A Case for TreeWell® System Installation: Addressing Underperforming Pump-and-Treat Systems
TreeWell units consist of trees planted in large holes lined with plastic root barriers that force the consumption of groundwater instead of percolating precipitation. Contaminant mass reduction occurs primarily through microbial metabolism — both aerobic and anaerobic — within the backfill soil surrounding the root zone. TreeWell systems have been used successfully to optimize or replace underperforming pump-and-treat systems by establishing hydraulic control and/or actively remediating groundwater.
Background: The Reality of Long-Term Pump-and-Treat Performance
Pump-and-treat (P&T) systems emerged in the 1970s and 1980s alongside emergent environmental regulations and quickly became one of the most widely implemented groundwater remediation technologies. Still today, they account for an estimated 20–30% of remedy selections (according to EPA datasets), with many systems operating for decades.
Early expectations for P&T systems were optimistic—cleanup timelines of 5–10 years, relatively homogeneous subsurface conditions, and efficient contaminant mass removal. However, long-term performance data, including findings from Truex et al. (2017), have consistently shown a different reality. While P&T systems often remove a significant portion of contaminant mass in the initial operational years, removal rates decline dramatically over time. Residual contamination persists due to slow mass transfer, back-diffusion, and complex hydrogeologic conditions.
As a result, many P&T systems fail to achieve aquifer restoration within anticipated timeframes. Instead, they transition into long-term containment strategies, often operating for decades while incurring substantial operation and maintenance (O&M) costs.
Persistent Challenges with Conventional P&T Systems
Responsible parties managing long-running P&T systems frequently encounter several key issues:
Diminishing remedy effectiveness over time
Escalating O&M costs associated with extended system operation
Complex and costly transitions to revised Remedial Action Objectives (RAOs) or cleanup standards
Discovery of previously uncharacterized contaminants (e.g., PFAS or 1,4-dioxane), requiring expensive system retrofits/modifications
These challenges highlight a fundamental limitation: conventional P&T systems are mechanical, energy-intensive, and not inherently adaptive to evolving site conditions.
TreeWell Technology: A Passive, Adaptive Alternative
TreeWell systems offer a fundamentally different approach. Rather than relying on mechanical extraction, TreeWell technology leverages engineered phytoremediation to provide passive hydraulic extraction and in-situ treatment enhancement.
In this system, trees function as solar-driven pumps as well as sources for biostimulation of in-situ microbial communities. Through evapotranspiration, they induce hydraulic gradients that influence groundwater flow and plume migration. For many organic contaminants, the TreeWell soil column acts as a treatment zone, where microbial communities in the rhizosphere and engineered soil backfill facilitate contaminant degradation. Trees provide root exudates that include carbohydrates and enzymes that support microbe metabolism. For specific contaminant profiles, bioaugmentation (direct injection of microbes) is also an option.
Importantly, TreeWell systems are not limited to traditional phytoextraction mechanisms. Their primary function is hydraulic control coupled with biologically mediated treatment (both in-situ and in plant)—greatly expanding their applicability across a range of contaminants and site conditions.
Regulatory Alignment and Practical Implementation
One of the most compelling advantages of TreeWell systems is that they may be able to be deployed without altering existing RAOs or cleanup standards. Instead, they can provide an alternative mechanism for achieving those objectives.
In practice, TreeWell systems can be integrated into existing remedies through a phased approach:
Phase 1 (Integration): TreeWell units are installed alongside existing P&T systems to demonstrate hydraulic capture and treatment performance.
Phase 2 (Optimization): Mechanical pumping is reduced or cycled as TreeWell units assume increasing hydraulic load.
Phase 3 (Transition): Mechanical infrastructure is decommissioned, leaving TreeWell as the primary passive remedy.
This staged transition minimizes regulatory friction and allows for performance validation before full adoption.
Adaptive Treatment of Emerging Contaminants
A critical advantage of TreeWell systems is their inherent adaptability. Unlike conventional systems that may require engineered retrofits to address newly identified contaminants, TreeWell systems rely on dynamic soil microbiomes. These microbial communities respond naturally to the presence of contaminants, adjusting degradation pathways often without the need for external intervention.
This capability is particularly valuable in addressing contaminants such as 1,4-dioxane, which has often been discovered after remedy implementation and can be costly to treat using traditional methods.
Conclusion
Decades of operational data have demonstrated that while pump-and-treat systems can be effective for initial mass removal, they are often inefficient as long-term solutions. TreeWell systems provide a viable, regulator-accepted alternative that aligns with existing cleanup goals while reducing costs, improving adaptability, and enabling a transition to sustainable, passive remediation.
For sites facing declining P&T performance or escalating lifecycle costs, TreeWell technology represents a practical and forward-looking path.
