Deployment of Phytoremediation at Argonne's 317/319 Areas

Problem/Opportunity

The 317/319 areas at Argonne National Laboratory-East (Argonne-E) are contaminated by volatile organic compounds (VOCs) in soil and groundwater and low levels of tritium in the groundwater from past waste disposal practices. As part of a nationwide effort to find more cost-effective and environmentally friendly remediation technologies, the U.S. Department of Energy (DOE), through the Accelerated Site Technology Deployment (ASTD) program, funded the deployment of a phytoremediation system in the 317/319 areas at Argonne-E. The main objectives of this deployment, which was selected in place of the baseline approach of an asphalt cap and extraction wells, are to hydraulically contain groundwater migration and to remove the VOCs and tritium within and downgradient of the source areas.

The Energy Systems (ES) Division joined in with the Environment, Safety and Health (ESH) and Environmental Assessment (EA) Divisions to plan and conduct the final corrective action in the 317/319 areas by deploying a phytoremediation installation. For this application, phytoremediation is defined as the engineered use of natural processes by which woody and herbaceous plants extract pore water and entrained chemical substances from subsurface soils, degrade, sequester, and transpire them (along with water vapor) into the atmosphere.

Approach

We have been developing phytoremediation technologies since the early 1990s, under a Cooperative Research and Development Agreement (CRADA) with Applied Natural Sciences, Inc. since 1994. For this project, we worked closely with ESH staff, responsible for site remediation activities, to ensure the scientific and technological soundness of the approach. We provided phytoremediation expertise in selecting the technological approach, planning the installation, and monitoring the performance of the selected remedy.

An aerial view of the phytoremediation system at Argonne National Laboratory-East (project funded by DOE)

The installed system consists of plantings of hybrid willows and special deep-rooted hybrid poplars covering approximately two hectares. Because of the hydrogeological setting of the site (a complex framework of glacial tills interlaced with sands, gravels, and silts), the patented TreeMediation® and TreeWell® processes developed by Applied Natural Sciences, Inc. were used to enhance the aggressive rooting ability of the hybrid poplars and target the contaminated groundwater of 10 meters (m) deep, excluding a clean, perched aquifer located approximately 5 m below ground. Hybrid willows were planted directly in the contaminated soil at the VOC source area to remove the excess moisture from the soil and to enhance the degradation of the VOCs.

Soon after DOE funded the project, the U.S. Environmental Protection Agency (EPA) and DOE agreed to include this remediation technology deployment in the projects evaluated by the EPA Superfund Innovative Technology Evaluation (SITE) Program. Under this program, the EPA will independently monitor and evaluate the technology's performance at the Argonne-E 317/319 site. These activities are conducted in addition to the scheduled monitoring activities conducted by Argonne.

Monitoring is a joint effort: ESH conducts compliance-related testing, the EPA SITE Program conducts an independent evaluation of the technology, and we conduct nontraditional monitoring activities aimed at understanding the fate of the contaminants in the plant-soil-groundwater system. The uptake and degradation of the VOCs is being monitored to determine cleanup rates and assess the advantages of the phytoremediation installation over natural attenuation of the contaminants. Because a portion of the contaminants may not undergo degradation within the leaves and be transpired into the air, VOC "emissions" from leaves are monitored as well.

The tritium contaminant is analyzed in plant tissue and transpirate both to understand its removal from the soil and as an excellent tracer to follow root growth into the contaminated aquifer.

Results

Approximately 800 trees were planted in the summer of 1999. These trees are expected to provide full, year-round hydraulic control by the year 2003 and be self-sustaining for the expected life of the engineered plantation.

We planned and conducted the first-year monitoring of the uptake and degradation of trichloroethene (TCE), tetrachloroethene (PCE), chloroform, and carbon tetrachloride in plant tissue collected from the trees growing into the contaminated soil.

Taking Plant Measurements at the 317 Area

Weoptimized a rapid method to measure chlorinated solvents and their degradation products in plant tissues. Trichloroacetic acid (TCAA), a known intermediate of the degradation of TCE and PCE, was analyzed throughout the vegetative season in addition to the parent compounds as an indicator of their degradation. Both parent compounds and TCAA were found in the plant samples (an indication that the trees are taking up the contaminants), with a prevalence of TCAA in leaf tissue and of the parent compounds in the branches. TCAA showed a trend toward accumulation in the leaf tissue as the vegetative season progressed. The levels of TCAA in the leaf samples were quite constant within a single tree but varied significantly as a function of the location of the tree within the contaminated area.

Samples of the air immediately surrounding a leafed branch were compared to air from the contaminated area and from other, uncontaminated areas within Argonne. While the air at the French Drain contained higher concentrations of VOCs than other clean areas on site, the presence of the leafed branches did not induce a measurable increase in the VOC concentration in the air, suggesting that most of the VOCs detected in the air come from direct venting off the soil.

Tritium levels in the leaves and transpirate of hybrid poplars planted in the hydraulic control area showed levels comparable to background, indicating that the trees have not yet reached the contaminated aquifer.

Future Plans

Because the phytoremediation system will reach its optimal growth stage and steady performance state in 2003, future plans are to evaluate the performance of the remediation system. Some of the questions raised by this objective cannot be answered by conventional, compliance-related monitoring, so a more hypothesis-driven approach will be adopted to find mechanistical evidence of the effect of the plants on the removal of the contaminants.

This field installation is also an excellent opportunity to leverage more research funds, because it is an ideal, relatively well-understood site to conduct much-needed fundamental and applied research on belowground soil and groundwater environmental topics, under actual field conditions. This type of facility is a rare commodity and should be used to attract funds and scientists nationwide.