Southern Louisiana
VOCs - CVOCs with petroleum Hydrocarbon
Managing Phytotoxic Concentrations
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Targeting Groundwater at Extreme Depths
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Managing Phytotoxic Concentrations --- Targeting Groundwater at Extreme Depths ---
Project Objective:
Evaluate the efficacy of a TreeWell® system for providing hydraulic control and remediation of groundwater impacted with high concentrations of VOCs including Dense Non-Aqueous Liquid (DNAPL) impacts. Target treatment depths ranged from 25 to 115 ft bgs.
Implementation:
The TreeMediation project included 90 TreeWell® units installed in three site areas over three phases.
Phase I (2015):
18 TreeWell units were installed at 20x20 ft spacing to a depth of ~25 ft. Each was planted with hackberry, Russian olive, live oak, and black locust trees. The units were divided into three soil treatments:Engineered soil with organic matter
Soil mixed with zero-valent iron (ZVI)
Soil mixed with ZVI and Daramend® (a proprietary electron donor)
Phase II (2017):
18 close-bottomed straw TreeWell units were added in another part of the site. Depths ranged from 44–115 ft.One set of shallow units was filled with soil and ZVI.
The remaining shallow and all deep units used engineered soil with organic matter.
Phase III (2019–2020):
51 straw TreeWell units were installed and planted in March 2020 with 35 willow and 25 black locust trees.
A monitoring network of piezometers and camera tubes was installed to track the system’s effects on contaminant biodegradation and groundwater flow.
Results:
From 2016 to 2021, the TreeWell® units supported healthy tree growth with no visible signs of stress or VOC-related phytotoxicity. Trees continued to thrive, with steady increases in height and canopy size—even in areas with high VOC concentrations.
Groundwater monitoring showed clear signs of in-situ VOC biodegradation. VOC levels consistently decreased as water moved upward through the TreeWell units. While backfill with ZVI or Daramend® slightly improved performance, even units with only loam soil showed effective VOC breakdown.
Importantly, the straw TreeWell units successfully reached groundwater at depths up to 115 feet, enabling deep contaminant treatment as designed.
Coastal Florida
1,4-Dioxane in Fractured Bedrock
Managing Phytotoxic Concentrations
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Underlying Clean Groundwater Zone
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Managing Phytotoxic Concentrations --- Underlying Clean Groundwater Zone ---
Objective:
Evaluate the efficacy of a TreeWell® system as a sustainable and cost-effective replacement for a costly and ineffective pump-and-treat system.
Site Conditions:
The source area remained active, and a plume of 1,4-dioxane was migrating offsite through a fractured bedrock aquifer. Shallow aquifer media (0–2 meters) consisted of clean, sandy soils, while the deeper zone (2–4.5 meters) contained contaminated groundwater within fractured bedrock.
Implementation:
To facilitate the installation of a targeted remediation system, a mature vegetative canopy was removed. A 154-unit TreeWell® system was subsequently installed to address groundwater contamination within fractured bedrock. The design focused on intercepting and treating a plume of 1,4-dioxane migrating offsite.
The system incorporated some native wetland tree species selected for their tolerance to site conditions and phytoremediation potential: Slash Pine (Pinus elliottii), Willow (Salix spp.), Sycamore (Platanus occidentalis), Cypress (Taxodium distichum), and Laurel Oak (Quercus laurifolia).
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Results:
Eliminated high-cost pump-and-treat system, saving the client approximately $300,000 per year in operational costs.
Replaced ineffective UV/peroxide treatment with a TreeWell® System, delivering measurable contaminant reduction.
Achieved active remediation and hydraulic control of the impacted zone beneath clean groundwater, with low O&M requirements.
Secured regulatory closure in record time:
No Further Action granted within three years.
Site Rehabilitation Completion Order (SRCO) issued by the Florida Department of Environmental Protection.
Resolved a 20-year remediation challenge through nature-based, cost-effective intervention.
The Netherlands
1,4-Dioxane
Managing Phytotoxic Concentrations
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Multiple Water Bearing Zones
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Managing Phytotoxic Concentrations --- Multiple Water Bearing Zones ---
Project Objective and Background:
The groundwater contamination with 1,4-dioxane (dioxane) at the former dioxane production plant, is being contained via a phyto-containment approach with 239 poplars in TreeWell® units. The principle of this approach is that the trees take up sufficient (dioxane-contaminated) groundwater to realize a hydraulic containment, thereby limiting and ultimately preventing further off-site migration of dioxane.
The plume was upto 1,500 mg/L and in two separate zones (Layer A: 4-6 mbgs, Layer B: 7-10 mbgs).
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Implementation:
This TreeWell System installation intially took place in March 2013 and included 239 Straw TreeWell Units (Layer A - 185 units and Layer B - 54 units) and was designed to account for groundwater flow across the planting area. A Straw TreeWell Unit, which is a closed-bottom version of a standard TreeWell unit, can be employed to address contaminated groundwater at extreme depths or to address specific contaminated water-bearing zones.
Results: Phytoremediation System Performance Summary (5-Year Review)
Overall healthy trees despite phytotoxic concentrations
Tree health and growth remained consistently satisfactory over five years. Most trees were in fair or better condition, with the healthiest found in the southwestern area—possibly due to elevated polyol concentrations. Poorer-performing trees appeared randomly distributed.
High mass uptake (50–100 kg/year)
Trees consistently took up significant amounts of dioxane, estimated at 50–100 kg per year through groundwater extraction.
Evapotranspiration <0.10% of mass entering system in GW
Dioxane volatilization was minimal. In 2019, only 0.03% of the estimated uptake was released via evapotranspiration, consistent with 2018 and lower than in earlier years.
1,4-Dioxane biodegradation
Microbiological evidence and lab studies confirmed dioxane biodegradation in the tree root zones, explaining the low volatilization rates.
Phyto was a cost-effective, sustainable, nature-based alternative for providing hydraulic control
The system effectively contained contaminant migration while avoiding the high costs and inefficiencies of pump-and-treat approaches.
Eastern Illinois
VOCs, carbon tetrachloride R11 and R12
Cost Savings
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Cost Savings --- ?? ---
Background and Objective:
The site is a former refrigerant manufacturing and packaging facility with carbon tetrachloride impacts in glacial till. Historically, a groundwater pump and treat system was operated as an interim remedy to control the groundwater plume and mitigate potential off-site migration.
Implementation:
The initial pilot system consisted of 51 TreeWell units (alternating willow and poplar trees) installed on a 20 by 20 foot spacing to approximately 25 ft bgs and the objective was plume control.
Based on the success of the first installation, a second installation of 28 additional TreeWell units (aspen and Norway spruce) was installed two years later to provide hydraulic capture of the contaminant plume.
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Results: