Phytoremediaton and Coal Combustion Residuals
What is fly ash or CCR?
Coal combustion residuals (CCR) is the preferred terminology for the waste material generated when coal is burned. These residuals have a characteristic set of contaminants including minerals and trace elements such as mercury, arsenic, selenium, lead, molybdenum and boron, which can have destructive effects on ecosystems and public health. Companies have stored billions of tons of CCR material across the US in solid waste impoundments, landfills, and water receiving bodies, and some releases from such CCR impoundments in recent years have resulted in increased scrutiny of groundwater and environmental impacts.
Can phytoremediation effectively address CCRs?
A significant concern in CCR remediation is often the presence of the metal boron in groundwater. Interestingly enough, although it can have toxic effects for humans at elevated concentrations, it is an essential micronutrient for plants. Trees require boron, store it in their biomass, and can suffer from boron deficiency. However, most trees do not have a natural defense against elevated boron levels—they take up as much as is in solution in groundwater and it can ultimately be toxic to all plants at elevated concentrations.
About 10 years ago, while doing exploratory work at a former CCR impoundment, we noticed that when the boron plume migrating away from the site reached a forested area, the boron concentration in the shallow groundwater (<10 ft bgs) dropped to non-detectable levels. Data suggested that native vegetation growing at the surface was making an impact. We also noted that the boron concentration was in the sweet spot where it was not toxic to the trees, and mature forests were taking it up without showing toxic effects. However, the primary boron plume was significantly deeper and would require more remediation than the surface vegetation could take up and sequester on its own.
Has phytoremediation been used to remediate boron from CCR impoundments?
ANS recently installed a TreeWell system to address boron in groundwater originating from a former CCR impoundment. Depth to groundwater was 20 feet. Groundwater and plant tissue data will be collected over the next few growing seasons to monitor effectiveness.
This has been an exciting project for a few reasons. Phytoremediation has rarely, if ever, been used to mitigate CCR impacts to deep groundwater. Also, TreeWell systems (which enable access to deep groundwater) have become an accepted tool to remediate volatile organic compounds (VOCs), but metals have generally not been a target. Remediating metals was the initial focus of the most widely known early phytoremediation projects in the 1990s. In a way, this latest project is a return to the early roots of phytoremediation.
A brief history of using phytoremediation to remediate metals
The 1990s were the early days of phytoremediation and the focus of early, well-funded start-ups was addressing metal-based contamination. In theory, the idea was that hyperaccumulators or genetically modified plants would sequester metals in their tissue. Then, the plants could be harvested and burned, and the resulting ash could be recycled or disposed of. In practice, there were a few notable barriers to success in remediating metals. At that time, phytoremediation methods could only address contamination in shallow soils. A competing remediation method—dig and haul—could cost-effectively and quickly remove all the soil and contaminants in the upper two to three feet of soil. Also, the bioavailability of metals in soil is generally low. Plants alone could only sequester about 50% of the undissolved metals present in those shallow soils. To make the metals more readily available for plant uptake, adding a chelating agent would have been necessary, but chelating agents risked mobilizing what had previously been immobile contamination, potentially exacerbating the problem. These issues severely limited the potential of plant-based remediation of metals.
A handful of practitioners were doing work with hybrid poplar trees and contaminants, but our focus had been addressing agricultural products, petroleum waste, and then chlorinated solvents. In relation to metals and phytoremediation, funding dried up and metals were avoided, but some of us kept working in phyto and contributing to the maturation of the field and its appropriate application with a range of other contaminants [read more on phyto’s history here].
Addressing metals: a type of full circle for phytoremediation
In the case of CCRs and boron, we can now address the two barriers early phytoremediation enthusiasts encountered. Boron is unique in CCR sites in that it is already dissolved in groundwater—it is readily accessible if the groundwater can be reached. Now, with TreeWell units, we regularly target groundwater at depths of 100 ft bgs or more, so we have moved well beyond addressing contaminants in only the shallow subsurface [read more here]. The advancements we’ve made as well as the unique scenario presented in the case of CCRs makes engineered phytoremediation a very promising remediation alternative.
