Grantee Research Project Results
2000 Progress Report: Understanding Seasonal Variation of Bioavailability of Residual NAPL in the Vadose Zone
EPA Grant Number: R827133Title: Understanding Seasonal Variation of Bioavailability of Residual NAPL in the Vadose Zone
Investigators: Holden, Patricia , Keller, Arturo A.
Institution: University of California - Santa Barbara
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1998 through September 30, 2001
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $425,000
RFA: EPA/DOE/NSF/ONR Joint Program on Bioremediation (1998) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
Objective:
The current regulatory trend towards accepting intrinsic bioremediation as a long-term management scheme for residual organic pollutants in the subsurface for fuel spills is based on statistical evaluations of plume behavior for a limited array of pollutants. The effectiveness of intrinsic bioremediation towards protecting water supplies, and hence human health, is not known for many pollutants. In addition, the patterns of biodegradation that occur over seasonal time scales, and hence, the seasonality of intrinsic bioremediation and its intermediate effectiveness is unknown. Residual nonaqueous phase liquids (NAPLs) in the vadose zone, if left in place, have the potential to mobilize with seasonal moisture fluctuations. These seasonal variations in mass transfer, along with the intrinsic microbial biodegradative response to moisture variations, will determine bioavailability, which is a measure of the potential effectiveness of intrinsic bioremediation. This research project focuses on determining the effect of seasonal fluctuations in moisture content and soil temperature on abiotic and biotic fate and transport of hydrocarbons in the subsurface.Progress Summary:
Abiotic Mass Transfer at the Pore Scale. A mathematical model has been developed that phenomenologically describes mass transfer (whether volatilization or dissolution) at the pore scale. The model can be scaled up to the dimensions of a laboratory core or a field site. The model predicts that Sh depends on Pe to a power of one-half when the flow rate of the flowing fluid (air or water) is very slow (Pe << 1). As Pe increases, the model predicts that the dependence of Sh on Pe increases, until at very large flow rates (Pe >> 1), Sh is directly proportional to Pe. Because Pe is the ratio between advective and diffusive processes, this indicates that at the higher flow rates typical of laboratory core experiments it would be likely to observe a dependence greater than 0.5. One would expect that for field studies of the rate of mass transfer, the correct exponent would be closer to 0.5, as most field sites operate at low Pe values. Under these conditions, the assumption that local equilibrium (LEA) exists seems to be justified. However, for active remediation (e.g., Soil Vapor Extraction, vigorous pump and treat, or soil flushing with surfactants that raise the solubility limit), LEA may not be valid, and Sh is likely to depend more on Pe (i.e., a higher exponent would be warranted, closer to the laboratory experiments).
The model includes considerations for advective transport around a NAPL blob, diffusive transport from the NAPL blob to the region where fresh water flows, and mixing of different pore water downgradient from the NAPL blob. From calculations of the conductance of water through the thin cross-sectional area within the pore body crevices surrounding the NAPL blob, advective transport through these crevices is not a very significant contributor to the overall mass transfer process, and most of the mass transfer is done through molecular diffusion from the NAPL blob to the external flowing fluid through the narrow pore throats. In addition to the dependence of Sh on Pe, it also depends on residual NAPL saturation, Sn, via the molecular diffusion process. The dependence of Sh on Sn is not as easily explained, given that the underlying relationship is between Sh and effective interfacial contact area of NAPL blobs, anw, which up to now had been difficult to characterize. However, an evaluation of the functional relationship indicates that as Sn becomes smaller, the rate of mass transfer (and thus Sh) increases, so there is an inverse relationship between Sh and Sn. Also, it is readily seen that as Sn becomes very small, the flowing fluid can more easily flow past the NAPL blob, and a larger interfacial area is contacted, also increasing the rate of mass transfer. Experimental evidence was generated that indicates the phenomenological model is correct. Even at the low Pe of micromodel experiments, advective transport through crevices is small relative to the diffusive transport, because the cross-sectional area for the flowing fluid is very small. The mass transfer process follows a power law behavior, with an increasing exponent as the end of the mass transfer occurs. A model also was developed for the dependence of anw on Sn, which is based on observations and generalization of the mass transfer process. This model predicts that at the pore scale, initially as Sn decreases, anw begins to increase, because the NAPL blob shrinks slightly and exposes more interfacial contact area. Once the NAPL blob has shrunk enough to become spherical (thereby reducing its surface energy), anw sharply increases, only to decrease as the spherical blob rapidly loses size.
Three-Phase Relative Permeability Relationships. The traditional approach to estimating three-phase relative permeabilities is to assume that they can be constructed from the measured two-phase relative permeabilities. A power law approach was used to model the relationship between NAPL saturation and previously reported NAPL relative permeability, finding good correlation. Good correlation was found between initial spreading coefficient and the power law exponent for three similar organic liquids. Preliminary evidence was gained using a numerical model that incorporates relative permeabilities that NAPL will travel rapidly through the vadose zone until it either reaches the water table or it becomes spread out into thin layers and discontinuous blobs. Once this condition is reached, the NAPL will continue to flow albeit very slowly. Water in the presence of "residual" NAPL or NAPL at high saturation follows a power law relationship as well. Gas relative permeability is essentially the same under two- and three-phase flow conditions. A linear fit seems more appropriate under these conditions to avoid extrapolating a very low gas saturation at low Sg, because in this case there is no possibility for the gas to flow out through thin films. The most important finding from these studies is that the three-phase oil relative permeability is a strong function of spreading coefficient at low NAPL saturations in the region where it is most important for long-term simulation. Water and gas three-phase relative permeabilities are similar to two-phase relative permeabilities in that they are bounded by trapped NAPL, gas or water; in general, the behavior can be modeled with a smooth functional relationship.
Biotic Mechanisms in Bioavailability. Three local (pore scale) mechanisms have been proposed to account for bioavailability and subsequent bacterial metabolism of sparingly soluble hydrocarbons: (1) dissolution and diffusion of dissolved hydrocarbons to cells with uptake via active or passive transmembrane transport; (2) invagination of hydrocarbon NAPL into cells with subsequent intracellular metabolism of the hydrocarbon inclusions; and (3) bacterial production of surface-active compounds such as surfactants (biosurfactants) and emulsifiers (bioemulsifiers) that increase the local pseudosolubility of hydrocarbons and thus improve mass transfer. Pseudomonas aeruginosa was studied in sand culture as a model system for understanding biosurfactant and bioemulsifier production in soils. Although surface-active compounds are known to accumulate in liquid culture when hexadecane is the carbon source and either N, P, or Fe are limiting, little is known about the accumulation of surface active compounds at the pore scale in unsaturated porous media. As surfactant accumulation affects interfacial tension that inherently affects NAPL spreading, the real importance of biosurfactant accumulation during conditions of intrinsic bioremediation must be understood. Experimental findings using four strains (one doubly proficient in producing bioemulsifying protein and biosurfactant, one singly deficient in producing bioemulsifying protein, one single mutant in biosurfactant, and one double mutant) confirmed the accumulation of surface active compounds under N-limitation in liquid culture. In sand culture, however, strains biodegraded hexadecane at equivalent rates regardless of the ability to produce surface active compounds in liquid culture.
The mathematical model of mass transfer at the pore scale suggests that solubility, not diffusion through thin films, limits bioavailability of hexadecane to bacteria in unsaturated porous media. Thus, this research project suggests that biosurfactant, if produced, is in insufficient quantities to affect high bioavailability of hexadecane to biodegrading bacteria in sand. This research additionally tests the guiding hypothesis of this proposal that biofilms of bacteria and extracellular polymeric substances (EPS) are the predominant growth habit for biodegradation bacteria in NAPL-contaminated three-phase systems. This hypothesis is being tested using ongoing experiments in liquid and sand culture to determine if hydrocarbon substrates stimulate EPS production under vadose-like cultivation conditions. The implications are central to modeling the rate of hydrocarbon biodegradation in unsaturated porous media. In an additional study, the physical characteristics of unsaturated Pseudomonas biofilms were explored at the air/biofilm interface to determine if surface topography and/or adhesive properties could explain previously observed diffusive mass transfer limitations through such biofilms. The findings conclude that, unlike biofilms cultivated under flowing fluid, unsaturated biofilms are quite smooth under a range of desiccation conditions. Similarly, surface forces were not strongly affected by desiccation. These findings suggest future models of mass transfer limitations across biofilm-air interface can treat resistance to intra-biofilm mass diffusion as more important than interfacial resistance.
Future Activities:
Future research will be directed at:
? Codifying the mathematical model derived in this study into a network model, to study the interactions between different pore bodies, and predict the aggregate behavior of NAPL blobs.
? Incorporating the mass transfer, such as UTCHEM, to study field sites where mass transfer limitations are important and evaluate the accuracy of the model in predicting the rate of mass transfer.
? Developing further the limitations that moisture content would pose on the rate of volatilization of NAPL, as the water fills some of the pore throats and constrains the rate at which the flowing gas phase can remove the diffusing organic molecules.
? Developing and testing a genetic reporter system that can be used to understand the expression of genes encoding surface active compound production in porous media.
? Determining the effect of C/N and carbon substrate quality on EPS production in unsaturated systems.
? Understanding the fundamental geometry of unsaturated biofilm cells that are cultivated on hexadecane.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 20 publications | 6 publications in selected types | All 6 journal articles |
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Auerbach ID, Sorensen C, Hansma HG, Holden PA. Physical morphology and surface properties of unsaturated Pseudomonas putida biofilms. Journal of Bacteriology 2000;182(13):3809-3815. |
R827133 (2000) |
not available |
Supplemental Keywords:
sediments, chemical transport, chemicals, toxics, PAH, bacteria, terrestrial, cleanup, environmental chemistry, biology, ecology, modeling, bioavailability., RFA, Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Hydrology, Ecosystem/Assessment/Indicators, Ecosystem Protection, exploratory research environmental biology, Chemical Mixtures - Environmental Exposure & Risk, Contaminated Sediments, Environmental Chemistry, Ecological Effects - Environmental Exposure & Risk, chemical mixtures, Ecological Effects - Human Health, Bioremediation, Groundwater remediation, Ecological Indicators, ecological effects, ecological exposure, fate and transport, ecology, NAPL, sediment, contaminated sediment, biodegradation, chemical transport, mass transfer, seasonal variation, bioremediation of soils, biological attenuation, vadose zone, exoplymeric substances, sediments, natural bioattenuation, groundwaterProgress 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.