SUMMARY OF THE REMEDIATION TECHNOLOGIES DEVELOPMENT FORUM
PHYTOREMEDIATION OF ORGANICS ACTION TEAM
VEGETATIVE CAP SUBGROUP
CONFERENCE CALL

July 24, 1997
11:00 a.m. to 12:30 p.m.

On July 24, 1997, members of the Phytoremediation of Organics Action Team, Vegetative Cap Subgroup, met in a conference call. The following members were present:

Tom Wong, Union Carbide Corporation (Subgroup Co-chair)
Steven Rock, EPA/NRMRL (Subgroup Co-chair)
Lucinda Jackson, Chevron
Bruce Pivetz, ManTech Environmental Research Services Corporation
Christina Negri, Argonne National Laboratory
Rebecca Jamison, EPA/Region 9
Jessica Palmiotti, EPA/Student Intern
Sam Gavande, Texas Natural Resource Conservation Commission (TNRCC)
Felix Flechas, EPA/Region 8
John Fletcher, University of Oklahoma

Also present were Susan Brager Murphy and Erik Balsley of Eastern Research Group, Inc. (ERG).

DATA AND TASKS THAT ARE NEEDED TO MEET OBJECTIVES

Wong reviewed the objectives of the subgroup:

• To demonstrate whether a vegetative cap is an acceptable final cover for all types of landfills

• Provide data on surface water infiltration through a vegetative cap

• Provide data on toxicity reduction using phytoremediation

• To gain regulatory acceptance of vegetative caps

Field Data Versus Lab Data

Sam Gavande described his past experiences with reforestation and revegetation projects for the United Nations. These projects tested species in the greenhouse under controlled conditions before introducing them into the field. Gavande said that meaningful results can be obtained within 1 to 3 years.

John Fletcher discussed the "direct approach" where nature is allowed to determine the species in a vegetative cap. Contaminated areas that were revegetated 10 to 15 years ago can act as a screen for plants that can grow and survive under these conditions. A succession of plant species exist in the cap over time until a stabilized situation arises.

Lucinda Jackson raised the issue of undesirable weed species entering a site. Fletcher pointed out that what is good for agriculture may not be good for hazardous waste sites. He pointed out that the succession of plants on a contaminated site will most likely begin with weeds, then move to grasses and then to woody plant species. Jackson pointed out that the use of weeds may involve working with local regulations on noxious weeds.

Wong described the results of a natural vegetative cap on the Union Carbide test site in Texas. The effects were stumbled upon during the course of geo-technical work. Wong said the upper 2 to 4 feet of the site have been converted to a "material like topsoil" with no noticeable chemical odors, although analysis showed the soil still contained high levels of PAHs. However, a TCLP extraction did not detect the chemicals.

Gavande said that a vegetative cap approach should not only reduce infiltration but also better control erosion. Wong said a "self-sustaining" cap was much better than a synthetic or other type of cap.

Steven Rock said that prior to EPA's restrictions on introducing hazardous wastes into municipal solid waste landfills there was no way to tell the difference between the types of landfills. He made the point that the purpose of vegetative caps and other types of caps is to protect ground water and if they cannot do that, they will not be acceptable as an alternative to EPA's accepted caps (Subtitle C and D).

Felix Flechas described recent negotiations between EPA's Region 8 and the Air Force to use vegetative caps, affirming the Region's position that vegetative covers are a remediation technology it would like to implement. The Region would like to see whether the moisture rejection capabilities of vegetative caps are the same as existing technologies (e.g., multilayered RCRA caps).

Fletcher said that older conventional caps will allow long-term performance to be evaluated. He said that tall grass prairie on top of clay soil that is not managed with burning can result in an invasion of woody plants. He said that there may be a number of clay-capped municipal sites where trees have penetrated the caps. To develop a better cap system, it would be useful to study the long-term performance of these sites.

Jackson said her site will not be planted until next spring, so there is some time to obtain baseline data. She has two sites: one will receive the vegetative cap, and the other has undergone phytoremediation. The phytoremediation area is split between natural and planted vegetation. The two areas are approximately 70 acres and are split by a road. One side has been vegetated for 3 to 4 years and the other has been plowed. There are differences between the soils in the two areas. The soil in the vegetated area looks like regular brown soil, while the soil in the other area is still oily. The site was an oxidation pond that was drained about 5 years ago and contains hydrocarbon contamination. The plant species present will be assessed shortly.

Wong suggested that the RTDF establish a 3-year project with the chance it could grow to be a 5-year project. Gavande said there was no conflict between lab and field data as each has its place. Negri pointed out the differences between field data from existing locations and new field tests that are run under controlled conditions. Wong mentioned that the possibility of setting up test plots has been discussed. Negri said that greenhouse tests can be used for microdetermination, whereas studies on water balance require a field study. The type of data to be collected should be determined by what the group chooses to study.

Bruce Pivetz said that a lot of data on toxicity reduction can be obtained from existing sites. He said that he would expect to see a reduction in toxicity in 3 to 5 years. Wong asked about measuring the extent of the reduction and whether it is possible to evaluate total reduction in that time or whether it would be necessary to project total reduction based on a trend.

Flechas mentioned that models can be used only to evaluate two alternatives and cannot be relied on to determine the final cover. He said that other models besides HELP should be included in the data analysis (e.g., unsaturated flow, comparison of models, limitations of models). He evaluated the effectiveness of a vegetative cap compared to a RCRA multilayer cap using a performance standard of one-tenth of a millimeter per year as an acceptable rate of infiltration. He believes the purpose of the cap is to isolate the waste and keep water out.

Plant Types

Rock mentioned that there is a lot of work that has been done on the evapotranspiration of plants, not specifically for capping. The USDA has done a lot of that research. Also, data are needed on infiltration rates for vegetative covers. Gavande mentioned that data are available on the types of species and their proportions along highways from the Texas highway department and conservation department. Wong asked if the data can be used to determine which plant species will maximize evapotranspiration and minimize infiltration. Gavande said that it would provide natural vegetation for a stable long-term cover over 60 to 80 percent of an area and reduce infiltration.

Wong returned the discussion to the issue of trees versus grasses. Gavande said that the TNRCC does not recommend trees as a cap unless they are shallow-rooted or are planted on a mound so the roots do not penetrate into landfill cover. Wong said that a vegetative cover may not require clay if it can achieve an adequate water balance on its own and that root penetration is desirable if phytoremediation is to be an integral component. Someone mentioned a company that sells poplar trees with 10- to 12-foot root systems as part of a cap. These caps are mostly in the northern plains states and have been proposed for sites as far east as Baltimore.

Gavande said he did not believe that using trees would meet EPA's landfill requirements. Jessica Palmiotti said that she has been told a mixture of plants such as shrubs and grasses should be used in a vegetative cap. The mixture should also include a variety of cold- and warm-season plants. She said that highway seeding may not be appropriate for landfills because highways receive more moisture from water drainage than landfills, which tend to be more dry. She said she saw a technology called VETSTEEC at a workshop in St. Louis that may be used to select plant species for vegetative covers. She said it is a plant species selection decision support system developed by Steven D. Warren of the U.S. Army Construction Engineering Research Lab. The system should be available August 1, 1997.

Wong said that corn is often mentioned as a possible plant to use in phytoremediation. He questions its applicability for his Texas site, which will not be converted into farmland. Rock mentioned that duckweed has been used in some projects, but neighbors expressed concern as it is a noxious weed.

Wong said that organics are taken up by plants and degraded in the soil at his Texas site. He has seen a healthy ecosystem develop and many animals have been spotted on the site. He said the results were different from sites where metals are returned to the site by the plants.

Climate

Wong said that he believes that a goal of the group should be to test the suitability of vegetative caps for an array of different climates. Rock said he thought it would be easier to start testing in semi-arid areas than in areas where it rained more. Fletcher said that vegetative caps need to be compared with conventional clay caps and that the Argonne site can be useful for looking at penetration. When clay caps fail or don't work well, then a better system is needed. If a natural community can be established as a cap, it can limit the amount of leaching and foster the degradation of waste.

The group discussed the 30-year monitoring period for capped landfills. Gavande said that vegetative caps can fail in arid regions if the rain is not uniformly distributed throughout the year. Wong said that vegetative caps can be useful in all climates. He said they can be designed for efficient runoff in rainy regions and for absorption in arid regions. He said a standard design may not always work.

Clay or Synthetic Layers as Part of a Vegetative Cap

Wong said that the group's notes indicate that Argonne is planning to study the impact of trees on a clay cap. The results may be useful in arguing that vegetative covers can be as effective in long-term performance as clay caps. Discussion then turned to whether clay caps and plant caps oppose each other since vegetative caps may degrade clay caps. Negri said she was not sure that vegetative covers will destroy clay caps since the vegetation will not reach into the cap unless it needs moisture for growth. The landfill she is studying has a topsoil cover as shallow as 2 feet. Fletcher mentioned this would be an interesting site for studying how the succession of plants affects the clay cover. Negri will do an ecological assessment of plants as part of the study.

Water Balance

Wong asked how to perform a reasonable water balance in the field that could be used in a model. Discussion turned to the use of lysimeters and TDRs for measuring water balance. Flechas said he used lysimeters because he found limitations with TDRs. He said lysimeters are used at Hanford and Sandia. Negri said she was planning to measure evapotranspiration by plants at a site with tritium and this will actually be done in the field.

Flechas said his newly capped landfill is being monitored for 30 years using 10 100-square-meter rectangular lysimeters, 30 inches below the surface on a 41-acre landfill. It will be covered by seeded indigenous prairie grass native to the high plains of Wyoming. Gavande said water balance calculations based on averages may not be sensitive enough. The work of Craig Benton and DOE on water balance calculations was mentioned, indicating that large lysimeters are needed in semi-arid areas to measure macropore imperfections that appear in any natural system.

NEXT STEPS

The next conference call is scheduled for August 14 at 11 a.m. (EDT). Wong asked ERG to e-mail copies of the minutes to participants.

A meeting of the Phytoremediation of Organics Action Team is tentatively set to take place in Cincinnati on September 24-25. Rock described his plans for the meeting. He intends to design a research program. He has received two proposals to evaluate model landfills or lots. He has also uncovered an old DOE study that began a decision system for landfill covers. It was not finished, as funding ended. The work was mostly done by Leonard Lane of the USDA in Tuscon, Arizona. Rock said there are many research efforts in the field and he would like to coordinate them. He mentioned Lucinda Jackson's cap as a source of information.