Grantee Research Project Results
Final Report: Traveling Wave Behavior During Subsurface Transport of Biologically Reactive Contaminants: Implications for In Situ Bioremediation
EPA Grant Number: R824785Title: Traveling Wave Behavior During Subsurface Transport of Biologically Reactive Contaminants: Implications for In Situ Bioremediation
Investigators: Valocchi, Albert J.
Institution: University of Illinois Urbana-Champaign
EPA Project Officer: Packard, Benjamin H
Project Period: January 1, 1996 through December 1, 1998 (Extended to December 31, 1999)
Project Amount: $200,000
RFA: Water and Watersheds (1995) RFA Text | Recipients Lists
Research Category: Watersheds , Water
Objective:
Clean up of polluted groundwater resources is a major environmental challenge of modern society. In the ground (in situ) remediation of contamination costs a fraction of other approaches. For many classes of organic pollutants it is possible to stimulate naturally occurring microorganisms to transform the pollutants biologically into harmless by-products. This process is known as in-situ bioremediation, which is one of the fastest growing sectors of the hazardous waste field. Engineered in-situ bioremediation often involves the injection of limiting nutrients and electron acceptors into aquifers containing the organic pollutants which serve as substrates to produce a biologically active zone (BAZ) where indigenous microorganisms significantly grow in the presence of the substrates and the amended compounds. Although there are numerous chemical and biological factors responsible, it is widely believed that aquifer heterogeneity and other transport-related factors play a key role in determining the effectiveness of in-situ bioremediation at any specific site.The objectives are to investigate how transport and mixing processes affect the overall performance of engineered in-situ bioremediation. Although these processes play a key role in the ultimate success of actual remediation projects, their significance cannot be ascertained through typical laboratory-scale studies. Therefore we are analyzing mathematical models for a typical bioremediation scenario in which a uniformly-distributed organic contaminant is degraded by indigenous soil microbes that are stimulated by an injected material (e.g., an electron acceptor such as oxygen). Our analysis starts with simple one-dimensional homogeneous systems and progresses to more realistic multi-dimensional heterogeneous aquifers.
Summary/Accomplishments (Outputs/Outcomes):
We have studied the following cases: one-dimensional uniform flow in a homogeneous aquifer; ideal radial flow from an injection well in a homogeneous aquifer; and two dimensional flow in stratified and randomly heterogeneous aquifers. For a wide variety of cases, we find that the system evolves to form traveling waves; that is, the spatial profiles of the organic pollutant, electron acceptor, and biomass attain constant shapes which travel in unison. These traveling waves form due to a balanced interaction between solute mixing processes and localized biodegradation processes. This results in a very localized reaction zone where the pollutant and electron acceptor mix together.For conditions when traveling waves exist we have derived simple formulas to calculate the long-term rate of pollutant removal due to biodegradation. The removal rate expressions are similar for all the different flow systems that we have examined. It is surprising and significant that complex multi-dimensional heterogeneous aquifers behave similar to simple one-dimensional homogeneous aquifers. The pollutant removal rate depends upon transport properties of the aquifer, but it does not depend upon the initial biomass concentration or upon the rate at which the biomass can degrade the pollutant. Results from the simple formulas were verified by comparison with numerical simulations.
The theoretical pollutant removal rate represents a large-time limiting result. Detailed numerical simulations show that it takes a certain initial time to establish traveling wave conditions; during the initial time phase the biodegradation rate depends in a complex fashion on all the reaction and transport parameters. The duration of this initial time phase depends upon the relative magnitude of transport and degradation processes; for the limiting case of very fast degradation kinetics, traveling wave behavior is established quickly after the onset of electron acceptor addition. Numerical simulations for stratified and randomly heterogeneous aquifers also reveal that small-scale variations in soil permeability play an important role in extending the duration of the initial time period. Flow heterogeneity creates interfaces where the injected electron acceptor and the background pollutant mixing is controlled by transverse hydrodynamic dispersion and molecular diffusion. This tends to enhance the pollutant removal rate above that predicted by the analytical result based upon traveling wave theory.
Conclusions:
The traveling wave framework is a useful simplifying tool for approximating the complexity of bioremediation modeling. Use of the analytical formulas reduces significantly the computational burden of estimating the efficiency of a remedial design. The results are significant because they indicate conditions for which the long-term pollutant removal rate does not depend upon the laboratory-determined rate of biodegradation. This helps explain the common observation that laboratory-determined degradation rates overestimate the degree of biodegradation attained in many field projects.Our detailed numerical simulations reveal that traveling waves are only attained under certain conditions. In practical contamination problems, we expect our theory to apply to readily degradable compounds, such as petroleum hydrocarbons. Limited field data available in the literature shows qualitative agreement with many of the features of traveling wave theory. Because of its simplicity, the theory is useful for gaining an initial understanding of the potential for in-situ bioremediation at a particular site. However, more detailed simulation and field studies are required to make engineering design decisions.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
| Other project views: | All 9 publications | 3 publications in selected types | All 3 journal articles |
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Oya S, Valocchi AJ. Characterization of traveling waves and analytical estimation of pollutant removal in one-dimensional subsurface bioremediation modeling. Water Resources Research 1997;33(5):1117-1127. |
R824785 (Final) |
not available |
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Oya S, Valocchi AJ. Analytical approximation of biodegradation rate for in situ bioremediation of groundwater under ideal radial flow conditions. Journal of Contaminant Hydrology 1998;31(3-4):275-293. |
R824785 (1998) R824785 (Final) |
not available |
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Oya S, Valocchi AJ. Transport and biodegradation of solutes in stratified aquifers under enhanced in situ bioremediation conditions. Water Resources Research 1998;34(12):3323-3334. |
R824785 (1998) R824785 (Final) |
not available |
Supplemental Keywords:
RFA, Scientific Discipline, Water, Waste, Mathematics, Remediation, Engineering, Chemistry, Biology, Watersheds, Environmental Chemistry, Groundwater remediation, Hydrology, Bioremediation, Physics, Water & Watershed, adsorption, traveling wave behavior, kinetic studies, organic solutes, environmental engineering, in-situ bioremediation, organic contaminants, in situ bioremediation, biodegradation, fate and transport, groundwater, subsurface systems, chemical transport, desorption rates, mathematical models, subsurface transport of contaminants, aquatic ecosystemsProgress 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.