Hydrogeology Research Group

DNAPL Migration and Biodegradation

Research Objective

This research addresses the nature and distribution of DNAPL sources in fractured clays and weathered shales and the potential for natural attenuation of plumes derived from these sources. Preliminary investigations show that although DNAPL migration is largely controlled by the fracture network, some DNAPLs may be capable of invading the pores of the fine-grained matrix as an immiscible phase. As well, theoretical investigations show that dissolution of the DNAPL and transfer to the fine-grained matrix by diffusion is likely much more rapid than previously expected. Both of these factors could make DNAPL source removal in fractured clay-rich deposits a difficult and slow process. This effects the entire issue of whether DNAPL source zone removal will be considered Technically Impracticable (TI) for methods other than excavation or very aggressive steam/heat/volatilization treatments. Natural attenuation of DNAPLs in fractured clays and shales is also expected to be influenced by the character of the source zone. Diffusion into the matrix will greatly slow the overall migration of dissolved phase DNAPL, allowing more time for degradation, but may also effect degradation rates because microbial communties in the small pores of the matrix are likely to be much different than in the fractures. Presently, natural attenuation of dissolved phase plumes derived from DNAPLs in fractured clay-rich deposits is still at an early stage of investigation, but there are some promising signs. The first field site to be investigated, at Oak Ridge National Laboratory, shows the presence of TCE degradation products in the shale bedrock just below the saprolite and activity of methanotrophic bacteria, which are often associated with degradation of DNAPLs. Field and laboratory investigations are underway to examine the processes and determine whether the products are indeed due to natural degradation.

Investigate DNAPL entry and pressure-saturation behavior for immiscible solvents in undisturbed columns of fractured glacial clay and weathered shales, to determine whether typical DNAPL contaminants can enter these materials, and determine how the residual will be distributed.
Determine whether dissolution and diffusion into the matrix has a significant effect on the fate of DNAPL residuals in fractures.
Assessment of the potential for natural attenuation of common solvents, especially TCE, in these deposits. This includes both field scale investigations at an existing contaminated site and laboratory scale investigations using undisturbed columns and microcosms.

Research Progress and Implications

Weathered and Fractured Shale Column Experiments

Air/water and DNAPL/water entry and pressure-saturation curves have been experimentally measured for an undisturbed column sample of fractured weathered shale from Oak Ridge National Laboratory (ORNL). The DNAPL used for the experiments was a non-hazardous compound manufactured under the trade name Fluorinert FC-40. Preliminary evaluation of the results indicate that air/water data can be used, along with appropriate scaling factors based on the differences in fluid properties, to estimate DNAPL behavior. This is significant, because it raises the possibility that field scale air/water measurements could be used as a non-hazardous means of predicting DNAPL/water relationships. At capillary pressure head values of 5-8 cm the DNAPL first entered the fractures and at 160-210 cm it began to enter the fine-grained matrix. These are well within the range of head values expected for even a small DNAPL spill, so it indicates that at many sites DNAPL is likely to enter both the fractures and the matrix.

Matrix Porosity and Pore Size Investigations in Weathered Shale

Investigations of porosity and pore size distribution using mercury and helium porosimetry methods were carried out using weathered shale matrix samples from the Oak Ridge field site. The study indicates that the effective porosity of the analyzed specimens ranges from 14.0% to 51.4% (based on mercury porosimetry), and from 18.2% to 58.0% (based on helium porosimetry). Pore-throat sizes of the saprolite specimens range from 3 nm to 8000 nm, which are smaller than the maximum size indicated by the column experiments, but this is likely related to the small size of the samples used for porosimetry. Pore structure distribution in the matrix will be investigated this summer using thin sections.

DNAPL Natural Attenuation Field Site in Shale and Weathered Shale

A field facility at the Oak Ridge National Laboratory is serving as an extraordinary example of how natural attenuation processes are eliminating the off-site transport of chlorinated organics in a fractured shale bedrock. The field facility consist of a 35 m long transect of multilevel sampling wells that follow strike parallel geology and extend from the waste burial trenches to a seep exiting into a perennial stream. The wells are situated within either a fast flowing fracture regime or a slow flowing matrix regime. The spatial and temporal variability of TCE, DCE, VC, ethylene, ethane, methane, and various geochemical redox indicators are being assessed within the field facility and within upgradient waste trenches located 12 m from the field site. VOC and dissolved gas concentrations are highest in two large waste trenches located directly upgradient the field facility which is consistent with groundwater flow following strike parallel fractures. The concentration of organics follows TCE~VC->1,2-DCE->ethylene. TCE concentrations are 10 fold higher in the waste trenches relative to downgradient sampling wells, whereas VC, 1,2-DCE, and ethylene concentrations were similar or slightly higher than the sampling wells. TCE concentrations disappear within 10 m from the end of the waste trenches and 1,2-DCE disappears just prior to the seep, 35 m downgradient the trench source. VC and ethylene are still present at the seep, with ethylene showing peak concentrations at this locale. These results are strong indicators that anaerobic reduction of the chlorinated organics is occurring. In addition, the presence of high concentrations of methane throughout the site is also an indication of anaerobic metabolism. Since ethylene concentrations are 100 fold larger than the chlorinated organics, the extent of anaerobic reduction is extraordinary. Another interesting finding is that the concentration of chlorinated organics is slightly higher in the matrix relative to the fracture regime, whereas the concentration of ethylene is lower in the matrix relative to the fracture regime. These results suggest that the fracture regime is slightly more effective in the anaerobic reduction of the chlorinated organics. Temporal variability of TCE and its degradation products is slight, with a general increasing concentration trend of chlorinated organics and dissolved gases as the site hydraulic gradient increases (a response to increased storm events during the winter and spring months). This may imply that the intrinsic bioremediation scenario at the site is less effective at higher discharge rates. Bacterial population data collected from the site indicate various populations of methanotrophs, total heterotrophs, and some sulfate reducing bacteria. The methanogenic population was not determined. The presence of methane oxidizers (methanotrophs) suggests possible zones of oxygenated groundwater which is confirmed with on- site DO measurements. Thus there are two mechanisms of biological removal of chlorinated organic compounds occurring at the site. One pathway involves anaerobic dechlorination of TCE to VC and ethylene, and the other pathway involves oxidation of TCE to ethane via methane oxidizing bacteria. Since both ethane and VC are present as possible degradation products of the TCE, it is probable that both mechanisms are operative. Various molecular techniques are being considered for provide insights into the diversity of bacteria at the site so that the mechanisms of TCE degradation can be resolved and exploited for future opportunities. Future efforts will also couple geochemical, hydrologic, and microbiological information to better understand why this site is so effective in the natural attenuation of chlorinated organic compounds.

Fractured Till Column Experiments

A cylindrical sample, 0.5 m in diameter and length, was obtained by excavation from 3.7 to 4.2 m depth below ground surface in a surficial deposit of fractured glaciolacustrine clay. The sample was enclosed in a triaxial cell in a cold room to impose conditions close to field temperature and stress. After hydraulic testing of the sample, which yielded a mean hydraulic fracture aperture of 5 to 6 mm, a column of immiscible-phase trichloroethene (TCE) imposed incrementally at the top of the sample provided an entry-pressure-derived aperture equal to 17 mm for a parallel-plate fracture. TCE DNAPL that flowed through the sample produced dissolved-phase diffusion haloes in the matrix that indicated the preferential fracture pathways. The sample was cut into five horizontal slices and the diffusion zones along visible natural fractures were measured using a miniature core sampling technique. The diffusion haloes that were used to determine the flow pathways of the DNAPL (discussed in the last progress report) were also used to provide evidence of matrix diffusion and to measure the amount of mass transfer by diffusion. The data was modelled using a simplex optimization routine (Devlin, 1994) which determined the least squares best-fit to the equation for 1-D diffusion assuming a constant source concentration at the fracture surface and assuming a constant retardation factor R accounting for sorption. Visible immiscible-phase TCE was seen on only a small part of one fracture at the top of the column. Quantification of total TCE mass in the diffusion zones indicates that the lack of visible DNAPL in the fractures when the column was taken apart was caused by diffusive mass transfer into the matrix.

Planned Activities

Laboratory scale studies of TCE degradation in microcosms and undisturbed columns of fractured weathered shale.
Laboratory scale studies of TCE entry pressure, residual concentration and diffusive disappearance in weathered shales and glacial tills.
Continued field investigations

Microbiological Characterization and Potential for Solvent Degradation in Saprolite

The microbiological characterization and potential degradation of DNAPL in saprolite are being investigated. Biodegradation of organic solvents will depend on whether the solvent exists as a DNAPL or in the dissolved phase and how it is distributed in the deposit (i.e. in fractures, in matrix, sorbed to soil organic, sorbed to soil minerals). Other factors influencing biodegradation of TCE in the soil includes contaminant bioavailablity, the presence of suitable microorganisms, suitable pH and necessary cofactors (nutrients, oxygen inducers, etc.), and the toxicity of the DNAPL and its daughter products. The biodegradation will initially be investigated in a series of batch microcosm experiments and samples of saprolite for initial batch microcosm studies have been collected and characterization begun. The testing will include total aerobic and anaerobic bacterial enumeration, DNA extraction and analysis to determine which degradative genes are present, and mRNA to determine which genes are active. All experiments will be done on bulk material as well as on fracture surfaces. These will be followed by a DNAPL spill in a large undisturbed column of saprolite, which will then be slowly flushed with clean groundwater for several months to a year. During this period the effluent will be monitored to determine dissolution rates and to look for degradation products. Collection and setup of a preliminary undisturbed saprolite column sample has been carried out and the system is being tested for hydraulic properties and chemical characteristics of the materials to ensure compatibility with the solvents (mainly TCE) used in the experiments. This includes testing and calibration of the necessary sampling and analytical equipment.

Natural Attenuation Field Site in Shale and Saprolite

TCE Entry and Dissolution Experiments in Till

A cylindrical sample, 0.5 m in diameter and 0.5 m in length, was excavated from a depth of 3.7 to 4.2 m below ground surface in a surficial deposit of desiccation-fractured glaciolacustrine clay. Hydraulic testing provided a saturated hydraulic conductivity of 7 x 10-10 m/s, which is only slightly larger than the matrix hydraulic conductivity. The cubic law applied to this flow test in the conventional manner gave a mean aperture of 4 to 6 �m for the four continuous vertical oxidation-stained fractures in the sample. A column of TCE DNAPL imposed incrementally at the top of the sample provided an entry-pressure-derived aperture equal to 17 �m for an idealized parallel plate fracture. After visible DNAPL breakthrough at the bottom of the sample at 12.5 days, a subsequent 10 day shut-in period was imposed to allow solute diffusion with no advection. The sample was then cut into five horizontal slices for subsampling using a miniature coring device to delineate the diffusion halos of aqueous plus sorbed TCE in the matrix along the fractures where DNAPL had flowed. These halos indicated that DNAPL flowed only through some of the visible, oxidation-stained fractures and that this flow was limited to preferential paths or channels within these fractures. Flow was isolated to only approximately 10% of the total fracture plane. Using the previous hydraulic test results and applying the cubic law only to the aperture segments of apparent DNAPL flow, an equivalent hydraulic aperture of 7.5 to 12 �m was obtained, which is greater than the conventional mean hydraulic aperture and smaller than the local aperture determined from the DNAPL entry pressure. These differences are large in the context of fluid flux, which is proportional to the aperture cubed. It is unlikely that the presence of such small fractures would be identified in the field using hydraulic test because the bulk hydraulic conductivity of the clay is so close to the matrix conductivity. However, the rapid passage of solvent DNAPL through the fracture network in the column indicates that small-aperture fractures that would probably go undetected at many field sites may allow passage of solvent DNAPL.

Summary of Research Accomplishments

completed measurements of matrix porosity and pore size distribution in shale saprolite using mercury and helium porosimetry
measured hydraulic aperture values and air/water entry aperture values and pressure-saturation behavior in a column sample of fractured shale saprolite. DNAPL/water measurements are in progress
found preliminary evidence that natural intrinsic bioremediation is controlling the fate and transport of DNAPL in a contaminated fractured shale bedrock at ORNL
measured TCE DNAPL entry pressure and resultant DNAPL entry aperture for column of fractured till and investigated DNAPL channeling within the conductive fractures
experimentally determined that a typical DNAPL, in this case TCE, can enter and rapidly travel through very small fractures (< 17 �m) in an undisturbed clay till, under applied heads that are typical of field conditions
found conclusive experimental evidence that dissolution of DNAPL residuals in fractures in clay till and significant diffusion into the clay-rich matrix can occur over relatively short periods (in this case 22.5 days)

Email questions, comments or concerns to: mlenczew@utk.edu