Grantee Research Project Results
Final Report: Bioavailability and Biostabilization of PCBs in Soil
EPA Grant Number: R825365Title: Bioavailability and Biostabilization of PCBs in Soil
Investigators: Luthy, Richard G. , Dzombak, David A. , McNamara, Sean W.
Institution: Carnegie Mellon University
EPA Project Officer: Aja, Hayley
Project Period: January 1, 1997 through December 31, 1999
Project Amount: $499,056
RFA: DOE/EPA/NSF/ONR Joint Program on Bioremediation (1996) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
Objective:
The objective of this research project was to develop an understanding of the relationship between the bioavailability and biostabilization of polychlorinated biphenyls (PCBs) in aged soils that have undergone active land biotreatment, but which still show a residual concentration. The intended result of this objective was to advance scientific knowledge for improved estimates of the potential for release of residual PCB congeners, including the concept of biostabilization, which envisions that contaminated materials can be actively biotreated to remove a large fraction of the potentially mobile and bioavailable organic contaminants. The key questions addressed were: (1) What is the rate of release of residual PCBs for aged soils that have undergone active land biotreatment? (2) What are the rates of intrinsic biodegradation for residual PCBs? and (3) What are the implications of these factors for environmentally acceptable endpoints and risks for residual PCBs?
This research was built on prior collaborative studies between Carnegie Mellon University and the Aluminum Company of America (Alcoa) on land biotreatment of soil/sludge mixtures contaminated with PCBs and hydraulic oils. The research took advantage of the significant investment in developing a field site to assess the biodegradation of PCBs in contaminated soil/sludge mixtures. The proposed work included: (1) monitoring of the transformation and degradation of PCBs in land treatment units during passive operations following active biotreatment; (2) assessing the fluxes of water and oxygen in land treatment units; (3) assessing the solubility and release rates of PCBs in the soil/sludge mixtures; (4) developing appropriate models for use in identifying the key physicochemical and microbial parameters governing the fate of the residual PCBs; and (5) using these results to examine the risk implications of biostabilized PCB contaminants in soil. The work emphasizes the study of aged contaminant mixtures in a field test for which the sequestration of PCBs may render such compounds only slowly available to microorganisms or the environment. The impact of the research linked a rational model for bioavailability for aged samples with practical measurements relevant to bioremediation engineering practice. The research assessed the extent that biostabilization reduces risks and the uncertainty in risk assessment.
Summary/Accomplishments (Outputs/Outcomes):
This research employed laboratory tests, extensive data compilations from field experiments, conceptual models, and computer modeling to determine the physiochemical factors and relationships affecting the potential release of residual PCBs (i.e., partitioning from the soil/sludge/sediment matrix to the aqueous phase) from biotreated materials. These included:
· The assessment of equilibrium partitioning models based on historical
sampling data, including field and lab measurements of PCB concentrations on
bulk soil and in soil-water leachate, of total organic carbon (TOC), and extractable
oil and grease concentrations;
· An investigation of the degradation rates of oil and grease and PCBs,
as homolog group or by structural characteristics, and the impact that differential
removal rates may have on the aqueous-phase availability of PCBs in land treatment
systems;
· The development and demonstration of a field sampling device for monitoring
the time-averaged aqueous-phase availability of PCBs and other chlorinated organic
compounds in unsaturated soil systems; and
· An assessment of the long term effectiveness of land biotreatment systems
in reducing PCB concentrations in the soil and soil-water leachate, and, hence,
soil risk.
The results of these investigations will be submitted for peer-review and three journal articles will be published. The results will: (1) summarize the performance of the land treatment units (LTUs) and address equilibrium partitioning models; (2) detail the design and hydraulic characteristics of the new pore-water sampling device; and (3) detail the chemical capture, recovery, and analysis of chlorinated organics from the sampling device, including its field demonstration in the LTU soils. The third research paper describes the equilibrium partitioning and degradation kinetics of PCBs in the LTU systems, with particular focus on the role of oil and grease in controlling partitioning.
Land Biotreatment of PCBs. This research focused on the operational characteristics and performance of three pilot-scale LTUs at the Alcoa's Massena, NY facility. These LTUs have been in operation since 1994 as part of a long term demonstration project on the biostabilization of PCBs in soils and sediments through biotreatment. The LTUs were loaded with sediment/sludge material from onsite settling and treatment lagoons associated with sanitary wastewater, stormwater, and industrial processes. These materials were mixed with clean sand such that initial concentrations in the LTU treatment zones ranged from approximately 10 to 110 mg/kg-dw total PCBs, 300 to 1700 mg/kg-dw total PAHs, and 4,000 to 9,000 mg/kg-dw total oil and grease. Each LTU began its operation with 2 to 3 months of active treatment, where the treatment zone was periodically tilled, and soil moisture and nutrients were monitored and controlled. The LTUs were then covered with a layer of topsoil and operated thereafter under passive treatment conditions with no engineering controls. Soil samples were taken at various times throughout the active and passive treatment phases and analyzed for soil nutrients, TOC, oil and grease, microbial populations, and the concentration of PAHs and PCBs on the bulk soil and in laboratory-derived soil-water leachate.
The results indicated that LTUs mainly are effective in the reduction of di- and tri-PCB homologs, and that most of these reductions can be expected to occur during the active treatment phase. This was especially true for materials that have been pretreated, showing enhanced microbial growth. Further reductions in PCBs occurred under passive remedial conditions, but the degradation rates associated with these reductions were quite small. Mono-ortho-substituted PCBs, including coplanar tetra-PCB congeners, were the only major subcategories besides the di- and tri-PCB homologs that showed appreciable degradation rates. Declines in PCB-degrading microbial populations during passive treatment appeared to be a major limiting factor in the continued reduction of PCBs in these systems.
In each LTU, oil and grease levels were reduced by approximately 50 percent during the active remedial phase, with continued reductions at a lower rate during the passive phase. Notably, the rate of oil and grease degradation during passive biotreatment was faster than that for most PCB homologs. This has major implications for PCB partitioning between the oil and grease and aqueous phase, as enrichment of PCBs in the organic phase resulted in higher aqueous phase equilibrium concentrations. This was observed in laboratory measurements of soil-water leachate for the LTU system that had the highest levels of oil and grease, but it also was observed in the lowest rates of PCB degradation.
Analysis of three equilibrium partitioning modeling approaches confirmed that oil and grease was a far better measure of PCB partitioning to the aqueous phase than empirical modeling based on TOC and solid phase PCB concentrations. Modeling based on PCB partitioning between a non-aqueous phase liquid (NAPL), oil and grease, and water phase using Raoult's Law of liquid solubilities, provided an order of magnitude estimation of actual PCB concentrations in the soil-water leachate, while modeling based on soil organic carbon and empirical organic carbon-water partitioning (i.e., foc-Koc models) typically overestimated aqueous phase concentrations by two to three orders of magnitude. The models demonstrated that PCBs could be expected to differentially partition between the oil and grease and the soil organic carbon by three or more orders of magnitude on an equivalent organic carbon basis. This implies that as long as residual levels of oil and grease remain in the soil matrix, PCB partitioning to the aqueous phase will be controlled primarily by the oil and is best modeled by Raoult's Law. This, combined with the differential rates of degradation between the oil and grease and PCBs, may lead to increased partitioning to the aqueous phase for higher-chlorinated PCBs in some land treatment systems, and as a result, may impact soil risk and treatability goals.
Despite limited PCB reductions in all LTUs, and enhanced aqueous phase partitioning in one LTU, the overall ecological toxicity of the LTU soils was reduced over time, as evidenced by Microtox, earthworm survival, seed germination, and root elongation studies. The exposure risk to human receptors was estimated to decrease in two of the LTUs, but not in the LTU with the high initial concentrations of PCBs and oil and grease. Dioxin-like congeners in the LTU soils were shown to significantly contribute to the overall human exposure risk.
Measuring Aqueous Phase Availability. A novel in situ pore-water sampling device was developed to measure the aqueous-phase availability of PCBs in the unsaturated land treatment systems. The sampling device was designed to integrate the mass of PCBs sampled in the LTU soil pore-water over extended sampling periods (weeks to months), and to quantify the volume of water from which that mass was collected. In this way, the sampling technique provides a field estimate of the time-averaged mass and volumetric flux rates of PCBs in the LTU soil column. It is anticipated that the use of this device in field investigations may result in better estimates than either solid-phase leachate procedures or sequestration/release models could possibly provide, of the average aqueous-phase availability of hydrophobic compounds in aged field soils. It also is anticipated that this technique will be useful in quantifying contaminant mobility, bioavailability, and the extent to which natural attenuation and biostabilization may limit soil risk.
The nominal sampler design consisted of a cylindrical, porous metal interface packed with granular activated carbon encapsulating the end of a fiberglass wick with 100-cm horizontal and 100-cm vertical lengths. This design was developed for deployment in the shallow land treatment units. The passive device samples soil water by using the capillarity of the fiberglass wick and elevation potential to create a pressure gradient, which draws water from the surrounding soil, through the cylindrical porous interface and activated carbon, and into and along the wick where it is conveyed to a vessel for collection. Aqueous PCBs are stripped from the water as it passes through the activated carbon, where they are preserved for analysis until a representatively sized sample of soil water has been collected. The PCBs are recovered from the activated carbon using pressurized solvent extraction and analyzed with standard gas chromatography (GC) methods.
The hydraulic characteristics of horizontally-installed, sorbent wick sampling devices were evaluated through wick tracer studies and laboratory soil column experiments to assess the influence of horizontal wick length and sampler interface design on sampling pore-water in unsaturated soils. Tracer dye studies indicated that the addition of a horizontal wick length lowers the maximum sampling velocity of horizontally installed systems by more than half that predicted in previous vertical wick-only investigations. The maximum sampling rate of horizontally installed wick systems declines exponentially with increasing horizontal wick length, while vertical length influences the range of soil-water pressures that may be sampled. The nominal design sampled pore water from clay loam laboratory columns at 8 to 14 mL h-1 under steady-state infiltration conditions and 2 to 5 mL h-1 under draining conditions across a 10-45 cm H2O soil-water pressure range; sampling rates in a medium grained sand were half that of clay loam. Analysis of observed sampling velocities, versus calculated soil water contents and hydraulic conductivities, indicated that the design performs best when the soil water content is greater than 0.15 and when unsaturated hydraulic conductivity is greater than 0.2 cm h-1. A hydraulic model was developed that estimates the sampling velocity of the nominal design based on sampler interface pressure, which was linearly correlated with soil pressure. This model can be used to optimize the wick lengths for specific field applications.
Laboratory experiments were conducted to evaluate the chemical capture and quantitative recovery of chlorinated organic compounds sampled from the pore water of unsaturated soils with the sorbent-wick sampling device. A pressurized fluid extraction procedure was developed that employed elevated temperature and benzene to recover target compounds from the sampler's granular activated carbon sorbent material. Laboratory column studies using Ottawa sand spiked with tri- and tetrachlorobenzenes were used to evaluate the accuracy of the sampling device in assessing average pore-water concentrations and to test the influence of vapor phase partitioning on sampling results. The findings from these experiments showed that the sorbent-wick sampler provided reliable and repeatable estimates of the pore water concentrations of the chlorobenzenes in the unsaturated soil columns at pore water concentrations as low as several parts per billion.
Field trials were conducted in one of the land biotreatment units to assess the availability of trace levels of aqueous-phase PCBs. This land treatment unit had undergone 3 months of active and 5 years of passive remediation at the time of installation. Results from the field sampler were compared with laboratory-derived soil-water leachate results. The comparison indicated that the samplers were able to replicate the aqueous-phase PCB homolog pattern obtained from more costly and time intensive soil-water extraction tests. Tetra- and ortho-substituted PCBs were dominant in sorbent wick samples and aqueous extracts; these PCBs appear relatively recalcitrant in land biotreatment units.
Conclusions:
In summary, this investigation expanded on both field and laboratory measures of PCB availability in land treatment systems. The results indicated that oil and grease would largely control PCB partitioning in LTU soils, and that aqueous phase concentrations may actually increase with land treatment. Understanding the interdependency of these factors may be important in meeting treatability goals.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 7 publications | 3 publications in selected types | All 3 journal articles |
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Type | Citation | ||
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McNamara SW, Luthy RG. Sorbent wicking device for sampling hydrophobic organic compounds in unsaturated soil pore water. I: Design and hydraulic characteristics. Journal of Environmental Engineering 2005;131(1):11-20. |
R825365 (Final) |
not available |
|
McNamara SW, Luthy RG. Sorbent wicking device for sampling hydrophobic organic compounds in unsaturated soil pore water. II: Chemical capture, recovery, and analysis. Journal of Environmental Engineering 2005;131(1):21-28. |
R825365 (Final) |
not available |
|
McNamara SW, Ghosh U, Dzombak DA, Weber AS, Smith JR, Luthy RG. Effect of oil on polychlorinated biphenyl phase partitioning during land biotreatment of impacted sediment. Journal of Environmental Engineering 2005;131(2):278-286. |
R825365 (Final) |
not available |
Supplemental Keywords:
polychlorinated biphenyls, PCB, non-aqueous phase liquids, NAPL, equilibrium partitioning, leachate, vadose zone, land treatment, bioremediation., RFA, Scientific Discipline, Toxics, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Contaminated Sediments, exploratory research environmental biology, Remediation, Environmental Chemistry, Ecosystem/Assessment/Indicators, Chemical Mixtures - Environmental Exposure & Risk, Ecosystem Protection, Chemistry, HAPS, chemical mixtures, Ecological Effects - Environmental Exposure & Risk, Ecological Effects - Human Health, Bioremediation, Biology, Engineering, Ecological Indicators, risk assessment, fate and transport, fate, biostabilization, biostabilization of PCBs, contaminant transport, contaminated sediment, PCBs, biodegradation, hydraulic oils, chemical transport, kinetic studies, land biotreatment, chemical contaminants, bioremediation of soils, contaminants in soil, environmentally acceptable endpoints, models, vadose zone, physicochemicalProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.