Stimulation of hybrid poplar growth in petroleum-contaminated soils through oxygen addition and soil nutrient amendments

Hybrid poplar trees (Populus deltoides x nigra DN34) were grown in a green-house using hydrocarbon-contaminated soil from a phytoremediation demonstration site in Health, Ohio. Two independent experiments investigated the effect of nutrient addition on poplar growth and the importance of oxygen addition to root development and plant growth. Biomass measurements, poplar height, and leaf color were used as indicators of plant health in the selection of a 10/5/5 NPK fertilizer applied at 1121 kg/ha (112 kg-N, 24.4 kg-P, 46.5 kg-K per ha) to enhance hybrid poplar growth at the Health site. Five passive methods of oxygen delivery were examined, including aeration tubes, gravel addition, and an Oxygen Release Compound (ORC). When ORC was placed in coffee filters above hydrocarbon-contaminated soil, a statistically significant increase of 145% was observed in poplar biomass growth, relative to unamended controls. The ORC in filters also stimulated significant increases in root density. A 15.2-cm interval of soil directly below ORC addition exhibited an increase from 2.6 +/- 1.0 mg/cm3 to 4.8 +/- 1.0 mg/cm3, showing stimulation of root growth in hydrocarbon-stained soil. The positive response of hybrid poplars to oxygen amendments suggests that overcoming oxygen limitation to plants should be considered in phytoremediation projects when soil contamination exerts a high biochemical oxygen demand, such as in former refinery sites.     

Plants‐associated microflora and the remediation of oil‐contaminated soil

The use of plants and their rhizospheric microorganisms is a promising emerging technology for remediating contaminated soils. The degradation of total petroleum hydrocarbon (TPH) in the rhizospheric and nonrhizospheric soil of three domestic plants, namely, alfalfa (Medicaga sativa) broad beans (Vicia faba) and ryegrass (Lolium perenne) was investigated. The experimental data from the studies of plantmicrobe‐soil interactions implicated the enhancement of TPH degradation by the rhizospheric microbial community. Although the three domestic plants exhibited normal growth in the presence of ～1.0% TPH, the degradation was more profound in the case of leguminous plants. The TPH degradation in the soil cultivated with broad beans and alfalfa was 36.6 and 35.8%, respectively, compared with 24% degradation in case of ryegrass. Such a high correlation between plant type and TPH degradation rates indicate that selection for enhanced rhizosphere degradation may be accomplished by selecting leguminous plants.     

TECHNICAL NOTE: NITROGEN FERTILIZATION EFFECTS ON THE DEGRADATION OF AGED DIESEL OIL IN COMPOSTED DRILLING WASTES

Hydrocarbon-contaminated wastes generated from oil and gas drilling activities may be used as a soil amendment once composted and further decomposition of residual hydrocarbons can be accomplished after the composts are applied to soils. To test if N fertilization may enhance hydrocarbon decomposition, we investigated the effects of N application on hydrocarbon degradation in different-aged composts (1-, 2-, 3-, and 4-year-old composts, coded as 1Y, 2Y, 3Y, and 4Y composts, respectively) through a pot experiment planted with white spruce (Picea glauca [Moench] Voss) seedlings. The percentage degradation of total petroleum hydrocarbon (TPH, C11 to C40) in the composts without N fertilization was correlated to initial NH₄ ⁺ concentrations (R = 0.99, P < 0.001). The percentage degradation of TPH was highest in the 3Y compost (41.1%) that had an initial level of 325.3 mg NH₄ ⁺-N kg^?1 and the lowest in the 1Y compost (9.3%) that had an initial level of 8.3 mg NH₄ ⁺-N kg^?1. The degradation of TPH was enhanced by N fertilization in the 1Y (from 9.3 to 15.3%) and 4Y composts (from 14.3 to 22.6%) that had low initial NH₄ ⁺ concentrations. Our results show that application of NH₄ ⁺-based fertilizers may enhance the degradation of TPH when initial NH₄ ⁺ concentrations in the compost are low.     

Technical note: fate and transport of jet fuel (JP-8) in soils with selected plants

Remediation of soil and groundwater contaminated by leaking fuel storage tanks may be assisted by plants, although plant effects on abiotic and biotic removal processes remain unclear. The objectives of this study were to investigate abiotic and biotic removal of JP-8, a kerosene-based jet fuel, in soils with plants, and to determine the effects of plant-induced water movement. Loss of JP-8 in a dry-soil, control column was 25% after 5 months, primarily due to volatilization and gas-phase diffusion. By comparison, managed treatments with simulated surface spills averaged 86% mass reduction at 5 months, indicating an important contribution of biodegradation. Overall JP-8 mass reduction was similar in surface and subsurface-irrigated systems, indicating water content, not mode of water application, influences bioremediation in near-surface systems. The JP-8 concentration reductions in soil columns contaminated above a simulated watertable were 36% after 3 months and 50% after 12 months for vegetated columns compared to 26% and 34% in unplanted columns. Downward movement of JP-8 in unplanted columns was double that in planted columns. Near the groundwater table, JP-8 persists longer than near the soil surface. Plants promote upward movement of water and help draw spilled JP-8 to aerobic near-surface soil.     

Selection of cold‐tolerant plants for growth in soils contaminated with organics

A mixture of organic chemicals (MOC) containing equal molar amounts of benzoic acid, hexadecane, 2,2‐dimethyl 4,n‐propyl‐benzene, phenanthrene, pyrene, and either cycloheptane or cis‐decahydronaphthalene (cis‐decalin) was applied to soil at rates of 0 to 8000 mg/kg. In a plant‐screening experiment, growth responses of four legume and five nonlegume species were determined at 10 and 25°C. The MOC applied at 2000 mg/kg reduced the growth of several species without resulting in significant seedling death. At 10°C, the growth of alpine bluegrass (Poa alpina L.) in the 1000 and 2000 mg/kg treatments of soil increased by more than 185%. In a plant growth response experiment, alpine bluegrass and alfalfa (Medicago sativa L.) were grown in soil that had been contaminated at rates of 0 and 2000 mg/kg. At 14 weeks, the shoot and root dry weights of alfalfa were 97% lower in the contaminated soil, while the shoot dry weight, root dry weight, and root length of alpine bluegrass were 135,235 and 268% higher, respectively. Except for pyrene, <23% of the compounds comprising the MOC remained in the soil after 4 weeks and <5% after 14 weeks. The disappearance of the MOC was not significantly influenced by the presence of alfalfa or alpine bluegrass.     

Bioremediation and phytoremediation of total petroleum hydrocarbons (TPH) under various conditions

Remediation of contaminated soils is often studied using fine-textured soils rather than low-fertility sandy soils, and few studies focus on recontamination events. This study compared aerobic and anaerobic treatments for remediation of freshly introduced used motor oil on a sandy soil previously phytoremediated and bioacclimated (microorganisms already adapted in the soil environment) with some residual total petroleum hydrocarbon (TPH) contamination. Vegetated and unvegetated conditions to remediate anthropogenic fill containing residual TPH that was spiked with nonaqueous phase liquids (NAPLs) were evaluated in a 90-day greenhouse pot study. Vegetated treatments used switchgrass (Panicum virgatum). The concentration of aerobic bacteria were orders of magnitude higher in vegetated treatments compared to unvegetated. Nevertheless, final TPH concentrations were low in all saturated soil treatments, and high in the presence of switchgrass. Concentrations were also low in unvegetated pots with fertilizer. Acclimated indigenous microbial communities were shown to be more effective in breaking down hydrocarbons than introducing microbes from the addition of plant treatments in sandy soils. Remediation of fresh introduced NAPLs on pre-phytoremediated and bioacclimated soil was most efficient in saturated, anaerobic environments, probably due to the already pre-established microbial associations, easily bioavailable contaminants, and optimized soil conditions for microbial establishment and survival.     

PHYTOREMEDIATION OF BTEX HYDROCARBONS: POTENTIAL IMPACTS OF DIURNAL GROUNDWATER FLUCTUATION ON MICROBIAL DEGRADATION

Volatile hydrocarbons have multiple potential fates in phytoremediation. This research investigated the relationship between biodegradation and plant uptake of BTEX compounds in laboratory and field settings. At a phytoremediation site, preliminary studies revealed minimal uptake into trees and enhanced degradation potential in the rhizosphere and in the bulk soil. Increased oxygen transport to the vadose zone caused by diurnal rise and fall of the water table was hypothesized to enhance degradation in the bulk soil. A detailed greenhouse study was then conducted to investigate potential bioremediation impacts using field-site soil and DN34 hybrid poplar trees.

In rhizosphere soils, the contaminated-planted reactor had significantly higher BTEX degrader populations versus the uncontaminated-planted reactor, as was anticipated. The bulk soil in the planted-contaminated reactor had increased degrader populations than the unplanted-contaminated soil or planted-uncontaminated soil, and planting increased degradation throughout the soil profile, not just in the limited volume of rhizosphere soils. Oxygen diffusive and advective transport into reactors was modeled and calculated. Oxygen input in planted reactors was at least 3 to 5 times higher than in unplanted reactors, and increasing oxygen input lead to increased degrader populations in a linear manner. These results combined with the knowledge that high-transpiration trees draw the contaminated groundwater to the capillary fringe and the rhizosphere indicate that phytoremediation can aid microbial degradation via multiple mechanisms: increasing degrader populations, increasing oxygen input via groundwater diurnal fluctuations, and transporting contaminants to the biologically-enriched soil profile.     

Degradation of crude oil in the rhizosphere of Sorghum bicolor

Dissipation of petroleum contaminants in the rhizosphere is likely the result of enhanced microbial degradation. Plant roots may encourage rhizosphere microbial activity through exudation of nutrients and by providing channels for increased water flow and gas diffusion. Phytoremediation of crude oil in soil was examined in this study using carefully selected plant species monitored over specific plant growth stages. Four sorghum (Sorghum bicolor L.) genotypes with differing root characteristics and levels of exudation were established in a sandy loam soil contaminated with 2700 mg crude oil/kg soil. Soils were sampled at three stages of plant growth: five leaf, flowering, and maturity. All vegetated treatments were associated with higher remediation efficiency, resulting in significantly lower total petroleum hydrocarbon concentrations than unvegetated controls. A relationship between root exudation and bioremediation efficiency was not apparent for these genotypes, although the presence of all sorghum genotypes resulted in significant removal of crude oil from the impacted soil.     

石油污染土壤植物-微生物修复研究进展

依据国内外近10年来有关石油污染土壤生物修复研究的成果,综合阐述了石油污染土壤的植物修复、微生物修复及植物-微生物联合修复方法研究,重点讨论植物-微生物联合作用,主要包括植物根际微生物、根分泌物以及菌根对石油污染物降解的影响,提出了污染土壤原位修复中需要重视的问题.     

Ecotoxicity Test Data for Total Petroleum Hydrocarbons in Soil: Plants and Soil-Dwelling Invertebrates

Ecotoxicity benchmarks for petroleum mixtures can be used in a screening-level ecological risk assessment. Data from studies evaluating the toxicity of total petroleum hydrocarbons (TPH) to plants and soil invertebrates were reviewed for possible application to soil benchmark development. Toxicity data included LOAECs; estimated EC25s, EC20s, and LC50s; effective concentrations that caused greater than a 20% level of effect; and NOAECs. The variabilities in petroleum material, chemical analytical methodology, age of hydrocarbon-soil contact, nutrient amendment, and measured effects levels did not permit much meaningful aggregation of the data. Tenth, twenty-fifth, and fiftieth percentiles of toxicity and no-effects data are presented for unaggregated results within studies. Effects on invertebrates often occurred at concentrations of TPH lower than those associated with effects on plants. Lighter mixtures generally were associated with lower ranges of effects concentrations than heavier crude oil. Few aged and non-aged samples were available from the same study, and these did not show obvious trends regarding toxicity. Similarly, the addition of nutrients to promote bioremediation was not observed across studies to alter effective or nontoxic concentrations in a systematic way. Existing toxicity data are not sufficient to establish broadly applicable TPH ecotoxicity screening benchmarks with much confidence, even for specific mixtures.     

Restoration of Petroleum-Contaminated Soil Using Phased Bioremediation

A conceptual approach is presented for the restoration of petroleum-contaminated sites by combining bioremediation with revegetation using native plants. Phased bioremediation includes active and passive treatment options for soil containing greater than 1% total petroleum hydrocarbons (TPHs). Phase I is used when initial soil TPH exceeds 1%. Phase I utilizes either active land treatment, with regular soil tillage, or passive bioremediation to attain a treatment endpoint of 1% soil TPH. Passive treatment utilizes static soil and TPH-tolerant plants. Phase II is utilized when soil contains 1% TPH or less. It combines passive bioremediation with revegetation using native plants to complete the site restoration process. The phased approach to bioremediation was developed from results of full-scale field bioremediation and laboratory treatability studies. This approach assumes that the kinetics of TPH biodegradation are initially rapid, followed by a much slower second stage. It provides active initial treatment, followed by lower-cost passive treatment. The selection of either active or passive treatment in Phase I depends on whether total cost or time of treatment is more important. Passive treatment, although less costly than active treatment, generally requires more time. Phased bioremediation may provide a flexible, cost-effective, and technically sound approach for restoration of petroleum-contaminated sites.

Vegetation used with passive bioremediation has several benefits. Plants stabilize soil, preventing erosion and thereby minimizing exposure to soil contaminants. Phytoremediation may also occur within the rhizosphere. The use of native plants has a strong ecological basis. They provide ecological diversity, are aesthetically pleasing and beneficial to wildlife, while requiring little maintenance. Phased bioremediation can provide a flexible, cost-effective, and technically sound approach for the restoration of petroleum-contaminated sites.     

Degradation of Polycyclic Aromatic Hydrocarbons in the Rhizosphere of Festuca arundinacea and Associated Microbial Community Changes

A phytoremediation growth chamber study was conducted to evaluate the contribution of soil microbial diversity to the contaminant degradation. Target contaminant removal from soil was assessed by monitoring concentrations of polycyclic aromatic hydrocarbons (PAHs), along with changes in the bacterial community structure over a time period of 10 months in the presence of tall fescue (Festuca arundinacea). Enhanced degradation of PAHs was observed in rhizosphere soil, with a maximum reduction in pyrene at a rate 36% higher than that noted for the unvegetated control. The dissipation of < 4-ring PAHs, 4-ring PAHs, and > 4-ring PAHs in unvegetated soil was 70%, 54%, and 49% respectively, whereas a higher dissipation rate was observed in tall fescue treated soil of 78%, 68%, and 61% at the end of the study. Microbial enumeration results showed greater total bacterial numbers and PAH-degrading bacteria in rhizosphere soil when compared to unvegetated soil. The results from the terminal restriction fragment length polymorphism (T-RFLP) analysis indicated that there was a shift in the rhizosphere bacterial community during the phytoremediation process.     

Distribution of Petroleum and Aromatic Hydrocarbons at a Former Crude Oil and Natural Gas Production Facility

Site characterization and remediation activities were performed at a former crude oil and natural gas production facility prior to redevelopment of the site. Field activities included delineation, excavation and segregation of approximately 1,250,000 m³ of soil impacted by total petroleum hydrocarbons (TPH) and the aromatic volatile organic compounds (VOCs) benzene, toluene, ethylbenzene, and xylenes (hereafter, collectively referred to as BTEX). Petroleum hydrocarbon chain length information was used to determine whether remediation was required in impacted areas, because the site-specific cleanup values for TPH compounds, established by the California State Regional Water Quality Control Board (RWQCB), were based on hydrocarbon chain length. Site-specific cleanup levels were also established by the RWQCB for BTEX. Subsurface investigation activities performed at the site indicated that the mean percentage of condensate and TPH compounds in the gasoline range was significantly greater at depths ranging from 4.6 to 18 m than in shallower samples. There was no significant difference in the mean concentration of BTEX compounds and mean percentage of diesel range and heavier hydrocarbons with depth. The occurrence of BTEX, diesel range, and heavier hydrocarbons at depth may result from preferential pathways for downward migration of contaminants, including blown out wells, abandoned wellbores, and the presence of faults. Vapor phase diffusion may also be a major transport mechanism controlling movement of BTEX compounds beneath the site.     

Enhancing the biodegradation of total petroleum hydrocarbons in oily sludge by a modified bioaugmentation strategy

The effect of successive inoculation with hydrocarbon-degrading bacteria on the dynamics of petroleum hydrocarbons degradation in soil was investigated in this study. Oily sludge was used as a source of mixed hydrocarbons pollutant. Two bacterial consortia composed of alkanes and polycyclic aromatic hydrocarbon degraders were constructed from bacteria isolated from soil and oily sludge. These consortia were applied to incubated microcosms either in one dose at the onset of the incubation or in two doses at the beginning and at day 62 of the incubation period, which lasted for 198 days. During this period, carbon mineralization was evaluated by respirometry while total petroleum hydrocarbons and its fractions were gravimetrically evaluated by extraction from soil and fractionation. Dosing the bacterial consortia resulted in more than 30% increase in the overall removal of total petroleum hydrocarbons from soil. While alkane removal was only slightly improved, aromatic and asphaltic hydrocarbon fraction removal was significantly enhanced by the addition of the second consortium. Polar compounds (resins) were enriched only as a result of aromatics and asphaltene utilization. Nonetheless, their concentration declined back to the original level by the end of the incubation period.     

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.