluate the important scale-up factor. The developed meso-scale system holds 4 kg of soil, with bioventing conditions controlled from a nutrient, airflow, and water content perspective. Three soils were tested, and categorized as loamy sand, silt loam, and a mixture. Results over a 30-day period showed a two-stage degradation pattern that encompassed first-order degradation rates as compared with the single-stage first-order degradation rate determined in the micro-scale study. For the first stage (0–8 days), the degradation rate for loamy soil was 0.598 day^?1, with the silty soil at 0.460 day^?1, and mixed soil at 0.477 day^?1. After 8 days, the degradation rate constant for the loamy soil dropped to 0.123 day^?1, with the silty soil dropping to 0.075 day^?1, and the degradation rate for the mixed soil dropping to 0.093 day^?1. Comparison of the measured degradation rate values with the results from the micro-scale experiments gave scale-up factors varying from 1.9 to 2.7 for the types of soil considered in the current study. These differences in degradation rates between the two scales show the importance of scale-up factors when transferring feasibility study results to the field.     

The Effect of Temperature on the Bioventing of Soil Contaminated with Toluene and Decane

The effect of temperature on evaporation and biodegradation rates during soil bioventing (SBV) was studied for a mixture of toluene and decane in bench-scale soil columns at a continuous air flow and consecutively at two different flow rates. The effect of temperature on SBV was monitored by GC headspace analysis of contaminant, CO₂ and O₂ concentrations in the soil gas over time. Separation of evaporation and biodegradation processes into three different phases based on their rates was used together with Q₁₀ and E₁₀ (values that give the factor by which biodegradation and evaporation rates increase when the temperature is raised by 10 degrees) to compare quantitatively the removal kinetics at 10 and 20°C. Adsorption of toluene and decane onto soil (a phase partitioning process) at 20 and 10°C was described with linear Freundlich isotherms. A temperature decrease from 20 to 10°C resulted in an increase of soil-air partitioning coefficients by a factor of 1.8 and of 2.1 for toluene and decane, respectively. The mean Q₁₀ value for the biodegradation of toluene was found to be 2.2 for a temperature rise from 10 to 20°C. A toluene content in the soil gas above 75% of the saturation concentration inhibited biodegradation at both temperatures. The SBV efficiency was dependent on temperature with respect to remediation time. SBV at 20°C resulted in a 99.8% and a 98.7% reduction of toluene and decane initial concentrations, respectively. To reach similar results at 10°C, about 1.6 times as much time and 1.4 times as much air were required; however, at both temperatures the total amounts of biodegraded hydrocarbons were approximately the same. The evaporation-to-biodegradation ratios at 20°C were 82.5:17.5 for toluene and 16:84 for decane, whereas at 10 °C they were 71:29 and 2:98, respectively. A comparison of Q₁₀ values showed that, except during the initial phase of SBV, only a modest decrease in biodegradation rates should be expected after a decrease in temperature from 20 to 10°C. Flow rate reduction had a significant impact on the toluene evaporation rate at a higher temperature, whereas for decane this rate was only slightly affected by temperature. In contrast to decane, the ratio between toluene vapor pressures at 20 and 10°C may be used to predict the removal of toluene by evaporation during the above-mentioned phases of SBV, when evaporation is important.     

Microwave-Assisted Process (MAPTM) for the Extraction of Contaminants from Soil

Laboratory-scale tests were performed to evaluate the use of Environment Canada's patented Microwave-Assisted Process (MAP_TM) for the extraction of petroleum hydrocarbons from contaminated soil. The purpose of these tests was to determine the potential for using the process for large-scale processing of contaminated soil. Tests were performed using three soil types: a certified sediment and certified soil, both contaminated with polycyclic aromatic hydrocarbons (PAHs), and spiked peat soil contaminated with long-chain petroleum hydrocarbons. The test methods used were based on existing MAP techniques that have been proven for the sample preparation of contaminated soils for analytical purposes. The parameters evaluated concentrated on those that are amenable to a continuous large-scale process running at atmospheric pressures. This meant using solvents that are inexpensive and readily available in large volumes, low solvent to material ratios, and optimized energy inputs. In general, it was found that microwaves could be used to enhance the solvent extraction of the contaminants from the soil and that the properties of the soil greatly affected the extent to which the contaminants were removed.     

Bioventing of diesel oil-contaminated soil: Comparison of degradation rates in soil based on actual oil concentration and on respirometric data

The effects of bioventing, nutrient addition and inoculation with an oil-degrading bacterium on biodegradation of diesel oil in unsaturated soil were investigated. A mesocosm system was constructed consisting of six soil compartments each containing 6 m³ of naturally contaminated soil mixed 11 with silica sand, resulting in a diesel oil content of approximately 2000 mg kg^–1. Biodegradation was monitored over 112 days by determining the actual diesel oil content of the soil and by respirometric tests. The best agreement between calculations of degradation rates based upon the two methods was in July, when venting in combination with nutrient addition resulted in degradation rates of 23 mg kg^–1 day^–1 based on actual oil concentration in the soil and 33 mg kg^–1 day^–1 calculated from respirometric data. In September, these rates decreased to 9 and 1.4 mg kg^–1 day^–1, and in October the degradation rates were 5 and 0.7 mg kg^–1 day^–1 based upon the two methods. The average ambient temperature during the respirometric tests was 14,10 and 2°C in July, September and October, respectively. The combination of venting and nutrient addition resulted in an average residual oil content of the soil of 380 mg kg^–1. Neither venting alone nor inoculation enhanced oil degradation. The respiratory quotient averaged 0.40. The oil composition changed following degradation resulting in the unresolved complex mixture constituting up to 96% of the total oil content at the end of the experimental period.     

Vapor Extraction/Bioventing Sequential Treatment of Soil Contaminated with Volatile and SemiVolatile Hydrocarbon Mixtures

A cost-effective removal strategy was studied in bench-scale columns that involved vapor extraction (SVE) and bioventing (SBV) sequential treatment of toluene- and decane-contaminated soil. By using GC analysis to measure hydrocarbon concentrations, CO₂, and O₂ content values in the outlet gas, the removal kinetics were determined as was the contribution of evaporation and biodegradation to the removal of contaminants from soil. The effect of operating mode on treatment performance was studied at a continuous air flow and consecutively at two different flow rates, and compared with an intermittent (pulse) flow rate. The two-rate flow was required due to the inhibitory effect of toluene on indigenous microorganisms at above 75% of the toluene saturation concentrations in the gas phase. The intermittent flow was controlled by the O₂ content values in the soil gas, which at above 4% did not limit biodegradation. To reach comparable removal efficiency at the constant flow, about three times less air was required for toluene than for decane. This air volume could be reduced, in the case of decane, by a factor of 1.6 and 2.9, at the two-rate and intermittent flow, respectively. A higher contribution of biodegradation to the overall removal of hydrocarbon will lower hydrocarbon concentrations in the off-gases to be treated. Together with the decreased amount of air used, this can reduce the overall remediation costs. The overall process can be better understood by determining the degree of contaminants removal by evaporation and biodegradation in the experimental set up.     

Comparison,Validation, and Use of Models for Predicting Indoor Air Quality from Soil and Groundwater Contamination

The use of models to predict indoor air quality and health risk for the soil vapor transport to indoor air pathway is commonplace; however, there is significant uncertainty surrounding processes and factors affecting this pathway, and the accuracy of models used. Available screening models were evaluated through a review of model characteristics and sensitivity, and through comparisons to measured conditions at field sites. Model simulations and comparisons to field data indicate that the vapor attenuation ratio (α) is highly sensitive to certain processes (e.g., biodegradation and ad-vection) and input parameters. Comparisons of model predicted to measured a values indicate that models based on the Johnson and Ettinger (1991) framework in most cases result in predictions that are conservative by up to one to two orders of magnitude for field sites that were assessed, providing that appropriate input parameters are used. However, for sites where the advection potential is high, these models may not be conservative. The potential for advective transport of vapors into building may be significant for sites with shallow contamination, high permeability soil and foundation and high building underpressurization. The paper concludes with possible tiered management framework for the soil vapor pathway.     

Field trials of a TiO2 pellet‐based photocatalytic reactor for off‐gas treatment at a soil vapor extraction well

A field trial of a pilot‐scale TiO₂ photocatalytic reactor for treatment of off‐gases from a soil vapor extraction (SVE) well at a chlorinated solvent spill site at the Savannah River Site near Aiken, SC, is described. Trichloroethylene (TCE), perchloroethylene (PCE), 1,1‐dichloroethylene (1,1 ‐DCE), and 1,1,1 ‐trichloroethane (1,1,1 ‐TCA) were treated at flow rates up to 6 1/min and space times of 5.1 x 10⁷ to 1.2 x 10⁹ g/mol. The TiO₂ used was in the form of porous pellets with a surface area of 150 m²/g. Operation of the reactor with the undiluted waste stream (5000 ppmv) at 80, 100, and 110°C yielded many undesirable byproducts, such as phosgene, chloroform, carbon tetrachloride, and penta‐ and hexachloroethane, even though the conversion of PCE and TCE approached 100%. After diluting the waste stream with ambient air to below 1000 ppmv and maintaining space times around 5 x 10⁸ g/mol, a conversion >99.5% was achieved with the production of only small amounts (<10 ppmv) of hexachloroethane. The reactor was Operated continuously for 8 d, with no noticeable deterioration in catalyst activity. In a subsequent laboratory study to determine the mass balance, CO₂ determined in the reactor effluent accounted for 80% of the influent carbon.     

Establishing Correlations and Scale-Up Factor for Estimating the Petroleum Biodegradation Rate in Soil

The proper design of a bioremediation strategy for petroleum-contaminated sites requires a reasonable estimate of the biodegradation rate constant, which is not easy due to spatial heterogeneity. Accordingly, predictive models were developed by completing a bioventing study at the meso-scale. Reactors holding 4 kg of disturbed soil were tested using five different types of soils. Using statistical analysis, a two-stage process was observed, with a fast rate of hydrocarbon degradation in the first 8 days and a slower rate in the remaining 22 days. Review of the correlations showed that the initial population of petroleum-degrading bacteria and increasing silt content had a positive effect on the degradation. A negative impact on the degradation rate was seen by increasing the fraction of organic matter and clay content. Comparison of previously completed micro-scale and meso-scale degradation rates gave a scale-up factor (SF) of 1.8 ± 0.5. Soils with an increased sand fraction had slightly higher SF values, whereas soils high in organic matter content had lower SF values. The measured SF values and developed correlations will help practitioners with site closure decisions, indicating the need for additional SF work to allow better transfer of meso-scale data to the field.     

Long-Term Evolution of Biodegradation and Volatilization Rates in a Crude Oil-Contaminated Aquifer

Volatilization and subsequent biodegradation near the water Table make up a coupled natural attenuation pathway that results in significant mass loss of hydrocarbons. Rates of biodegradation and volatilization were documented twice 12 years apart at a crude-oil spill site near Bemidji, Minnesota. Biodegradation rates were determined by calibrating a gas transport model to O₂, CO₂, and CH₄ gas-concentration data in the unsaturated zone. Reaction stoichiometry was assumed in converting O₂ and CO₂ gas-flux estimates to rates of aerobic biodegradation and CH₄ gas-flux estimates to rates of methanogenesis. Model results indicate that the coupled pathway has resulted in significant hydrocarbon mass loss at the site, and it was estimated that approximately 10.52 kg/day were lost in 1985 and 1.99 kg/day in 1997. In 1985 3% of total volatile hydrocarbons diffusing from the floating oil were biodegraded in the lower 1 m of the unsaturated zone and increased to 52% by 1997. Rates of hydrocarbon biodegradation above the center of the floating oil were relatively stable from 1985 to 1997, as the primary metabolic pathway shifted from aerobic to methanogenic biodegradation. Model results indicate that in 1997 biodegradation under methanogenenic conditions represented approximately one-half of total hydrocarbon biodegradation in the lower 1 m of the unsaturated zone. Further downgradient, where substrate concentrations have greatly increased, total biodegradation rates increased by greater than an order of magnitude from 0.04 to 0.43 g/m²-day. It appears that volatilization is the primary mechanism for attenuation in early stages of plume evolution, while biodegradation dominates in later stages.     

Outgassing Losses of Toluene and m-Xylene Evaluated by 14C-Based Mass Balances for Laboratory Bioventing Simulations

Mixtures of toluene, ethylbenzene, and the xylenes spiked with ¹⁴C-labeled toluene or m-xylene were added to bench-scale bioventing simulation columns filled with hydrocarbon-contaminated subsurface soils. After 2 to 4 weeks of incubation during which air was pumped through the column at rates of at least 2?ml·min^?1·kg^?1 between 54 and 84% of the radiolabel was recovered in traps as outgassed parent compound from four columns sterilized with gamma-irradiation. In contrast, seven nonsterilized but otherwise identically treated (except for inorganic nitrogen addition) columns lost less than 0.4% (and one column lost 0.7%) of the radiolabel through outgassing of the parent compound. Nonsterilized columns lost 40 to 61% of the radiolabel as ¹⁴CO₂, whereas gamma-irradiated columns usually lost only trace amounts of ¹⁴C in this form. Biologically active columns also retained much larger fractions than sterilized columns of the radiolabel in the subsoil in forms, possibly microbial biomass, from which it could be recovered by wet oxidation. Addition of 10 or 40?mg/kg of mineral nitrogen had no consistent effect on bioventing performance.     

Laboratory studies of air stripping of VOC‐contaminated soils

Bench‐scale laboratory experiments were conducted to evaluate the effectiveness of air stripping for in situ remediation of benzene‐contaminated soils. Several parametric studies were performed to evaluate the effect of soil grain size, air injection flow rate, and air inlet temperature on the benzene recovery efficiency.

An increase in soil grain size produced a significant increase in benzene recovery efficiency especially during the early stages of air injection. After 2 h of treatment, an increase in soil grain size from D₅₀ = 0.31 mm to D₅₀ = 1.20 mm resulted in an increase in benzene recovery efficiency from 49 to 65%. When the air‐flow rate was increased from 5 l/min to 10 l/min, the benzene recovery efficiency increased from 56 to 70% after 4 h of venting operation. Maximum recovery of benzene was reached after approximately 37 h of soil venting at a flow rate of 5 l/min and after approximately 24 h at a flow rate of 10 l/min. Preheating the air to 45°C at the inlet resulted in an increase in recovery efficiency from 70 to 90% after 5 h of air stripping.     

Multi-Species Ecotoxicity Assessment of Petroleum-Contaminated Soil

In 1992, a study was begun to compare the effect of landfarming vs. natural attenuation on the restoration of soil that had been contaminated with crude oil. Each of three lysimeters was filled with a sandy loam topsoil, and crude oil was applied to two of the lysimeters. One of the contaminated lysimeters was tilled, watered, and received a one-time application of fertilizer (N, P, K). No amendments were added to the second contaminated lysimeter, and the third was left uncontaminated. The lysimeters were monitored for 6 months and then left unattended. In 1995 and again in 1997 we sampled these lysimeters to evaluate the long-term effects of contamination and bioremediation. In 1995 we found marked effects on soil chemistry, bacterial, fungal, nematode, and plant populations and a higher rate of bioremediation in the fertilized-contaminated lysimeter (Lawlor et al., 1997). Data from 1997 and previously unreported data from 1995 are the subject of the current report. In 1997, low densities of hydrocarbon-degrading bacteria were found in all the lysimeters and little loss of TPH from the two contaminated lysimeters, suggesting a decreased rate of bioremediation. Nevertheless, there were increases in diversity and number of functional groups of bacteria, nematodes, and native plant species. However, molecular analyses revealed marked differences remained in the composition of dominant eubacterial species, and tests of soybeans indicated field conditions remained unsuitable for these plants.     

BTEX Remediation under Challenging Site Conditions Using In-Situ Ozone Injection and Soil Vapor Extraction Technologies: A Case Study

Remediation was successfully completed in a petroleum-contaminated site using ozone sparging combined with soil vapor extraction technologies. The site contained high levels of BTEX contamination in dissolved, adsorbed, and free phases. The presence of fine-grained soil, smear zone contamination, and shallow groundwater posed challenging site conditions for this remediation. Active remediation was performed for approximately 18 months followed by one year of post-remediation monitoring. Application of soil vapor extraction technology effectively removed free phase contamination. Biodegradation during active and post-remediation periods also played a role in bringing down the contaminant levels and achieving closure of the release. The cost of this remediation approach was estimated to be $2.04 per cubic foot ($72.08 per cubic meter) and found to be cost-effective. Timely completion of the project prevented the spreading of contaminants towards the down-gradient residential and school properties.     

Gas phase photocatalytic degradation on TIO2 pellets of volatile chlorinated organic compounds from a soil vapor extraction well

The degradation of trichloroethylene (TCE, Cl₂C=CHCl) and tetrachloroethylene (PCE, Cl₂C=CCI₂) in a gas stream from a soil vapor extraction (SVE) well was demonstrated with an annular photocatalytic reactor packed with porous TiO₂ pellets in a field trial at the Savannah River Site in Aiken, SC. The TiO₂ pellets were prepared using a sol‐gel method. The experiments were performed at 55 to 60°C using space times of 10⁸ to 10¹⁰ g ? s ? mol^‐1 for TCE and PCE. Chloroform (CHCl₃) and carbon tetrachloride (CCl₄) were detected as minor products from side reactions. On a molar basis, the amounts CCl₄ and CHCl₃ produced were about 2 and 0.2% of the reactants, respectively.     

Field Validation of Helium as a Tracer Gas During Soil Vapor Sample Collection

This paper reports the results of a field study to evaluate the use of helium as a tracer gas during soil vapor sampling. The authors found that the helium tracer method recommended in regulatory guidance can detect atmospheric leakage. However, the degree of leakage can be underestimated due to: 1) losses of helium from the tracer reservoir during sampling; 2) the entry of atmospheric air from an area outside the tracer reservoir; 3) temporal variability of leakage; and 4) flow-related reduction in helium detector response. Frequent leakage was observed in soil vapor probes installed using conventional direct push techniques. Probes installed at a depth of 8 ft exhibited less leakage than those installed at 4 ft. Probes installed in glacial till exhibited greater and more frequent leakage than those installed in sandy loam. Based on the frequency of observed leakage, refinement and standardization of tracer methods, with routine incorporation of tracers and in-line pressure and flow monitoring, is recommended.     

Estimation of Cd,Pb, and Zn Bioavailability in Smelter-Contaminated Soils by a Sequential Extraction Procedure

Chemical fractionation methods may be capable of providing an inexpensive estimate of contaminant bioavailability and risk in smelter-contaminated soil. In this study, the relationship between metal fractionation and methods used to estimate bioavailability of these metal contaminants in soil was evaluated. The Potentially BioAvailable Sequential Extraction (PBASE) was used for Cd, Pb, and Zn fractionation in 12 soils contaminated from Pb and Zn mining and smelting activities. The PBASE procedure is a four-step sequential extraction: extraction 1 (E1) is 0.5 M Ca(NO₃)₂, E2 is 1.0 M NaOAc, E3 is 0.1 M Na₂EDTA, and E4 is 4 M HNO₃. Metal bioavailability for two human exposure pathways, plant uptake (phytoavailability) and incidental ingestion (gastrointestinal, Gl, availability), was estimated using a lettuce (Lactuca sativa L.) bioassay and the in vitro-Gl Physiologically Based Extraction Test(PBET). Metal in the PBASE E1 fraction was correlated with lettuce Cd (P < 0.001) and Zn (P < 0.05) and was the best predictor of Cd and Zn phytoavailability. Only total metal content or the sum of all PBASE fractions, ΣE_1–4, were correlated (P < 0.001) with PBET gastric phase for Pb. The sum of the first two PBASE fractions, ΣE_1–2, was strongly correlated (P < 0.001) with Pb extracted by the PBET intestinal phase. The PBASE extraction method can provide information on Cd and Zn phytoavailability and Gl availability of Pb in smelter-contaminated soils.     

Arsenic Pollution of a Loam Soil: Retention Form and Decontamination

A study was conducted to assess the retention form of arsenic in soil and to evaluate the use of phosphate for releasing it from the soil. In this study, a loam soil was artificially polluted with arsenate at pH 5.5, which is one of the pH values at which maximum arsenic adsorption occurred. The soil was kept for 2.5 months under wet conditions to allow for stabilization. The soil was maintained under aerobic condition and losses of arsenic by volatilization were determined to be minimal. The soil was then sequentially extracted with a series of chemicals to identify the soil fractions in which the arsenic was bound. The percentage of arsenic found in the Fe bound-exchangeable, reducible-residual, Al bound exchangeable, residual, calcium bound exchangeable, and easily exchangeable forms was 31.6, 27.3, 25.2, 5.5, 4.9, and 4.7%, respectively. A batch experiment showed that at 20°C, 80% of the bound arsenic was removed by phosphate in the pH range of 5 to 7. A power function model was found to fit the data with a desorption rate constant of 402?mg/kg As h^?1.     

A Comparison of Ultrasonication and Soxhlet Methods for DDT Extraction from Soil

The goal of this study was to determine the efficacy of ultrasonication extraction of 1,1,1-trichloro-2,2-bis[p-chlorophenyl]ethane (DDT), 1,1-dichloro-2,2-bis[p-chlorophenyl]ethane (DDD), and 2,2-bis[p-chlorophenyl]1,1-dichloro-ethylene (DDE) residues in soil for the purposes of saving time, minimizing generation of hazardous solvent wastes, and reducing costs associated with monitoring contaminant concentrations at remediation sites. An ultrasonic extraction method was developed for DDT, DDD, and DDE residues in soil, and the efficiency of extraction using an ultrasonic cavitator was compared to the traditional soxhlet method by GC-MS. Un-contaminated soil was spiked with analytes DDT, DDD, and DDE at 0.1,1.0,10.0, and 100.0?mg/ kg. Experiments were performed in triplicate, and recoveries of analytes were determined and statistically compared. Results indicate that ultrasonic extraction is a suitable preparatory method for analysis of DDT, DDD, and DDE residues in soil. For spike concentrations of 1?mg/kg to 100?mg/kg, ultrasonication extraction resulted in recoveries in excess of 80% in all but one case. Most recoveries obtained by ultrasonication extraction were statistically indistinguishable from or slightly lower than recoveries obtained by soxhlet extraction. In addition, the lower temperatures employed in ultrasonication extraction may have reduced the amount of thermal degradation of DDT to DDE, a phenomenon that could occur during soxhlet extraction.     

从土壤中提取DNA方法比较

设计并比较了3种直接从土壤中提取DNA的方法。试验结果表明:3种方法都可以从土壤中提取到分子量大于23.13 kb的DNA的片段,每克干土DNA的提取量为2.5~31μg,不同方法间在DNA产量、纯度等方面存在较大差异。