Phytoremediation leads to successful closure of legacy CVOC brownfield

At a former industrial site in east-central Illinois, more than three decades (1960s–1990s) worth of historical use of chlorinated VOCs (TCE, TCA, and associated daughter products) has caused significant groundwater contamination.

Ramboll originally identified and defined six source areas that created two shallow groundwater plumes within a heterogenous geologic setting located up-gradient of an adjacent residential area.

Several interim treatment methodologies have been previously implemented at the site, including in situ injection of bioaugmented liquid activated carbon and construction of a zero-valent iron permeable reactive barrier. Although successful at mitigating leading-edge plume conditions, these focused measures were not sufficient to achieve final regulatory closure of the full site.

In order to achieve regulatory closure, our experts considered remedial design options and decided on the nature-based solution (NbS) of phytoremediation. Phytoremediation uses plants to clean up contaminated environments by storing the contaminants in the roots, stems, or leaves; convert them to less harmful chemicals within the plant; converting them to vapors, which are emitted into the air; or sorbing contaminants onto their roots where microbes can break them down.

We designed and implemented the strategic installation of endophyte-assisted poplar and willow trees to capture, treat and significantly diminish chlorinated VOC groundwater impacts and received approval from Illinois EPA to move forward with the large-scale implementation of phytoremediation for the two plumes.

We deployed the NbS at the site in two separate plantings. Each consisted of approximately 300 trees in a total of 11 individual treatment plots covering an area of one acre within the 27-acre site. The plantings consisted of tree poles with the majority located within asphalt or concrete paved areas. Plots were designed to focus primarily on the six delineated source areas and to additionally provide control and treatment of the two groundwater plumes resulting from these sources.

The survival rate for each planting was nearly 100% with significant growth noted despite many of the plots being installed within paved areas and/or in areas exhibiting elevated concentrations of chlorinated VOCs. Approximately two years after planting, the trees ranged in height from approximately 20 to 25 feet with diameters of 4 to 8 inches at breast height. The inoculation of trees with proprietary bacterial endophytes appeared to allow several plots to survive and maintain effectiveness in what could otherwise have been considered adverse conditions for tree survival and growth.

Within three years the site was determined to have achieved risk-based groundwater remedial objectives and a closure report was submitted to Illinois EPA. Our NbS resulted in measured reductions of up to 95% during the final confirmation sampling event. These significant and sustained reductions in measured concentration of chlorinated VOCs combined with the implementation of focused institutional controls resulted in the achievement of risk-based remedial objectives and a determination of no further action from Illinois EPA.

Since the closure approval, the site has been purchased by the local city government with the intention of future redevelopment for industrial/commercial purposes. In the interim, the trees are being maintained and serve to further remediate the groundwater as well as provide a visually aesthetic view of the currently unoccupied property from adjacent roadways and residential properties.