Published in March 2020 Focus on the Environment Newsletter
As the name implies, pump and treat (P&T) is a relatively simple remediation technology where groundwater containing dissolve contaminants is pumped from the aquifer and directed to some form of treatment. The advantages of P&T are two-fold, it provides hydraulic containment and control of the contaminated groundwater plume while simultaneously reducing the dissolved contaminant concentrations. These attributes made P&T an attractive and popular alternative back in the 1980’s and early 1990’s when groundwater remediation was in its infancy. As a result, P&T systems were routinely recommended, approved and implemented, via the technology screening process, as both the interim and final groundwater remedial measure.
Following startup, most P&T systems performed as predicted and proved successful at both containing and reducing source concentrations. Over time, however, the nice linear contaminant removal trends became asymptotic as the contaminant removal rate decreased. These asymptotic or “tailing” concentration trends became synonymous with P&T and marked where the system reached the point of diminishing returns. Adding insult to injury, the concentration where the tailing contaminant trend plateaued often exceeded the site cleanup level or remedial action objective (RAO). As a result, the original estimated times to achieve the RAOs came and went, and it became evident that most P&T systems would need to operate much longer than expected. To be fair, this was less a failure in the technology than it was in the failure to understand and account for the complex mass transfer processes that govern contaminant transport – a hindsight that provides little if any solace to those faced with operating their P&T systems in perpetuity.
Along with the extended time projections to attain RAOs came ever increasing operation and maintenance (O&M) requirements and costs. P&T systems installed as interim measures were often designed based on incomplete/inadequate site characterization and understanding, which often resulted in flawed P&T design and poor system performance. This subsequently made them ill-suited to serve their subsequent long-term role as the final remedy. In addition, system design was often focused solely on the target contaminant(s), with little thought given to the natural groundwater quality. P&T system design typically did not account for the operational challenges resulting from the physical, chemical or biological encrustation/plugging of the well screen, pump, discharge line, and treatment system from naturally occurring minerals and bacteria. As a result, P&T system performance typically decreases with time and use, resulting in increased energy use and lower pumping rates. Declining performance and increased operational costs are especially troublesome for P&T systems because they are typically required to operate continuously at designated pumping rates to maintain adequate plume capture.
It is important to realize that much of the stigma associated with P&T as an O&M nightmare is born out of the poor design and operation of P&T systems and not the technology itself. After all, the water supply industry has successfully utilized P&T technology for over 100 years to provide groundwater as a reliable source of drinking water for their customers. And although they face the same challenges with regard to mineral precipitation and biofouling, they have learned to successfully design, manage and operate their systems to achieve maximum long-term efficiency.
If you currently own or operate a P&T system and are faced with the ongoing expense and hassle of operating it over the long term, then it may be time to take a fresh look at the problem. The first step in this process is to evaluate whether the P&T system can be replaced with an alternative treatment technology. For example, in-situ oxidative and bioremediation technologies have been utilized successfully at many petroleum hydrocarbon and chlorinated solvent sites as cost-effective replacement technologies for original P&T systems.
If alternative technologies aren’t an option, there are still some steps you can take to potentially improve the overall efficiency of the P&T system. The process begins with a simple reassessment of the site conceptual model. The data collected since system startup can be reviewed to refine/update the site conceptual model and identify modifications (pumping rates, well locations) that will improve the P&T system performance, while still maintaining capture.
The P&T system should be inspected to identify and replace any faulty or inefficient equipment. In addition, any components that are not used and/or no longer necessary should be removed. Remediation wells that require frequent cleaning to maintain required pumping rates are candidates for replacement when the maintenance costs exceed the cost for a replacement well. Any new replacement wells should be installed in the best location to intercept/capture the plume and designed for the site-specific aquifer conditions and minimum pumping rate to maximize efficiency.
Finally, because well performance typically decreases with time and use, a simple, regularly scheduled data collection and analysis plan should be implemented to track system performance with time. The resultant data trends provide the operator with the ability to better forecast P&T system maintenance requirements, thereby minimizing the cost and inconvenience of unscheduled down-time and repairs and improving the cost effectiveness and efficiency of the maintenance efforts.
Unfortunately, P&T as a remediation strategy is not going away anytime soon. If you are currently operating a P&T system, however, it may be time to take a fresh look and see what if anything can be done to optimize the system and reduce long-term O&M requirements and costs. Please feel free to contact me if you feel a fresh look may be warranted.
Doug is a licensed professional geologist in Indiana and North Carolina. As a consulting hydrogeologist, Doug specializes in aquifer characterization and yield determinations, well and wellfield performance evaluations, and the design and testing of both vertical well and horizontal collector well systems. Additional areas of expertise include environmental assessment and remedial system evaluation and design. He has worked throughout the United States on a wide variety of groundwater supply and environmental contamination related projects. Doug’s wide-ranging expertise and extensive experience in the groundwater supply industry add another dimension to Cox-Colvin’s technical staff and provide additional opportunity to support our clients in meeting their needs and reaching their business goals.