2002 Progress Report: Incorporating Natural Attenuation Into Design and Management Strategies For Contaminated Sites

EPA Grant Number: R828770C004
Subproject: this is subproject number 004 , established and managed by the Center Director under grant R828770
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: HSRC (2001) - Midwest Hazardous Substance Research Center
Center Director: Banks, M. Katherine
Title: Incorporating Natural Attenuation Into Design and Management Strategies For Contaminated Sites
Investigators: Novak, John T. , Widdowson, Mark
Institution: Virginia Polytechnic Institute and State University
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 2001 through September 30, 2004
Project Period Covered by this Report: October 1, 2001 through September 30, 2002
Project Amount: Refer to main center abstract for funding details.
RFA: Hazardous Substance Research Centers - HSRC (2001) RFA Text |  Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management


Monitored natural attenuation (MNA) is recognized as a feasible site remediation approach, in which it can be shown that MNA provides a satisfactory level of risk reduction. One of the important uses of MNA is as a plume management strategy as a follow up to source remediation. The coupling of source remediation with MNA is an approach that only can be addressed by modeling. In effect, this approach seeks to answer the following question: Given the natural attenuation capacity of an aquifer, how clean does the source area need to be? Once that question is answered, natural attenuation coupled with source remediation can be compared to other remediation approaches. Similarly, phytoremediation, as an enhanced natural attenuation process, can be incorporated into models that judge if the degradation and evapotranspiration rates for specific plant systems are adequate at a given site to reduce the risk to acceptable levels; this too can be incorporated into models that incorporate other treatment options. The four objectives of this research project are to: (1) monitor the progress of MNA at two sites to assess the long-term efficacy of this remediation strategy, and to obtain data for the rates of remediation; (2) develop and demonstrate a model protocol and user interface for MNA with phytoremediation, and MNA with source remediation strategies at a number of demonstration sites; (3) obtain laboratory data in conjunction with the field data, which help to better clarify the mechanisms associated with the degradation/transformation rates of the contaminants; and (4) develop and improve modules for simulating natural attenuation and phytoremediation in the transport model SEAM3D.

Progress Summary:

Monitoring and Assessment of MNA

In Year 1, field work at the chlorinated solvent site included locating the dense non-aqueous phase liquid (DNAPL), which at this site is tetrachloroethene (PCE), source areas using a Membrane Interface Probe/Field Gas Chromatography (GC) and simultaneously conducting a complete round of groundwater sampling. We began additional microcosm experiments to assess the effects of bioaugmentation and source treatment on the rate of PCE degradation, using aquifer sediment collected from the site. Source delineation was conducted in conjunction with Remedial Investigations/Feasibility Studies (RI/FS) performed by CH2M Hill and Virginia Polytechnic Institute and State University (VT) as a precursor to biological source treatment. Figure 1 shows the two distinct source areas (PCE "hot spots"). The level of PCE concentrations varied with depth, showing more heavily contaminated zones in both the upper and lower regions of the 17-foot thick unconfined aquifer. However, PCE concentrations in the source areas were well below solubility, indicating that the residual DNAPL is depleting and undergoing decay.

Figure 1. Total Chlorinated Ethene Plume (mg/L) and MIP/Geoprobe Sampling Locations at the Chlorinated Solvent MNA Study Site

We performed assessment of the DNAPL source history and projection of time of remediation for each source area, using SEAM3D to simulate non-aqueous phase liquid (NAPL) dissolution and aqueous phase transport for site conditions. Figure 2 shows that the time of source depletion and the PCE equilibrium concentration are sensitive to the percent of PCE in the NAPL at the time of release (in this case, dry cleaning wastewater and sludge). Comparison with historic data allowed us to assess source mass and composition for predictive modeling. We used these results to assemble input parameters for a site model needed to assess MNA and source treatment options.

At the creosote-contaminated phytoremediation site, the research team collected two rounds (late-winter and mid-summer) of groundwater samples to assess contamination changes and hydrologic data. Figure 3 depicts the average total polycyclic aromatic hydrocarbon (PAH) concentration in the shallow groundwater over time, beginning in March 1998 (approximately 1 year after the phytoremediation system of poplar trees was installed) and continuing into the 2 most recent years (March 2001 and 2002). These results indicate the effectiveness of the phytoremediation system, starting with the second growing systems and the beginning of a gradual decline in concentrations.

Figure 2. Simulated Equilibrium PCE Concentration Versus Time at the DNAPL Source for a Constant Starting Mass and Variable PCE Composition

Figure 3. Total PAH Concentration in the Shallow Groundwater at the Oneida, TN Sites

Develop and Demonstrate Model Protocol and User Interface

We developed and tested an initial version of the modeling protocol at several Department of Defense (DoD) chlorinated solvent sites, employing a user interface developed in conjunction with the U.S. Geological Survey under a U.S. Navy-funded project. The primary remedial action at these sites was source treatment combined with MNA. The modeling protocol currently is under peer review.

Clarification of Degradation Mechanisms

We designed and implemented field and laboratory experiments at the Oneida phytoremediation site with the following objectives:

· Determine in situ degradation rates to ultimately optimize the design of future phytoremediation projects in contaminated areas and predict the remediation timeframe.

· Delineate the individual mechanisms responsible for phytoremediation of PAHs to optimize remediation technology.

Push-Pull Tests

Push-pull tests consist of injecting a contaminant and a tracer into a well and following injection, extracting groundwater from the well. Samples are collected during the injection and the extraction procedures to assess degradation. At the creosote-contaminated Oneida site, multiple methods have been developed, but success thus far has been limited because of a low groundwater table as a result of the summer drought. A series of 10 wells were installed, resulting in a shallow and deep well pair at five locations. We collected soil samples from each foot and analyzed them for PAH content. We collected extra soil from the screened areas and preserved them for microcosm experiments. One well pair was a control well outside the plume and we placed the remaining wells at locations near trees and away from trees to assess the role of trees in degradation. Initially, naphthalene, the most degradable of the PAH compounds found in creosote, was injected along with a bromide tracer and 2,4,5-trichlorophenol (2,4,5-TCP). When compared to naphthalene, 2,4,5-TCP had a similar Kow (octanol-water partitioning coefficient), and it served as a means to assess sorption during the test, because 2,4,5-TCP is extremely recalcitrant. We conducted multiple push-pull tests, but variation in the resulting breakthrough curves caused development of an alternative method that is more applicable to all contaminated sites. Recently, only naphthalene and bromide were injected and dissolved oxygen was monitored to assess a degradation rate; however, because of an abnormally low groundwater table, the test could not be completed. Push-pull tests cannot be performed during the summer because of the low groundwater table; future push-pull tests are planned following fall rains and the recovery of the groundwater table. Ultimately, the push-pull tests plan to compare fall, winter, spring, and early summer degradation rates, as well as day/night degradation variation. The installed wells also enable the comparison of the effects of areas with and without trees, providing for an improved understanding of the role that symbiosis at the mycorrhizae has in the overall degradation of PAHs at the site.

Microcosm Study

A microcosm study is being conducted as a supplement to the naphthalene degradation rates expected from field push-pull tests to provide relative microbial degradation rates corresponding to the five locations at the site. We set up aerobic microcosms with soil from the five shallow well screens and anaerobic microcosms with soil from the five deeper well screens. The initial naphthalene concentration in the microcosms was approximately 500 mg/kg soil-which varied depending on background concentration in the soils. The aerobic microcosms were run for 35 days and the anaerobic microcosms now have run for 85 days. The aerobic microcosms had some variation and have been set up again with an improved procedure. Preliminary degradation rates and lag phases for the anaerobic microcosms are listed in Table 1.

Table 1. Anaerobic Naphthalene Degradation Rates and Lag Periods From Laboratory Microcosm Experiments. They were conducted using sediment collected at the Oneida site.

  Lag Phase Zero Order First Order
No trees, no contamination, anaerobic 30 days 5.5 0.01
Trees, prior contamination, anaerobic 20 days 5.9 0.017
Big tree, contamination, anaerobic 0 days 11 0.073
No trees, contamination, anaerobic Not degraded -- --
Trees, contamination, anaerobic 0 days 13.3 --

Develop and Improve Models for Simulating NA and Phytoremediaiton

The focus of work in this area remained on the validation of modules for NAPL dissolution and biotransformation/biodegradation of chloroethenes to site and laboratory data. An example of this work is shown in Figure 4, which is a plot of steady-state trichloroethene (TCE), dichloroethene (cis-DCE), and vinyl chloride (VC) monitoring well concentrations along the centerline of a plume at a U.S. Navy site versus model-simulated TCE (top), cis-DCE (middle), and VC (bottom) concentrations. These results demonstrate the validity of SEAM3D to simulate chloroethene biotransformations, depending on site-specific conditions.


Figure 4. Calibrated Model Results Showing Concentration Versus Distance Along the Plume Centerline for TCE (top), cis-DCE (middle), and VC (bottom). Open symbols represent observed values and solid lines depict simulation results.

Future Activities:

In the coming year, the activities described above will continue, including MNA monitoring at the chlorinated solvent site in Virginia Beach. Depending on the results of the RI/FS, biological source treatment will be designed and implemented at the pilot scale. We will validate site models at several other DoD sites with chlorinated solvents. This approach to evaluation of MNA will be presented in a number of venues including national meetings, peer-reviewed journals, and at several U.S. Environmental Protection Agency (EPA) laboratories and regional offices. Future activities for the PAH-contaminated MNA/phytoremediation site are described below:

Bench-Scale Push-Pull Tests. Concurrently, work is underway to develop lab-scale push-pull tests using soil and poplar cuttings obtained from the site. We are developing a flow-through system that will inject naphthalene, bromide, naphthalene degredation, and oxygen uptake. This system will assess degredation rates. Six distinct experimental setups will be created in triplicate. Three soil variations will be set up with and without a tree: one with autoclaved, noncontaminated soil, another with highly contaminated soil, and one with soil from a low-contamination area. This experimental setup enables a complete understanding of lab-scale degradation that provides a more controlled environment than in situ testing, while providing the addition of trees that cannot be assessed with microcosms. Collectively, the in situ push-pull tests, the lab scale push-pull tests, and the microcosms may provide an ideal approach to assessing and comparing the validity of results for degradation rates at the Oneida site.

Radioactive Labeling Uptake Study. We are planning laboratory scale studies using poplar cuttings and through the monitoring of the fate of C14-labeled PAHs. The relative contribution to the overall degradation rate can be quantified for plant uptake and degradation, rhizosphere degradation, and immobilization into the soil humus. Furthermore, this research project may be able to characterize PAH metabolites accumulated in plant tissue. Development of methods and an experimental plan for uptake studies using naphthalene, as well as a 2- and 3-ring PAH, is underway.

Microbial Characterization. Parallel to obtaining degradation rates and quantifying the relative contributions of different mechanisms, we will conduct a characterization of the microbial community in rhizosphere soil versus nonrhizosphere soil. Nonmolecular PAH-degrader screening methods, as well as molecular methods (PCR and nucleic acid probes) and methods based on enzyme-assays currently are being reviewed.

Journal Articles:

No journal articles submitted with this report: View all 6 publications for this subproject

Supplemental Keywords:

monitored natural attenuation, MNA, biodegradation, phytoremediation, reductive dechlorination, chlorinated solvents, tetrachloroethene, PCE, trichloroethene, TCE, vinyl chloride, polycyclic aromatic hydrocarbons, PAHs, dense non-aqueous phase liquid, DNAPL., RFA, Scientific Discipline, Waste, Water, Contaminated Sediments, Remediation, Environmental Chemistry, Hazardous Waste, Environmental Monitoring, Hazardous, Environmental Engineering, contaminant transport, contaminant dynamics, contaminated waste sites, contaminants, contaminated sites, contaminated sediment, transport contaminants, modeling, contaminant transport model, groundwater hydrology models, natural attenuation, groundwater remediation, transport models, contaminated groundwater, monitored natural attenuation, hazardous waste sites, transport modeling, ecological research

Relevant Websites:

http://bridge.ecn.purdue.edu/~mhsrc/ Exit

Progress and Final Reports:

Original Abstract
  • 2003 Progress Report
  • Final

  • Main Center Abstract and Reports:

    R828770    HSRC (2001) - Midwest Hazardous Substance Research Center

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R828770C001 Technical Outreach Services for Communities
    R828770C002 Technical Outreach Services for Native American Communities
    R828770C003 Sustainable Remediation
    R828770C004 Incorporating Natural Attenuation Into Design and Management Strategies For Contaminated Sites
    R828770C005 Metals Removal by Constructed Wetlands
    R828770C006 Adaptation of Subsurface Microbial Biofilm Communities in Response to Chemical Stressors
    R828770C007 Dewatering, Remediation, and Evaluation of Dredged Sediments
    R828770C008 Interaction of Various Plant Species with Microbial PCB-Degraders in Contaminated Soils
    R828770C009 Microbial Indicators of Bioremediation Potential and Success