Grantee Research Project Results
Final Report: Bioavailability and Biostabilization of Multicomponent Nonaqueous Phase Liquids in the Subsurface
EPA Grant Number: R825961Title: Bioavailability and Biostabilization of Multicomponent Nonaqueous Phase Liquids in the Subsurface
Investigators: Illangasekare, Tissa , Bielefeldt, Angela , Ramaswami, Anuradha
Institution: University of Colorado at Boulder , University of Colorado at Denver , Colorado School of Mines
Current Institution: University of Colorado at Boulder , Colorado School of Mines , University of Colorado at Denver
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1997 through September 30, 2000
Project Amount: $433,441
RFA: EPA/DOE/NSF/ONR - Joint Program On Bioremediation (1997) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
Objective:
The overall objective of this research project was to increase our understanding of the key factors that control the bioavilability and biostabilization potential of high molecular weight hydrophobic organic contaminants sequestered within multicomponent dense, non-aqueous phase liquids (DNAPLs) entrapped in heterogeneous aquifers. These contaminants are halogenated organic compounds (HOCs), such as polycyclic aromatic hydrocarbons (PAHs) or poly chlorinated biphenyls (PCBs). The project is motivated by the large number of superfund sites in the nation, including former manufactured gas plant sites that are contaminated with PAHs and PCBs. The hypothesis evaluated is that contaminant plumes, from the slow dissolution of HOCs from DNAPL pools in the subsurface, can be biologically stabilized by low-levels of microbial activity in the vicinity of the DNAPL sources. Microbial biodegradation of HOCs, coupled with biomass growth in the aquifer that alters the hydrodynamics of HOC transport near the DNAPL pools, contributes to biostabilization. The net effect is a reduction in HOC flux, leaving the source zone with the flowing groundwater and associated health risks without the technologically impractical task of complete destruction of the DNAPL mass released at the spill site. Information on biostabilization potential was gathered in batch tests, laboratory flow tanks, and via mathematical models.Summary/Accomplishments (Outputs/Outcomes):
The research project was conducted in five phases. Each is summarized with major findings below:
Phase 1: Biostabilization Screening Tests. Bench-scale screening tests were developed for rapid identification of those DNAPLs that are amenable to biostabilization. The screening protocols compared four criteria - microbial activity, composition of the DNAPL residue, aqueous phase contaminant concentrations, and aggregate aqueous phase toxicity-across unbiotreated controls and in mixed versus unmixed biometers, the latter system representing slow dissolution from DNAPL pools in the quiescent subsurface. The protocols were developed and evaluated with two multi-component DNAPLs: coal tar (PAH mixture) and aroclor (PCB mixture). Unmixed coal tar biometers, characterized by slow mass transfer and low-level microbial activity, exhibited reduced aqueous phase contaminant concentrations and aggregate toxicity, as well as stable DNAPL composition, consistently indicating favorable potential for in situ biostabilization. Aroclor 1242 was not effectively biostabilized in either mixed or unmixed systems, with increased bioactivity but inconsistent effects on aqueous contaminant concentrations and toxicity. Subsequent biostabilization studies were therefore limited to coal tar as the model DNAPL that showed consistent potential for biostabilization in bench-scale tests.
Table 1. Summary of Parameters and Comparisons Indicating Potential (Favorable or Unfavorable) for Biostabilization.
Parameter (1) | Comparisons (2) | Favorable Results: Indication of high potential for biostabilization (3) | Unfavorable Results: Indication against attempting in situ stabilization (4) |
Microbial Activity | Mixed biometer versus unmixed biometer. | Unmixed biometers have lower stable microbe concentrations compared to mixed respiring biometer microbes. | Microbe concentrations are variable, microbes are dead. |
DNAPL Composition | Mixed biometer versus unmixed biometer versus un-biotreated DNAPL. | Biometers show stable DNAPL composition, compared to un-biotreated control. Mixed biometers may show depletion of more soluble DNAPL constituents. | DNAPL composition does not stabilize. |
Aqueous Phase HOC Concentration | Mixed biometer versus unmixed biometer versus un-biotreated control. | HOC concentration in unmixed biometers < mixed biometers < un-biotreated controls. | Biotreated DNAPL yields higher HOC concentrations than un-biotreated control. |
Aggregate Aqueous Phase Toxicity | Mixed biometer versus unmixed biometer versus un-biotreated control. | Unmixed biometers show lower toxicity than mixed biometers, both lower than un-biotreated control. | Biotreated DNAPL yields greater toxicity than un-biotreated DNAPL. |
Phase 2: Measuring and Modeling Biodegradation Kinetics for PAH Mixtures Released from Coal Tar DNAPL. The objective in this phase was to describe the biodegradation of multiple PAHs released from coal tar, including the overall biogrowth associated with such multi-substrate degradation, from individual PAH biokinetic data. A pseudo component approach was utilized to describe the biodegradation of a complex aqueous phase mixture of PAHs employing biokinetic parameters measured for a small subset of five representative PAHs. The representative PAHs chosen were: styrene and benzene for 1 ring compounds, naphthalene for two-ring non-methylated compounds, 2-methyl naphthalene for methylated two ring compounds, and phenanthrene, representing three ring and larger PAHs released from coal tar. A multi-substrate model that incorporated competitive substrate inhibition and accrued microbial growth from all PAH components effectively described the biodegradation behavior of the PAH mixture. The multi-substrate model utilized Monod kinetic parameters determined for the 5 representative PAHs in individual biokinetic tests, along with pseudo-component concentrations in water. The multi-substrate pseudo-component method offers a relatively simple but effective tool to predict the overall biodegradation of various components of a PAH mixture, as well as the overall biogrowth that results from such biodegradation.
Phase 3: Abiotic Mass Transfer and Bioavailability Tests in Flow Cells. These tests were conducted in abiotic systems to determine the impact of media properties, fluid flow rate, and compositional changes on the rate of release of PAHs from DNAPL pools. At the beginning of this phase of research, the feasibility of using intermediate scale test tanks with dimensions 8 ft. long and 2 ft. high was investigated. A number of attempts were made to create a coal tar pool at the bottom of the tank. These experiments failed as it was infeasible to create stable coal tar pools in large tanks. Hence, the experiments were conducted in much smaller cells. Flow-through tests were first conducted in a well-mixed cell containing water and an embedded coal tar DNAPL pool. The water that flowed through the system caused changes in DNAPL composition and associated equilibrium aqueous phase PAH concentrations as various PAH components were depleted from coal tar. The measured change in equilibrium aqueous phase concentrations for each PAH component was largely proportional to the mole fraction of the PAH components left in the coal tar. This enabled the use of Raoult's law with the ideal DNAPL assumption for modeling dynamic compositional changes in coal tar-water equilibria. Flow-through tests were then conducted in porous packed columns containing an embedded coal tar DNAPL pool at the base. The columns were operated at three different flow rates, packed with two different sand grain sizes and embedded with two pool lengths to evaluate, in independent tests, the impact of these parameters on the kinetics of PAH dissolution from the DNAPL pool. A numerical model was developed with ModIME, an integrated modular modeling environment that couples MODFLOW with MT3D, to describe the dissolution of PAHs from the DNAPL pool. The model incorporated a quasi-steady state constant concentration boundary condition at the DNAPL-water interface, with transverse dispersion resulting in PAH transport into the water just above the pool. Further plume development occurred by advective-dispersive transport in pore water. A mass balance was used to update the composition of the DNAPL pool and the equilibrium aqueous interfacial concentration boundary condition, due to PAH depletion from the coal tar over the long-term.
Phase 4: Bioclogging Tests. The effects of entrapped PAH-degrading biomass on the hydrodynamic properties (conductivity and dispersivity) of homogenous sands were determined in this phase of the project. This information was needed to model biostabilization in Phase 5, which occurs by a combination of the biokinetic processed studies in Phase 2, the mass transfer processes described in Phase 3, and the bioclogging phenomena evaluated in Phase 4. Bioclogging was studied under endogenous conditions with no substrate present, because the low-levels of recalcitrant HOCs released slowly from DNAPL pools shown in Phase 1 caused little biogrowth. Under these low bio-growth conditions, microbial entrapment had minimal effects on hydraulic conductivity and porosity of the medium, although the longitudinal dispersivity increased significantly in some porous packed columns. The increase in longitudinal dispersivity was associated with microscale heterogeneities in bacterial deposition on the grains within the column. The breakthrough of bacteria in the porous packed column was studied as a function of water flow rate. It could be described by a relatively simple, physically-based particle deposition model incorporating collection efficiency on porous packed spheres.
Phase 5: Measuring and Modeling Coal Tar Biostabilization. Coal tar biostabilization was studied in bench-scale porous packed columns containing an embedded coal tar DNAPL pool. In contrast to the Phase 3 abiotic tests, the columns were seeded uniformly with biomass at the start of the tests. Biostabilization occurs as a result of three coupled processes: (1) mass transfer of PAHs from the coal tar pool to water; (2) biodegradation of multiple PAHs released into water from coal tar; and (3) any bioclogging effects due to the consequent bacterial growth and transport in the porous medium. The progress of biostabilization is monitored by measuring and comparing effluent PAH concentrations in biotic and abiotic systems operated under different flow rates. This test is a long-term test that has not yet terminated. At the end of the test, dispersivity changes and biogrowth deposition within the column will be measured, as well as the changes in DNAPL composition in the biotic and abiotic systems. Experimental observations will be compared with a simplified 1-dimensional DNAPL biostabilization model developed upon the Phase 3 ModIME framework. A comprehensive 2-dimensional DNAPL biostabilization model is still under development. We are expecting that this model will be tested and experimentally validated. This model, once validated, will allow us to extrapolate and evaluate more complex 2 or 3-dimansional flow fields encountered in the field.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
Other project views: | All 36 publications | 6 publications in selected types | All 4 journal articles |
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Type | Citation | ||
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Bielefeldt AR, Riffel AM, Ramaswami A, Illangasekare T. Assessing multicomponent DNAPL biostabilization potential. II: Aroclor 1242. Journal of Environmental Engineering 2001;127(12):1073-1079. |
R825961 (2000) R825961 (Final) |
not available |
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Iselyn M, et al. Measuring and modeling biodegradation of multiple PAHs released from coal tar. American Society of Civil Engineers Journal of Environmental Engineering. |
R825961 (Final) |
not available |
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Ramaswami A, Johansen PK, Isleyen M, Bielefeldt AR, Illangasekare T. Assessing multicomponent DNAPL biostabilization I: Coal tar. American Society of Civil Engineers Journal of Environmental Engineering 2001;127(12):1065-1072. |
R825961 (2000) R825961 (Final) |
not available |
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Richard DL, Ramaswami A, Bielefeldt A, Illangasekare T. Impact of microbial transport on porous media properties: assessment of no-growth conditions. Journal of Contaminant Hydrology. |
R825961 (Final) |
not available |
Supplemental Keywords:
groundwater, bioavailability, coal tar, PCBs, PAHs, DNAPL, bioremediation, modeling., RFA, Scientific Discipline, Toxics, Waste, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Environmental Chemistry, Chemistry, State, HAPS, Fate & Transport, Environmental Microbiology, Microbiology, chemical mixtures, Bioremediation, Groundwater remediation, Engineering, Environmental Engineering, fate and transport, risk-based decisions, NAPL, DNAPL, biostabilization, contaminated sediment, biodegradation, multicomponent nonaqueous phase liquids, chemical transport, subsurface systems, HOCs, contaminants in soil, Maryland (MD), chemical releases, contaminant release, sediments, California (CA)Relevant Websites:
http://stripe.colorado.edu/~bielefel/research.html Exit
http://www.mines.edu/fs_home/tillanga Exit
http://www.cudenver.edu/~aramaswa Exit
Progress 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.