Skeptical About Phytoremediation?

Unrealistic expectations and poor project implementation have made some of phytoremediation's staunchest skeptics. A dose of skepticism and appreciation for phytoremediation’s processes have fueled our creative remedial work. Our resulting TreeWell system has expanded phytoremediation’s efficacy while level-headedly respecting limitations.

Still skeptical about phytoremediation?

 Phytoremediation, a commonly used, nature-based remediation alternative, is an approach to stabilizing, reducing, or removing contaminants in soil or groundwater by using plants and associated soil microbes. But even those who are familiar with the term are sometimes skeptical of its efficacy in challenging remedial situations. Could hydraulic control of contaminant plumes and contaminant mass removal really be possible with phytoremediation?

At Applied Natural Sciences, we believe that phytoremediation works, and we have spent decades refining our understanding and methods for addressing soil and groundwater contaminants using the power of nature. Through our work, we have encountered the limitations of traditional phytoremediation, and we have developed unique engineered phytoremediation strategies to enhance this nature-based solution’s applicability.

When phyto succeeds…and fails

Traditional phytoremediation involves the direct planting of trees, shrubs, or grasses into affected soil. Much research has been done on hyperaccumulators, phreatophytes, and other plants with special traits that can help address the specific remediation challenges of a given site. The roots of the selected plants attract and cultivate a rich microbial environment, and when appropriate plant species are selected and the environmental conditions are right, the plants and microbes can degrade a range of chemicals. Common contaminants of concern in active phytoremediation sites include VOCs, metals, and SVOCs, and a new wave of work is being done to pursue PFAS degradation or containment.

The primary limitations for traditional phytoremediation include the nature and concentration of the contaminant (potential for phytotoxicity), and the depth of the contaminants within the soil or groundwater, and in the saturated zone groundwater water flow rates. The reality that contamination may exist in specific groundwater horizons to the exclusion of other (potentially overlying) horizons also adds a layer of complexity. So, fundamentally, traditional phytoremediation is a nonstarter for remediation of sites with contamination present at phytotoxic concentrations, occurring at greater than 15 feet below the surface, or where the water flow rate is too fast to be captured by the plants.

Engineered phytoremediation and TreeWell® Systems

Failed projects stemming from unrealistic expectations have made some of phytoremediation's staunchest skeptics. A dose of skepticism and appreciation for phytoremediation’s processes have fueled our creative remedial work. Our resulting TreeWell system has expanded phytoremediation’s efficacy while level-headedly respecting limitations. TreeWell systems differ from traditional phytoremediation approaches in their capacity to overcome issues that restrict deep rooting such as unfavorable soil conditions or complex stratigraphy.  By “engineering” the growth of roots, trees can be made to reliably treat or utilize groundwater at depths of 50 feet or more.  This approach also makes it possible to direct the root growth to target a particular horizon in the aquifer of interest while excluding other water sources, and our systems have enabled trees to thrive in what would normally be considered phytotoxic conditions.

A key factor in expanding the applicability of phytoremediation with TreeWell systems comes with greater control and attention to detail in the rhizosphere. Precision in plant growth in the root zone makes targeted horizons in a range of hydrogeologic conditions and even high contaminant concentrations possible with the added benefit of no irrigation necessary and no surface or rainwater confounding.

How does a TreeWell System work?

The installation of a TreeWell unit begins by excavating a borehole to the horizon of interest. A specialized Root_Sleeve™ liner is placed in the hole to serve as a well casing that excludes non-targeted surface water and encourages vertically downward root growth towards the targeted depth. Then the hole is backfilled with engineered topsoil & selected amendments. Finally, trees specifically selected for the site’s conditions are planted.

The trees in TreeWell units act as pumps and bioreactors. The tree naturally draws contaminated groundwater upwards toward the roots while roots are encouraged to grow downwards toward contaminated water. Simultaneously, microbes degrade contaminants before even having contact with roots. Residual contaminants may then be treated within the tree and/or transpired into the atmosphere where they can be photo-oxidized. TreeWell technologies can also be coupled with various other remedial technologies including in-situ­ chemical reduction (ISCR, including ZVI, EZVI, Daramend®) to help address source area or elevated/recalcitrant contaminant conditions.

Could TreeWell-based phytoremediation work for that site?

Hydraulic Control/Containment

TreeWell systems are designed to capitalize on the high water-use capacity of selected trees and other vegetation to consume water and simultaneously process soluble contaminants. In soil or soil-like matrices, the trees’ extensive root systems function to extract contaminants, even those in micropores that are often unavailable to traditional pumping/extraction systems. Our systems differ from traditional phytoremediation approaches in their capacity to overcome issues that restrict deep rooting such as unfavorable soil conditions or complex stratigraphy, whereby our trees can reliably treat groundwater at depths of 50 feet or more.

Contaminant Mass Removal

TreeWell systems function to remove contaminants from the groundwater primarily in three ways:

·        Phytoextraction, or direct uptake of contaminants by the plant, is achieved by promoting the development of roots to the targeted depth, enabling the uptake of groundwater from the zone(s) of contamination by the plants.

·        Rhizodegradation, defined as degradation of contaminants in the area around the root system, is achieved, in part, by the root system’s production of favorable microbial growth conditions and increased microbial activity which enhances the degradation of contaminants.

·        Enhanced degradation in the saturated zone of the TreeWell column via anaerobic processes optimized by treatment media.

Phytoremediation’s Success

TreeWell units have proven to be effective in a range of challenging conditions. With our unique form of engineered phytoremediation, which we have been refining for 30 years, substantial data have been collected demonstrating contaminant degradation in our systems. Our systems have successfully replaced multiple mechanical remedial systems, and allowed sites to secure regulatory closure, resulting in substantial remedial life cycle cost savings for our clients.

Our TreeWell systems have demonstrated effectiveness in addressing a wide range of contaminants, providing our clients with a cost-effective, low maintenance, sustainable, nature-based remedial alternative for many challenging sites.